What makes a galaxy dead or alive is simple: internal stores of gas.

The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to the nearby, more massive M81. As one of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. The young stars within it have formed from gas reservoirs still present within this galaxy, indicating an “alive” galaxy.

Inside living galaxies, gas is required to enable the formation of new stars.

The enormous bar at the core of galaxy NGC 1300 spans many tens of thousands of light-years, nearly the full width of the galaxy. While many spiral galaxies contain large, prominent bars such as this one, our Milky Way’s central bar is far more modest, extending only about a third of the way out to the Sun’s position. The pink regions found along the spiral arms are evidence of new star formation, triggered by the interaction of internal gas and the density waves of the internal structure.

When massive gas clouds gravitationally collapse, new stars inevitably arise.

Star-forming regions, like this one in the Carina Nebula, can form a huge variety of stellar masses if they can collapse quickly enough. Inside the ‘caterpillar’ is a proto-star, but it is in the final stages of formation, as external radiation evaporates the gas away more quickly than the newly-forming star can accrue it. Within the first ~2 million years of this star’s birth, protoplanets should already begin arising within the accompanying protoplanetary disk.

As matter fragments, the various clumps grow rapidly, forming new stars and massive star clusters.

The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1-10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars.

Many events trigger galactic star-formation, including:

The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way.

• internal dynamics,

The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk

• external gravitational tugs,

The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth.

• or even galactic mergers.

Zw II 96 in the constellation of Delphinus, the Dolphin, is an example of a galaxy merger located some 500 million light-years away. Star formation is triggered by these classes of events, and can use up large amounts of gas within each of the progenitor galaxies, rather than a steady stream of low-level star formation found in isolated galaxies. Note the streams of stars between the interacting galaxies, which can either become part of a population of stars in the post-merger galaxy’s stellar halo, or could get expelled from the post-merger galaxy entirely, roaming the intergalactic medium.

Isolated galaxies are more likely to form stars quiescently: at slow, steady rates over long timescales.

The isolated galaxy MCG+01-02-015, all by its lonesome for over 100,000,000 light years in all directions, is presently thought to be the loneliest galaxy in the Universe. The features seen in this galaxy are consistent with it being a massive spiral that formed from a long series of minor mergers, but that has never experienced a major merger, and where star-forming activity has been relatively quiet for the past several billion years. A galaxy such as this may continue forming new stars in an ongoing fashion for much longer than the present age of the Universe.

However, once a galaxy becomes gas-depleted or even gas-free, star-formation ceases within it.

This nearby galaxy, NGC 1277, although it may appear similar to other typical galaxies found in the Universe, is remarkable for being composed primarily of older stars. Both its intrinsic stellar population and its globular clusters are all very red in color, indicating that it hasn’t formed new stars in ~10 billion years. Some of the earliest living planets and worlds may have arisen in “red and dead” galaxies such as these.

Without gaseous material, there’s no “fuel” for future generations of stars.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Even after galaxy clusters form, surrounding galaxies and galaxy groups, including initially Milky Way-like galaxies, get drawn in. Over time, they will lose their gas and eventually cease forming new stars.

This is often the fate of even initially Milky Way-like galaxies as they fall into galaxy clusters.

Galaxy clusters, like Abell S740, are the largest bound structures in the Universe. When spirals merge, for example, a large number of new stars form, but either post-merger or by speeding through the intra-cluster medium, gas can be stripped away, leading to the end of star formation in that galaxy and, eventually, a red-and-dead final structure.

Inside rich clusters, galactic mergers are common, with major mergers often leading to galaxy-wide starbursts.

The galaxy NGC 7727 shows extended spiral arms: likely the aftermath of a recent major merger between two comparably massive galaxies. The presence of two supermassive black holes inside this galaxy, as well as the extended streams of gas and stars, show one possible outcome of a major merger of two similar-mass, initially gas-rich galaxies.

These violent episodes of star-formation generate incredible winds, expelling large reservoirs of gas.

This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds.

Furthermore, galaxy clusters contain a gas-rich intracluster medium.

Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, eliminating its ability to form new stars. Phenomena such as this allow us to conclude that the galaxy, the cluster, and the gas within it are all made of matter, not antimatter.

Rapidly speeding through it strips a galaxy’s gas away.

When galaxies like the spiral galaxy at right, D100, speed through a rich environment (like the Coma Cluster, which D100 is a member of), the friction with the environment can cause gas stripping, leading to the formation of stars and increasing the dark matter-to-normal matter ratio of the host galaxy. The central feature is evidence of ram pressure stripping as the galaxy speeds through the intracluster medium, quickly losing its capacity to form new stars. The galaxy next to it is simply an older version, having already become ‘red-and-dead’ many billions of years ago by a likely similar process.

The end state of a gas-free galaxy is a giant elliptical where only old stars survive.

A map of neutral hydrogen (in red) overlaid on the galaxy D100 in the Coma Cluster shows how much gas is being quickly stripped from this galaxy as it travels through the cluster. Galaxies found in environments like this one become ‘red-and-dead’ far more quickly than galaxies in less dense regions of space.

With many mergers and constant gas stripping, galaxy clusters are graveyards for Milky Way-like galaxies.

The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The infrared data comes from the Spitzer Space telescope, while ground-based data comes from the Sloan Digital Sky Survey. The Coma Cluster is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. Gas-free, red-and-dead elliptical galaxies are very common, especially toward the cluster center, in large galaxy clusters such as this one.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

This article Galaxy clusters are graveyards for Milky Way-like galaxies is featured on Big Think.

Here on Earth, one of the greatest geological wonders of all is the Grand Canyon. Carved by the Colorado River over millions of years, which connected multiple older segments of the canyon together, the full extent of this giant, steep-sided valley is now remarkable and impressive. Spanning 446 kilometers (277 miles) in length, the canyon is up to 29 kilometers (18 miles) wide and up to 1.857 kilometers (1.153 miles) deep. The advance, retreat, and melting of glaciers, combined with the release of enormous amounts of water, have exposed a wide variety of rocks formed throughout Earth’s geological history, including formations as many as 2 billion years old.

And yet, the full extent of Earth’s Grand Canyon pales in comparison to the grandest canyon in all the Solar System: Valles Marineris on Mars. Mars, a much smaller planet than Earth with a very different geological past, might not seem like the ideal candidate for such a gigantic feature, and yet not only is it present, it was likely created in a very different fashion than the Grand Canyon was on Earth. But how, precisely, did it form? That’s what Rosa Been wants to know, asking:

“You know how Mars has a huge scar in it? Like 6 miles deep. I was curious what caused it. I’ve heard it could be an asteroid or solar flare kinda thing etc.”

It could have been a lot of things, and in reality, it was probably formed by many processes combined. But the greatest lesson of all for its formation may not come from Earth’s Grand Canyon at all, but a very different feature. Here’s the most likely story we’ve been able to piece together.

This colorized topographic map of Mars, made with Mars Orbital Laser Altimetry (MOLA) data, showcases a difference of more than 20 kilometers between the deepest depths, such as in the deep outflow channels of Valles Marineris that lead into the northern hemisphere’s deep (oceanic) basin, and the highest heights of the mountaintops of the high mountains in the Tharsis region of Mars.

What you see, above, is a topographic map of Mars. Although there are many notable features, there are a few prominent ones that are relevant when it comes to discussing the grandest canyon of them all, Valles Marineris, which appears just south of the Martian equator and just slightly to the east of what’s known as the Tharsis bulge on Mars: the high-elevation region (in red) that’s home to many of the largest volcanic mountains in all the Solar System, including Olympus Mons, which is the lone white-capped mountain at the western edge of the Tharsis bulge. On either side of the Tharsis region are “dark blue” regions, which correspond to low-elevation regions that likely once were home to ancient Martian oceans.

While today, Mars is a cold, dry, desert world, where liquid water cannot persist on its surface due to the overwhelmingly low atmospheric pressure – just 1/140th of Earth’s at the Martian surface – in the past, Mars was very different. Although we still have enormous uncertainty about many aspects of our Solar System’s past, we now know enough to reconstruct a very interesting, detail-rich history of Mars from many lines of evidence. While a lot of what follows is a speculative story, this is presently the best consensus picture of how Mars came to be the way it is today, and in particular, how the “grandest canyon” in all the Solar System came to be on Mars.

Although we now believe we understand how the Sun and our Solar System formed, this early view of our past, protoplanetary stage is an illustration only. While many protoplanets existed in the early stages of our system’s formation long ago, today, only eight planets survive. Most of them possess moons, and there are also small rocky, metallic, and icy bodies distributed across various belts and clouds in the Solar System as well.

In the beginning, the Solar System took shape from a pre-solar nebula and a protoplanetary disk surrounding it. At the center of this nebula was a proto-star that would grow into our Sun, while the disk developed instabilities within it. Over time, nuclear fusion ignited in the proto-star’s core, transforming it into a full-fledged star: our Sun. The instabilities within the protoplanetary disk, likely over the span of just a few short million years, began to form what would become the cores of many protoplanets and, eventually, full-fledged planets. We are relatively certain that there were more than eight such worlds, initially, and that the extras were either ejected, hurled into the Sun, or collided with other bodies in events that created larger-mass planets and that gave rise to moons.

While the most well-known of these early planetary collisions was between the young Earth and a now-deceased world that we’ve named Theia – resulting in the kick-up of debris that would coalesce into our Moon – there were other collisions that occurred as well. Out in the distant reaches of the Kuiper belt, a world collided with Pluto, giving rise to Charon and the other four, smaller moons that orbit Pluto. It’s possible that collisions occurred on Venus and Uranus, perhaps giving them their unusual axial tilts. And on Mars, a giant collision also took place, leading to the formation of not merely its two surviving moons, Phobos and Deimos, but a third, larger, innermost moon as well: one that was transient, and destroyed not long after it formed.

Rather than only the two Martian moons we see today, Phobos and Deimos, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. The idea is that Mars’s once-innermost moon was destroyed and fell back onto Mars long ago. This hypothetical transient moon of Mars, proposed in a 2016 paper, is now the leading idea in the formation of Mars’s moons, and helps explain the enormous differences in topography between Mars’s northern and southern hemispheres.

That Moon, most likely, was first destroyed by the gravitational tidal forces exerted by the parent planet, Mars, where it was stretched out into a ring of debris. Unlike the modern Martian atmosphere, early Mars possessed a thicker planetary atmosphere more similar to that of a young Earth, likely augmented by the volatile gases emitted by the same continuous volcanic activity that persists even today on Mars. That ring-like debris from its larger moon then began interacting with the Martian atmosphere, where the drag forces eventually brought the entire ring – a full large moon’s worth – back down onto Mars, where it struck the surface and altered the landscape of the planet. The northern hemisphere of Mars, perhaps as a result of these processes, is at a significantly lower elevation than the southern hemisphere.

Just as early Earth had copious amounts of water on its surface, it was very likely that so did early Mars. While the combination of Earth’s size and mass, its active core, its volcanic activity, and the lubricating effects of surface water all contribute to Earth’s plate tectonics, tectonics on Mars operated very differently, even in these early stages. As a much smaller planet, Mars cooled far more quickly than the Earth did, while volcanic activity created the Tharsis bulge. This combination of impacts, external heating, internal volcanic activity, and planetary cooling likely created conditions that led to an important event: the formation of a strike-slip fault zone on Mars.

The Dead Sea Rift, also known as the Dead Sea Transform, is a rift that occurs between the African plate to the west and the Arabian plate to the east. The yellow arrows indicate current motion of the Arabian plate relative to the African plate at a rate expressed in millimeters-per-year.

Here on Earth, we have familiarity with a very analogous tectonic feature: the Dead Sea Rift, which is actually a fault system that runs for around 1000 kilometers between two adjacent plates: the African Plate to the west and the Arabian plate to the east. Over time, the two plates have displaced relative to one another by more than 100 kilometers, and have experienced this relative motion only in geologically recent times: over no more than the past 50 million years or so. Rift zones occur all over an active planet with plate tectonics routinely, and represent a “pulling apart” of two portions of the lithosphere. Lake Baikal, the deepest, largest, oldest lake on Earth, formed because of this type of rifting phenomenon.

For a very long time – all of the 20th century and more than the first decade of the 21st – the explanations for the formation of Valles Marineris were inadequate, relying on ideas like:

• erosion from water,
• permafrost melting in glacier-rich regions,
• the sudden withdrawal of subsurface magma,
• or tensional fracturing which caused solid rock to split.

But in 2012, a scientific study came along that changed everything: using surveyed data of Mars from space, UCLA scientist An Yin (who died in 2023 at the relatively young age of 64) determined that features on one side of this valley, including features left by ancient crater impacts that still survive, were displaced from matching features on the valley’s opposite side of between 150-160 kilometers.

An ancient impact basin on Mars, outlined with a black circle, has been shifted/offset due to the relative motion of the top and bottom sides of Valles Marineris. This 150-160 km offset, first identified in 2012 by An Yin, is some of the strongest evidence about the origin of Valles Marineris and the early geologic history of Mars.

That rifting behavior along a fault line was the first step in the creation of Valles Marineris, but that’s only the beginning of the story. As Mars still possessed liquid water well into its history – for perhaps the first 1.5 billion years of our neighboring planet’s existence – erosion and the collapsing of rift walls are generally accepted to have then enlarged and expanded the long, narrow valley that initially rifted apart. (There is a possible analogy to the East African Rift here on Earth.)

Beyond the forces of erosion, there were also landslides that no doubt occurred, perhaps being connected to the watery past conditions on Mars but also subsequently, including long after the planet lost the last of its liquid surface water. This provides a possible explanation as to why, when we examine the walls on either side of Valles Marineris, we see that they appear to show evidence of enormous numbers of deposits on the floor of this canyon.

Landslides could also have contributed to further expanding the width of the canyon (at the expense of its depth, suggesting that it may once have been even deeper than it is today), which could have been further exacerbated by both marsquakes and by subsequent, late-time impact events. The formation of the relatively recent crater Oudemans, for example, may have triggered one of the landslides that occurred within Valles Marineris long after its initial formation.

The impact crater Oudemans, shown here with its location overlapping with the highest-elevation basin of Valles Marineris in the inset image, may have been one of a few impact craters primarily responsible for late-time landslides that helped widen and fill in the deepest depths of the Solar System’s largest canyon.

However, there’s a tremendous difference between tectonics on Mars and tectonics on Earth, which is extremely important for understanding why Valles Marineris has persisted for so long and grown so large, whereas practically all of the rift valleys on Earth, as well as even our largest canyons, are under 100 million years old. Here on Earth, our planet’s lithosphere – the crust and the upper mantle – is fragmented into many large plates, which “float” atop the asthenosphere (the lower mantle). As those plates collide, spread apart, slide across one another, and otherwise generally move, features like mountains, volcanoes, and rifts form on our world.

On Mars, however, there’s evidence that even though it possesses tectonics, the notion of many separate, mobile, large plates is incongruent with the overall geological history of the planet. The three biggest geological features on Mars are as follows.

1. Its northern lowlands, including indicators that the crust of Mars’s northern hemisphere is very thin and was resurfaced (by lava) relatively recently compared to the older southern hemisphere.
2. Its southern highlands, whose surface is older than the northern lowlands and contains the oldest, most ancient impact craters found on Mars.
3. And the Tharsis bulge: the equatorial highlands that contain Olympus Mons and several other large mountains: among the largest in the Solar System.

When we put all of these pieces of information together, we can tell a story – perhaps a likely, but unproven, past history of Mars – that explains the formation of Valles Marineris.

A large impact from an asteroid billions of years ago may have created the moons of Mars, including an inner, larger one that no longer exists today. Although this is not a likely explanation, on its own, for the formation of the Tharsis region and Valles Marineris, it may be primarily responsible for the northern/southern hemisphere dichotomy that plays a major evolutionary role in the red planet’s geological history.

First, the giant impact that created Mars’s moons occurred, and then the largest, innermost moon fell back onto Mars. It’s plausible, but not necessarily certain, that this created the dichotomy between the northern and southern hemisphere. Then, a combination of volcanic activity and the “floating” of the thickened crust atop the mantle – known as isostatic uplift – occurred over what would become the Tharsis region of Mars.

However, because Mars’s lithosphere was not mobile, meaning that the volcanic “hot spots” weren’t moving relative to the surface features on the planet, depressions began to form and the crust began to spread out in that region, widening the Tharsis bulge and the plateau-like region it inhabited. This may have caused the Tharsis bulge to begin shifting relative to the volcanic hot spots beneath it, and may have gone as far as to shift the entire Martian crust and/or lithosphere relative to those volcanic hot spots.

Because the stability of the crust atop the mantle is dependent on being in what’s known as isostatic equilibrium, like a boat floating atop the ocean, shifting the position of the crust relative to the mantle beneath it results in instability. And, with so much mass to carry, the weak regions will begin to fracture.

This color-coded map of Mars highlights the enormous scar, or canyon, known as Valles Marineris. With a maximum depth of approximately 7 kilometers (23,000 feet), there is no single feature in the Solar System that has a combined length, width, and depth more extreme than Valles Marineris. Initially, a fracture in the crust, likely induced from stresses arising from the uplifted Tharsis region, gave rise to the first, most important steps in forming this rift valley.

One of these fractures – perhaps the largest fracture of any location in the Solar System, even including the cooling-induced fractures that formed on Mercury – extended for some 4000 kilometers (2500 miles) across the surface of Mars, creating the initial rift that would lead to the modern-day Valles Marineris. The new locations of the volcanic hotspots would lead to the major Martian volcanoes that came to exist today, including Olympus Mons, Alba Mons, and the three Tharsis Montes, all of which are still actively growing even at present.

Additional volcanic and tectonic activity led to the further rifting of Valles Marineris, including the 150-160 kilometer “shift” identified between the northern and southern portions by An Yin. For as long as water flowed on Mars, Valles Marineris probably provided the main channel by which ices, snows, and other forms of water would flow eastward into a one-time ocean, creating a network of outflow channels that are still visible in modern-day altimetry data.

Finally, even after Mars became a dry, barren planet – after its core dynamo died, after its atmosphere was stripped away, and after liquid water became impossible on its surface – further landslides, possibly driven by quakes, tectonic activity, and/or subsequent impacts, created the landslides that further widened the grandest canyon of them all, at the expense of filling in its deepest depths, rendering them shallower than they were previously.

A breathtaking view of Mars from space, showcasing the majestic landscapes reminiscent of the Mars Grand Canyon.

You have to realize just how impressively large Valles Marineris is to fully appreciate it. From end-to-end, it’s about 4000 kilometers (2500 miles) across, giving it a similar extent to the continental United States or the continent of Australia. At its widest, it’s approximately 200 kilometers (120 miles) from the uppermost high-walled rim to the lowermost rim on the opposite side of the valley: more than six times the maximum width of the Grand Canyon on Earth. And, despite the fact that landslides have filled in the deepest depths of this valley over billions of years, it’s still some 7 kilometers (23,000 feet) deep: deeper than all but the absolute deepest ocean trenches on Earth.

It’s even more impressive when you consider that Mars itself is a much smaller planet than Earth; with a Martian circumference of “only” 21,000 kilometers (about half that of Earth), Valles Marineris spans about 20% of the planet’s full physical extent. With a lower mass than Earth, Mars has a much smaller force of gravity at its surface than our planet does, allowing mountains to reach higher and valleys to form deeper than they can stably form on Earth. It isn’t a surprise that Mars would have larger mountains and deeper valleys than Earth, but the full extent of the grandest canyon of them all, Valles Marineris, never fails to impress!

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: Why is there a grand canyon on Mars? is featured on Big Think.

The Milky Way galaxy may be just one among trillions present within the observable Universe, but it’s uniquely special for personal reasons to us: it’s our cosmic home. It’s the fertile soil from which our Sun and Solar System, including the bodies that would eventually become planet Earth, sprung some 4.6 billion years ago. All told, it’s composed of a few hundred billion stars, about a trillion solar masses worth of dark matter, a supermassive central black hole of about 4 million solar masses, and a plethora of gas and dust. And that’s no outlier; we’re actually somewhat typical of modern galaxies, with perhaps a hundred billion others similar to our own. We’re neither among the biggest nor the smallest of galaxies, nor are we in an ultra-massive cluster or found in isolation, but rather a modest galaxy group, where we’re the second-largest member.

What does make us special, though, is how evolved our galactic home has become. Some galaxies grow up quickly, exhausting their gaseous fuel and becoming “red and dead” when they lose the ability to form new stars. Some galaxies undergo major mergers, often transforming from gas-rich spirals into gas-free ellipticals in the aftermath of those collisions. Still others experience enormous tidal disruptions, leading to sweeping, distended spiral arms. Not the Milky Way, though. We grew up in exactly a typical fashion. Here’s how we got here.

The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth.

At the present time, galaxies like the Milky Way are incredibly common. Here are some properties that Milky Way-like galaxies typically display:

• they contain hundreds of billions of stars,
• concentrated into a disk-like, or pancake-like shape,
• surrounded by globular clusters in a halo-like distribution,
• containing spiral arms that extend radially outward for tens of thousands of light-years in either a flocculent or multi-arm pattern,
• often with a central bar-like feature emanating from a region containing a bulge of stars,
• with a tremendous amount of gas and dust concentrated in the galactic plane,
• and young star-forming regions found where the gas and dust is densest: along the density waves within the spiral arms.

Such a behemoth, with so much total mass, exerts a tremendous gravitational pull acting on everything else nearby. You can recognize a galaxy like this from afar, from the copious amounts of starlight streaming out of it. But it couldn’t have been this way forever. What we know is our Universe began with the Big Bang some 13.8 billion years ago, and galaxies couldn’t have always been this way. In fact, if we look back far enough, we can see the differences between modern and ancient Milky Way-like galaxies start to appear.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this effect goes to the extreme. As far back as we’ve ever seen, galaxies obey these rules.

Compared to the Milky Way and other Milky Way-like galaxies that we see today, galaxies from long ago and far away were:

• younger, as evidenced by an increase in young stars,
• bluer, since the bluest stars die the fastest,
• smaller, because galaxies merge together and attract more matter over time,
• and less spiral-like, because we only see the brightest parts of the most active, distant, star-forming galaxies.

Our galaxy today, in other words, is the result of 13.8 billion years of cosmic evolution, where large numbers of small proto-galaxies merged together, forming one larger-than-average galaxy, while continuously attracting additional matter into itself. We are what remains after countless other galaxies have been swallowed by our own.

The story of how we built our Milky Way is like building a giant structure out of LEGOs. Only, instead of the LEGOs remaining the same over time, they’re actively changing form as we assemble our structure. It would be like starting with all the pieces to put together 100 different X-Wing LEGO fighters, and winding up with a Star Destroyer when we were done. One of the ways that we can learn this is by surveying the globular clusters within our galactic halo, and finding that many of them have properties that correlate with them infalling into and joining the Milky Way at various critical points in cosmic history: evidence of hierarchical mergers in our cosmic past.

The merger history of the Milky Way reconstructed, along with the stellar mass added to our galaxy and the number of globular clusters originating from each merger. This reconstruction, however, has substantial uncertainties to it, as shown by the curves associated with each merger event. For example, the latest study, based on subgiant stars instead of globular clusters (as shown here), places the Gaia-Enceladus merger as potentially even earlier than the Kraken merger.

Galaxies, you see, don’t just grow by attracting other galaxies and merging together to form larger ones. Galaxies also experience evolution on their own, meaning that they:

• rotate,
• form stars,
• funnel matter in toward the center,
• generate density waves along their spiral arms,
• attract additional matter from outside the galaxy along cosmic filaments,
• and change shape and orientation based on the other galaxies and matter that falls into them.

While the earliest proto-galaxies that eventually grew into the Milky Way may have formed somewhere around 200-250 million years after the Big Bang, cosmic evolution continued all throughout that time.

The first stage that led to the earliest proto-galaxies was the formation of the earliest stars and star clusters, which took around 100 million years, and formed out of the pristine material (hydrogen and helium) left over from the Big Bang. These star clusters evolved quickly, resulting in a very rapid end-of-life for their stars. When those stars died, they polluted the interstellar medium with heavy elements that then gave rise to the second generation of stars. By the time that 200-to-300 million years had gone by, numerous star clusters had merged together with one another and drawn in material from their intergalactic surroundings, giving rise to the very first full-fledged galaxies.

Galaxies that are currently undergoing gravitational interactions or mergers are almost always also forming new, bright, blue stars. Simple collapse is the way to form stars at first, but most of the star formation we see today results from a more violent process. The irregular or perturbed shapes of such galaxies are a key signature that this is what’s occurring, and the evidence for these mergers can go back as far as our telescopes can see at present.

The cosmic web then begins to take shape. As more time goes by, gravitation, limited by the speed of light, can extend its reach greater and greater distances, causing larger-scale clumps of matter to fall in toward one another. When a clump that’s smaller than the early galaxy falls in, it gets tidally torn apart and funneled into the galaxy’s interior gently and slowly, where its constituent material simply gets absorbed over time.

These events, known as minor mergers, are common, and any galaxy that falls into a larger one with up to about a third of the larger one’s mass falls into this category. Any internal structures in the larger galaxy, such as spiral arms, star-forming regions, a bar, or a bulge, should all remain intact. Meanwhile, the additional gas and dust added by the “swallowed” galaxy provides new fuel for new generations of stars. Star formation usually intensifies during merger events, even minor ones. For the first 2 or 3 billion years of cosmic history, this process was exceedingly common.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2-10, located 30 million light years away. This galaxy will likely evolve, post-merger, into another disk galaxy if copious amounts of gas remains within it, or into an elliptical if all or nearly all of the gas is expelled by the current starburst.

As time goes on and the Universe expands, however, mergers become, on average, less common but more major. Galaxies clump and cluster together into groups of many different sizes, but occasionally can form large galaxy clusters with hundreds or even thousands of times the mass of our own Local Group. These dense galaxy clusters are some of the most spectacular sights in the Universe, but they’re also relatively rare: the majority of mass and the majority of galaxies are found in small groups like our own, not in the massive clusters that we see so prevalently in our Universe. By the time the first 4 or 5 billion years of cosmic history had gone by, it became clear we’d never become part of a massive cluster; nearby galactic groups such as the Leo Group, the M81 Group, and the Virgo Cluster were all beyond our reach.

It’s important, however, that if we want to keep our galaxy Milky Way-like, with lots of gas, a set of spiral arms, and ongoing star-formation inside, that these mergers remain small. If we experience a major merger, where two similarly-sized galaxies collide, they can induce an enormous burst of star formation, which can:

• use up a large amount of the available star-forming gas,
• expel the remainder of the gas from the post-merger galaxy,
• and “mix up” the stellar matter in the galaxy as well.

When major mergers occur, there’s a chance that all three of the aforementioned events will occur as a result. It may not be the result of most major mergers, but in a rich environment where major mergers are common, giant ellipticals are also common; where major mergers are rare, disk galaxies, such as spirals, are far more ubiquitous.

The ultramassive, merging dynamical galaxy cluster Abell 370, with gravitational mass (mostly dark matter) inferred in blue. Many elliptical galaxies are found inside massive clusters like this, as the result of major mergers that occurred billions of years ago. There are still a large number of spirals, too, as the total mass of this galaxy cluster may exceed a thousand times that of the Local Group.

The Milky Way that we inhabit, however, has clearly remained a disk galaxy with multiple spiral arms and ongoing star-formation still occurring inside: we have not become a giant elliptical, which tells us that there have not been very many (if any) major mergers that have occurred in our galactic past. If such an event had occurred, we might have expected that we’d see a large population of stars that all appeared to form throughout the Milky Way at some specific time long ago. There is no such population in our own galaxy, but there is one in our neighbor Andromeda: an indication that perhaps, around 2 or 3 billion years ago, what was once the Local Group’s third largest galaxy, behind only Andromeda and ourselves, merged with Andromeda to produce the modern-day “big sister” galaxy right next door that we can see with the naked eye in our own night skies.

It takes a lot of mass gathered together in two adjacent places, built up over time, to create a major merger. So long as a galaxy is massive enough (as in Milky-Way sized or comparable) and gas-rich, there will be material available to support and enable the formation of new stars. So long as galaxies have angular momentum and a preferred rotation axis (which they inevitably do in the absence of a major merger), and so long as they have experienced enough time to settle down into a stable shape (which, 13.8 billion years into our cosmic history, they all would have had), we expect them to have a spiral shape and structure internally.

The isolated galaxy MCG+01-02-015, all by its lonesome for over 100,000,000 light years in all directions, is presently thought to be the loneliest galaxy in the Universe. The features seen in this galaxy are consistent with it being a massive spiral that formed from a long series of minor mergers, but that has never experienced a major merger, and where star-forming activity has been relatively quiet for the past several billion years. A galaxy such as this may continue forming new stars in an ongoing fashion for much longer than the present age of the Universe.

Our Milky Way likely grew from a series of proto-galaxies that settled down into a spiral shape, then gradually gobbled up many of the smaller galaxies present in its vicinity: galaxies that were once a part of the Local Group, but whose existence has been erased by the cosmic violence of galactic cannibalism. We didn’t even gather the majority of those small galaxies or of the matter in our Local Group; that honor goes to our neighbor, Andromeda. Nor are we done with the merger process: there are satellite galaxies merging with us today, and a few galaxies on our outskirts, like the two Magellanic Clouds, that will likely be devoured in the next few hundred million years (perhaps surviving for up to a couple of billion years) or so.

In biology, the creatures that survive obey the law of natural selection: the survival of the fittest, and the ones that are the most adaptable to changing conditions. For galaxies, however, the story is very different. The cosmic story that brought the Milky Way to be is one of survival of the largest and most massive: the objects with the greatest inertia, and the greatest resistance to being significantly disrupted by an encounter with another galaxy. When it comes to dominating the galaxy population over long periods of time, mass is the overwhelming factor in determining what, and who, survives.

Initially, clumps of matter form in a relatively random, asymmetric shape: something that I often think of as “potato-like,” but that astrophysicists more often approximate as a triaxial ellipsoid: a distribution of matter where one of the three 3D axes is shorter than the other two. As time goes on, the shortest axis collapses first, causing the normal matter within it to “pancake,” or go splat. This winds up forming a flat, disk-like shape, and that disk has angular momentum, and so rotates. As it rotates, the structure within it, such as spiral arms, begins to wind up. As a consequence, a disk galaxy’s spiral arms became more pronounced and developed over time, accumulating more and more turns within them.

Spurs emerge from those arms, and gravitational interactions both internally and from external influences lead to the formation of stars along the tail ends of a galaxy. As additional gas flows into the galaxy’s outskirts, it eventually winds up getting funneled toward the center. As galaxies continue to evolve, they also develop many recognizable features:

• a central bulge forms where the matter is densest,
• a central bar develops and grows over time,
• the dynamics of gas and stars causes the galaxy to compress along the short axis and extend farther in the disk-like direction, becoming an even thinner disk,
• and finally, as gravity does the inevitable, all the galaxies bound together within a group or cluster will eventually merge.

In some sense, we already know that our fate is sealed. The Milky Way itself is destined, approximately 4 billion years from now, to begin merging with Andromeda, and then in another 3 billion years, that merger process is expected to reach completion: resulting in a new, larger, single galaxy that already has the rather poetic name of Milkdromeda.

A series of stills showing a visualization of the Milky Way-Andromeda merger and how the sky will appear different from Earth as it happens. This merger will begin occurring roughly 4 billion years in the future, with a huge burst of star formation leading to a depleted, gas-poor, more evolved galaxy ~7 billion years from now. Despite the enormous scales and numbers of stars involved, only approximately 1-in-100 billion stars will collide or merge during this event. The final form of the galaxy, despite the illustration here, is more likely to be a gas-rich, disk-possessing galaxy than the elliptical one shown, as only a small percentage of major mergers lead to a red-and-dead, gas-free elliptical final state.

The cosmic story that led to the Milky Way is a story of constant, but not necessarily ultra-violent, evolution. We likely formed from hundreds or even thousands of smaller, early-stage proto-galaxies and early galaxies that merged together. The spiral arms that we see today were likely formed many times by interactions, only to re-form from the rotating, gas-rich nature of an evolving, gas-rich, disk galaxy. Star formation occurred inside in waves, often triggered by minor mergers or gravitational interactions, but also occurring during quiet periods in our galaxy’s life: quiescent star formation. Finally, these waves of star-formation, as stars live-and-die, bring along increases in supernova rates, stellar cataclysms, and heavy metal enrichment of the interstellar medium.

These changes don’t occur all-at-once and abruptly, but rather in a continuous fashion. They were not just a part of our cosmic past, but are still occurring, and will come to an extremely spectacular conclusion just a few billions of years in the future, as all the galaxies of the Local Group eventually will coalesce and merge together. Every single galaxy has its own unique cosmic story, and the Milky Way is just one typical example of a somewhat mature, larger-than-average but not among the largest galaxies, found within the Universe. What’s important to recognize is that the story hasn’t ended yet. As grown up as we are, the Milky Way, our cosmic home, is still evolving.

This article What was it like when the Milky Way grew up? is featured on Big Think.

One of the strangest facts about the Universe is how dramatically it has changed over time. Today, we see a Universe filled with:

• hundreds of billions of large galaxies,
• containing hundreds of billions of stars apiece,
• surrounded by trillions of even smaller galaxies,
• all clumped and clustered together into a massive, filament-like structure known as the cosmic web.

Closer back in time to the Big Bang, however, none of those entities were present. There were no stars, no galaxies, and no larger-scale structures at all. Instead, everything was extremely smooth and uniform, with very little clumping or clustering to speak of. At the earliest times of all, the densest regions were only thousandths of a percent denser than average, and the regions of lowest density were only a few thousandths of a percent less dense than the cosmic average.

Qualitatively, that allows us to put a relatively simple picture together that puts all of these facts in their proper context. The Universe was born with only very tiny imperfections, but gravitation causes those density imperfections to grow, all while the Universe expands. Depending on how, where, and how quickly gravity wins by overcoming this cosmic expansion, we wind up with these enormous galaxies and galaxy clusters separated by regions containing practically nothing: cosmic voids.

The cosmic structure that we see today didn’t form all at once, however, with the largest structures forming last. This is the cosmic reason why.

The evolution of large-scale structure in the Universe, from an early, uniform state to the clustered Universe we know today. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. Note the fact that small-scale structure appears early on in all cases, while structure on larger scales does not arise until much later.

Imagine the Universe as it was in these early stages. It’s full of matter (both normal and dark) and radiation (in the form of neutrinos and photons) that is distributed almost perfectly evenly everywhere you look. In the aftermath of the Big Bang, a typically overdense region had 100.003% the average density, while a typically underdense one had 99.997% the average density. When we describe the early Universe as uniform, this is the level of uniformity we achieved, and this amount of non-uniformity appears as the “seeds” of all cosmic structures.

These overdensities and underdensities were almost exactly the same on all scales: from tiny ones to enormous ones and everywhere in between. Whether you looked at a region that’s:

• a few kilometers,
• or a few light-years,
• or a few million or billion light-years

in size, the density fluctuations that you’ll find are all the same magnitude. You’ll keep finding that same ratio, with only a 1-part-in-30,000 fluctuation from the mean, describes the overdense and underdense regions the Universe began with on all cosmic scales.

The overdense regions that the Universe was born with grow and grow over time, but are limited in their growth by the initial small magnitudes of the overdensities, the cosmic scale on which the overdensities are found (and the time it takes the gravitational force to traverse them), and also by the presence of radiation that’s still energetic, which prevents structure from growing any faster. It takes tens-to-hundreds of millions of years to form the first stars; small-scale clumps of matter exist long before that, however.

But that initially uniform (or, rather, almost perfectly uniform) state doesn’t remain that way for long. Gravity immediately begins preferentially attracting mass into the overdense regions compared to all the others. The underdense regions more readily give up their matter to the surrounding, comparatively more dense regions, and so material begins to flow along gradients: from the less dense regions into the more dense regions in their immediate vicinities. Over time, this will add up, and the regions of greatest initial densities will accumulate matter from their surroundings more quickly, and more successfully, than all others.

Yet even though the law of gravity is universal, and the same on all scales, the Universe doesn’t form star clusters, galaxies, and galaxy clusters all at once. Instead, structure formation in the Universe is hierarchical: with the smallest-scale structures like stars and star clusters forming first, and with galaxies, galaxy groups and clusters, and even larger cosmic structures only forming much later. In fact, from a quantitative perspective, it takes under 100 million years for the first stars to form, but billions of years — more than ten times as long — before we form the massive galaxy clusters populating the Universe.

The fluctuations in the cosmic microwave background, as measured by COBE (on large scales), WMAP (on intermediate scales), and Planck (on small scales), are all consistent with not only arising from a scale-invariant set of quantum fluctuations, but of being so low in magnitude that they could not possibly have arisen from an arbitrarily hot, dense state. The horizontal line represents the initial spectrum of fluctuations (from inflation), while the wiggly one represents how gravity and radiation/matter interactions have shaped the expanding Universe in the early stages.

This might seem counterintuitive, as:

• the seeds of structure exist on all scales, even from the beginning,
• the laws and rules of gravity are always in place,
• and it’s inescapable, as anything with mass or energy, in any form, experiences gravitation.

However, there’s a simple reason for the existence of hierarchical structure formation that shows up in even the earliest, very first picture we have from the infant Universe: the cosmic microwave background and the fluctuations inherent to it. Even though gravitation is an infinite-range force, it doesn’t propagate at infinite speeds. It propagates only at the speed of light, meaning that if you want to have an impact on a region of space that takes you 100 million years to reach at the speed of light (e.g., something that’s 100 million light-years away), it cannot feel your presence until 100 million years of time have passed.

This is why, in the graph of the spectrum of fluctuations within the cosmic microwave background, above, the largest scales (at left) have temperature fluctuations that are completely flat: gravitation hasn’t impacted them yet. The 380,000 years that have passed from the time the hot Big Bang first occurred until the cosmic microwave background gets emitted, even within the expanding Universe, hasn’t provided enough time for those large cosmic scales to even experience the influence of gravity.

That first, massive peak in the graph is where gravitational contraction is just taking place now, but there hasn’t been enough collapse to trigger pushback on the part of radiation. And the peaks-and-valleys beyond that represent a splashing around on scales smaller than the current cosmic horizon: where gravitational contraction has not only occurred, but where radiation has “pushed” that once-contracted matter back out again at least once.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Even after galaxy clusters form, surrounding galaxies and galaxy groups, including initially Milky Way-like galaxies, get drawn in. Over time, they will lose their gas and eventually cease forming new stars.

This all translates into a detailed roadmap for how the large-scale structure in the Universe forms. We can break it down into a few general rules.

• Structure will form on smaller scales first: stars before galaxies, galaxies before clusters, clusters before superclusters.
• That characteristic scale where the density fluctuations have the greatest magnitude — corresponding to that first “acoustic peak” in the graph of the cosmic microwave background — will correspond to a distance scale, today, where we’re more likely to see galaxy correlations than on either shorter or longer scales.
• If there’s some sort of acceleration phase that arises later in the Universe (e.g., dark energy), it will cause a cutoff in structure formation: a maximum, largest scale for any structures that can stably exist.
• And once you become gravitationally bound, you should remain gravitationally bound even as the expansion of the Universe continues without bound.

Based on our observations of both the nearby and the distant Universe, all of these predictions have been borne out spectacularly.

An illustration of the first stars turning on in the Universe. Without metals to cool down the clumps of gas that lead to the formation of the first stars, only the largest clumps within a large-mass cloud will wind up becoming stars: fewer in number but greater in mass than today’s stars.

The first stars, as we understand them, appear when the Universe is between 50 and 100 million years old. Their formation requires that many millions of solar masses (but not much more than a billion) gather together in one region of space, initiating gravitational collapse, in order for some of that material to fragment and lead to the formation of individual stars. This is an exceptionally large amount of material, which is special for the primordial material that the Universe was born with (rather than the heavy-element-rich material that arises once prior generations of stars have lived-and-died), which means that even the densest regions of all won’t develop stars until many tens of millions of years have passed.

It will then take additional lengths of time for these individual star clusters to both:

• merge together,
• and to accrete matter from the surrounding regions,

in order to create the first galaxies. After those galaxies come into existence, it will take several hundreds of millions of years for significant numbers of those galaxies to merge together to create evolved galaxies and galaxy groups, and for those groups to merge together to form galaxy clusters. This is what we mean when we talk about the cosmic web and the large-scale structure of the Universe forming hierarchically: it has to build itself up, from small scales (where gravity takes action first) to large ones.

The white boxes outline member galaxies that are part of the most-distant proto-cluster of galaxies ever identified: A2744z7p9OD, found just 650 million years after the Big Bang. These objects are not yet gravitationally bound together, but will become so over time, and will wind up forming a galaxy cluster upward of 1 quadrillion solar masses. Owing to the incredible observations of JWST, we can identify these proto-clusters even when they’re still in the process of forming.

Even though this is how structure forms in the Universe, where it ultimately gives rise to a network of filaments that intersect at various nexuses, that network doesn’t appear everywhere at once, but rather appears on smaller scales first. The larger scales don’t exhibit structure, or rather, the growth of structure beyond the magnitude of its initial seeds, until the Universe has aged further. It takes an extremely large amount of time in order for a gravitational signal to traverse the large distances separating the clumps of matter that wind up forming structure on the largest scales of all: hundreds of millions or even billions of light-years.

By the present time, we have an observable Universe that’s a whopping ~92 billion light years in diameter. Given how large the Universe was at an age of 380,000 years, and the size/scale of the acoustic “peak” that we saw imprinted in the cosmic microwave background, that sets the scale at which we’re more likely to see galaxy-galaxy correlations than others. That acoustic scale, today, is at a distance of about 500 million light years, which means if you put your finger down on any galaxy and look a certain distance away, you’re more likely to find another galaxy 500 million light-years away than you are either 400 or 600 million light-years away. It also explains the first cosmic bounce we’ve ever discovered: a bubble-like galaxy-rich structure spanning approximately 500 million light-years in radius.

The structure Ho’oleilana, a candidate for an individual baryon acoustic oscillation, can be identified visually by the human eye as a circular feature around 500 million light-years across. The red circle, shown in animation, makes the presence of this acoustic oscillation even clearer.

Furthermore, the large-scale features we recognize as galaxy clusters shouldn’t be — and, in fact, aren’t — present at the earliest stages of cosmic history. For many hundreds of millions of years, there should be no galaxy clusters at all, only proto-clusters still in the process of forming, at best. If we want to see large groups of galaxies all clustered together in the same region of space, and all gravitationally bound together on top of that, it should take billions of years of cosmic evolution before they appear.

Large collections of galaxies will inevitably clump and cluster together, as gravitation on those larger cosmic scales will eventually catch up, drawing them in toward their collective center of mass. It takes time for these larger, more massive, grander-scale structures to form, but here in the Universe, saying “we’ve got all the time in the world” is a gross understatement.

For galaxy clusters, the ones that appear at these early times should be lower in mass than the ones that show up at later times, as even on these large scales, there’s still ongoing gravitational growth over time. By and large, this is borne out spectacularly by observations, with the earliest massive galaxy clusters known appearing well after massive galaxies are plentiful. As we look close by, we find galaxy clusters that are still more massive and contain even more galaxies than the earlier, more distant ones.

The giant, nearby galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5 million light years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all. The farther away we look, the lower in mass galaxy clusters are, while the earliest proto-cluster we find has only a few handfuls of galaxies inside it, and still requires nearly a billion years to begin taking shape.

Most spectacularly of all, there seems to be a limit to the size and mass of structures. You may have heard of our local supercluster: Laniakea, which contains the Milky Way, the Local Group, the Virgo cluster, and many other clusters and groups that appear to be arranged in a spindly, web-like structure. If you were to map it all out, you might be tempted to conclude that Laniakea is real, and that this massive object is an even larger structure than the big galaxy clusters we see across the Universe.

Yet, Laniakea itself is nothing more than a phantasm. It is only an apparent structure; it isn’t gravitationally bound. On the largest cosmic scales, dark energy dominates the gravitational force, and has been doing so for the past 6 billion years. If an object hadn’t gravitationally grown to a sufficiently great density so that it would collapse under its own power by that epoch in cosmic history, it would never make it to that point.

Laniakea, like all enormous supercluster-scale structures, is presently being torn apart by the expansion of the Universe. It takes, on average, about 2-to-3 billion years for these large galaxy clusters to grow to sufficient densities to gravitationally collapse. The most massive ones might contain many thousands of Milky Way-sized galaxies today, but there are no behemoths spanning tens of billions of light-years or containing tens of thousands of Milky Ways inside them. (Claims to the contrary are based on dubious evidence, and are likely completely wrong.) The accelerated expansion of the Universe is simply too much for gravitation to overcome.

The Laniakea supercluster, containing the Milky Way (red dot), is home to our Local Group and so much more. Our location lies on the outskirts of the Virgo Cluster (large white collection near the Milky Way). Despite the deceptive looks of the image, this isn’t a real structure, as dark energy will drive most of these clumps apart, fragmenting them as time goes on.

Although the seeds necessary for cosmic structure were planted in the very earliest stages of the Universe — all the way back during the phase of cosmic inflation that preceded and set up the Big Bang — it takes time and the right resources for those seeds to grow to fruition. The seeds for small-scale structure germinate first, as the gravitational force propagates at the speed of light, growing overdense regions into the earliest star clusters after only a few tens of millions of years. As time goes on, the seeds for galaxy-scale structure grow too, taking hundreds of millions of years to bring about galaxies within the Universe.

But galaxy clusters, growing from the same magnitude seeds on larger distance scales, take billions of years to fully form. As time continues to pass, more galaxy clusters form, with increasingly greater masses, and some of these galaxy clusters even attract and merge together, leading to the grandest cosmic smash-ups of all. But by the time the Universe is 7.8 billion years old, the Universe’s expansion has already begun to accelerate owing to the presence of dark energy, explaining why there are no bound structures larger than galaxy clusters within the Universe.

The cosmic web is no longer growing as it once was, but is primarily being torn apart, as the filaments within it stretch and stretch owing to the presence of dark energy. Enjoy what we have while it still persists; the observable Universe will never be this richly structured again!

This article What was it like when the cosmic web formed? is featured on Big Think.

How large is the largest structure in the Universe? According to our best understanding of the Universe, the mix of radiation, normal matter, dark matter, and dark energy that we have should lead to a rich cosmic web of large-scale structure, where massive objects — like galaxies — cluster together along filaments, and where those filaments intersect, we should expect to find massive galaxy clusters. In the space between filaments should be great cosmic voids: regions with a great underdensity of galaxies and other forms of matter. We should expect structures, such as walls of galaxies, as large as 1-2 billion light-years to form, while the voids might be as large as 3-4 billion light-years across. On larger scales, we expect the Universe to appear relatively uniform.

But a recently claimed discovery of a giant, ring-like structure some 1.3 billion light-years across and 4 billion light-years in circumference — nicknamed “the Big Ring” — now makes it the seventh claimed structure, along with:

• the Hercules-Corona Borealis Great Wall,
• the Giant GRB (gamma-ray burst) ring,
• the Huge-LQG (large quasar group),
• the Giant Arc,
• U1.11 LQG,
• and the original Clowes-Campusano LQG from all the way back in 1991,

ranging from ~2 billion to ~10 billion light-years in size. While the scientists involved with these structures express little skepticism over their reality, pointing to statistical tests that favor their existence and a violation of the cosmological principle, the majority of the astrophysics community remains unconvinced. Here’s why everyone should take the claims that these structures actually exist with a grain (or more) of salt.

Galaxies can be found along, nearby, and within cosmic filaments. There is often both neutral and ionized matter within the haloes of these galaxies as well as along their line-of-sights, so when that light arrives, those absorption features seen in their spectra can tell us what the density and temperature of matter was in their own circumgalactic mediums, as well as for intervening galaxies and our own Milky Way. The galaxies and gas, which emit and absorb light, are biased, imperfect tracers of the underlying mass distribution, which includes dark matter.

Maps and measurements

The most important rule that any battlefield general often learns — sometimes the hard way — is the lesson that the map is not the terrain. Here in our Universe, that’s absolutely true as well. The maps that we make are fundamentally limited by what our instruments are capable of observing over the time that we’re making these observations. And by the very nature of the enterprise of astronomy, those observations have inherent biases built-in to them.

The simplest of these biases to understand is what’s known as Malmquist bias: the notion that the easiest objects to see are inherently the ones that appear brightest. This means that the intrinsically brightest objects, as well as the intrinsically closest objects (since an object’s apparent brightness falls off as one over its distance squared), are the easiest to see. And there’s an additional caveat here: you’re only seeing the brightest, closest objects in the particular wavelength range of light over which you’re observing.

Malmquist bias is the reason why Hubble found so many more nearby and intermediate galaxies than ultra-distant galaxies, and it’s also the same reason why the ~6000 or so stars visible to the naked human eye are biased toward both nearby and very bright, massive, or evolved stars: we most easily see the things that appear brightest to our instruments.

The Milky Way, as seen at La Silla observatory, is a stunning, awe-inspiring sight to anyone, and offers a spectacular view of a great many stars in our galaxy. There are approximately 6000 stars visible to the naked human eye, but the ones we can see are intrinsically brighter, bluer, and/or closer than the more typical, numerous stars of the Milky Way. We can only see the ones that are the easiest for our eyes to detect.

If we wanted to overcome our biases, what we’d most like to do is measure:

• as much of the sky as possible,
• in as comprehensive a set of wavelengths as possible,
• as deeply and as faintly as possible,

in an effort to map out the Universe’s structure without succumbing to the biases inherent to having a limited amount of data.

One way to attempt to create an unbiased (or, at least, a less biased) map of the structure in our Universe would be to make a deep and comprehensive galaxy catalog, where the three-dimensional positions of galaxies could be mapped out to the greatest precisions possible, to as faint a set of magnitudes as possible, all while accounting for how the wavelength of a galaxy’s light changes with respect to its distance from us owing to the expansion of the Universe.

This method — the best one that we possess for creating a (relatively) unbiased structure map of the Universe — has two small problems still associated with them.

FOGs, or Fingers of God, are known to appear in redshift space. Because galaxies in clusters can get extra redshifts or blueshifts due to the gravitational influence of its surrounding masses, those galaxy positions that we infer from redshift will be distorted along our line-of-sight, leading to the Fingers of God effect. When we perform our corrections and move from redshift space (left) to real space (right), the FOGs disappear.

The first problem is illustrated above. When we observe a distant galaxy, we don’t directly measure its distance, but rather measure the light from it and how that light is “redshifted,” or stretched by the expansion of the Universe. The problem is that galaxies also have their light get redshifted (or blueshifted) by the gravitational influence of the other, nearby objects that tug on them. When a large clump of matter (or a great void) is nearby, they can add what’s called a peculiar velocity to that galaxy of up to hundreds or even thousands of kilometers-per-second to their motion, which can bias the inferred distance to them by up to tens or even a couple of hundred million light-years. We have to correct for those gravitational influences: by going from “redshift space” to “real space,” which has its own set of challenges.

And secondly, we have to recognize that we are not capable of making a comprehensive mass map of the Universe through this method, but only a comprehensive map of where the starlight — the stuff visible through our telescopes — comes from. If there are clumps of gas without stars out there, or clumps of dark matter that won’t cause normal matter to collapse or stars to form, these surveys will miss them. The way to overcome that bias would be to construct a comprehensive 3D lensing map of the Universe: something currently beyond our modern scientific capabilities.

The Sloan Great Wall is one of the largest apparent, though likely transient, structures in the Universe, at some 1.37 billion light-years across. It may just be a chance alignment of multiple superclusters, but it’s definitely not a single, gravitationally bound structure, as dark energy is in the process of driving it apart. The galaxies of the Sloan Great Wall are depicted at right.

The largest unambiguous structures

When we see a large wall-like or filamentary structure in the Universe, made by mapping out galaxies, we can be certain that these are tracing out at least part of the matter distribution in the Universe. We’ve found, over the past ~40 years, lots of structures ranging from a few hundred million light-years up to about 1.4 billion light-years in physical extent. (Plus or minus that ~200 million light-year figure from the effect of peculiar velocities.) While some of these are actually gravitationally bound structures, the largest of them — including our local supercluster, Laniakea — were formed by gravity and the evolution of the Universe, but aren’t actual structures. Instead, these are phantasmal pseudo-structures: made of many separate bound clumps of matter that will forever be driven away from one another owing to the presence of dark energy.

While the human tendency to “connect the dots” wherever we see adjacent points of light near each other may sometimes lead us to declare that we’ve found a structure when in fact we’ve found:

• a pseudo-structure,
• an association,
• or just non-existent patterns in the noise,

we have to keep our minds open to the possibility that there really could be structures that exist on larger scales than we expect. Large-scale violations of homogeneity (the idea that the Universe is the same everywhere) or isotropy (the idea that the Universe is the same in all directions) would severely challenge the way we conceive of our Universe, so it’s very important that we take any assertions of structures so large that they shouldn’t exist very seriously, but also we should demand a demonstration that these are real structures, not just apparent associations.

The “Big Ring” of ionized magnesium absorbers, in blue, is shown alongside the “Giant Arc” of ionized magnesium absorbers, in red. While it’s plausible that some or all of these objects are related as part of a connected structure, it’s also possible that every single point is unrelated to every other point in this image; there is no “mapping” of the Universe that has convincingly established the reality of these structures. They may simply be unconnected points that our minds, and eyes, are too tempted to do anything other than connect.

The alleged quasar and gamma-ray “structures”

We have other ways of looking at the sky, however, than simply by using optical/infrared light to search for galaxies. Among them, we can:

• use X-ray and radio data to search for active black holes,
• use the 21-centimeter emission line from hydrogen to identify current/recent star-forming regions and galaxies,
• use absorption features (such as that from neutral hydrogen) to identify intervening clouds of matter,

along with many other ways of probing the Universe.

Two methods that are interesting — but importantly, that are not comprehensive — are to look for quasars and gamma-ray bursts. Quasars are the brightest, most luminous class of supermassive black holes in the Universe, and are typically caused by normal matter falling into a supermassive black hole, heating up, getting accelerated, and emitting radiation all across the electromagnetic spectrum. Gamma-ray bursts, meanwhile, are extremely energetic stellar cataclysms arising from supernovae and neutron star collisions (and possibly other events as well) that last anywhere from just a few milliseconds to many hours.

On August 17, 2017, the Laser Interferometer Gravitational-wave Observatory detected gravitational waves from a neutron star collision. Within 12 hours, observatories had identified the source of the event within the rather mundane galaxy NGC 4993, shown in this Hubble Space Telescope image, and located an associated stellar cataclysm called a kilonova (box), caused by the collision of two neutron stars. Note that a kilonova is only one possible origin of gamma-ray bursts, and cannot account for all of them. Inset: Hubble observed the kilonova fade (in optical light) over the course of six days.

Most galaxies, when we look at them, have neither a quasar at their cores nor have they ever had an observed gamma-ray burst occur within them: these are rare events. Some gamma-ray bursts have been localized to their host galaxies, and appear to inhabit relatively unremarkable galaxies, as though they appear randomly and rarely. Quasars often result from the mergers of massive galaxies, but also are relatively rare: most galaxies that merge do not become quasars.

The first identified grouping of objects that appeared to violate the cosmological principle was the Clowes–Campusano LQG, found in 1991 by Roger Clowes and Luis Campusano. This grouping of 34 quasars, however, could just be 34 random points that are not part of the same structure; we have never mapped out that region and found galaxies in a filamentary network there. Similar LQGs, U1.11 and the Huge LQG, were also discovered by the same group. The gamma-ray burst groupings (discovered by a different group of scientists), such as the Hercules-Corona Borealis great wall and the Giant GRB Ring, have the same deficiencies: they are a few individual points, but whether they represent a connected structure or just are unrelated points is not determined.

Similarly, the two newest structures, the Giant Arc and the Big Ring, were identified by connecting a number of nearby points, picked out by the same group (of Clowes and Campusano) using absorption features. These points were identified by the (biased) signature of singly ionized magnesium: a feature that can be mapped by looking at the absorption features of background quasars as their light passes through the intervening matter when it’s been heated sufficiently, likely by a nearby star-forming region.

The blue points, in the background, represent identified quasars, which “backlight” the clouds of matter in front of them. The grey points represent absorption signatures of that quasar light by singly ionized magnesium, which is only produced in the presence of heated gas. The alleged “Big Ring” is shown near the center of the image, and makes more of a helix-like shape in 3D space than a ring. The distance between even adjacent sources, here, is greater than the distance from the Milky Way to the Virgo cluster.

The “rare find” problem

Imagine you were surveying houses in the United States, and rather than mapping out every house there was, you simply mapped out only the houses that were pink mansions. You can easily find several examples, as this description fits one out of every few million buildings. However, if you mapped them out and (erroneously) connected the dots, you might find what appeared to be a pattern to their locations.

Because you are only mapping out such a small set of examples of houses-and-buildings, however, and because that sample is biased, you have to recognize that the map of pink mansions has nothing to do with the true distribution of houses-and-buildings in the United States. Drawing conclusions about how housing, and buildings in general, are structured in the United States from a map where only pink mansions are highlighted is a foolish, poorly thought-out idea.

Similarly, a map of:

• quasar groupings,
• gamma-ray bursts and their associations,
• or ionized magnesium absorption features,

is a terrible proxy for mapping out the large-scale structure of the Universe. These rare finds are not indicative of the underlying, true distribution, and until we map that distribution, we have no reason to believe that these are “real” structures in any sense of the word.

The coathanger, also known as Brocchi’s cluster, is an asterism of 10 stars (circled in red) shaped like a coathanger. While many have called it a cluster in the past, the only star cluster in this image is circled in yellow, and is nearly 3000 light-years farther away than the most distant of the 10 stars comprising the coathanger, all of which are unrelated.

Statistics and our eyes: deceptive and unreliable tools

Take a look, above, at what your eyes would tell you is clearly a coathanger in the sky. This grouping of stars looks like it could be a star cluster, all of approximately the same apparent brightness (within 2 visual magnitudes) and all within a narrow ~1.3° region of the sky. If you were to ask, statistically, what the likelihood of finding 10 stars with these similar properties to one another in the same region of sky were, you would find that this is an outlier of more than 5σ (five-sigma): past the “gold standard” threshold for announcing a new discovery in astronomy and physics.

But a close analysis of these 10 stars that compose the coathanger, also known as Brocchi’s cluster, shows that it really is a chance alignment of unrelated stars. These stars range from 235 to 1735 light-years away, with no two stars closer than ~20 light-years to one another. There is a star cluster in that region of sky, but much farther away: between 4,000 and 5,000 light-years away, all unrelated to the foreground stars, which are all unrelated to one another. We have called it a “cluster” for most of the time it’s been known, but it isn’t a cluster at all. These objects are unrelated, and while both our eyes and statistical analyses may erroneously lead to the opposite conclusion, it just goes to show that a visual inspection and a statistical analysis is no substitute for comprehensive data.

This association of nine gamma-ray bursts, all located around 9 billion light-years away but separated by more than 5 billion light-years from end-to-end, were seen over a period of 8 years: from 2004 through 2012. The “connected superstructure” is only an artist’s representation; it is undetermined whether there is any physical relation between the various gamma-ray bursts and unknown whether any two of them are even part of the same cosmic structure.

So where does that leave us, as far as the “Giant Arc,” the “Big Ring,” and the other so-called cosmic structures that are “too large to exist?”

If you’re not wedded to being responsible about your science, you’ll talk about the cosmological principle, and how these “structures” violate it. You’ll talk about the scale of homogeneity and how these structures are too big to exist within our Universe, and how that means our concordance model — of the inflationary hot Big Bang with dark matter and dark energy dominating the Universe — must be wrong. You’ll appeal to alternative, even non-viable models and declare that all of modern cosmology’s successes are irrelevant when considering these structures. (And, as a pet peeve of mine, you’ll issue a press release about it without even having a supporting paper available so that others can review your work.)

But if you want to show that these are actually structures, you’ll work to map out the full suite of matter-and-galaxies in this region, and demonstrate that these are not just “points” in the cosmic web, but actual tracers of galactic filaments and genuine structure. Arguing over statistics and types of tests and analyses will never resolve the situation, just as no statistical test could ever tell you the truth about Brocchi’s cluster or the underlying housing distribution based on pink mansions. It takes comprehensive quality data to determine whether a grouping or association is actually a real structure. Until that data arrives, we should all remain appropriately skeptical of claims that these so-called “structures” are anything more than coincidences.

This article Giant ring? Giant arc? These “structures” may not even be real is featured on Big Think.

Humanity once thought our Solar System was typical.

Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. For a long time, we assumed this configuration was typical and common. Today, we know better.

The other stars, presumably, were Sun-like objects, but very far away.

The brightness distance relationship, and how the flux from a light source falls off as one over the distance squared. The earliest estimates for the distances to the stars assumed they were intrinsically as bright as the Sun, and that their faint appearance was solely caused by their great distance from us.

We soon learned that stars and stellar systems varied tremendously.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. In terms of size, the smallest M-class stars are still about 12% the diameter of the Sun, but the largest main sequence stars can be dozens of times the Sun’s size, with evolved red supergiants (not shown) reaching hundreds or even 1000+ times the size of the Sun. A star’s (main sequence) lifetime, color, temperature, and luminosity are all primarily determined by a single property: mass.

Individual stars come in many different masses, temperatures, and colors.

Binary systems typically have unequal masses, unequal brightnesses, and orbit a barycenter that lies outside of both stars. Only if the alignment with respect to us is sufficiently edge-on, at right, will it appear as an eclipsing binary. Wide binaries, with separations of thousands of astronomical units (AUs), are exceptionally difficult to characterize. Approximately 35% of all stars are found in binary systems, with half in singlet systems and the remainder in trinary or even richer multi-star systems.

While our Solar System has just one star, half of all stellar systems have multiples.

The richest star system among the more familiar stars is Castor: the 24th brightest star in the sky and an intrinsically sextuple system. Unlike our Sun, which is the only star in our system, practically half of all stars have one or more companions in their stellar systems.

Surveying nearby stars reveals that 48% of them are bound in multi-star systems.

The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1-10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars.

But what about the heaviest, most massive stars of all?

This fragment of the young star-forming region NGC 2014 showcases many stars that are bluer, more massive, and much shorter lived than our Sun. However, the fainter, redder, less luminous stars are far more numerous, making us wonder just what “typical” truly is for a star. NGC 2014 is also found in the Large Magellanic Cloud: over 160,000 light-years away.

They’re too short-lived to perform an accurate census of them.

The flame Nebula, shown here in a combination of X-ray data (from Chandra) and infrared light (from Spitzer), showcases a young, massive star cluster at the center, which carves out a spectacular shape in the surrounding gaseous material that was used for star-formation. Direct observations of the hottest, brightest, most massive stars that form inside these regions are difficult, as there are frequently large amounts of (visible) light-blocking matter intervening.

The environments in which they form, star-forming regions, are often opaque.

The Atacama Large Millimetre/submillimetre Array, or ALMA, is the most powerful, highest-resolution array of radio telescopes in the world. Although it only has the light-gathering power of all its dishes, combined, it has the resolution of the space between the dishes, making it capable of resolving details no other observatory can see.

But ALMA, the Atacama Large Millimetre-submillimetre Array, can finally peer inside.

In very-long baseline interferometry (VLBI), the radio signals are recorded at each of the individual telescopes before being shipped to a central location. Each data point that’s received is stamped with an extremely accurate, high-frequency atomic clock alongside the data in order to help scientists get the synchronization of the observations correct.

This array of radio telescopes, through interferometry, performs an incredible trick.

This image of the black hole, event horizon, and beginning of the launched jet comes from a 6.5 billion solar mass black hole at the center of galaxy Messier 87 (M87). The radio astronomy technique of very-long baseline interferometry was essential to the construction of each aspect of this detailed image.

It gathers light with only the individual dishes, but its resolving power covers the space between them.

In the center of the remnant of SN 1987A, ALMA, with its incredible resolution and long-wavelength capabilities, was able to observe a particularly hot spot within the gas and dust of SN 1987A. The extra heat is thought by many to be an indicator of a young neutron star, which would make this the youngest neutron star ever discovered.

As a result, it can image at higher resolution than any other observatory.

At “low” resolution, ALMA, observing the protostar cluster G333.23-0.06, can pick out dense cores of matter and identify regions where various new stars are in the process of forming. Even though these regions cannot be identified at optical wavelengths, ALMA’s high resolution and radio-wavelength sensitivity make it ideal for this task.

Recently, ALMA observed the high-mass stellar protocluster G333.23-0.06.

The dense cores of protostar cluster G333.23–0.06, as identified by ALMA, show strong evidence for large levels of multiplicity within these cores. Binary cores are common, and groups of multiple binaries, forming quaternary systems, are also quite common. Triplet and quintuplet systems are also found inside, while, for these high-mass clumps, singlet stars turn out to be quite rare.

Singlet stars were rare, but binaries were overwhelmingly common.

If the kinetic energy of members of the same star system is below the gravitational energy, systems can be considered to be gravitationally bound, and the systems within G333.23–0.06 where that is determined to be the case are shown here. Particularly at the high-mass end, for stars of 5 solar masses and up, multi-star systems are overwhelmingly not just common, but perhaps even the norm.

Triplet, quadruplet, and even quintuplet systems were spotted directly.

In this close-up view of a dense core of matter in the star-forming region G333.23–0.06, a variety of dense clumps of matter have been probed with ALMA, revealing (insets, going clockwise from top left) a quaternary, binary, quintuple, and triplet star system within it. This indicates these star systems were born as multiples, rather than capturing other members later on.

The lack of a disk suggests core fragmentation as the formation mechanism.

This ALMA observation of a high-mass protostar cluster, G351.77-0.54, has gotten down to ~120 AU spatial resolution, corresponding to 0.06 arc-seconds at the distance of these protostars. The gaseous material is fragmenting into at least four separate cores, a hint (now with further evidence) that core fragmentation, rather than anything having to do with a disk, is a major player in determining how many stars form in these high-mass star-forming regions.

Like dogs, not humans, a single, high-mass stellar “child” is rare.

Three separate regions illustrate various stages of a newly forming star’s life, which are totally obscured in the optical and can only be seen in the infrared. At left, a protostar emits radiation that’s shrouded in light-blocking dust. In the center, a ‘yellowball’ announces the start of nuclear fusion, but still cannot be seen in the optical due to all the surrounding matter. At right, a more evolved star has begun to blow an ionized bubble in the surrounding region. For high-mass stars, we now know that forming a singlet system, as opposed to a multi-star system, is a relative rarity.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

This article Star clusters give birth like dogs, not humans, ALMA shows is featured on Big Think.

One of the most fascinating facts about the natural world is that so many entities within it — both biologically and purely physically — obey a specific set of patterns and ratios. Many galaxies exhibit spiral shapes and structures, as do a wide variety of plant structures: pinecones, pineapples, and sunflower heads among them. Ammonites, shelled animals that went extinct more than 60 million years ago, also show that spiral pattern, where one of the key features of spirals is that the next “wind” around outside the prior one displays a specific length ratio to the size of the prior, interior winding.

That ratio, in any such structure, is often extremely close to the ratio of two adjacent numbers found in the Fibonacci sequence. This mathematical sequence, often taught to children, simply starts with the numbers “0” and “1” and then gets the next term in the sequence by adding the two prior terms together. It’s arguably the most famous mathematical sequence of all, but what explains the sequence’s pattern, and is it truly, inextricably linked to nature? That’s what Ragtag Media wrote in to ask, inquiring:

“Is there a Fibonacci sequence with regards to the way galaxies develop?”

Indeed, simply looking at the “spiral” structures in galaxies might appear to be Fibonacci-like, but is that real, or just our minds making superfluous connections where only an apparent link exists?

This MIRI view, from JWST and the PHANGS collaboration, of spiral galaxy NGC 1566 shows heated, dusty and nuclear features that are entirely invisible to other observatories observing in optical/UV and even radio wavelengths. This dust filament network is ubiquitous in spirals, but the spirals themselves do not follow the pattern one might expect from the golden ratio or the Fibonacci sequence.

Galactic and other physical spirals

When it comes to spirals that naturally occur in the purely physical sciences, “spiral galaxies” are undoubtedly the most famous among them. Somewhere just over half of all known large, nearby, massive galaxies have spiral shapes and structures within them, but when we examine them mathematically, it turns out that there are very few of them that exhibit a Fibonacci-like pattern.

Importantly, a Fibonacci-like pattern is what we call “self-similar,” where if you zoom out, and look at it on larger scales, that same structural pattern repeats itself when you zoom in to smaller scales. The spiral structures seen in galaxies don’t do that in two separate ways, as:

• the interiors of spiral galaxies rarely spiral in all the way to the center, but rather terminate in an asymmetric galactic bulge or bar,
• and the exteriors of these galaxies — in which the stars, gas, and dust are largely confined to a disk — are better approximated by the curve of a circle than by any “spiraling-out” structure.

Remember that the spiral arms within a galaxy are caused by density waves, and by the galaxy “winding up” over time. There are a few notable features that exist in some spiral galaxies that exhibit a Fibonacci-like pattern over those in-between regions, but this is not the norm.

While some grand design spiral galaxies may have features that do appear to follow the Fibonacci pattern, this does not represent most spiral galaxies nor most features even within galaxies that do roughly obey that pattern. While the Fibonacci sequence does appear in many places reliably, spiral galaxies aren’t one of them.

The few spirals that do show that Fibonacci-like pattern are a part of a class of spirals known as Grand Design Spiral Galaxies, and these represent only about 1-in-10 spiral galaxies, as opposed to the most common types with multi-arm spirals (including the Milky Way) and the second most common type with subtle, many-laned spiral structure known as flocculent spiral galaxies. These “grand design” spirals are almost exclusively galaxies that have recently undergone or are currently undergoing a gravitational interaction with a nearby companion galaxy, and it’s only that external gravitational influence that pulls the outermost arms and features into shapes that are more consistent with ratios found within the Fibonacci sequence.

While there are many spiral shapes that occur from purely physical, non-biological processes in nature — from whirlpools and vortices that form in bodies of water to the aerial shapes of hurricane clouds and clear lanes — none of these spirals are Fibonacci-like when it comes to the actual mathematical details of their structures on a sustained basis. You may be able to take a “snapshot” where one or more of the features exhibits ratios that are consistent with the ratios found in the Fibonacci sequence for a particular moment, but those structures don’t endure and persist. The Fibonacci-like patterns seen in spiral galaxies are inventions of our eyes, rather than a physical truth of the Universe.

This satellite photo of Hurricane Katrina appears to show a spiral-like structure to its clouds. While it may be tempting to conflate this pattern with the Fibonacci spiral, the association is spurious, as neither this nor most hurricanes has a spiral pattern that obeys the golden ratio.

Biological Fibonacci spirals

However, the Fibonacci-like patterns and ratios found in many biological organisms, including in plants, truly are related to the Fibonacci sequence: both in a mathematically rigorous fashion and also for an evolutionary reason that makes perfect sense. Let’s tackle the biological properties first, and return to the mathematics.

Biologically, let’s imagine you’re a plant: a primitive plant at that. You have the ability to generate your own energy from sunlight, soil nutrients, water, and carbon dioxide, and make sugars (stored energy) through the process of photosynthesis that takes place in your leaves. When you sprout from a seed, you have to put leaves out, and somewhere — buried in your genetic code — will be a piece of information telling you what angle to put your “next leaf” out at relative to the previous leaf.

You can go the simplistic route that a plant like a clover takes, and simply put out three leaves at an angle of 120° to one another: making a triangular pattern. The problem with that route is that it’s efficient, but not scalable. We don’t have giant clover trees because you can’t “scale” that up: when you put out three leaves at 120° angles relative to each other, there’s no place to put a “next” leaf that isn’t either very efficiently blocked by the prior leaves, or won’t be an efficient blocker for the prior leaves collecting sunlight.

Leaf arrangements on plants can come with either one leaf per node (alternate) or two leaves per node (whorled), with the simple distichous pattern being found in bamboo, but the more efficient Fibonacci spiral pattern, following the golden ratio, appearing in many plants, including succulents such as aloe.

But what if you wanted to encode the most efficient way to put your “next leaf” out, based on wherever you put the previous leaf? Sure, for a total of three leaves, 120° is mathematically perfect, but for an arbitrary number of leaves, it won’t do you any good. Imagine that you’re a plant, growing upward, and you’ve just put out your first leaf. As you grow upward and you go to put out your second leaf, what angle should it go out at so that not only the first and second leaves, but also the third, fourth, fifth, sixth, and so on, will all get the maximum amount of sunlight when they’re all in place?

The answer is that, for each leaf, the next leaf should be put out right around 61.8% of a complete circle away from the prior leaf. For a circle with 360° in it, that corresponds to an angle of 222.5°, and the exact number that corresponds to is what mathematicians define as –ψ, which equals (√5 – 1)/2, or approximately 0.61803398875. The positive version of that, (√5 + 1)/2, is known as φ, or the golden ratio, and is 1/(-ψ), which happens to equal 1 + (-ψ) as well: the ratio between each Fibonacci number and its predecessor. If you keep putting out leaves at that key angle, 222.5°, relative to the prior leaf, you’ll wind up with your leaf patterns forming a Fibonacci spiral. That same mathematical property, encoded into pineapples, pinecones, and more, explains why biological organisms often display numbers found in the Fibonacci sequence.

Here, a mother chamomile plant is shown with a flower in full bloom. The interior of the flower has many components, arranged in a pattern that is traced out (inset) in blue and cyan in either direction, with 21 blue lines and 13 cyan lines. Both of those are successive Fibonacci numbers.

The mathematics of Fibonacci

But the big question isn’t “why the Fibonacci sequence is found in nature,” but rather, “what is it that determines the Fibonacci sequence” in the first place? It’s pretty easy to calculate the Fibonacci numbers; all you have to do is start with the first two numbers: “0” and “1” to begin, and then make the next term out of the prior two terms combined. The first few terms of the Fibonacci sequence are then:

• 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, etc.,

where you can verify that each successive term can be arrived at by looking at the two terms prior.

The Fibonacci spiral, shown here, is generated by turning each number in the sequence into a square, then generating the next number by placing an adjacent square with the length of its side determined by the sum of the previous two squares, in a counterclockwise pattern around the last ones. For large numbers, these ratios approach a value known as the golden ratio.

Now, let’s look at the ratios of any two successive terms relative to one another, and figure out what the ratio of the “next” term is to the “prior” term, starting with the two “1s” in the sequence (so that we don’t wind up dividing by zero).

• 1 ÷ 1 = 1.0,
• 2 ÷ 1 = 2.0,
• 3 ÷ 2 = 1.5,
• 5 ÷ 3 = 1.66666…,
• 8 ÷ 5 = 1.6,
• 13 ÷ 8 = 1.625,
• 21 ÷ 13 = 1.61538462…,
• 34 ÷ 21 = 1.61904762…,

and so on. As you can see, terms oscillate between being slightly less than φ, the golden ratio, and slightly greater than φ, but approach it, both from above and below, as we go to greater and greater terms.

The ratio of each term in the Fibonacci sequence to the prior term rapidly approaches a value known as the golden ratio: from above and from below with each successive term. The approximation gets better and better very quickly, so that the deviation from the true value is almost indiscernible by eye after just a few terms are calculated.

By the time you get up to very large terms, it’s easy to see how close you get to the golden ratio. If we were to look at the final three numbers written above — 2584, 4181, and 6765 — you can see that the ratios:

• 4181 ÷ 2584 = 1.61803405573, and
• 6765 ÷ 4181 = 1.61803396317,

very quickly and very closely approach the golden ratio, φ, itself. If we go up just a few more numbers, to 10946, 17711, 28657, 46368, and then 75025 (the last Fibonacci number under 100,000), we find that the ratio 75025 ÷ 46368 = 1.61803398896: an estimate for the golden ratio that only differs once you expand out to the 11th significant digit.

It turns out there’s nothing special about the starting point of the Fibonacci sequence, either. You can start with any two non-negative numbers that you like where at least one of them is non-zero: they need not be “0” and “1,” they need not be whole numbers, they need not be close together. All you need to do is follow the same formula, where you take the first two numbers and add them together to make the next (third) number, and then add that number with the previous to make the next subsequent number, and so on. No matter which numbers you start with, the ratio of any two successive numbers will quickly approach φ, the golden ratio.

The logarithmic golden spiral, also known as a Fibonacci spiral, is a self-similar, self-repeating pattern that arises by constructing each successive term in the sequence by summing the prior two terms. Although this is only encoded approximately in nature, the mathematical relationship underlying it is exact.

A generating Fibonacci fraction

It’s enough to make one wonder: is there a way to simply generate any-and-all of the Fibonacci numbers without having to sum up each of the previous terms to get there? It turns out there is, and it’s an incredible mathematical curiosity. The key, believe it or not, is the 11th number in the Fibonacci sequence: 89.

What’s so special about the number 89? On the surface, not so much. The two ratios that it’s a part of, 89 ÷ 55 and 144 ÷ 89, don’t appear special: they come out to 1.6181818… and 1.6179775…, respectively. But if you instead take a very different ratio, of the number 1 to the number 89, you notice something a little bit odd. If you write it out as an expanded decimal, you’ll find the first few numbers of the Fibonacci sequence just appear, and appear in order.

• 1/89 = 0.011235955…,

in which we can see the first few recognizable numbers easily: 0, 1, 1, 2, 3, and 5. You might look at the “9” and think it goes awry there, but remember that the next two numbers are 8 and 13, and so we involve some sort of “carrying over” that can transform what we’d hope to be an “8” into a “9” if we do our addition properly. There’s a clever trick we can then leverage to test for the Fibonacci pattern a little more expansively.

The first eight terms of the Fibonacci sequence, with the third-through-eighth terms shown generated by summing each of the prior two terms. This simple sequence, named after a 13th century mathematician but known for over 1000 years prior, shows up in nature, but has its roots in mathematics.

Instead of using the number 89, let’s remember we’re in base 10, so let’s help ourselves out by appending a “9” on either side of that number 89, to instead create the fraction 1/9899. When we expand that out — to more digits this time — here’s what we find as its decimal expansion:

• 1/9899 = 0.0001010203050813213455…,

and suddenly we see many more Fibonacci numbers emerging. What if we tried adding a few more 9s to either side? Say, 1/99989999? Now the decimal expansion becomes:

• 1/99989999 = 0.00000001000100020003000500080013002100340055008901440233037706100987159725844181…,

and we can see that more and more terms are emerging before we run into “carrying” errors. Add more 9s on either side of your denominator, and you’ve got a formula for generating the Fibonacci numbers, in order, appearing in your fraction, as far as you dare to expand it out. You can simply read the numbers off, and by adding as many 9s as you like, in equal numbers, on either side of “89” in the denominator, a decimal expansion will give you all the Fibonacci numbers, guaranteed, with fewer digits in it than the number of “9s” you put on either side of the fraction’s denominator.

From expanding the fraction 1/9999899999, where you have four “9s” on either side of the key number “89” in the denominator, you can read off the first 24 Fibonacci numbers in order, with every 5 digits, until the problem of “carrying over” starts to pollute your sequence.

But it all comes back to “89” for a profound reason. Imagine that we added up the terms in the Fibonacci sequence by dividing each term by 10⁽ⁿ⁺¹⁾, where n is the number of that term. In other words, that means our additive sequence looks like:

• 0.0 + 0.01 + 0.001 + 0.0002 + 0.00003 + 0.000005 + 0.0000008 + 0.00000013 + 0.000000021 + 0.0000000034 + 0.00000000055 + 0.000000000089 + 0.0000000000144 + ….,

and so on. Now, let’s do a little math trick: we’ll multiply that sequence by 10, and then subtract the original sequence from it (giving us nine times the original sequence). That looks like this (ignoring the first term, which equals zero):

• 0.1 + 0.01 + 0.002 + 0.0003 + 0.00005 + 0.000008 + 0.0000013 + 0.00000021 + 0.000000034 + 0.0000000055 + 0.00000000089 + 0.000000000144 + … – (0.01 + 0.001 + 0.0002 + 0.00003 + 0.000005 + 0.0000008 + 0.00000013 + 0.000000021 + 0.0000000034 + 0.00000000055 + 0.000000000089 + 0.0000000000144 + …),

which, if we take the first term separate and then group each subsequent term together, gives us:

• 0.1 + (0 + 0.001 + 0.0001 + 0.00002 + 0.000003 + 0.0000005 + 0.00000008 + 0.000000013 + …),

which tells us that nine times the original sequence equals 0.1 + one-tenth of the original sequence!

Or, in other words, that the original sequence, i.e., the sum of the numbers in the Fibonacci sequence sorted by decimal places, equals 0.1/8.9, or 1/89. And that’s why the Fibonacci sequence isn’t inherent to nature, but rather, to pure mathematics instead. It appears in nature because the golden ratio has a biological utility, but wherever it appears in the physical sciences, including in some spiral galaxies, it’s only by pure coincidence!

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: What explains the Fibonacci sequence? is featured on Big Think.

It’s almost unfathomable how far humanity has come in just the past few decades in terms of discovering potentially habitable worlds. As recently as 1990, there were no known, confirmed planets beyond the ones here in our Solar System; as of today, there are more than 5000 confirmed exoplanets, from super-Jupiters all the way down to sub-Earth sized worlds. Many of the smaller ones are around stable, Sun-like stars; many of them are thought to have thin atmospheres; many likely possess the heavy elements needed for life processes; many are part of multi-planet systems, with the potential for Earth-like temperatures and pressures at their surface.

While Earth might be the template for what we think of as an ideal, habitable world, we can still envision a wide variety of circumstances that are very different from our own that might also support life on a long-term basis. It took more than 9 billion years of cosmic evolution before Earth formed, however. It’s wildly unreasonable to assume that the Universe required all of that time to create the necessary conditions for habitability.

When we look at the minimum recipe for habitability, with the fewest assumptions, those conditions could have originated far earlier. The raw, chemical ingredients for life are a part of that puzzle, but can’t be the entire story. But in order to form a habitable planet, we have to look more deeply. Here’s what we need to know about the earliest habitable worlds.

With recent discoveries, we have a tremendous amount of knowledge about the number of planets out there, including what stars they’re around and what sizes and distances from their star they possess. But when it comes to the question of whether they’re inhabited, we have no information at all.

The first thing you need, if you want to have an inhabited world, is the right type of star for it to orbit around. There could be all sorts of scenarios where life on a planet can survive even around an active, violent star, and remain habitable despite the hostility.

The lowest mass stars, the red dwarf stars like Proxima Centauri, emit flares and high-energy radiation in bursts. Any potentially habitable planet would be at risk of losing its atmosphere until that violent stellar activity settles down: a process that, for the lowest-mass stars, takes longer than the present age of the Universe. While you can imagine a scenario where:

• a magnetic field,
• a thick atmosphere,
• and life that was smart enough to seek refuge during flaring, radiative events,

might all combine to make such a world habitable on a sustained basis, it’s very difficult to imagine a thriving ecosystem on such a world during the early cosmic stages.

In other words, if we’re going to look for the first possibly inhabited planets, we should be looking around the stars that reach that “point of stability,” where they won’t be stripping an otherwise good candidate-for-life planet’s atmosphere away, and irradiating their surfaces with X-rays and flares, the soonest.

This image shows a temperature profile of star HD 12545, which unlike our Sun, doesn’t just have a small number of tiny sunspots on it, but is dominated by a massive, star-spanning starspot that covers approximately 25% of its surface. Many stars, including low-mass, young, and rapidly rotating stars, have enormous sunspots that can play a major role in the habitability of their systems: disfavoring them as good candidates for life.

As it turns out, the more massive a star is, the more quickly it settles down into a state where it:

• doesn’t flare as frequently,
• doesn’t emit the highest-energy X-ray radiation in copious quantities,
• and doesn’t give off winds, particles, and other atmospheric-stripping effects,

in great bursts that would hinder an orbiting planet’s habitability. In addition, the more massive a star is, the greater the amount of overall energy it will give off. Combining these facts together could suggest that, if energy is what it takes to power life, perhaps these large volumes of space around these stars, where the temperature will be “just right” for liquid water on an Earth-like planet’s surface, might be the best place to look.

But if your star is too short-lived, habitability is impossible. Additionally, if your stellar system is too pristine, meaning it contains too few heavy elements, habitability will also be impossible, as the raw ingredients that are required for life processes simply won’t be present. If we want a planet that can house life, we need:

• a not-too-massive star that will live long enough for an orbiting world to evolve life on it,
• in a stellar system that’s enriched enough for a rocky world to form with the right chemical ingredients for life,
• but also a massive-enough star so that stellar activity won’t effectively sterilize any life that attempts to form.

The star forming region Sh 2-106 showcases an interesting set of phenomena, including illuminated gas, a bright central star that provides that illumination, and blue reflections off of gas that has yet to be blown away. The various stars in this region likely come from a combination of stars of many different pasts and generational histories, but none of them are pristine: they all contain significant quantities of heavy elements in them. That is one of the necessary ingredients for rocky planets and potential habitability.

The first generation of stars, known as Population III stars, is right out with these constraints. On average, these stars are very massive: 10+ times the mass of the Sun, meaning that they don’t live for ~10 billion years, but only for ~10 million years apiece: a timescale that’s too short to meet our necessary conditions. In addition, that first generation of stars is pristine, meaning that there are practically no heavy elements inside. (If we look at the amounts of elements like oxygen and carbon produced during the Big Bang, it’s down by a factor of many trillions below what the solar abundance of oxygen and carbon, the third and fourth most common elements in the Universe, are observed to be today.)

In fact, we now know what the rough threshold, in terms of chemical enrichment, is for planets to exist around stars: between about 10-25% of the fraction of heavy elements found in the Sun. Of the more than 5000 exoplanets that have been discovered, only:

• 1.8% of them are found around stars with a quarter or fewer of the Sun’s heavy elements,
• 0.2% of them are found around stars with a tenth or fewer of the Sun’s heavy elements,
• and 0 of them are found around stars with 5% or fewer of the Sun’s heavy elements.

Significant chemical enrichment is needed for planets at all, and that takes not only time, but multiple generations of stars.

This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. Stars in red, orange, and yellow are all rich enough in heavy elements that they should have planets; green and cyan-coded stars should only rarely have planets, and stars coded blue or violet should have absolutely no planets at all around them. Note that the central plane of the galactic disk, extending all the way into the galactic core, has the potential for habitable, rocky planets.

In a stellar system that is enriched enough and whose stars are long-lived enough, we also have to recognize that cratering/impact rates, or a “heavy bombardment” period, will also disfavor the sustainability of life while it occurs. From the best observations we have today, of our own Solar System as well as other stellar systems, it takes about half-a-billion years, give or take, for those conditions to occur. In other words, we not only need prior generations of stars to live-and-die to create sufficiently enriched material to form rocky worlds, we also need time to pass on those worlds.

And why would we need rocky worlds? As far as we understand life, a world that houses it would need:

• an energy gradient, where it has a non-uniform energy input,
• the capability of maintaining a substantial-enough atmosphere,
• liquid water in some form on the surface,
• and the right raw ingredients so that life, given the right confluence of circumstances, can survive and thrive.

A rocky planet of large enough size, forming with the right atmospheric density, and orbiting its world at the right distance, has a chance. (A rocky moon of a gas giant would also have a chance.) Given all the planets that could possibly form around a new star and the astronomical number of stars formed in each galaxy, the first few of these conditions are easy to meet.

This depiction of an Earth-like exoplanet showcases a rocky world with a thin atmosphere in its parent star’s habitable zone. It has oceans and continents and clouds, and could possess macroscopic life forms on its surface. At a distance of multiple light-years away, it would take a gargantuan telescope to image them, and it would only be able to see the world as it was in the distant past, not as it is right now.

Orbiting a star will provide an energy gradient, as could orbiting a planet, a gas giant possessing a large moon, or simply a world that’s sufficiently geologically active. Whether from solar input or hydrothermal/geothermal activity, a non-uniform energy input is easy. With enough of the elements such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and a few others, a substantial atmosphere will allow liquid water on the surface. Planets with these conditions should come into existence after only just a few hundred million years, but those worlds will also need enough time for their initially violent environment to settle down.

These needed elements must also be accessible, and so that typically requires an aqueous environment, where elements like phosphorus and sulfur can undergo reactions with carbon, oxygen, and nitrogen atoms, so that the right chemicals can be formed. It’s better if these environments are self-contained, such as in a tidepool, in a freshwater store on land, or some other shallow environment, as collisions between atoms and molecules are what lead to interesting binding patterns and structures. Without those conditions, it’s difficult to see how the right biochemical reactions that we require to have life processes can occur.

This aerial view of Grand Prismatic Spring in Yellowstone National Park is one of the most iconic hydrothermal features on land in the world. The colors are due to the various organisms living under these extreme conditions, and depend on the amount of sunlight that reaches the various parts of the springs. Hydrothermal fields like this are some of the best candidate locations for life to have first arisen on a young Earth, and may be home to abundant life on a variety of exoplanets.

It’s also worth talking about where these conditions will arise first. In locations found on the outskirts of large galaxies, it might take many billions of years for enough generations of stars to live-and-die to get up to that necessary abundance. But in the hearts of these massive galaxies, where star formation occurs frequently, continuously, and out of the recycled remnants of prior generations of supernovae, planetary nebulae, and neutron star mergers, that abundance can rise quickly.

Even in our own galaxy, globular cluster Messier 69 shows evidence that stars had up to 22% of our Sun’s heavy element content by the time the Universe is just 700 million years old: approaching the conditions needed for rocky planets and life processes in under a billion years of cosmic history.

The centers of active galaxies, however, is a relatively difficult place for a planet to be considered habitable, as the accompanying cosmic fireworks are a hazard that must be reckoned with. Wherever you have stars continuously forming, you have a spectacular slew of cosmic fireworks. Gamma ray bursts, supernovae, black hole formation, quasars, and collapsing molecular clouds make for an environment that is, at best, precarious for life to arise and sustain in.

This spectacular image was created with composite Spitzer and Hubble data, and shows a tidally distorted galaxy, rich in gas and actively forming new stars, merging with an old, gas-free elliptical galaxy made up of older stars. Poetically, this is called ‘the penguin and the egg,’ where the active star-forming regions of the Penguin may create a hostile environment for life, whereas the calm environment of the Egg may be among the best places for sustained life to emerge and thrive.

To have an environment where we can confidently state that life arises and maintains itself, we need for the star-forming process to come to an abrupt end. We need something to put a stop to star formation, which in turn puts the kibosh on the activity that is most threatening to habitability on a world. That is likely to be feedback from stars, which will clear away the central environment of gas and dust, and soon after, the central black hole, rather than being in an active state, will go quiet. It is for these reasons that the earliest, most sustainably habitable planets might not be in a galaxy like ours, but rather in a red-and-dead galaxy that ceased forming stars billions of years ago.

When we look at galaxies today, some 99.9% of them still have populations of gas and dust in them, which will lead to new generations of stars and constant, ongoing star formation. But about 1-in-1000 galaxies stopped forming new stars some 10 billion years ago or more. When their external fuel ran out, which could occur in the aftermath of a catastrophic major galactic merger, star formation abruptly comes to an end. Without new stars forming, the more massive, bluer ones simply end their lives when they run out of fuel, leaving the cooler, redder stars as the only survivors. These galaxies are, today, known as “red and dead” galaxies as a result, because all of their stars are stable, old, and unimpeded by the violence that new star formation brings.

One of them, galaxy NGC 1277, can even be found in our relative cosmic backyard.

This nearby galaxy, NGC 1277, although it may appear similar to other typical galaxies found in the Universe, is remarkable for being composed primarily of older stars. Both its intrinsic stellar population and its globular clusters are all very red in color, indicating that it hasn’t formed new stars in ~10 billion years. Some of the earliest living planets and worlds may have arisen in “red and dead” galaxies such as these.

The recipe for a habitable planet, at the earliest, might be to

• form stars rapidly,
• over and over,
• in a very dense region of a large galaxy,
• followed by a major galactic merger,
• resulting in a massive, gas-and-dust-clearing starburst,
• followed by a sudden end to star formation that persists for the indefinite future.

This could get us up to stars-with-planets, with solar-like heavy element abundances, where the planets (or moons, or other worlds) orbiting those stars have settled down into a state without frequent giant impacts, in just over a billion years after the start of the hot Big Bang.

Once those conditions are in place, it’s likely that complex molecules will form, smash together, and achieve a wide variety of configurations. When two conditions are met simultaneously by the first complex molecules:

1. they can metabolize some source of energy or nutrient from their environment,
2. and they can replicate and/or reproduce themselves,

then we can state, even though it’s a primitive form of life that many biologists won’t recognize as “alive,” that the first arrival of life from non-life has arisen. The worlds where that life can sustain itself, survive, reproduce, and remain for long periods of time, rather than dying out almost immediately, will become the first inhabited worlds in the Universe.

Intelligent aliens, if they exist in the galaxy or the Universe, might be detectable from a variety of signals: electromagnetic, from planet modification, or because they’re spacefaring. But we haven’t found any evidence for an inhabited alien planet so far. We may truly be alone in the Universe, but the honest answer is we don’t know enough about the relevant probabilities to draw definitive conclusions. Our first discovery of life beyond Earth still awaits.

While it may have taken over 9 billion years for the Earth to form, and several hundred million years for life to arise and begin thriving on our world, we can be certain that similar worlds, with similar life-friendly conditions, were in place just over a billion years into the history of the Universe. It’s an extremely rapid, optimistic estimate, but given that there are trillions of galaxies in the Universe today, some of them are going to be cosmic oddities and statistical outliers in a fashion that makes life’s arrival more favorable than in average places. The only questions that remain are those of:

• abundance (how often do potentially habitable worlds with all the right ingredients and conditions arise),
• likelihood (how often, among those potentially habitable worlds, do they wind up becoming inhabited),
• and timescales (how long does it take for such worlds to come into existence, and how long does it take life to arise on them).

It still remains plausible that life may even arise in the Universe before the billion-year threshold is reached, but our goal is sustained, continuous habitability, not just life merely arising, only to almost-immediately go extinct. By the time the Universe is a shade under two billion years old — just 13-14% its current age — there should exist many galaxies within it where Sun-like stars, Earth-like planets, and all the right ingredients and conditions for life’s emergence and persistence should exist and remain. The key step left to discover is the one we still remain ignorant about: quantifying the prevalence of life on planets beyond Earth. Once the first living worlds arise, we can be certain that many more will soon follow, likely all throughout the Universe.

This article What was it like when the first living worlds formed? is featured on Big Think.

At the center of practically every galaxy today isn’t just a collection of stars, gas, and dust, but a monster behemoth: a supermassive black hole. Ranging from millions to billions of solar masses, these cosmic monstrosities are responsible for some of the most violent, energetic events in the known Universe.

• When a star or other massive object passes too close to one, the black hole’s gravity can violently tear it apart: a tidal disruption event.
• When gas or other matter gets accreted around that black hole, the acceleration of that matter produces jets of radiation and particles: an active galactic nucleus, blazar, or quasar, depending on how we view it.
• And when another black hole merges with a supermassive one, it generates incredibly energetic gravitational waves: perhaps the most energetic events in all the cosmos.

Today, even the most massive of the known black holes represent only about 0.1% of the stellar mass of the galaxy: just one-thousandth of the amount of mass found by summing up all the stars in the galactic environment surrounding it. For a long time, astronomers have wondered just how these supermassive black holes came to be: did they form from earlier generations of stars, or was something else needed to explain them? With a large suite of new data now available owing to the advent of JWST, the answer now seems certain: stars, alone, can’t explain these black holes. Here’s the evidence that leads us to that conclusion.

This illustration of a tidal disruption event shows the fate of a massive, large astronomical body that has the misfortune of coming too close to a black hole. It will get stretched and compressed in one dimension, shredding it, accelerating its matter, and alternately devouring and ejecting the debris that arises from it. Black holes with accretion disks are often highly asymmetrical in their properties, but far more luminous than inactive black holes that lack them.

The first thing we have to understand is that there are two main ways that black holes can grow:

1. by the steady, gradual accretion and infall of matter, such as gas,
2. and by individual events such as mergers and the swallowing of massive objects, such as stars, stellar remnants, and other black holes.

The first one, in general, is the primary method that leads to growth over time, while the second can lead to growth in leaps, jumps, and bursts, particularly when violent events such as galactic mergers occur. (And, specifically, when two supermassive black holes, one originating from each progenitor galaxy, merge together.)

When you have an accreting black hole, such as an active galactic nucleus or a quasar, there has to be a balance between the outward force/pressure of the radiation and winds that emerge from the object and the gravitational force of matter that falls inward. If there’s too much infall of matter, then there will be a rise in the amount of radiation and winds, and that will blow the infalling matter back out: a phenomenon that occurs when black hole growth proceeds at a rate that exceeds a certain theoretical limit. Because of this relation, there’s a limit to how quickly objects like black holes can grow, and hence, if we extrapolate backward in time, there’s a limit to how large an initial “seed” for these supermassive black holes must have been.

If you begin with an initial, seed black hole when the Universe was only 100 million years old, there’s a limit to the rate at which it can grow: the Eddington limit. If seeds of several tens-of-thousands of solar masses arise early on and these SMBH seeds grow rapidly thereafter, there may be no conflict with what’s observed, after all.

For a black hole of even tens of billions of solar masses today, that doesn’t necessarily pose any sort of problem, because today’s black holes have had 13.8 billion years of cosmic history over which they grow. But if we look back in time, we would have naively expected that the most massive black holes that we would have seen at early times would be far less massive than the enormous black holes spotted today: in the billions or even tens of billions of solar masses.

In the pre-JWST era, it was rare to find astronomical objects — things like galaxies and quasars — from the first ~1.5 billion years of the Universe’s history (beyond a redshift of z = 4, in astronomy-speak), as those objects were not only incredibly far away, but were fainter, lower in mass, and had their light severely redshifted by the expansion of the Universe. The few that we did find, if they had enough activity coming from their centers, gave enough information to infer the masses of the supermassive black holes they housed from data in the non-visible portion of the spectrum: such as at infrared or X-ray wavelengths.

Quite surprisingly, we began to find that, even before the Universe was a mere 1 billion years old, some of these black holes had masses that rivaled or even exceeded a billion solar masses.

If the Universe was born with primordial black holes, a completely non-standard scenario, and if those black holes served as the seeds of the supermassive black holes that permeate our Universe, there will be signatures that future observatories, like the James Webb Space Telescope, will be sensitive to.

That led to a big puzzle: how did black holes get so massive so quickly? After all, there were three major options to consider that would still be consistent with the other data we had about the Universe from observations of cosmic structure, galaxy evolution, the leftover radiation from the Big Bang, and more.

1. The Universe was born without black holes, and none arose until the first stars lived-and-died, with the most massive among them leaving black hole remnants behind.
2. The Universe was born without black holes, but formed them not only from stars, but from clumps of matter — such as streams of gas — that directly collapsed to form event horizons around them.
3. The Universe was born with either primordial black holes or incredibly massive, overdense seeds that would swiftly collapse to create black holes, long before any stars had the opportunity to form.

The first option is a certainty: as soon as our Universe made stars, many of those first stars would die in a catastrophic supernova explosion, their cores would collapse, and many of those stars would leave a black hole behind as a remnant. Those black holes, however, would only have masses around 100 times that of the Sun. The second option is an intriguing possibility, bolstered by recent theoretical simulations by Muhammad Latif and Daniel Whalen, that could lead to more massive seeds: up to 10,000 or even perhaps 100,000 solar masses each. The third option, while exotic, would perhaps require invocation if still more massive seeds were needed.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the growth of galactic structures.

The key to uncovering the past history of the Universe, and to understanding the origin of the heaviest supermassive black holes, is not rooted in some theoretical calculations; it’s driven by observations. If we want to know how these black holes arose, and how they grew so big so quickly, there are two key classes of observations we need to make.

One is to look farther back in space and time to the earliest times in the Universe — back to galaxies from the first 1.0-1.5 billion years of cosmic history, the first 0.8-1.0 billion years, the first 0.6-0.8 billion years, and even before the first 0.6 billion years — and to measure the masses of supermassive black holes found at these early stages of cosmic history.

And the second is to take those galaxies where a black hole’s mass can be extracted and to measure the mass present in stars in those galaxies, and to see whether the relationship between the mass of the central, supermassive black hole and the mass present in stars is the same as it is at late times — in that same ration of about 1:1000 — or whether that mass ratio evolves toward more equal values.

With both of those pieces of information, we should be able to tell whether the black hole-stellar mass ratio remains constant, which would point to a “stellar seed” origin for these black holes, or whether there are an abundance of what we call overmassive black holes: where the black holes that are observed would need to have grown from something more massive than the black hole left behind by a star.

A very distant galaxy, found in the background of JWST’s image of galaxy cluster Abell 2744 (Pandora’s cluster), emits copious amounts of X-rays, consistent with a black hole of between 10 and 100 million solar masses. The galaxy itself has only about that much mass in stars, making this the first “missing link” in discovering the connection between black hole and galaxy growth in the early Universe.

Now that we’re in the JWST era, we’ve discovered our first black holes from when the Universe was less than 600 million (0.6 billion) years of age: from when the Universe was just 4.3% of its current age or younger. In fact, we now have three detected black holes from that era.

• The supermassive black hole in CEERS 1019, which has a mass of around 9 million solar masses: about 1% of the total stellar mass at that time, which comes from around 570 million years after the Big Bang.
• The supermassive black hole UHZ1, which comes from just 470 million years after the Big Bang: although its mass is uncertain, between 10-100 million solar masses, the stellar mass within that galaxy is comparable: also between 10-100 million solar masses, implying the black hole-stellar mass ratio is at least 10%, and could be even 100% or more.
• And the most distant black hole yet detected, in the galaxy GN-z11, comes in at right around 2-3 million solar masses, but has an impressive ~1 billion solar masses worth of stars inside of it. The black hole-stellar mass ratio is only 0.2-0.3%, and comes from 440 million years after the Big Bang.

However, these are only a few data points, and the less data you have, the less reliable your results are going to be, especially when drawing conclusions about the entire Universe.

Whereas the blue and red-dotted lines at the lower-right of this diagram indicate the populations of modern day galaxies with black holes and stars in them, the JWST data from examining early galaxies, shown in multicolored data points elsewhere on the graph, indicate a severe departure from the modern-day relationship. This has severe implications for the seeds and origins of supermassive black holes.

Thankfully, in a new paper published in the Astrophysical Journal Letters, lead author Fabio Pacucci was able to identify an enormous number of galaxies — primarily from JWST’s CEERS and JADES surveys — that allow us to extract estimates for both the stellar masses of the host galaxies and also reliable estimates (with quantified uncertainties) for the masses of the central, supermassive black holes housed inside of them.

Today, there are big, massive galaxies like Messier 87: the central galaxy of the impressive Virgo Cluster of galaxies. There are over a trillion stars inside of it, containing a total stellar mass of between 2-and-3 trillion solar masses. It also has the largest supermassive black hole known in our vicinity: of about 6.5 billion solar masses. The black hole-stellar mass ratio of 0.2% is fairly typical of galaxies with the biggest black holes in them: around that 1-to-1000 ratio.

On the other hand, there are also galaxies with more modest supermassive black holes. Andromeda, the largest galaxy in the Local Group, has more like a 1-to-10,000 ratio. Our own galaxy, the Milky Way, has more like a 1-to-100,000 ratio, with a black hole of just ~4.3 million solar masses despite having around ~400 billion solar masses worth of stars. Finding this fainter, lower-mass population will be difficult in the early Universe, so we might expect to find only the biggest, brightest examples at early times: consistent with that 1-to-1000 ratio.

Instead of a black hole-stellar mass ratio of about 1:1000, as seen in modern times, the JWST galaxies from the first 1.5 billion years of cosmic history exhibit a black hole-stellar mass ratio more consistent with 1:10 or 1:100, indicating that more massive seed black holes were required to explain the observations.

But that isn’t what Pacucci’s study, using the most comprehensive JWST data available at the start of 2024, found at all. Instead of a 1-to-1000 ratio, they found that, overall, there was more like a 1-to-10 or 1-to-100 ratio: where black holes were ten-to-a-hundred times as massive, compared to their galaxy’s stellar masses, as the modern-day counterparts that we find today. Even if we restrict ourselves to the most extreme supermassive black holes known today, we are seeing that early on, black holes were much more overmassive back then as compared to now.

This has a lot of implications on a lot of fronts, but the biggest one is this: seeing black holes that are more massive early on, as compared to the stellar masses we find in the galaxies that house them at those early times, is strong evidence that — at least in these galaxies — the black holes that we are seeing did not arise from the stars themselves, but rather from more massive seeds than stars, alone, can provide. The fact that these black holes are “only” a factor of 10, 100, or even 1000 times larger than they are at present suggests that they didn’t form from stars, but rather from the direct collapse process of clouds of normal matter, further suggesting that invoking something exotic, like primordial black holes, is unnecessary.

Whereas “light” black hole seeds, such as black holes that arise from the first generations of stars that form, could theoretically explain many of the supermassive black holes that exist at both early and late times, there is now strong evidence, especially early on, that at least some population of heavy seeds, of around ~10,000-100,000 solar masses, is needed to explain the early, overmassive black holes spotted by JWST.

This new study, synthesizing large amounts of JWST data from the first 1.5 billion years of cosmic history, reveals a large population of overmassive black holes. Whereas the identification of UHZ1, the first severely overmassive black hole, provided evidence for heavy black hole seeds that were formed by direct collapse, the current study both strengthens the evidence for that scenario and also shows that this scenario may be the norm for giving rise to the most massive black holes of all in the modern-day Universe.

This scenario was theoretically predicted back in the 2010s, and to see the evidence coming in for it today is vindication for the astronomers and astrophysicists who pushed for this once-controversial scenario back in the days when data was sparse. The fact is that we can now confirm that stars, alone, are unable to explain the origins of all of the supermassive black holes that we find in the Universe, and that some other mechanism — a mechanism that is consistent with what’s predicted by direct collapse scenarios — is required to give rise to the ones we find early on.

This set of illustrations explains how a large black hole can form from the direct collapse of a massive cloud of gas a few hundred million years after the Big Bang. Cold streams of gas can lead to the direct collapse of a “seed” black hole of several tens of thousands (at least) solar masses, which can form even prior to any stars forming in the surrounding young galaxy. As the galaxy and black hole grow, eventually the stellar mass content will outweigh the more slowly-growing black hole.

We still don’t know whether there are other, smaller black holes out there that have yet to be revealed, particularly among the faintest, lowest-mass galaxies that strain the limits of what JWST can detect. There could yet be multiple populations of galaxies with black holes: some of which were formed by direct collapse, and others that formed from the remnants of the first stars, as we are only beginning to take a census of these objects.

However, the era of claiming that nothing more than stars — living and dying as normal, with some of the most massive ones having their cores collapse down to black holes — are required to give rise to the supermassive black holes we find at later times is now over. Something more is required: perhaps it’s direct collapse, which is the leading suspect, or perhaps it’s something even more exotic. Perhaps stars can still explain many of the supermassive black holes that arise in the Universe, or perhaps stars are only a sub-dominant effect, and that direct collapse is responsible for the majority of supermassive black holes in the Universe. With the JWST era now in full swing, the good news is, we’re likely to soon find out.

This article Stars alone can’t explain black holes, JWST data reveals is featured on Big Think.

When it comes to living your best life, Albert Einstein — notorious as the greatest physicist and genius of his time, and possibly of all-time — probably isn’t the first name you think of in terms of life advice. You most likely know of Einstein as a pioneer in revolutionizing how we perceive the Universe, having given us advances such as:

• the constancy of the speed of light,
• the fact that distances and times are not absolute, but relative for each and every observer,
• his most famous equation, E = mc²,
• the photoelectric effect,
• the theory of gravity, general relativity, that overthrew Newtonian gravity,
• and Einstein-Rosen bridges, or as they’re better known, wormholes.

But Einstein was more than just a famous physicist: he was a pacifist, a political activist, an active anti-racist, and one of the most iconic and celebrated figures in all of history.

He was also known for his unconventional behavior in a variety of ways that flouted social norms, including his unkempt hair, his witty humor, and his unrelenting hatred of socks. But less well-known is Einstein’s freely-given life advice to many of his friends, acquaintances, and contemporaries, which are perhaps even more relevant today, in the 21st century, than when he initially doled out his words of wisdom and compassion. Taken from the book The Einstein Effect, written by the official social media manager of the Einstein estate, Benyamin Cohen, these rules for a better life go far beyond physics and are relevant to us all. Here are, perhaps, the best and most universally applicable lessons from Einstein himself.

Einstein, shown here in 1940 receiving American Citizenship, was known around the world for his disheveled appearance and always wearing the same few sets of clothes, perhaps even better than he was known for his scientific theories.

Rule #1: Expend your efforts on the things that matter.

When you think of Einstein’s appearance, the word “disheveled” may come to mind. His overgrown, uncombed hair, his ratty, worn-out, often smelly clothing, his shoes without socks, etc., all were notoriously slovenly. But none of that bothered Einstein, who in his later years wore what could be considered almost a uniform: a signature grey suit, sans the traditional sport coat, with a leather jacket in its place. (And, of course, with shoes and no socks.)

This idea, of wearing simple but functional clothing that puts the wearer at ease with themselves, has been made famous in recent years by tech entrepreneurs who have their own signature style:

• Steve Jobs and his infamous blue jeans and black turtlenecks (a style copied by Elizabeth Holmes),
• Jeff Bezos, who wears blue jeans with short-sleeve, monochrome, collared shirts,
• Mark Zuckerberg, who prefers blue jeans and T-shirts,
• Satya Nadella, who typically wears slacks, polo shirts, and Lanvin shoes,
• and Jack Dorsey, whose all-black outfits often include a hat, hoodie, or jacket,

is prized for one reason above all others: efficiency.

This 1937 photo shows Einstein in his New Jersey home with violinist Bronislaw Huberman. Einstein is wearing his favorite outfit: a suit with his Levi’s leather jacket and shoes with no socks.

If you have a lot of decisions to make each day, or a lot of work that requires mental effort in any sense, cutting down on your overall mental load is of paramount importance if you want to avoid what’s known as decision fatigue: where our ability to make good decisions degrades as we become more tired from relentlessly having to make choices.

As fashion journalist Elyssa Goodman wrote, “Uniform dressing has roots in not just physical but mental efficiency. People who have to make immense decisions every day will sometimes choose a consistent ensemble because it allows them to avoid decision fatigue, where making too many unrelated decisions can actually cause one’s productivity to fall off.”

It’s a way to economize your efforts: to put them where they’re most needed, at the expense of not wasting them on spurious or unimportant matters. In other words, choosing not to put effort into the things that are superfluous to what’s actually important to you is a way to become more mentally efficient, which frees up your mind to focus on what actually matters most to you. Einstein’s lack of effort into his personal presentation extended to his disdain for going to the barber, as well as his often nearly-illegible penmanship. But the rewards, of focusing his mind on what was truly important to him, led him to a rich, fulfilling life.

This 1930 photograph shows Albert Einstein sailing with his step-daughter Ilse and her husband Rudolf Kayser in Germany, less than 3 years before he fled his home country for the United States.

Rule #2: Do things you love, even if you’re terrible at them.

While many of Einstein’s passions extended far beyond physics — including a love of baked goods and a penchant for playing the violin — perhaps the one he enjoyed the most was sailing. As Einstein wrote, “A cruise in the sea is an excellent opportunity for maximum calm and reflection on ideas from a different perspective.” His second wife (and cousin), Elsa, added that “There is no other place where my husband is so relaxed, sweet, serene, and detached from routine distractions; the ship carries him far away.” By focusing on something mundane, Einstein’s mind was free to wander, frequently leading him to exciting new ideas.

Einstein, however, was completely inept at sailing, and was at best a wildly inattentive sailor. He would frequently lose his direction, run his boat aground, or have his mast fall. Other sailing vessels frequently had to beware of Einstein’s ship, as he was a hazard to himself and others, refusing to wear a life vest despite being unable to swim. Boaters and even children routinely rescued him, and having his boat towed back to shore was a frequent occurrence. But the serenity Einstein experienced while sailing was unparalleled, giving him a mental freedom that we should all aspire to for ourselves.

This 1934 photograph shows Einstein in front of a blackboard, deriving special relativity for a group of students and onlookers. Although special relativity is now taken for granted, it was revolutionary when Einstein first put it forth, and it isn’t even his most famous equation; E = mc² is.

Rule #3: Have a puzzle mindset.

Think about the problems that we face, both as individuals and collectively, as a civilization. These could be financial, environmental, health-related, or political, for example, as those arenas affect us all. Do you view these problems as crises? If you do, you probably feel despair at them, as there’s very little that’s empowering about facing a crisis. But if you view them as a puzzle, you might be inclined to think about a fresh approach to solving them. In this regard, Einstein was pretty much the prototype individual for someone who viewed every difficulty he faced as a puzzle to be solved: in physics and beyond.

Consider his oft-misunderstood but most famous quote, “Imagination is more important than knowledge.” While many people had looked at the puzzle of objects moving near the speed of light before — including other geniuses like FitzGerald, Maxwell, Lorentz, and Poincaré — it was Einstein’s unique perspective that allowed him to approach that problem in a way that led him to the revolution of special relativity. With a flexible, non-rigid worldview, Einstein would easily challenge assumptions that others couldn’t move past, allowing him to conceive of ideas that others would unceremoniously reject out-of-hand.

The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein’s equivalence principle. If inertial mass and gravitational mass are identical, there will be no difference between these two scenarios. This has been verified to ~1 part in one trillion for matter, and was the thought (Einstein called it “his happiest thought”) that led Einstein to develop his general theory of relativity.

Einstein was no stranger to having strongly held convictions about both life and physical reality, but each of his opinions, even those he was most certain of, were no more sacred to him than a mundane hypothesis. When one has a hypothesis, or idea, the goal isn’t simply to find out whether that hypothesis is right or wrong; in some sense, that’s the least interesting part of the endeavor. The search for the answers, including figuring out how to perform the critical test and interrogate the Universe itself in an effective manner, was what truly got Einstein excited.

His thought-experiments were among the most creative approaches ever taken by physicists, and that line of thought has been adopted by a great many scientists ever since who wish to avoid what’s known as cognitive entrenchment. What would a light-wave look like if you could follow it by traveling at the same speed it traveled at? How would the light from a distant star be deflected by the Sun’s gravity during a total solar eclipse? What experiments could one perform to determine whether our quantum reality is pre-determined by variables we cannot observe directly? Unlike a preacher who claims to be infallible, a prosecutor who wants to convince you of their perspective, or a politician who just wants to win your approval, having a puzzle mindset — i.e., the mind of a scientist — is the only one that can lead you to novel discoveries, including quite unexpected ones.

Niels Bohr and Albert Einstein, discussing a great many topics in the home of Paul Ehrenfest in 1925. The Bohr-Einstein debates were one of the most influential occurrences during the development of quantum mechanics. Today, Bohr is best known for his quantum contributions, but Einstein is better-known for his contributions to relativity and mass-energy equivalence. Both were known for thinking long and hard about the most difficult puzzles the Universe had to offer.

Rule #4: Think deeply, both long and hard, about things that truly fascinate you.

Over the course of his long life, Einstein received many letters: from those who knew him well to perfect strangers. When one such letter arrived on Einstein’s desk in 1946, asking the genius what they should do with their life, the response was as astute as it was compassionate. “The main thing is this. If you have come across a question that interests you deeply, stick to it for years and do never try to content yourself with the solution of superficial problems promising relatively easy success.”

And if you fail to arrive at the solution you’ve been chasing, don’t despair. As Einstein wrote to his friend David Bohm, “You should not be depressed by the enormity of the problem. If God has created the world, his primary worry was certainly not to make its understanding easy for us.” Although Einstein was most famous for the problems he did solve, there were plenty whose solutions eluded him all his life: from finding a deterministic explanation for the observed quantum behavior to the attempt to unify all of physics (including gravity and the other forces) into one overarching framework.

Although many have tried-and-failed (and continue to try-and-fail) to solve these and other puzzles, the greatest joy and fulfillment is often to be found in the struggle itself.

This political cartoon, published in 1933, shows Einstein shedding his pacifist wings to roll up his sleeves and take up a sword labeled “preparedness.” Einstein would at this point call upon the friends of civilization all across the world to unite against Nazi militarism.

Rule #5: Don’t let politics fill you with either rage or despair.

Einstein kept up with many friends and members of the public, but also with his extended family. In correspondence with his cousin Lina Einstein, he offered a lesson that many of us would do well to heed. “About politics to be sure, I still get dutifully angry, but I do not bat my wings anymore, I only ruffle my feathers.”

How many of us have seen a friend, acquaintance, or even total stranger make a statement that filled us with outrage, and flew off the handle, filled with righteous indignation, and launched into a tirade as a result? While that might fulfill some primitive need in us to speak our mind and challenge what we see as an unacceptable narrative, how often was such a response actually effective in achieving any of our goals?

Sometimes, it truly is important to intervene and go all-out: what Einstein refers to as “batting our wings.” But at other times, in a lesson that King Bumi from Avatar: The Last Airbender would heartily approve of, sometimes the best response is to sit back, observe, think, and wait for the opportune, strategic moment to take action down the road: “ruffling our feathers” for the time being. It’s often a wise course of action, although for Einstein’s ill-fated cousin, Lina, it’s worth mentioning that she died in the Nazi gas chambers in 1942. (Update: That was a different “cousin Lina” to Einstein. The Lina he gave the advice to, his cousin Carolina, left Europe in the 1930s and emigrated to Uruguay, where she lived out the rest of her days.)

Albert Einstein (right) is shown with physicists Robert Millikan (left) and Georges Lemaître (center) several years after admitting his biggest blunder. If you think that modern critics are harsh, one can only imagine how Lemaître must have felt to receive a letter from Einstein calling his physics abominable! Fortunately, just as Einstein was not dissuaded by the prevailing authorities of his time, Lemaître and others were not deterred by Einstein’s declarations of unsoundness.

Rule #6: Blind obedience to authority is the greatest enemy of the truth.

Many of us, upon hearing something that we are certain is either absurd, flawed, or hopelessly corrupt, immediately and vociferously make up our minds to oppose them, regardless of what the full suite of evidence actually indicates. Once we abandon our critical thinking faculties because we are certain we know the answer, we tend to simply go along with those who agree with us and oppose those who espouse anything different. To Einstein, this represented the death of the rational mind, which he called “collective insanity” or a “herd mind.” Today, we would likely call it groupthink, and Einstein noted that it was often driven by a prominent figure spouting propaganda.

Scientists, including formerly reputable ones like Johannes Stark (Nobel Laureate and founder of the Stark effect), formed an anti-relativity society that discredited Einstein and his theory. Fueled by nationalism and anti-semitism, Einstein and his ideas became a target, with one line of attack claiming relativity was wrong and dangerous, and another line claiming it was brilliant but that Einstein stole the idea from “real” (non-Jewish) scientists. It was this course of action that eventually led to Einstein having a bounty placed on his head, leading to him fleeing Germany for the United States. While Einstein initially thought these machinations were silly, ridiculous, and harmless, he later concluded that “Blind obedience to authority is the greatest enemy of the truth.” In the era of fake news, this lesson is more important to assimilate than ever.

During the 1940s, Einstein himself gave a number of lectures to students who would have, in the past, never have had access to a speaker such as himself. Einstein made it a point to be generous with his time and with affording others access to him, and was a prominent supporter of civil rights for all.

Rule #7: Science, truth, and education are for everyone, not just the privileged few.

Einstein was often very critical of the United States Government, even after emigrating in the 1930s and gaining his citizenship in 1940. The history of slavery and ongoing segregation and racism, in particular, resonated with him the same way that anti-Semitism did: as fundamentally dehumanizing as it was baseless. The FBI began a file on Einstein in 1932, and it had burgeoned to more than 1400 pages by the time Einstein died in 1955, and Einstein’s anti-racist actions were deemed fundamentally un-American by many (including Senator Joseph McCarthy), but Einstein would not be deterred.

In 1937, Einstein invited black opera star Marion Anderson to stay at his house when she was refused lodging at the local (segregated) hotel in Princeton. In 1946, Einstein took the revolutionary action of simply visiting Lincoln University — the first degree-granting black college in the United States — and lectured, speaking with students and answering questions. Delivering an address to the student body, Einstein said:

“My trip to this institution was on behalf of a worthwhile cause. There is a separation of colored people from white people in the United States. That separation is not a disease of colored people. It’s a disease of white people.”

In 1953, Einstein defended the academic freedom of William Frauenglass, a teacher who taught about easing interracial tensions, in a letter published by The New York Times. The following year, he further pushed for “the right to search for truth and to publish and teach what one holds to be true.” In this day and age, we can be certain that Einstein would have pushed for science, truth, and education to be available to everyone. While certain physical properties may be relative, like space and time, the joys, knowledge, and truths uncovered by science belong to no one race, nation, or faction, but rather to all of humanity.

The quotes contained within were curated and lifted from Benyamin Cohen’s new book, The Einstein Effect, and this piece contains affiliate links to that book.

This article Einstein’s 7 rules for a better life is featured on Big Think.

Our Solar System formed some ~4.6 billion years ago.

Although we now believe we understand how the Sun and our Solar System formed, this early view of our past, protoplanetary stage is an illustration only. While many protoplanets existed in the early stages of our system’s formation long ago, today, only eight planets survive. Most of them possess moons, and there are also small rocky, metallic, and icy bodies distributed across various belts and clouds in the Solar System as well.

Eventually, eight planets emerged, with a giant impact creating Earth’s Moon.

When two large bodies collide, as they very likely did between proto-Earth and a hypothesized Mars-sized world known as Theia in the early Solar System, they’ll generally merge to form one more massive body as a result, but the debris kicked up from the collision can coalesce into one or more large moons. This was likely the case not only for Earth, but for Mars and Pluto and their lunar systems as well.

Here are 7 ways the Earth has subsequently changed.

Early on, even small-mass planets like Earth had a large hydrogen and helium envelope. Due to their low gravity, solar wind and radiation swiftly stripped that primitive atmosphere away.

1.) Atmospheric composition. Early on, hydrogen and helium dominated.

Volcanic activity present on Earth, including from the earliest times, released large quantities of solid and gaseous material into our atmosphere, including nitrogen, carbon dioxide, and water, which transformed our young hydrogen/helium atmosphere into a nitrogen/CO2/H2O rich atmosphere, which would further be transformed by biological processes.

Volcanic and biological activities were transformative.

Today, our nitrogen/oxygen atmosphere has hints of water, argon, and carbon dioxide.

While modern Earth has had plate tectonic activity for at least the past 2 billion and potentially as much as 4.3 billion years or more, the earliest phases of our planet’s history are expected to have lacked plate tectonics, as it only developed once water arrived and enough differentiation had taken place.

2.) Plate tectonics. Early Earth was lava-rich, possessing poorly-differentiated internal layers.

These tiny zircon crystals, which are only as thick as a human hair, are over 4 billion years old and hold an enormous amount of chemical information about early Earth. The silicon, oxygen, and trace element and isotope contents in these zircons and their parental magmas suggest that plate tectonics existed on Earth more than 4 billion years ago.

With severe energy gradients, a mobile lithosphere, and liquid water, plate tectonics are undeniable today.

Tidal rhythmites, such as the Touchet formation shown here, can allow us to determine what the rate of Earth’s rotation was in the past. During the emergence of the dinosaurs, our day was closer to 23 hours long, not 24. Back billions of years ago, shortly after the formation of the Moon, a day was closer to a mere 6-to-8 hours, rather than 24.

3.) Length of a day. In ancient times, Earth rotated 360° in just 6-8 hours.

The Moon exerts a tidal force on the Earth, which not only causes our tides, but causes braking of the Earth’s rotation, and a subsequent lengthening of the day. The asymmetrical nature of Earth, compounded by the effects of the Moon’s and Sun’s gravitational pulls, causes the Earth to spin more slowly. To compensate and conserve angular momentum, the Moon must spiral outward. It is for this reason that Earth will no longer have total solar eclipses after another 600 million years, and that the length of each day is getting longer as time progresses.

The amount of time a “day” takes continually lengthens, at ~24 hours presently.

A synestia doesn’t just consist of this puffy ring/torus of debris around a joint planetary core, but also rises to temperatures in excess of 1000 K, causing it to emit substantial amounts of its own infrared radiation, with peaks in different parts of the infrared spectrum dependent on the exact temperature and temperature profile of the system in question. The heat from the early Moon, just 24,000 km away initially, would have played a role in heating the lunar-facing side of the Earth.

4.) Distance to the Moon. Upon initially forming, the Moon was just 24,000 km away.

This unfamiliar view shows the size of the Earth and Moon, plus the distance from the Earth to the Moon, to actual scale. The Earth is about 12,700 km in diameter with the Moon being a little over a quarter of the Earth’s size, but the present Earth-Moon distance averages out to an enormous 384,000 km: just over 30 times the Earth’s diameter.

Tidal braking causes outspiraling, leading to its modern distance of 384,000 km.

Artist’s concept of meteors impacting ancient Earth. Some scientists think such impacts may have delivered water, amino acids, and other molecules useful to emerging life on Earth, as the evidence is strong that the impact and cratering rate across the Solar System was much higher than present for the first 0.6-0.7 billion years of our Solar System’s history.

5.) Frequency of impacts. Ancient impacts were ubiquitous across the Solar System.

This two-faced mosaic from NASA’s Lunar Reconnaissance Orbiter shows the near side (L) and the far side (R) of the Moon with modern technology. By looking at the ratios and sizes of craters found on the Moon with respect to the age of that portion of the Moon, Mars, Mercury, and Earth, we can learn how cratering rates have varied over the Solar System’s history.

Martian and lunar data show an incredible decline in crater-causing impacts.

Early on, shortly after the Earth first formed, life likely arose in the waters of our planet. The evidence we have that all life that’s extant today can be traced back to a universal common ancestor is very strong, but the early stages of our planet, for perhaps the first 1-to-1.5 billion years, remains largely obscure. While life arose early on, there is no evidence that Earth came into existence with life already on it.

6.) Presence of life. Initially, Earth was completely uninhabited.

This photograph shows chloroplasts within the plant cells of the organism Plagiomnium affine. The photosynthetic conversion of carbon dioxide, water, and sunlight into sugars, producing oxygen as a waste product, is one biological process that has been truly transformative for Earth’s atmosphere and biosphere.

For 3.8+ billion years, however, life has transformed Earth’s biosphere.

Eventually, the evolution of the Sun will be the death of all life on Earth. Long before we reach the red giant stage, stellar evolution will cause the Sun’s luminosity to increase significantly enough to boil Earth’s oceans, which will surely eradicate humanity, if not all life on Earth. The exact rate of increase of the Sun’s size, as well as the details about its mass loss in stages, are still not perfectly known.

7.) Influence of the Sun. Solar luminosity has increased 40% over the past 4.5 billion years.

After the protostar that would become the Sun contracts and cools sufficiently, nuclear fusion initiates, but the Sun’s luminosity and energy output, once settling down to a level value about 50 million years after its formation, gradually increases over time. 4.5 billion years ago, it was only ~70% the luminosity it is today.

In another 1-2 billion years, Earth’s oceans will unceremoniously boil away.

Today on Earth, ocean water only boils, typically, when lava or some other superheated material enters it. But in the far future, the Sun’s energy will be enough to do it, and on a global scale. After 1-2 billion years of further solar evolution, Earth will lose all of its liquid water to the gaseous phase, and life is expected to end on our world at that time.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

This article 7 ways the Earth has changed from ancient to modern times is featured on Big Think.

Today, our Universe is filled with stars and galaxies, and is not only expanding, but the Universe’s expansion is accelerating. If we were to break up the Universe into the different types of energy that compose it, we’d find that it was dominated by dark energy, which makes up 68% of the Universe’s energy density. Next would be dark matter, as it composes some 27% of the Universe’s energy density, followed by normal matter (protons, neutrons, and electrons), that makes up about 4.9% of all that’s out there. The other 0.1%? That’s made of things like neutrinos and photons, where all photons and the fastest-moving neutrinos both behave as forms of radiation.

But if you think about how all that radiation came to exist, an enormous amount of it is left over from the Big Bang, and was generated when massive particle-antiparticle pairs annihilated. So why, if there were so many more particle-antiparticle pairs that annihilated as compared with the matter particles that get “left over” as normal matter, doesn’t radiation play a bigger role in the Universe? That’s what Terry Bollinger wants to know, writing in to ask:

“Since the early hot universe was an almost equal mix of matter and antimatter, shouldn’t the total gravitational mass of the cosmic microwave and neutrino backgrounds be billions of times greater than that of fermionic matter?”

The quick answer – before we unpack the in-depth explanation behind it – is that it was greater, once. But no longer. Here’s the science behind why.

Just as raisins within a leavening ball of dough will appear to recede from one another as the dough expands, so too will galaxies within the Universe expand away from one another as the fabric of space itself expands. The fact that all methods of measuring the expanding Universe don’t give the same rate of expansion is troublesome, and may point to a problem with how we presently model the expansion of the Universe.

What you see, above, is an illustration of the expanding Universe. It’s also an illustration of an unleavened ball of dough with raisins randomly sprinkled throughout it. If you were to take this ball of unleavened dough up to the International Space Station – where it would experience effective weightlessness – and let it leaven with time, you’d unsurprisingly find that the dough would expand: roughly equally in all three dimensions. The raisins, meanwhile, won’t themselves expand, but will rather remain embedded in the dough exactly where they were located initially, while the dough between the raisins expands between them.

That’s what the expanding Universe is like: where the “dough” represents the fabric of space and the “raisins” represent individual, gravitationally bound cosmic structures, things like stars, galaxies, groups of galaxies, and even massive clusters or groups of clusters of galaxies. The individual structures, like raisins, don’t expand, but the space that separates the various bound structures from one another, like the leavening dough, does expand. Over time, this causes the Universe to become sparser, less densely populated, and more dilute as the expansion relentlessly continues.

When light is emitted from a source, it has a particular wavelength. The longer it must travel through the expanding Universe before being absorbed by an observer, the greater the amount that the wavelength of that light will be redshifted, or stretched to longer values, compared to the wavelength it has when it was emitted.

That only works, however, because galaxies, like raisins, are governed by a different set of dynamics (being held together in a specific configuration by the gravitational or electromagnetic force, respectively) than the dynamics that govern either radiation or dark energy. Above, you can see what happens within the expanding Universe as:

• one galaxy emits light,
• toward another galaxy,
• and then travels toward it through the fabric of space,
• which is also expanding,
• and whose expansion also stretches the wavelength of the radiation that travels through it,
• until the light finally arrives at the destination galaxy.

Remember that light’s energy, E, is defined by its wavelength, λ, and the constants c (the speed of light) and h (Planck’s constant), via the formula, E = hc / λ. As the Universe expands, say, by a factor of 2, then the wavelength gets stretched to be twice as long as it was previously, which causes the energy of that particular quantum of radiation to then be halved. (As double the wavelength, for radiation, renders it having only half the energy.) Whereas the matter (raisins/galaxies) doesn’t itself change as the Universe expands, the properties of the radiation itself, including its wavelength and energy, do change and evolve.

While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.

There’s a third type of important energy in the Universe that’s neither matter nor radiation of any class: dark energy. Matter particles themselves don’t change as the Universe expands, but the overall density of matter decreases, as the same number of particles occupies larger and larger cosmic volumes as time goes on. Quanta of radiation do change as the Universe expands, in the sense that their wavelength stretches and their individual energy drops, but they also drop in number density just as matter’s number density drops, causing the energy in radiation to fall off even faster than matter’s energy density with continued expansion.

But dark energy doesn’t do any of that, because it doesn’t behave as either matter or radiation. Instead, it behaves as space: simply creating more and more of itself in between the various quanta of matter (and antimatter) and radiation that exist within the expanding Universe. Because space is just space, its energy density doesn’t drop as the Universe expands, but rather remains constant and doesn’t dilute. This is why, at late times, dark energy comes to dominate the energy budget of the Universe, whereas at earlier times (more than about 6 billion years ago), matter was the dominant form of energy.

Various components of and contributors to the Universe’s energy density, and when they might dominate. Note that radiation is dominant over matter for roughly the first 9,000 years, then matter dominates, and finally, a cosmological constant emerges. (The others do not exist in appreciable amounts.) However, dark energy may not be a cosmological constant, exactly.

But if we extrapolate these trends even farther back in cosmic time than that, look at what occurs according to the graph above. Yes, at very late times, dark energy becomes the only important component to the Universe, as not only do the components that exist (radiation, neutrinos, dark matter, normal matter) continually fall off in density as time goes on, but even theoretical components that might have existed but don’t appear to be present in our Universe (cosmic strings, domain walls, cosmic curvature) would continue to drop off as well. The density of all the other components drops, but dark energy’s density remains unchanged.

But as we go earlier, the matter in the Universe – including normal matter and dark matter (and, at late times/low speeds, neutrinos) – was denser and more closely packed, and that means its energy density was higher and greater in the past. The radiation density, however, rises even more rapidly at early times than the matter density does, as radiation doesn’t just get more closely packed in the early, small-volume Universe, but each individual quantum possesses:

• a shorter wavelength,
• a higher frequency,
• and a greater amount of energy,

the smaller and smaller we extrapolate the size of our Universe to have been early on.

A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but where distant galaxies are accelerating in their recession today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time. The fact that the Universe follows the curve it does is indicative of the presence, and late-time dominance, of dark energy.

Whereas today, the Universe is mostly dark energy, it still has a significant but sub-dominant amount of normal matter and dark matter, and only has a tiny amount of neutrinos and photon radiation, this was not the case early on. Back when the Universe was first forming neutral atoms – when the cosmic microwave background was released at an age of just 380,000 years after the hot Big Bang – the Universe was very different: mostly dark matter with some normal matter, but also where photons and neutrinos were important at around the ~10% level as well.

We don’t have mere “snapshots” that provide us with data, either. One thing that’s wonderful about the expanding Universe is that the thing that determines what the expansion rate is, at any moment in time, is the total, overall energy density – across all forms of energy – present in the Universe at that instant. By measuring the redshift-distance relation across our cosmic history, we can determine very accurately what must be the contents of our Universe smoothly throughout time. The same measurements that reveal the presence of dark energy also reveal the energy contents of our Universe, both now and at all moments in our cosmic past.

The matter and energy content in the Universe at the present time (left) and at earlier times (right). Note how dark matter and dark energy dominate today, but that normal matter is still around. At early times, normal matter and dark matter were still important, but dark energy was negligible, while photons and neutrinos were also quite important. The expansion rate is determined by the actual, instantaneous value for density, not by the distribution of the pie chart.

That’s super important! Once we know what the Universe is made out of, precisely, at any moment in time, as well as what the expansion rate was at any moment in time as well, we can immediately figure out what the relative energy densities of every component of the Universe were at any given moment in cosmic time.

The easiest way to do this, both mathematically as well as from an astrophysics perspective, is to make our best measurements of both what’s in the Universe and how fast it’s expanding today, at the present time, and then to extrapolate backward. If we do this, we find that our Universe today is roughly:

• expanding at around 70 km/s/Mpc (with a little bit of tension),
• made 68% of dark energy,
• 27% of dark matter,
• 4.9% of normal matter,
• 0.1% of neutrinos,
• 0.01% of photons,

and a negligible, possibly even 0 amount of other forms of energy (including curvature, cosmic strings, domain walls, and so on). Matter became more important earlier on, from when the Universe was around 10,000 to ~7.8 billion years of age, and then at the earliest times, radiation – mostly in the form of photons but also with a substantial neutrino contribution – was dominant.

The relative importance of different energy components in the Universe at various times in the past. Note that when dark energy reaches a number near 100% in the future, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time. Owing to dark energy, distant galaxies are already speeding up in their apparent recession speed from us. Way off the scale of this diagram, to the left, is when the inflationary epoch ended and the hot Big Bang began. Dark energy’s energy density is ~123 orders of magnitude lower than the theoretical expectation.

It might leave you wondering: what happened to all of that energy that was once present in the form of radiation? There are a few different ways of looking at it, all of which are correct in some sense.

• You can claim that the radiation exerts a pressure on the imaginary line that defines the boundary of the observable Universe, and it loses energy in correspondence with the work-energy theorem as it does work in causing the Universe to expand.
• You can claim that the expansion of the Universe is adiabatic – which guarantees constant entropy, but not energy, temperature, pressure, or volume – and so the loss of energy comes about as a consequence of the nature of cosmic expansion.
• Or, you can simply and correctly assert that energy is not conserved in an expanding Universe. In physics, in order to have what’s known as a conserved quantity (something that can be transferred or converted from one form to another, but cannot be created or destroyed), there needs to be an associated symmetry from which that conserved quantity can be derived. The symmetry that corresponds to conservation of energy is time-translation invariance, which means things are the same at all moments in time, and the expanding Universe breaks time-translation invariance, and hence, energy is not conserved.

This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. The expanding Universe is not time-translation invariant.

But let’s work through how energy was distributed in the early Universe to try and make sense of things explicitly. At the earliest times, there was only radiation: before the electroweak symmetry broke, particles did not yet have rest mass, and so the Universe was ~100% radiation. After the Higgs symmetry breaks, many particles and antiparticles gain positive, non-zero rest masses, and they begin behaving as matter as soon as their kinetic energy (the energy of their motion) drops below their rest-mass energy.

However, as the Universe cools, there’s also less energy-per-collision that occurs between any two quanta to produce new particle-antiparticle pairs, which occurs via Einstein’s E = mc². This means that when the energy-per-particle roughly drops below the rest mass energy of a given species, the particles and antiparticles of those species will annihilate away, typically to radiation (such as photons and neutrinos/antineutrinos at these early stages), leaving only whatever “excess” existed of matter over antimatter.

It’s true that, back when the Universe was in its first few seconds, matter and antimatter were both copious at various moments, but always annihilated away to lighter particles and pure radiation. By the time the last antimatter annihilates away (positrons, annihilating with excess electrons), the radiation energy density is billions of times the matter energy density, and around a decillion (~10³³) times the dark energy density.

This diagram shows, to scale, how spacetime evolves/expands in equal time increments if your Universe is dominated by matter, radiation, or the energy inherent to space itself (i.e., during inflation or dark energy dominance), with the latter corresponding to the inflationary phase that preceded and set up the hot Big Bang. Although all of these model universes expand toward infinite size, they approach it at different rates, with the “space itself” solution approaching infinity in a fundamentally more quick fashion than the other two.

And then the Universe expands. Even if you ignore everything else – all the structures that form, all the nuclear reactions that occur, all the stars that live and die, etc. – the expanding Universe will continue to expand, and the relative energy densities of the various species will continue to evolve according to their nature.

• For every multiplicative factor that the Universe expands by, let’s call it a, radiation will have its density drop by a factor of ~1/a⁴, where ~1/a comes from the radiation’s wavelength stretching and where another ~1/a³ comes from the fact that the volume of space expands in three dimensions.
• For matter, its density drops by a factor of ~1/a³, as the 3D volume that space occupies will increase by a factor of a in each dimension, but those are the only changes.
• And for dark energy, its density remains constant, and has no a-dependence at all. (If you insist, its evolution is proportional to a⁰.)

From the time that the Universe was one second old until today, 13.8 billion years later, the Universe has expanded in each dimension by a factor of around 4 billion, meaning that the radiation energy density has dropped by a factor of more than 10³⁸, and the matter energy density has dropped by a factor of nearly 10²⁹. The dark energy density? It hasn’t changed at all.

So yes, the total energy density in radiation was once billions of times greater than the matter energy density, but today, it’s less than 0.1% of the total matter density. Time, evolution, and cosmic expansion, unless you’re dark energy, will eventually dilute absolutely everything else. For radiation, the worst of it has already occurred.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: Why doesn’t radiation dominate the Universe? is featured on Big Think.

Over the past 13.8 billion years, the Universe has evolved from a hot, dense, largely uniform early state to a clumpy, clustered, star-and-galaxy-rich state, where the typical interstellar and intergalactic distances are absolutely tremendous. The stars that exist today, importantly, are different from the stars that were created in the earliest stages of the Universe. Whereas the stars that are forming today are composed of all the recycled material that was once inside one-or-more stars and returned to the interstellar medium, the stars that were made early on were pristine: made of up primarily of hydrogen and helium alone: the material that existed shortly after the hot Big Bang.

Whenever we look at a star, we gain information about the cumulative history of the Universe up until the moment that particular star formed: of all the generations that lived-and-died prior to its formation. But early on, when the first few generations of stars were forming, it’s possible that the chemical enrichment of the “next generation” of stars may have primarily arisen from just one single, massive source. If this is the case, even for a few stars, we should find a population of very old stars with unusual compositions: with highly unusual element ratios compared to the more common varieties that formed from material that was well-mixed within the interstellar medium. With one remarkable discovery, a single star is helping us rewrite our cosmic history.

Star forming regions are driven by gravitational collapse, turbulent flows, and energy arising from radiation, carving small-scale and large-scale features into spiral galaxies. On stellar/star-forming region scales up to galactic and galaxy group/cluster scales, its effects can be felt and seen by cosmic structures.

When we tell the history of star-formation in the Universe, it normally proceeds as follows. At the very beginning, there are no stars at all, as protons and neutrons, and then atomic nuclei, and then neutral atoms form. It takes tens or even hundreds of millions of years for these pristine clouds of gas to gravitationally grow massive enough so they can collapse, and when they do, they trigger the formation of the first generation of stars: stars initially made of hydrogen and helium alone.

But then the first generation of stars dies, including many of them in stellar cataclysms like supernovae, and all of the nuclear reactions that occurred within that star – the slow-burning fusion reactions that occurred in the star’s core during its lifetime, plus the rapid reactions that occur during a cataclysmic event – are cumulatively reflected in the material returned by that former star to the interstellar medium.

Over time, that enriched, ejected material then mixes with the remainder of the interstellar medium, which serves as a great cosmic recycling system. From that now-enriched interstellar material, new generations of stars will form, with the interstellar medium gradually getting more progressively enriched with time.

The star forming region Sh 2-106 showcases an interesting set of phenomena, including illuminated gas, a bright central star that provides that illumination, and blue reflections off of gas that has yet to be blown away. The various stars in this region likely come from a combination of stars of many different pasts and generational histories, but none of them are pristine: they all contain significant quantities of heavy elements in them. That is one of the necessary ingredients for rocky planets and potential habitability.

This is interesting from the perspective of learning how the Universe, and the stars within it, evolved and grew up: from early times until the present day. We exist today in the here-and-now, but the stars that we can see aren’t only the stars that are forming right now; they’re all the stars formed throughout the Universe’s history that are still burning and shining with respect to our observing perspective. If our goals include gaining an understanding of the earliest stars in the Universe, we have a couple of options:

1. we can look to the greatest distances possible, and try to detect the earliest stellar populations back at the epochs when those stars were just in the process of forming,
2. or we can look around us, nearby, at the oldest, most pristine relic stars that we can find, and try to learn what the Universe was like at the time of their formation by examining their properties today.

While the JWST era has given us an incredible wealth of data about the early Universe, it’s only in a few rare instances, such as the stars Quyllur and Earendel, that individual stars can even be resolved at all. Looking nearby, instead, has huge advantages from an observational perspective because of how much more you can measure as far as properties of that star is concerned. Instead of aggregate properties of a population of stars, you can make detailed measurements of individual stars themselves.

The visible light spectrum of the Sun, which helps us understand not only its temperature and ionization, but the abundances of the elements present. The long, thick lines are hydrogen and helium, but every other line is from a heavy element that must have been created in a previous-generation star, rather than the hot Big Bang.

In general, there’s a single property you can measure to determine whether a star is “more pristine” versus “more enriched” in terms of the elements that make them up: metallicity. To an astronomer, any element that’s heavier than either hydrogen or helium is known as a metal, even supremely non-metallic elements such as oxygen, chlorine, and heavy noble gases. Because we’ve exquisitely measured the presence of practically all elements found in the Sun, we generally compare a star’s metallicity to that of the Sun in terms of its element ratios: iron (Fe) to hydrogen (H), oxygen (O) to hydrogen, or carbon (C) to hydrogen, for example.

However, where will the most enriched stars be found? And where will the most pristine stars be found? The highest-metallicity stars, the most enriched ones, are going to be found where the greatest number of previous generations of stars have formed, lived, and died: toward the centers and in the disks of the most massive, gas-rich galaxies. On the other hand, the lowest-metallicity stars, the most pristine ones, will be found where the fewest generations of stars have formed: in galactic halos, in ancient globular clusters, and in the far-flung outskirts of space. For the Milky Way, our home galaxy, we’ve even mapped out stellar metallicities as a function of their location in space.

This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. Stars in red, orange, and yellow are all rich enough in heavy elements that they should have planets; green and cyan-coded stars should only rarely have planets, and stars coded blue or violet should have absolutely no planets at all around them. Note that the central plane of the galactic disk, extending all the way into the galactic core, has the potential for habitable, rocky planets.

At the present time, a suite of ground-based and space-based surveys have brought our astronomical knowledge to new, unprecedented heights. With tremendous sky coverage and impressive resolution, as well as wide-field cameras and optical setups, astronomers have surveyed not only large areas of the sky to very faint magnitudes, but have used these novel capabilities to measure many properties of enormous numbers of stars: hundreds of thousands in some surveys, millions in others, and even more than a billion stars in the case of the European Space Agency’s Gaia mission.

The reason you want to examine so many stars is because if you’re looking for outliers – for stars that don’t fit into the same broad categories that the vast majority of their brethren do – you have to survey a great number of them to even have a chance of finding the rare ones. The Sloan Digital Sky Survey (SDSS), for example, has been observing large areas of the sky for two decades now, with increasingly updated cameras and instruments. One of the stars it identified has properties that really stick out as unusual: J0931+0038, or J0931 for short, which was first identified by SDSS-V Milky Way Mapper data, which itself was designed to measure and analyze data from around 5 million separate Milky Way stars.

Artist’s illustration (left) of the interior of a massive star in the final stages, pre-supernova, of silicon-burning. (Silicon-burning is where iron, nickel, and cobalt form in the core.) A Chandra image (right) of the Cassiopeia A supernova remnant today shows elements like Iron (in blue), sulfur (green), and magnesium (red). Ejected stellar material can glow due to heat in the infrared for tens of thousands of years, and the ejecta from supernovae can be asymmetric and can have segregated elements within it, as shown here. In the right environment, this asymmetric material can be unevenly incorporated into future generations of stars.

This star, J0931, was initially flagged by SDSS for having a low metallicity: or a very low abundance of heavy elements. The amount of iron in the star, relative to the amount of hydrogen in the star, indicates that there’s only about ~1.7% of the ratio of iron found in the Sun: a low abundance of heavy elements, but not quite pristine. This led to astronomers following up on it, spectroscopically and with high resolution, by the large and powerful Magellan telescope: a 6.5 meter diameter telescope.

Surprisingly, the elements found within this star – if you look at them element-by-element – didn’t exhibit the typical abundances we normally find in stars when we looked at their abundances relative to one another. Some elements were greatly abundant compared to the amount of iron found, including enhanced abundances of strontium (Sr), Yttrium (Y), Manganese (Mn), Nickel (Ni), and Zinc (Zn), among others. Other elements, such as sodium (Na), titanium (Ti), scandium (Sc), and barium (Ba), showed tremendous deficiencies compared to most other typical stars. It’s almost as though this one star, J0931, is telling us multiple different cosmic stories at once.

Lesson #1: All stars aren’t enriched uniformly; at least some stars are partially enriched in certain elements and partially deficient in others.

This graph shows various spectral features seen by the Magellan telescope within the star J0931. While there are many elements that exhibit similar signatures to either a normal star, shown in purple, or to a r-process enhanced (e.g., enhanced by supernova-generated neutrons) star, shown in blue, the star J0931 (in black) is either severely enhanced or deficient when it comes to a significant number of elements.

However, we have to accept that this star exists, and even if there are multiple contributing factors to how it obtained the properties it now displays, there’s only one cosmic story that led to its existence. In a new paper just accepted into the Astrophysical Journal, astronomer Alex Ji and his collaborators put forth a fascinating potential explanation that could account for this system with just a single cosmic story: this star formed very early on in the Universe’s history, from nearly-pristine material that had only been slightly enriched.

But nearby, shortly before this star formed, a lone, early, and asymmetric supernova event occurred, seeding the interstellar medium irregularly, with some elements preferentially becoming enriched in certain directions over others, while some failed to become as enriched in those same directions. If there was some sort of directional emission of the detritus arising from the supernova, and then stars swiftly formed from the material that included this irregularly enriched and recycled stellar debris, it would be possible to form stars such as J0931: stars that were not formed from a well-mixed sample of the interstellar medium, but from an unusual sample with some enrichments and some deficiencies over the cosmic average at the time.

The gray lines with error bars represent the ranges for nearly 5000 normal stars whose heavy element abundance ratios range from 0.03% of the Sun’s all the way to 3.2% of the Sun’s. The red points, which show the spectroscopically determined elemental abundances of various elements in J0931, show both enhancements and severe deficiencies relative to that sample.

The reason so few stars are expected to display properties like J0931 is because, over time, three things happen.

1. Ejecta from various cataclysmic events, even highly asymmetrical ones, will eventually become well-mixed with the rest of the interstellar medium over time.
2. Most of the stars that do form in the Universe do so in the aftermath of many violent events, and also form in preferentially enriched areas of the Universe.
3. And that even initially pristine regions that have never formed stars, over time, will get “mixed up” with the recycled, polluted material.

Therefore, there’s only a brief window of time where these heavily, asymmetrically enriched/enhanced stars can form, and only the earliest of these windows will give rise to low-metallicity stars like J0931 that also display these properties.

Moreover, whenever stars form, they form with a wide variety of masses, and with higher mass comes a shorter lifetime. It’s only the least massive stars – and if we’re talking about stars that formed 12-13 billion years ago, that’s only the stars that are less massive than the Sun – which will still persist even today.

Lesson #2: Supernova events can produce an asymmetric distribution of elements, and new stars can then form with that unusual distribution imprinted onto them.

The red points on the graph show the observed abundances of various elements in J0931, while the blue points show the predicted element abundances generated by a single hypernova explosion whose progenitor star was 80 solar masses. As good as this model is for explaining most of the elemental abundances, however, it still cannot explain them all.

When we look at the precise distribution of elements that exists within J0931, the situation gets even more troubling, as we find very quickly that we don’t have a supernova model that can predict all of the observed elemental abundances simultaneously, alongside one another. A supernova or hypernova that arose from an ~80 solar mass star could do the trick for many of the elements, but not all of them. When you view the data all together, it is suggestive of a scenario where the material that formed this star was composed of:

• a mix of previously enriched, well-mixed, nearly-but-not-quite pristine material,
• that was augmented by an asymmetric predecessor supernova,

which could then explain the odd-even element relative observed abundances of the different elements.

However, there’s again a puzzle that emerges with this scenario, as our best models for stellar evolution indicate that stars of between about 40 and 140 solar masses shouldn’t go supernova at all, but would rather directly collapse to black holes without a stellar cataclysm of any type. It’s possible that this star has something new to teach us: which is that nature, at least sometimes for stars in this mass range, does something different than our naive expectations would lead us to believe.

Lesson #3: Our simplest models for when stars undergo direct collapse versus when they explode in a supernova do not match up with all of our observations, and will need refinement/improvement moving forward.

Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. We must also consider that effects other than mass and metallicity may indeed play major roles in determining the fate of massive stars, including in whether they can contribute to enriching the interstellar medium.

There are other lines of evidence that suggest that a star’s fate is not exclusively determined by factors like mass and metallicity, which is commonly assumed. We observed a star of 25 solar masses simply appear to “wink out” of existence, with no remnant, cataclysm, or emissions of any type left behind, even though stars of that mass should go supernova, not directly collapse. In the early Universe, we have evidence from observed black holes that there are probably clumps of matter in the 10,000-100,000 solar mass range that simply collapse to form the seeds of supermassive black holes, without any sort of intermediate stage where they formed stars.

It’s possible, perhaps even probable, that what we are seeing is hinting to us that our present understanding of stellar lives and deaths, particularly at high masses and in the more-pristine past, is far too naive and requires significant refinement. Perhaps much of the astrophysics behind the massive objects created in the early Universe is determined by factors we have not been modeling correctly, and that asymmetric enrichment of early stars was more common than we presently realize. Regardless of that, identifying J0931, and hopefully more stars like it, truly is shining a light onto how heavy elements were created at incredibly early times in our cosmic history, which leads to the final unexpected lesson.

Lesson #4: By identifying the oldest, lowest-metallicity stars and measuring them with high-resolution spectroscopy, covering a wide variety of elements, we can learn lessons about how nucleosynthesis worked in the most massive stars from the early Universe.

This article 4 unexpected lessons from the Milky Way’s weirdest star is featured on Big Think.

It was only a little over 30 years ago, in the early 1990s, that humanity detected our first planets in orbit around stars other than the Sun: the exoplanets. The earliest ones discovered were a bit of a surprise: they were all massive, in tight orbits around their parent stars, and extremely hot: a class known as hot Jupiters. Since that time, we’ve discovered more than 5000 exoplanets, ranging from sub-Earth sized all the way up to super-Jupiters, with a huge variety found in between. However, two puzzles have arisen:

• the fact that there are “hot Earths” and “hot Jupiters,” but no “hot Neptunes” in between them,
• and the fact that there are plenty of Earth-like planets up to about 140% the radius of Earth, and plenty of Neptune-like planets down to about half the size of Neptune (about 200% the radius of Earth), but preciously few planets in that in-between range: a puzzle known as the radius gap.

Although there are suspects in the quest to solve these cosmic puzzles, the direct supporting evidence has been elusive up until now. However, a new paper that investigates the remarkable properties of evaporating exoplanet WASP-69b may provide the key clue that leads to a solution for both. Here’s how.

The idea of the radial velocity method is that if a star has an unseen, massive companion, whether an exoplanet or a black hole, observing its motion and position over time, if possible, should reveal the companion and its properties. This remains true, even if there’s no detectable light emitted from the companion itself.

The first successful methods for finding exoplanets involved measuring the light from a parent star very exquisitely. If there are planets orbiting the star in question, the star isn’t only going to gravitationally pull on that planet, but the planet will gravitationally pull on the star, causing the star to move in an elliptical pattern around the mutual center-of-mass of the star-planet system. That causes the star to appear to “wobble” with respect to us, as it will periodically move toward-and-away from us leading to its light redshifting and blueshifting in a periodic fashion. This detection method, known as either the radial velocity or stellar wobble method, is most sensitive to high-mass planets in close orbits to their parent stars.

Similarly, a planet that transits across the face of the star as seen along our line-of-sight to it will block a fraction of that star’s light, causing a periodic flux dip in the light seen from the star over time. This is again most sensitive to planets that block a large fraction of their parent star’s light with a higher frequency. These two methods, now being joined by others such as direct imaging and microlensing, have revealed the overwhelming majority of the more-than-5000 exoplanets presently known, and already, the exoplanet population shows a few surprises.

The mass, period, and discovery/measurement method used to determine the properties of the first 5000+ (technically, 5005) exoplanets ever discovered. Although there are planets of all sizes and periods, we are presently biased toward larger, heavier planets that orbit smaller stars at shorter orbital distances. The outer planets in most stellar systems remain largely undiscovered, but those that have been discovered, largely through direct imaging, are difficult to explain the way we think most exoplanets form: via the core accretion scenario.

In particular, there are two puzzles that appear when we examine the data in detail.

1. There are plenty of hot Earth-sized planets, and also plenty of hot Jupiter-sized planets, with hot Earth-sized planets common around low-mass stars and hot Jupiter-sized planets common around higher mass stars. However, there are pretty much no classes of stars that show the existence of hot Neptune-sized planets around them. It’s almost as though there’s a process at play that forbids the existence of lower-mass, gas-rich worlds, and we don’t know exactly why.
2. There are also lots of Earths and super-Earths found in the exoplanet data, including all radii from all types of stars (at least, as far as we can measure them), as well as lots of Neptunes and mini-Neptunes that exist, particularly at distances that lead to the planet not being “too hot,” or too close to their parent star. However, there aren’t very many of the “in between” sized planets, which is known as either the radius gap or the Fulton gap in the literature.

There are a few ideas floating around out there that attempt to explain this puzzle, but ideas in science aren’t worth very much unless we can test and confront them with actual, meaningful data.

When you plot the radius of known exoplanets (planet size) against their periods around their parent stars, two notable “gaps” appear in the planet population. There are hot Jupiters and hot Earths, but no hot Neptunes, as well as a gap between rocky-like and Neptune-like planets at even greater orbital periods.

One interesting hypothesis, however, could explain both of these phenomena at once: the process of photoevaporation. Photoevaporation is a fancy way to say:

• there are a lot of energetic emissions that come from stars, including ultraviolet light and energetic particles,
• and those energetic emissions are going to not only heat any planets they encounter, but will collide with any particles that exist within those planetary atmospheres,
• and that if those atmospheric particles get energized beyond a certain threshold, they’re going to get kicked up to speeds that exceed the escape velocity of that world,
• and hence, the atmosphere will begin to be stripped away.

There could be a critical threshold in both cases. For the hottest planets, either they’ll simply become stripped planetary cores, incapable of holding onto any substantial atmosphere, or they’ll be massive enough that they can hang onto large volatile (hydrogen and helium rich) envelopes, but not in between. And for less hot planets, either you aren’t massive enough to hold onto a volatile envelope, in which case you’re a rocky, Earth-like planet, or you are massive enough, in which case you’re an inflated, Neptune-like planet. If you’re even a little less massive than the Neptune-like worlds, your volatiles are photoevaporated away, and you’d transition into an Earth-like state.

When starlight passes through a transiting exoplanet’s atmosphere, signatures are imprinted. Depending on the wavelength and intensity of both emission and absorption features, the presence or absence of various atomic and molecular species within an exoplanet’s atmosphere can be revealed through the technique of transit spectroscopy. JWST cannot get spectra for Earth-sized planets around Sun-like stars, but Habitable Worlds Observatory finally will.

It’s a challenge to observe these types of planets during the stages where they’re still forming, as the observational difficulties have not yet been overcome. But there is a very reasonable way to investigate the issue: by looking at the least massive hot Jupiter planets found around relatively young, massive stars, and to try and measure and quantify the effects of photoevaporation. If we can understand how these worlds, the least massive ones that still have volatiles, lose their atmospheric mass owing to the influence of their parent stars, perhaps we can learn lessons about photoevaporation and how it plays (or doesn’t play) a role in explaining these two big planetary puzzles.

In other words, what we want to do is to study systems that are experiencing photoevaporation, in real-time and in situ, to learn how efficiently parent stars can strip their orbiting planets’ volatile atmospheres away. One intriguing system to study is the WASP-69 system. WASP-69 is a K-class star, a little bit smaller and less massive than the Sun, but a little bit younger: at just 2 billion years of age. WASP-69 also has an exoplanet that orbits it: WASP-69b, roughly a Saturn-mass, slightly larger-than-Jupiter in radius exoplanet, that’s close enough to its parent star, and hence hot enough, to experience photoevaporation.

Photoevaporation is the process by which, when a planet is too close to its parent star, its atmosphere gets hot, and the stellar emissions can strip particles out of the atmosphere, leading to partial photoevaporation. For a low-mass, high-volatile planet, the entire atmosphere could easily be stripped away.

Why would a planet only about the mass of Saturn actually be larger than Jupiter? The answer comes from the parent star itself: because when you heat gas up, even in an exoplanet’s atmosphere, it expands, causing the exoplanet to become “puffier” in terms of its radius. The puffier your atmosphere is, the farther away the most tenuously-held atmospheric particles are from the planet’s center-of-mass, making its escape velocity lower. The hotter your planet is and the lower the escape velocity from it is, the greater the rate of photoevaporation will be.

If we can measure the photoevaporation rate directly, that can provide us with valuable information that teaches us which sorts of planets (and planetary atmospheres) can persist within exoplanetary systems in a stable fashion, and which sorts of planets (and planetary atmospheres) will be inherently unstable, and won’t persist for billions of years.

Because WASP-69b is an exoplanet that transits in front of its parent star, you can not only measure the properties of the exo-atmosphere during the transit, but whether there are any features that appear either before or after the transit itself if you can obtain good enough data. And fortunately, Dakotah Tyler and his team were able to get Keck telescope time to take exactly this type of data for themselves.

The raw signal from Keck’s spectroscopic observations shows a strong light-blocking and absorption signature at the precise wavelength that neutral helium absorbs at. Looking at the time series of this data, as well as wavelength-specific properties, can further reveal details about photoevaporation on WASP-69b.

What did they see when they actually took this valuable data?

Importantly, they took not only photometric (raw light) data, but spectroscopic data, where the light from the parent star is broken up into its individual wavelengths. As the planet first makes contact with the limb of the star, relative to our line-of-sight, some of the starlight is not only blocked by the disk of the planet, but some of it filters through the atmosphere of the planet itself. When there’s neutral gas, such as hydrogen or helium, in between the star and our line-of-sight, a characteristic absorption signal – observable at an element-specific wavelength – will appear in the data.

When Tyler and his team acquired the data, they found something fascinating that had only been suggested, never robustly seen, by previous studies.

• Prior to the transit, no absorption signal was seen, just as expected.
• During the transit, an absorption profile consistent with plenty of neutral helium in this atmosphere is shown to exist: confirming that this is a volatile rich gas envelope.
• But even after the transit has completed, a significant amount of neutral helium in a signal still persists for quite a long time, confirming the existence of copious amounts of neutral gas well-separated from the planet itself.

The best way to interpret this data? The atmosphere of WASP-69b is being photoevaporated, and that photoevaporation process is creating an enormous, massive, comet-like tail emerging from this hot exoplanet.

Whether you look at the helium absorption feature from the rest frame of the parent star (left) or the exoplanet (right), helium signatures can be seen absorbing that starlight both during the transit as well as for more than an hour afterward, but not before. This is a strong indicator of a large, dense, comet-like tail.

The neutral helium signal seen is incredibly informative. In detail, it tells us a number of fascinating facts.

• First off, material is being lost spectacularly fast from this exoplanet: at the rate of about one Earth-mass of atmosphere every ~1 billion years.
• Second off, that mass is departing this planet in a giant wake, extending for more than seven times the radius of the planet, or for a distance in excess of 350,000 miles (around 580,000 km) behind the planet itself.
• And third, because this is spectroscopic data, they can determine that this helium is being stripped away with a relative velocity of 23 km/s – around 50,000 miles-per-hour (80,000 km/hr) – which is consistent with photoevaporated material that then interacts with the star’s outgoing winds.

This data is enough to draw some fascinating inferences. It tells us that hot Jupiters and even hot Saturns are stable on scales of the lifetime of a typical star, as this exoplanet, which weighs in at around 90 times the mass of the Earth, is only losing ~1 Earth mass of atmosphere for every billion years of time that passes. Additionally, it tells us that “rocky cores,” or stripped planetary cores, would also be stable on the lifetime of a typical star, as the heavy elements making up a planetary surface wouldn’t be photoevaporated in the same way.

When exoplanet WASP-69b transits in front of its parent star, from the perspective of Keck Observatory, not only can the signature of helium absorption be seen during the transit itself, but even after the transit has completed, the helium absorption signature remains, demonstrating the existence of a comet-like tail trailing behind the planet itself.

In other words, this data is very suggestive of the idea that the reason “hot Neptunes” don’t exist is because, if they’re hot, their atmospheres will be quite inflated, leading to very low escape velocities for the top layers of their atmospheres. Lower escape velocity means that the rates of atmospheric stripping will be greater, and that may very well mean that unless there’s a large amount of mass – enough to build up to a Saturn/Jupiter-sized planet instead of a Neptune-sized one – the atmospheres of Neptune-like planets might be entirely stripped away if they’re in close orbit around their parent stars, forbidding the existence of hot Neptunes.

The radius gap may also be explicable in this fashion as well, although via a slightly different mechanism. When you’re first forming a planet, if you’re far enough away from your parent star, the type of planet you wind up with depends on how much mass you accrue, as well as how fast you accrue it. Below a certain mass, around ~2 Earth masses, you won’t have enough gravity to hold onto your hydrogen and helium (volatiles) even at substantial distances from the parent star; you cannot become Neptune-like. But above that mass threshold, you could hold onto them at great enough distances, and so only at small distances (and hot temperatures) will you not retain a volatile envelope, and so you’re destined to become either a mini-Neptune or a full-fledged Neptune-like world.

These new observations of WASP-69b show, definitively, how one of the lowest-mass “hot Jupiters” around a relatively nearby star – just 160 light-years away – is actively experiencing photoevaporation. The atmosphere gets inflated by the heat from the parent star, the volatile-rich atmosphere experiences stripping due to the energetic particles and radiation that create outflows, and those volatiles are not only blown off of the host planet itself, but create a comet-like tail behind the planet: perhaps a tail that’s even much longer than what’s observed, as an imperfect alignment between the planet and our line-of-sight to the star could admit such a possibility.

The implications of this observation are staggering, however. They could explain why hot Jupiters (and hot Saturns) exist in great abundances around stars, but that hot Neptunes don’t while solid, planetary cores do again. They are consistent with explaining the radius gap with the notion that either you can successfully hold onto your volatiles, making you Neptune-like, or not, making you Earth-like (or at least a solid-surfaced planet), and that “yes or no” question is why there are precious few exoplanets between 140% and 200% the radius of Earth.

But most importantly, what was only a theoretical scenario now has direct, powerful observational evidence supporting it. Perhaps, with future observations, we’ll definitively solve the “hot Neptune” deficit and the “radius gap” problems once and for all!

This article Evaporating exoplanet WASP-69b solves two planetary puzzles is featured on Big Think.

Imagine, if you dare, what the Universe was like before any stars had ever formed within it. All of the normal, atom-based matter within it was pristine, but the regions that had slightly more matter than normal would start attracting everything in their vicinity. Over time, they would build up larger and larger amounts of mass, until a critical threshold was reached that triggered gravitational collapse. Once that occurred, you’d make stars, and be well on your way to building up what would someday turn into a modern-day galaxy. These early “first galaxies,” still beyond the reach of even JWST, must have been quite abundant early on.

But are there any local, nearby analogues? Are there large, galaxy-scale populations of gas, at present, where maybe only a small number of stars exist within that clump at all? Could there even be enormous clouds of matter that have yet to collapse and form stars, bound together in a great, dark, star-free halo?

Previous large surveys failed to reveal any objects like this, finding only low-surface brightness galaxies that are either very tiny and low-mass or otherwise are caused by the gravitational interaction of two nearby galaxies. But an accidental discovery of an object known as J0613+52, potentially the first dark, primordial galaxy, may hold a treasure trove of cosmic riches within it.

The very first stars to form in the Universe were different than the stars today: metal-free, extremely massive, and destined for a supernova surrounded by a cocoon of gas. There was a time, prior to the formation of stars where only clumps of matter, unable to cool and collapse, remained in large, diffuse clouds. It is possible that clouds that grow slowly enough may even persist to very late cosmic times.

How many galaxies?

One of the biggest astronomy goals that people had, for a long time, was to answer the question, “How many galaxies are there within the observable Universe?” Back in the late 20th century, Carl Sagan told us there were “billions and billions,” estimating that there were around 100 billion galaxies within our observable cosmic volume. This turned out to be true to an extent: if we restricted ourselves to large, bright, massive galaxies, like those comparable to our own Milky Way.

In recent decades, however, we’ve taken multiple steps forward to better refine our answer. Improvements in the sensitivity of our instruments, as well as increases in wavelength coverage for various sky surveys, have revealed large numbers of galaxies that were previously unknown: galaxies that were smaller, fainter, and lower-in-surface-brightness than were known prior. It caused us to dramatically revise the number of galaxies expected in the Universe: from the hundreds of billions up into the trillions.

We now believe that there are anywhere from 30 to 100 small, faint, low-surface-brightness galaxies for every Milky Way-like galaxy out there, placing the number of estimated galaxies between 6 and 20 trillion at present, a number that would have boggled even Carl Sagan’s mind.

Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. As we discover smaller, fainter galaxies with fewer numbers of stars, we begin to recognize just how common these small galaxies are; there may be as many as 100 for every galaxy similar to the Milky Way.

The faint population

But what are these galaxies – the ones that don’t look like ours – actually like? They come in a few different varieties.

There are low-surface brightness galaxies, generally small and low in mass, that have a very small number of stars for their overall mass. They are thought, in general, to be remnants of what happens after a burst of star-formation occurs within a dark matter halo: some gas gets expelled, leaving a small number of stars behind inside a relatively large-mass, gravitationally bound system.

There are ultra-diffuse galaxies, which can come in small-to-intermediate sizes and masses, that have a small number of stars for the volume of space these galaxies occupy. They are thought, in general, to be the result of gravitational interactions, where either multiple galaxies have merged together or where an external gravitational influence has altered the structure of the previously undisturbed galaxy.

But in theory, there should be a third population as well: isolated dark galaxies, with relatively large masses, but absolutely no stars inside as well.

This Hubble Space Telescope image showcases the isolated void galaxy MCG+01-02-015, located near the center of an otherwise galaxy-free void. With no other galaxies known within 100 million light-years of it, it’s perhaps the loneliest galaxy in the known Universe. In other isolated regions, a much lower-mass collection of matter may not yet have collapsed to form stars, raising the possibility that a “dark galaxy” could exist even at late times.

If you have too little mass within a clump of matter, it won’t be enough to gravitationally collapse, and so it’s reasonable to think it may be possible for isolated clumps of matter to persist all throughout cosmic time. If that clump gets too massive, however, and it can cool and radiate its internal/heat energy away, then it will gravitationally collapse and trigger the formation of stars. This tells us that there may be clumps of matter up to a certain mass that have never formed stars, but that beyond a certain mass threshold, you can expect that stars will inevitably exist within any clump of matter so massive.

That’s the expectation.

But do pristine clumps of matter exist at present? Can a galaxy-size mass of gas remain gravitationally/thermally stable over all the billions of years of cosmic history that have ensued? Or will they inevitably form at least some stars, creating an upper limit to their gas-to-stars ratio or a lower limit to the density of gas they possess? And what conditions or mechanisms determine whether we have a clump of gas with no stars – i.e., a primordial, dark galaxy – or just a faint, low-surface-brightness galaxy to reckon with?

This wide-field view shows the sky around the dwarf galaxy WLM in the constellation of Cetus (The Sea Monster). This picture was created from images forming part of the Digitized Sky Survey 2. The bluish clump in the center of the image is galaxy WLM; the bright, colored, spikey points, including the red and yellow ones, are simply foreground stars within our own Milky Way. Dwarf galaxies commonly come in a wide variety of morphologies, including the low-surface brightness variety.

The search

The reason galaxies are called “low surface brightness” is because even at their centers, which are normally the brightest part of a galaxy, their brightness is at least one astronomical magnitude (about a factor of 2.5) fainter than the overall brightness intrinsic to the night sky itself. You can still measure the stars within them, but often it pays to look for other types of signals that might show up: signals like those imprinted by moving neutral hydrogen gas, which is observable in radio light. Since the 1970s with the advent of the (now defunct) Arecibo observatory, this has been leveraged to great success.

If you want to find a primordial, dark galaxy, it makes sense to start by using some of the same methods that you use to find low-surface-brightness and ultra-diffuse galaxies: look for faint, telltale signals that don’t necessarily rely on visible light. With Arecibo, the most successful survey as far as finding hydrogen-rich ultra-diffuse dwarf galaxies has been the ALFALFA survey, which found an estimated 30,000 extragalactic sources. Unfortunately, exactly zero of those sources were the vaunted primordial, dark galaxies that astronomers were seeking. They all had at least some level of stars inside of them, providing a clue that perhaps the answer is, “These objects don’t actually exist at present.”

This “almost dark” galaxy, nicknamed Nube, is an incredibly diffuse galaxy found within a grouping of many other galaxies. It is thought that this ultra-diffuse galaxy, which has only a small smattering of stars inside a large mass of neutral hydrogen, owes its properties due to environmental factors.

A happy accident

In 2021 and 2023, two “almost dark” galaxies were found: AGC 229101 (within the ALFALFA survey), and Nube, found within the IAC Stripe82 Legacy Project. They were the closest things we had found to a true “dark galaxy,” but both still have stars inside. Now, in 2024, the situation has changed, with the accidental but profound discovery of an object known as J0613+52: perhaps the first true dark galaxy ever spotted.

The accident was laughable in retrospect: as part of a survey of neutral hydrogen gas of known low-surface brightness galaxies, three different telescopes – the National Science Foundation’s Green Bank Telescope, the Arecibo Telescope, and the Nançay Radio Telescope in France – were used to point at and measure the hydrogen within about 350 independent galaxies. When analyzing the collected data, a discrepancy emerged: one of the galaxies showed wildly inconsistent data between the data collected from Green Bank and the data collected from Nançay.

Upon closer inspection, it turned out that there was an error in the source catalog used for pointing, and the Green Bank Telescope was, perhaps unceremoniously, pointed at entirely the wrong region of sky: a region only notable for its unremarkable nature, with no known nebulae or extragalactic sources within it.

A region of sky known only for its unremarkable nature: with no bright or discernible extragalactic sources within it, may in fact house the first primordial dark galaxy, as neutral hydrogen gas has been detected where the colored (but not the white) circles are drawn.

And yet, there was a signal coming from part of that region! The circles in white, above, indicate regions where no “signal” was found, indicating the (expected) absence of neutral hydrogen. But in the colored regions, there was neutral hydrogen found, with the line thickness indicating the strength of the observed signal. Where colors are seen, the “redder” colors indicate where this hydrogen appears to be moving away from us, while the “bluer” regions indicate where this hydrogen appears to be moving relatively toward us: perhaps a likely indication that this clump of matter is rotating.

When we turn these into quantitative numbers, it turns out the inferred rotational speeds are actually quite large, and even comparable to speeds found within the Milky Way: at around 200 km/s. The clump of gas must be quite distant – estimated at around 270 million light-years away – and also with a large amount of neutral hydrogen within it: between 1 and 2 billion solar masses worth of gas found inside.

As you can see, below, from the “bump” feature found in the colored circles where a neutral hydrogen signal has been detected, there really does appear to be something “real” found in this region of space, as opposed to elsewhere.

The spectrum of the various regions of sky where the alleged “dark galaxy” was found as well as beyond its detected extent. The monochrome spectra show no evidence for neutral hydrogen; the colored spectra show an enormous neutral hydrogen signal, with the various colors indicating the relative motion (blue = toward, red = away) of the gas relative to ourselves.

A true dark galaxy?

But unlike both of the “near dark galaxies” found, as well as many of the low-surface-brightness and the vast majority of ultra-diffuse galaxies, not only is there no detectable optical counterpart found in any survey that’s looked at this region of the sky, but there are no “nearby” galaxies to this cloud of gas at all: not for hundreds of millions of light-years!

This is a puzzle. Massive low-surface-brightness galaxies are thought to be formed through the interaction or infall of two smaller galaxies. Ultra-diffuse-galaxies, particularly the ones rich in hydrogen gas, can be created through mergers, infall into a larger group or galaxy, or the gravitational effect of tidal stripping. All of these mechanisms involve an interaction with a nearby, massive neighbor.

And yet, this dark cloud of massive amounts of hydrogen gas, J0613+52, appears to be incredibly isolated. This object, if it has any stars, must be incredibly faint, and even for the largest telescopes, it may take tens or even hundreds of hours of observing time to reveal if there are:

• faint, unseen companions nearby,
• small populations of stars within it that just have not been revealed yet,
• or the presence of other elements and compounds, such as carbon monoxide molecular gas.

However, this gas is not only definitively there, there’s an awful lot of it!

The rings highlight the location where an actual signal of neutral hydrogen gas was observed at radio wavelengths, indicating the possible presence of a dark galaxy. No extragalactic sources are known (or visible) within this region of sky.

Isolated and diffuse, but pristine?

This is a huge open question: can such a large mass of gas remain in a diffuse, uncollapsed state for so long? Perhaps, even for all 13.8 billion years that our Universe has experienced, going all the way back to the start of the hot Big Bang? In most cases, clouds will accumulate enough matter to collapse, and when they do, they’ll trigger the first burst of star formation in that region of space, polluting the interstellar medium from the detritus of the first stars to die, creating a bevy of enriched elements that leave a telltale spectroscopic signature behind, rich in oxygen, carbon, and other heavy elements.

If J0613+52 truly is a pristine galaxy, then when we take deep enough observations to reveal whatever background galaxies and quasars are present along this line-of-sight, we’ll be able to perform what’s known as absorption spectroscopy: looking at the light from background objects that filters through (and gets absorbed by) the intervening gas. This type of deep optical/infrared imaging can reveal the presence or absence of these heavy elements, the presence or absence of carbon monoxide, the presence or absence of any stars inside, and can tell us whether we truly have an undisturbed, very slowly evolving galaxy right here in the local Universe.

This view, of the nearby Taurus Molecular Cloud of gas, is more typical of the types of gas found in the modern Universe: polluted, found alongside stars, and clumped together within galaxies that are themselves in the vicinity of other galaxies. J0613+52 represents an entirely new, possibly pristine, class of isolated, dark galaxy.

A window into the cosmic unknown

It’s important to note that even if it turns out that there is some faint population of stars inside this object, it truly does represent a window into the cosmic unknown. We have never before found a dark (or even an almost-dark) galaxy that was so isolated from all other cosmic structures at such late times in the Universe, nor one with such large amounts of fast-moving neutral hydrogen gas within it. The three approaches taken to follow up on this object should be:

• to perform deep, multi-wavelength imaging of this region of sky with other instruments,
• to follow up with a larger-aperture, higher-resolution radio telescope to better map out the gas in this object,
• and to perform a high-resolution, systematic survey of the entire sky, searching in earnest to see if there are any other dark galaxies (or dark galaxy candidates) out there.

We know that these types of structures were far more common in the early Universe, and likely describe some of the first structures that formed: beyond even the current reach of JWST. If we want to understand what existed at the earliest times in our Universe’s history, the best thing we can do is to find the closest local analogue systems, and study them in gory detail. With the discovery of J0613+52 now in hand, the next scientific steps will provide a window into our cosmic past that astronomers have never had access to before.

This article The first dark, primordial galaxy has gas, but no stars is featured on Big Think.

The Moon is the closest natural object to Earth.

Japan’s Kaguya probe went to and orbited the Moon, which enabled magnificent views of the Earth seen over the lunar surface. Here, the Moon is photographed along its day/night boundary, the terminator, while Earth appears in a half-full phase. From the near side of the Moon, the Earth is always visible; both are the result of the aftermath of an early, giant impact between a Mars-sized protoplanet and a proto-Earth.

Its orbital distance ranges from 356,000 to 407,000 km.

A perigee full Moon compared with an apogee full Moon, where the former is 14% larger and the latter is 12% smaller than the other. The longest lunar eclipses possible correspond to the smallest apogee full Moons of all. At apogee, the Moon is not only farther and appears smaller, but also moves at its slowest in its orbit around Earth, and takes the longest amount of time for a round-trip signal to traverse that distance.

Simply folding a paper in half enough times would eventually reach the Moon.

Whenever you take a sheet of paper and fold it in half, you double its thickness and double the number of sheets in the stack. This happens not only for the first fold, but for each subsequent fold, compounded atop all prior ones.

But how many? The answer lies in the mathematics of exponential growth.

After one “doubling time” has passed, the initial population of any exponentially growing collection has increased by a factor of 2. After another doubling time, there’s another doubling, for a total factor of 4. After 16 doublings, the initial population would have increased by a factor of 2 to the 16 power, or 65,536. Exponential growth, whenever it occurs and regardless of whether it occurs in time, space, or by any other metric, is as catastrophic as it is relentless.

To start, you first need to know how thick a single sheet of paper is.

A ream of paper is normally packaged in a stack of 500 sheets. Each such ream is normally about 2 inches, or 5 centimeters, thick.

Paper is sold in reams of 500 pages, typically ~5 cm (2 inches) thick.

A single sheet of paper is quite thin: typically around just 0.1 millimeters (or 0.004 inches) thick. This is comparable to the thickness of a single human hair.

That implies a single sheet is ~0.1 mm (0.004 inches) thick.

Each time you fold a sheet of paper, you increase the number of sheets in the stack by a factor of two, while simultaneously increasing the thickness of the stack by two as well.

Each time you fold a piece of paper, you double its thickness.

A sheet of paper folded anywhere from one through six times, with the relative smaller area and increased thickness corresponding to the number of folds inherent to the paper.

Fold it once, twice, and then three times, and it becomes 0.2, 0.4, and then 0.8 mm thick.

This animation features satellite images of the far side of the Moon, illuminated by the Sun, as it crosses between the DSCOVR spacecraft’s Earth Polychromatic Imaging Camera (EPIC) and telescope, and the Earth — one million miles (1.6 million km) away. The far side of the Moon is vastly different from the near side. The Moon itself is located an average of 384,000 km away from Earth, but due to its elliptical orbit, can get more than 20,000 km closer or farther than that figure.

To reach the Moon, however, it must become at least 356,000 km thick.

This unfamiliar view shows the size of the Earth and Moon, plus the distance from the Earth to the Moon, to actual scale. The Earth is about 12,700 km in diameter with the Moon being a little over a quarter of the Earth’s size, but the present Earth-Moon distance averages out to an enormous 384,000 km: just over 30 times the Earth’s diameter.

That’s a factor of 3.56 trillion thicker than a single sheet of paper.

To fold a paper as many times as possible, the best strategy is to take the largest single sheet of paper you can access and fold it upon itself as many times as you can.

Each successive fold cumulatively doubles its previous thickness.

The greater the number of times you fold a piece of paper onto itself, the greater the thickness and number of sheets will be, with each fold further doubling the thickness and number of sheets in the stack over the prior fold.

After 10 folds, its thickness increases by a factor of 1024 (just over 1000).

Each successive folding of a paper (or set of papers) will double the number of sheets in the stack, and will double the thickness of the stack from the prior fold. Once 10 folds have accumulated, the number of sheets will have risen from 1 to 1024: since 2^10 = 1024.

After 20, 30, and 40 folds, the paper becomes over a million, a billion, and then a trillion times thicker than the original.

The exponential function, e^x, where e is the transcendental number that is the base of natural logarithms, is the only function whose slope at every point along the curve, as shown here, is equal to the value of the function itself. Numbers other than e can be exponentiated as well, and although “exponential growth” is a property common to all exponentiated numbers greater than 1, but the slope of such a curve will not be exactly equal to the value of the function.

Only 42 folds equates to a thickness of 439,804.6511104 km: enough to reach the Moon.

Although the Earth might be large and massive compared to the Moon, both bodies are very small compared to the distance between them. It takes about 1.25 seconds for light to travel one-way from the Earth to the Moon, and the Earth-Moon separation is about 30 times the Earth’s diameter. A paper folded 42 times would be thicker than the distance that separates the Earth from the Moon, even at its farthest.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Thanks to Lewis & Clark College’s Prof. Michael Broide for teaching the author this lesson in 2009.

This article How many times must you fold a paper to reach the Moon? is featured on Big Think.

Happy new year, everyone, and with a new year comes a spectacular new podcast! We normally cover an intricate and underappreciated aspect of astrophysics on the podcast, but I had the opportunity to bring on a true expert in the field of quantum computing and just couldn’t pass it up.

You’ve likely heard a lot of noise about quantum computers and the benefits that they’re poised to bring, with buzzwords like “P=NP,” “quantum supremacy,” and “quantum advantage” tossed around, but a lot of what you’re likely to hear is hype, not actual science. Good thing I was able to get Dr. Riccardo Manenti as a guest for our podcast!

Riccardo is the author of a state-of-the-art textbook on quantum computers, has his PhD from Oxford in Quantum Computing, and has been working for Quantum Computing startup Rigetti for several years now. Join us as he helps demystify some of the recent progress and problems right here on the cutting edge of this promising new arena of physics, right here on the Starts With A Bang podcast!

This article Starts With A Bang podcast #101 – Quantum Computing is featured on Big Think.

From the perspective of a human being, it’s almost impossible to fathom just how mind-bogglingly large the cosmic scales are. An average human is a little less than 2 meters in height, but planet Earth is more than 12,000 kilometers in diameter. The Sun is larger, at 1.4 million kilometers across, and incredibly far away: some 150 million kilometers distant. And these numbers, grand though they may seem, are paltry compared to what lies beyond our inner Solar System. Neptune is more than 4 billion kilometers away; the next-nearest star, Proxima Centauri, is 4.2 light-years (or ~40 trillion km) distant, while the Milky Way itself is more than 100,000 light-years across.

All of that doesn’t even take us beyond our own galaxy, to the trillions of others stretched across the observable Universe. Yes, it’s incredibly hard to fathom, but what if we shrunk those scales down to something more familiar? That’s the idea of Patreon supporter Pete Smoyer, who wants to know:

“If a grain of sand was a scale model of a star in a galaxy, how far apart would each grain of sand / star be on average? At this scale, how far would the Andromeda galaxy be from the Milky Way?”

Let’s take the analogy literally, using our own Sun as the template for a star. If the Universe were shrunk down so that the Sun was a grain of sand, what would the Universe look like?

This image shows the core of globular cluster Terzan 5, just 22,000 light-years away in our own Milky Way, with a wide variety of colors and masses inherent to the stars within. With millions of stars within only a few tens of light-years of one another, this dense collection of stars is still incredibly sparse, with hundreds of billions of kilometers separating the average star from its nearest neighbor.

The Sun as a grain of sand

Our Sun is the largest, most massive, and also most naturally perfect spherical object found in our Solar System. Its vital stats are as follows:

• Age: 4.6 billion years.
• Mass: 2 × 10³⁰ kg.
• Diameter: 1.4 × 10⁹ meters (or 1,400,000 km).
• Distance from Earth: 1.496 × 10¹² meters (or 150,000,000 km).

What would happen if we were to attempt to shrink it down, even if just in our minds, to the size of a single grain of sand?

Sand, as far as we understand it, is determined by what we call grain size. Whenever you have a sample of sediment, the average size of the grains, also known as the particle size, determines what classification it falls into. Above 2 mm (or 0.08 inches) in size, the grains/particles are known as gravel: too big to be classified as sand. Below 0.0625 mm (or 0.002 inches) in size, the grains/particles are known as silt, with particles smaller than silt forming clay, and particles smaller than clay forming a colloid when mixed with water.

But sand itself is between gravel and silt: comprised of particles between 0.0625 mm and 2 mm (0.002 and 0.08 inches), which means we’ll be shrinking the Sun down by around a factor of a trillion.

Grains of sand can be measured on a scale that ranges from under 100 microns all the way up to about 2 millimeters. Grains of particles that are larger than sand are known as gravel; smaller than sand is what’s known as silt.

The Sun and stars as sand

We have to remember, though, that not all stars are identical to one another, just as all grains of sand aren’t identical. The smallest, lowest mass stars are red dwarfs, which are around 7.5-8% the mass of the Sun and can be nearly as small as the planet Jupiter, with the smallest known red dwarf coming in at 12% the diameter of the Sun: just 20% larger than Jupiter.

On the other hand, stars can be much more massive, and also much larger, than our Sun. The most massive star known is R136a1, found in the Tarantula Nebula, which comes in at 260 times the mass of the Sun. Although this class of stars, the blue supergiants, can be large in terms of physical size, they “only” reach up to about 25 times the Sun’s diameter. The largest known stars are evolved, red supergiant stars, and they can be several hundred, or even up to ~1000, times the size of the Sun.

In other words, just as grains of sand come in a variety of sizes, so do the stars themselves. If we ignore the puffiest, most evolved stars and treat the smallest sand grain as the smallest star, this causes the Sun to come out at around 0.5 millimeters in diameter as a grain of sand: precisely 2.8 trillion times smaller than its actual physical size.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. In terms of size, the smallest M-class stars are still about 12% the diameter of the Sun, but the largest main sequence stars can be dozens of times the Sun’s size, with evolved red supergiants (not shown) reaching hundreds or even 1000+ times the size of the Sun. A star’s (main sequence) lifetime, color, temperature, and luminosity are all primarily determined by a single property: mass.

A microscopic Earth

Shrinking the Sun down to these small scales would entail imagining the rest of the Universe scaled down as well, in the same proportion as we scaled down the Sun to make it commensurate with the size of a grain of sand. If the Sun would be 0.5 millimeters in diameter, then that would make the entirety of planet Earth about 4.6 microns across: a little bit smaller than a red blood cell, but a little bit larger than a single bacterium. The continent of Australia would be just over 1 micron in size. The third-largest impact crater on our planet, Sudbury Basin in North America, would be about the size of the Zika virus: just 45 nanometers, or smaller than the wavelength of visible light.

As for human beings?

We’d be smaller than an atom. A full-grown human would be around 600 femtometers in size: about the same size as 40 uranium nuclei lined up end-to-end against one another. The physical size of your thumb’s topmost bone — known as the distal phalanx — would correspond to about 1.7 femtometers: the size of a single proton. Our own planet, and everything on it, would be minuscule.

The inner Solar System, including the planets, asteroids, gas giants, Kuiper belt, and more, is minuscule in scale when compared to the extent of the Oort Cloud. Sedna, the only large object with a very distant aphelion, may be part of the innermost portion of the inner Oort Cloud, but even that is disputed. On a linear scale, depicting the entire Solar System in a single image is incredibly limiting; to characterize the orbit of a faraway bound object requires years or even centuries of data.

A tiny Solar System

If the Sun were a grain of sand, what would that mean for the scale of the Solar System? Earth’s orbital distance in its motion around the Sun occurs at a radial separation of about 150 million kilometers. If the Sun were a grain of sand, that would correspond to a distance of about 5.3 centimeters, or a little more than 2 inches.

Jupiter, our largest planet, would be just below the size of the smallest sand grains to fall into the “silt” category, but would trace out an orbit separated by 28 centimeters (almost a foot) from the Sun. Neptune, our most distant planet, would be about a third the size of Jupiter, but would trace out an orbit that’s 1.6 meters (about 5’3″) wide.

Our Kuiper belt would go out about 2.4 meters (almost 8 feet), before we run into the edge of the Kuiper cliff that gives way to the scattered disk and then the Oort cloud. Compared to all of the other distances so far, the Oort cloud is tremendous: beginning about 50-100 meters from the grain-of-sand that represents our Sun and extending to more than 10 kilometers away.

This graphic shows the location of the closest star systems beyond the Solar System, centered on the Sun. If you can double the radius out to what you can see and measure, you encompass eight times the volume, which is why the ability to see farther even by a little bit vastly increases your chances to find something remarkable, even if it’s a rare type of system you’re seeking. Roughly every few hundred thousand years, a star will pass within 120,000 AU (a little less than 2 light-years) of Earth: coming within the estimated size of the Oort cloud.

The distance to the nearest stars

If the Sun were a single grain of sand — remember, we chose a grain that was representative of its actual size relative to the other stars, of 0.5 millimeters in diameter — then how far away would the other stars in our galaxy be?

Proxima Centauri, the closest star, would be about 14 kilometers away, and would be among the smallest grains of sand that wouldn’t be downgraded to be classified as silt. A little bit farther, at around 15 kilometers distant, would lie two Sun-sized grains of sand: representing the stars Alpha Centauri A and B.

Within about 30 kilometers, there are only around 10 total grains of sand, but that number begins to rise rapidly with increasing distance.

• Within 45 kilometers, there are 33 additional grains of sand beyond our Sun.
• Within around 200 kilometers, there are around 2000 grains of sand.
• And within 250 kilometers, that number rises to around 3000 grains of sand.

That last figure corresponds to the number of stars found within a distance of around 82 light-years (or 25 parsecs) from our Sun, and represents the best number available today for the number of known stars in our local vicinity.

The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way.

The scale of the Milky Way

The Milky Way galaxy, however, is much larger than this. With up to an estimated 400 billion stars inside — corresponding to 400 billion grains of sand — its stars are distributed into a spiral-lined disk that’s only a couple of thousand light-years thick, but that spans a little over 100,000 light-years in diameter.

When we scale the Sun down to a grain of sand, the dimensions for the Milky Way change dramatically. The diameter of the Milky Way would then be about 300,000 kilometers, or just slightly less than the distance from the Earth to the Moon. (Incidentally, it’s also the distance that light travels in one second.) For thickness, the Milky Way would be about 6,000 kilometers thick at the distance of the Sun, which would be approximately 70,000 kilometers from the Milky Way’s center.

Whereas on Earth, you’d only need to scoop out about 8 cubic meters of sand from the beach to wind up with 400 billion grains of sand, for the Milky Way, you’d have to cover around 4 × 10²³ cubic meters of volume to distribute those 400 billion grains of sand across in order to fit them all in, to scale. On Earth, the average grain of sand is in physical contact with at least one other grain of sand; in space, if the Sun were a grain of sand, the average separation distance between sand grains would be measured in kilometers.

This ultra-distant view of the Universe comes from a portion of the JADES survey, leveraging JWST’s capabilities. Although there are trillions upon trillions of stars producing the light powering these galaxies, they extend back for tens of billions of light-years in space. In reality, the density of stars in space is incredibly low.

Isolated beaches

Here on Earth, sand is only found along coastlines where rock has eroded and broken down over geologically long periods of time. Weathering breaks large rocks down into smaller ones, and minerals, such as quartz and feldspar, eventually wear down into the sand grains we’re familiar with.

Just as sand is only found where conditions are right, largely along coastal areas and only where inland rocks are routinely transported into larger bodies of water (lakes, seas, and oceans), in our Universe, stars are almost exclusively found where large collections of matter have gathered together: in galaxies.

Around 100 years ago, we were only just discovering that the grand spirals and ellipticals in the night sky were galaxies unto themselves; the idea that they were was called the “island universe” hypothesis. Given that we’re using sand to represent the stars, it makes sense to return to that analogy, as these cosmic beaches, replete with grains of sand, are isolated concentrations that can interact, cluster, and even merge together, but in general are separated by vast, enormous distances across the cosmos: millions of light-years under most typical circumstances.

This multiwavelength view of the two largest, brightest galaxies in the M81 group shows stars, plasmas, and neutral hydrogen gas. The gas bridge connecting these two galaxies infalls onto both members, triggering the formation of new stars. If each star were shrunk down to be a grain of sand, this group would be 36 million km away, but the two galaxies would be separated only by a little over 400,000 km: the Earth-Moon distance.

From the Milky Way to Andromeda (the two largest, most star-rich galaxies in the Local Group), for instance, you’d have to travel a whopping 7.5 million kilometers, and that’s if the Sun and all the stars within them were shrunk down to a grain of sand. The next-nearest large galaxies beyond the Local Group, such as the stunning pair of Bode’s galaxy and the Cigar galaxy, are more like 36 million kilometers away from us, but are only separated from one another by around 400,000 kilometers: an example of galaxies that are interacting, tightly grouped, and are on the path toward merging.

The Virgo cluster of galaxies, the largest rich cluster of galaxies to us, would be around 150 million kilometers away (about the same as the Earth-Sun distance if they weren’t shrunk down), and contains over 1000 large galaxies within a sphere around ~15 million kilometers in size.

And if you wished to go to the edge of the observable Universe, containing the trillions upon trillions of “island universes” within it, you’d have to venture out a little over 100 billion kilometers (around 130 billion kilometers) away: about the equivalent of the distance that Sedna, the only body in hydrostatic equilibrium within our Solar System that has such a large eccentricity, will reach at aphelion, its farthest distance from the Sun.

This artist’s conception shows a logarithmic view of the observable Universe. The Solar System gives way to the Milky Way, which gives way to nearby galaxies which then give way to the large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. If we were to shrink the Sun down to a grain of sand, then the observable Universe would only be a little over a hundred billion kilometers in radius, containing around 2 sextillion “grains of sand” (stars) strewn across that enormous volume.

Although shrinking the Sun down to a grain of sand — and shrinking all other cosmic distances and scales commensurately — might help reduce the absolute scale of the Universe to more familiar terms, it remains as sparsely-populated as ever. Consider that, if you were to take all the grains of sand contained in all the beaches and coasts and bodies of water on Earth, you’d wind up with a large number: 7.5 quintillion (7.5 × 10¹⁸). If you then multiplied the number of grains of sand by 300, and scattered them across the Solar System from here to the farthest orbital reaches of Sedna, you’d have an idea about how rare it would be to find a star within the Universe.

Yes, it’s true that our Universe is vast, impressive, and filled with enormous numbers of stars and galaxies. But the act of taking just 300 times the number of grains of sand on Earth and scattering them across a sphere nearly 1000 times the Earth-Sun distance in radius, where each grain of sand is analogous to a star and that sphere is analogous to the volume of the observable Universe, helps illustrate just how “empty” our Universe truly is. If the Sun were a grain of sand, a volume of space the size of the entire Earth would contain fewer than 1 billion grains of sand within it. The Universe, no matter how you view it, is already incredibly empty.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: What if the Sun were a grain of sand? is featured on Big Think.

When it comes to planet Earth, the most important source of light, heat, and energy actually comes from beyond our world. It’s the Sun that is the driver of the Earth’s energy balance, rather than the internal heat given off by the planet itself from sources like gravitational contraction and radioactive decays. The energy from the Sun keeps temperatures from freezing all across the planet, providing us with temperatures that allow liquid water on Earth’s surface, and that are essential to the life processes of nearly every organism extant on our world today.

And yet, it’s only within the last 200 years that humanity has even understood how much energy, overall, the Sun actually produces. Considering all of the scientific advances that came afterward, including the development of stellar, quantum, and nuclear physics, as well as the understanding of the subatomic fusion reactions that power the Sun, it might seem like a trivial matter to simply answer the question of “How much energy does the Sun produce?” But looks can be deceiving. If you didn’t already know (or hadn’t already googled) the answer to that question, how would you figure it out? Here’s how humanity did it.

When sunlight strikes the Earth’s surface, it has already been processed: by not only its journey from the Sun to the Earth, but by Earth’s atmosphere, clouds, and all objects that absorb or emit light along its journey to us. Here’s how we did our best to overcome those limitations and measure the Sun’s power output.

The Solar System is not enough

You might think to yourself that simply knowing a few physical properties about the Sun, such as:

• how big it is,
• how massive it is,
• and how far away from Earth it is,

would go a long way toward delivering the answer to such a question. After all, with even extremely primitive tools (like your naked eye and a sextant), you can determine how large, in terms of angular size, the Sun is. Since ancient times, it’s been known that the Sun is approximately half-a-degree across from end-to-end, with more modern measurements confirming that its angular size varies from 31.46 to 32.53 arcminutes over the course of a year. (Where 60 arcminutes equates to one degree.)

But that’s approximately the same angular size that the Moon takes up, and one is very close and relatively small while the other one is enormous and much farther away. We’ve been able to know the Moon’s size for around 2000 years, because once you know the size and shape of the Earth, viewing the Earth’s shadow on the Moon (during the partial phases of a lunar eclipse) allows you to infer the Moon’s size relative to the Earth. With just a little bit of geometry, you can then figure out the physical size of the Moon. This method was first used by Aristarchus in the 3rd century B.C.E., but it’s of no help in determining the distance to the Sun.

Animation showing the umbral phase of the November 19, 2021 partial lunar eclipse. At 9:03 AM UT, maximum eclipse is reached, where only 0.9% of the Moon remains illuminated by direct sunlight. The umbral phase lasts over 3.5 hours: the longest this century for a partial eclipse. Reconstructing the size of Earth’s shadow relative to the physical size of the Moon is the oldest method for measuring both the size of the Moon as well as the distance to it: a method first leveraged by Aristarchus back in the 3rd Century BCE.

The next great leap in determining the physical properties of the Sun wouldn’t arrive until the 17th century, when first Kepler and then Newton came along to put the Solar System in order. By determining the orbital dynamics of the planets, Kepler was able to arrive at three important laws that governed the planets’ motions around the Sun.

1. Planets move about the Sun in elliptical orbits, with the Sun at one focus of that ellipse.
2. Planets sweep out equal areas in equal times with respect to the Sun, moving more rapidly close to perihelion and more slowly close to aphelion.
3. And the square of a planet’s orbital period is proportional to the cube of the semimajor axis of the ellipse that its orbit traces out.

When Newton came along toward the end of the 17th century and put forth his law of universal gravitation, he placed Kepler’s qualitative and proportional laws of motion onto a more solid, quantitative mathematical footing. By introducing the notion of a universal gravitational constant and by showing that gravity was consistent with obeying an inverse-square force law, Newton finally, based on the measurement that Earth’s orbit takes one year to complete, gave us an accurate way to measure the distance to the Sun.

The orbits of the planets in the inner Solar System aren’t exactly circular, but are elliptical, as are the orbits of all bodies gravitationally bound to the Sun. Planets move more quickly at perihelion (closest to the Sun) than at aphelion (farthest from the Sun), conserving angular momentum and obeying Kepler’s laws of motion, which were put on a more solid, generalized mathematical footing by Newton. Once the quantitative relationships between orbital period and distance were uncovered, it became possible to know the distance from the Earth to the Sun.

Measuring the distance to the Sun gave us a vitally important piece of information, as we now know, based on the already-measured angular size of the Sun, how physically large it must be. Since the Sun is a sphere — the most perfect sphere in the Solar System, in fact — knowing its distance and angular size tells us its radius and, therefore, its surface area. The only piece of information that was left ambiguous was its mass, as Newton’s laws could only give us the product of his universal gravitational constant, G, and some particular mass (like the mass of the Sun or Earth), but was unable to disentangle them. That advance wouldn’t arrive until 1798, when Henry Cavendish was able to experimentally measure the gravitational constant directly.

If we want to know how much energy the Sun produces, knowing the distance from the Earth to the Sun is a huge asset, since we know how sunlight (like all forms of light) spreads out: like the surface area of a sphere. At double the distance, the Sun’s incident energy on a target will be quartered. Based on how much of the Sun’s energy is absorbed at the distance of Earth over a particular area, we can then calculate the total energy (and power) outputted by the Sun. Knowing all about the Solar System can get us very far in the quest to measure the Sun’s energy output, but measurements here on Earth are still needed.

The way that sunlight spreads out as a function of distance means that the farther away from a power source you are, the energy that you intercept drops off as one over the distance squared. This also illustrates, if you view the squares from the perspective of the original source, how larger objects at greater distances will appear to take up the same angular size in the sky. This relation is perfectly true in a Universe governed by Euclidean geometry, and is an excellent approximation for our Solar System.

Sunlight is more than visible light

Today, we take for granted that there’s much more to light than the tiny portion of the spectrum that’s visible to our eyes. But back hundreds of years ago, this wasn’t obvious in any easily provable way. Hot objects, which today we know emit large amounts of infrared radiation, were thought to possess a quantity of heat energy that had no light-based counterparts. At high energies, gamma-rays, X-rays, and ultraviolet light were not known, while at longer wavelengths and lower energies, there was not yet evidence for microwave or radio waves.

The first evidence for the existence of light beyond visible light — especially for light beyond the visible part of the spectrum as being part of sunlight — was uncovered quite by accident. Astronomer William Herschel, the famed discoverer of Uranus (which he found in 1781), suddenly found himself the recipient of accolades, resources, and wealth: largely gifted by King George III of England. He began to devote himself to astronomy full-time, completing what was then the world’s largest telescope later in the 1780s, discovering Titania and Oberon in 1787 (Uranus’s two largest moons), and then turned his attention to another astronomical object of interest: the Sun.

The behavior of a beam of sunlight, perhaps the greatest example of white light, as it passes through a prism demonstrates how light of different energies moves at different speeds through a medium, but how they all move at the same speed through a vacuum, which is why the light that doesn’t pass through a refractive medium remains white in color. Note that the visible light shown here is only illustrative of the sunlight that we can see; other wavelengths of light still exist beyond the portion that’s visible.

Herschel was extremely interested in viewing the Sun directly, but was aware of the damage (and pain) it could cause to one’s eyes. He was aware that through a telescope, the magnified light would significantly amplify the damage it could cause, but that by passing that light through a filter, he could reduce the intensity of the light his eyes would receive. In 1794, he presented a paper to the Royal Society where he recorded his experience viewing the Sun (and other stars), with a telescope but with a variety of filters applied.

He noted, quite remarkably, that:

• some of the filters greatly reduced the amount of light emitted by the Sun, but did little to alter its sensation of heat,
• some of the filters greatly reduced the amount of heat felt from the Sun, but did little to reduce the amount of visible light,
• and some filters greatly reduced both.

Herschel then speculated that both the heating power and the illuminating power of various colors of sunlight might be different from one another. To test this out, Herschel built what astronomers know today as a spectrometer: a device to break up light into its component wavelengths, where he could measure how much power-and-energy were inherent to each individual wavelength, which corresponded to the colors he could see.

The three types of cone cells found in human eyes, S, M, and L, shown with the wavelength range that they respond to: short, medium, and long wavelengths. Some humans lack one type of cone, rendering them color blind, while a few people have four types of cones and can see more colors than the rest of us: tetrachromats. The greatest sensitivity of human eyes to the intensity of light occurs between 500 and 600 nanometers, with the response dropping off rapidly at the most extreme red and violet wavelengths.

It was in 1800 that Herschel set up this spectrometer and shone sunlight through it, with the prism-like effect of the spectrometer creating a beautiful rainbow of colors. He would then set up a slit to allow a narrow range of color wavelengths through, where that light would fall onto a thermometer. Right next to it, but in complete shadow of this light, was the “control” thermometer. By observing the difference in temperatures induced by the light, Herschel was able to say something about the “heating power” of the various colors of light, which told him something about the overall energy.

He quantified how violet, green, and red light behaved, among others, and noted that heating increased for redder colors of light as compared to the bluer colors. He then went further: beyond where the visible light portion of the spectrum ended. Lo and behold, even as the light was no longer visible, the heating continued to increase: demonstrating the presence of invisible light, in the form of what we today call infrared radiation. Herschel’s data was good enough that he was even able to measure the spectral intensity of infrared radiation, including its peak and subsequent fall-off. The data from his final paper on the topic, written in 1800, is shown below, with annotations by Jack White.

This graph shows the intensity of visible light, by brightness, as a function of color on the right, while the shaded region shows the temperature-recorded heat intensity of light at both visible and infrared wavelengths. This 1800 publication, by William Herschel, represented the most robust demonstration of the existence of “invisible light” at the time.

Solving for the power of the Sun

But how much energy, total, was coming from the Sun? It was not yet generally accepted that light was a wave, so the correspondence of color with wavelength wasn’t known. With that modern knowledge, we know that the crown glass crystal that Herschel used likely absorbed a significant amount of shorter-wavelength light, and that the human eye responds much more severely to yellow and green wavelengths of light than to red wavelengths, implying that his measurements were not solely measuring the energy coming from the Sun, and certainly not in an unbiased way.

But we could imagine how that energy could be measured, and in fact two people put forth the idea: John Herschel, William’s son, and Claude Pouillet. All you had to do, they reasoned, was:

• create an open vessel that was filled with a high specific-heat material that was a good absorber of heat, like water,
• that would be held within a highly reflective material, like silver, that reflected sunlight of all wavelengths equally well,
• along with a device for measuring heat/temperature gain (like a thermometer),
• plus a way to calculate heat losses and dispersion from atmospheric effects,

and you could then infer how much energy was in sunlight incident on that particular area. Imagine expanding that area to the size of Earth’s orbit around the Sun, and you can calculate the energy emitted by the entire Sun itself.

This schematic drawing shows an actinometer developed by Jules Violle, which was intended to deduce the temperature of the Sun at its surface. Building upon earlier designs, including those of Pouillet and Herschel, allowed us to not only measure the total power-per-square-meter arriving from the Sun, but more intricate properties as well.

The earliest device that could do this, developed by John Herschel in 1825 (just three years after his father, William, passed away), was known as an actinometer. In 1833, Herschel traveled to South Africa for several years, and there was using reflective materials to build his own solar cooker, which proved much more effective at the near-equatorial latitudes in South Africa as compared to those back at home in England (33° to 51°, for comparison). Because the Earth is tilted on its axis by 23.5°, Herschel realized that he would have an incredible opportunity in South Africa that he wouldn’t have in England: to observe the Sun when it was nearly directly overhead, during the December solstice.

He brought his actinometer out and performed the experiment: setting up this initially cool-water vessel (kept in the shade) until the Sun was almost perfect overhead, at midday on the solstice. Then, he removed the shade, and watched the temperature rise during those critical moments, allowing him to calculate the energy output by the Sun over the time that the water was heated. Energy over time, in physics, is what’s known as power, and Herschel’s measurement worked out to a little more than one kilowatt of solar energy for each square meter of Earth that sunlight strikes.

The Sun’s light is due to nuclear fusion, which primarily converts hydrogen into helium. When we measure the rotation rate of the Sun, we find that it’s one of the slowest rotators in the entire Solar System, taking from 25-to-33 days to make one 360-degree rotation, dependent on latitude. Emitting a near-constant 3.8 × 10^26 W of power, the Sun is the brightest thing most of us will ever see. Although many other sources are intrinsically brighter, they’re much farther away.

By extrapolating the surface area over which sunlight struck his container to cover the total amount of sky at Earth’s orbital distance from the Sun, Herschel was able, for the first time, to estimate the outputted power of the Sun itself: that value works out to somewhere around 4 × 10²⁶ watts. (Herschel’s value was a little bit lower, as he underestimated the amount absorbed/reflected by the atmosphere.) That means, with each second that goes by, the Sun:

• fuses around 10³⁸ protons within its core into heavier elements,
• transforms 620 million tonnes of hydrogen,
• into 616 million tonnes of helium,
• and releases the energy-equivalent of 4 million tonnes of matter via Einstein’s E = mc².

It turns out that even though the Sun’s peak wavelength occurs in the visible light portion of the spectrum, the majority of the Sun’s total energy really is emitted at infrared wavelengths. This technology was swiftly put to use. By 1870, sunlight was used to create the first solar-powered motors, which developed into different engine configurations and became more efficient throughout the decade. By 1880, the first solar cell was developed, and in the 1950s, NASA began using solar power in space. All of today’s modern applications of solar power rely on knowing how much energy is outputted by the Sun, a piece of knowledge that’s less than 200 years old. Remarkably, now you know how to figure that value out for yourself!

This article How much energy does the Sun produce? is featured on Big Think.

Galileo died on January 8, 1642, and on Christmas Day of that same year, Isaac Newton was born. Back in 1616, two extremely famous playwrights (among their other literary endeavors), Miguel de Cervantes and William Shakespeare, died just one day apart: April 22 for Cervantes, and April 23 for Shakespeare. And the famed Plymouth Rock, a granite slab in honor of the first solid ground that the Pilgrims stepped on upon disembarking from the Mayflower, is presently inscribed with “1620” in honor of the year that legendary step was taken: December 26, 1620.

But, upon closer inspection, none of these facts are true. While physicists often refer to Christmas Day as “Newtonmas” in honor of Isaac Newton’s birth, that’s only true because England, unlike the rest of the world, hadn’t yet switched over to the Gregorian calendar; Newton’s actual birthday was January 4, 1643 according to our modern timekeeping practices. Cervantes and Shakespeare actually died 11 full days apart, not just one, as their countries were on different calendars at the time. And the Plymouth Rock landing actually occurred on January 5 of 1621 according to our modern calendar, not “1620” as written.

The reason for all of this historical confusion? A flawed calendar that dates all the way back to Julius Caesar. Here’s the science behind how humanity’s most enduring calendar, the Julian calendar, failed us all.

This image shows a clever one-page calendar view for the current year (which is also a leap year): 2024. Note that the monthly patterns differ from how they behave in a non-leap year, displaying a new pattern unique to leap years, corresponding to the fact that February has 29 days instead of 28.

Imagine that you weren’t here on planet Earth, but rather were in an unusual position: millions of miles up above the Sun’s north pole, looking down on the Solar System. As you observed the Earth, the third planet from the Sun, orbiting around it, you’d easily be able to measure how much time it took for that planet to complete a full 360° revolution around the Sun. That’s one way to measure a year, and to astronomers, that’s known as a sidereal (sy-DEER-ee-al) year, which, unsurprisingly, takes a little bit over 365 “days,” where a day is defined by the amount of time it takes Earth to spin 360° about its axis.

Unfortunately, that simple definition doesn’t quite match up with how we experience the year here on Earth. To any inhabitant of planet Earth, it isn’t a 360° revolution that determines the year, but rather the recurrence of the seasons: the conditions that a terrestrial observer on Earth experiences. That lines up with what astronomers call the tropical year, which recurs from:

• spring equinox to spring equinox,
• summer solstice to summer solstice,
• autumnal equinox to autumnal equinox,
• or winter solstice to winter solstice.

Basically, if you took a look at Earth’s axis and said, “this is how it’s oriented, with respect to the Sun, right at this moment,” a single tropical year would mark the very next time that the Earth’s axis returned to that exact same orientation.

Just 800 years ago, perihelion and the winter solstice aligned. Due to the precession of Earth’s orbit, they are slowly drifting apart, completing a full cycle every 21,000 years. Over time, the Earth drifts slightly farther from the Sun, the precession period increases, and the eccentricity varies as well. The most accurate measure of the “year,” as experienced by Earthlings, is to go from either equinox-to-equinox or solstice-to-solstice.

If you wanted to construct a calendar that kept time accurately over long periods of time, you’d have to ask yourself the right question: how long, precisely, is the tropical year, and what calendar system would lead to a definition of a “year” that matched up to a tropical year over long periods of time?

The length of the tropical year has been measured to extraordinary precision, and is known accurately to a fraction-of-a-second. In terms of the amount of time it takes to make up one Tropical Year today, it’s precisely 365.24219 days. In more conventional terms, that’s 365 days, 5 hours, 48 minutes, and 45 seconds.

This, quite notably, is about 20 minutes and 24.5 seconds shorter than a sidereal year, as the Earth’s equinoxes (i.e., the orientation of our axial tilt) very slowly precesses with respect to the Earth’s orbit around the Sun. On timescales in excess of 20,000 years, Earth’s axial orientation shifts in a circular pattern, which causes where our celestial “north” and “south” poles are to change over time. While today, Polaris marks the North Star (to within 1°), the pole stars in both hemispheres have changed significantly, and periodically, over time. 5000 years ago, when the Egyptian pyramids were being constructed, the star Thuban, in the constellation of Draco, was the northern hemisphere’s pole star instead.

Today, in the year 2024, Polaris makes an excellent “North star,” as it’s located within about 1 degree of the celestial north pole. As Earth’s axis precesses, the presence and properties of a pole star change; 5000 years ago, the star Thuban in the constellation of Draco was an excellent pole star for humanity.

However, these details — and these levels of precision — are relatively new assets to human knowledge. Thousands of years ago, it was known that the lunar calendar (based on the number of full moons in a year) didn’t line up with the solar calendar (our proxy, before we knew about the structure of the Solar System, for the tropical year), and so most calendrical systems typically consisted of 12 full moons, or lunar “months,” with an occasional 13th “intercalary” month inserted into the year to keep the lunar and solar calendars aligned.

The problem with this system, back in the era of the Roman Republic, was actually political: abuses of power led people with political aims to either grant or deny the insertion of an intercalary month dependent on who was in power. In the year 46 B.C.E., then-consul Julius Caesar proposed a reform: let the calendar be governed by the Sun, i.e., a solar calendar, to free it from the concerns of human tampering. The result, taking place on January 1, 45 B.C.E., was the adoption of the Julian calendar, which assigned 365 and ¼ days to the year: with 365 days to most years and an extra “leap day” to every fourth year. This approximation wasn’t a Roman invention, but had been known to the Egyptians for more than 1000 years already back in Caesar’s time.

To travel once around Earth’s orbit in a path around the Sun is a journey of 940 million kilometers. The extra 3 million kilometers that Earth travels through space, per day, ensures that rotating by 360 degrees on our axis won’t restore the Sun to the same relative position in the sky from day to day. This is why our day is longer than 23 hours and 56 minutes and 4.09 seconds, which is the time required for the spheroidal Earth to spin a full 360 degrees. Similarly, a “year” is not the time it takes for Earth to make a full 360 degree revolution around the Sun, but is determined by the return of its axis to the same position relative to the Sun to the year before.

For approximately the next 1600 years, a large portion of the world, including nearly all of what we know as the “western world,” adopted the Julian calendar (or a nearly-identical variant of it) in an effort to keep time in a uniform fashion. However, by the middle of the second millennium of the Common Era, it had become notable that the way Earth experienced the year — governed by solstices and equinoxes — had drifted considerably with respect to their initial calendar dates within the Julian calendar.

• The winter solstice, originally occurring on the 25th of December in the Julian calendar, was now occurring in the first half of the month.
• The spring equinox, originally occurring in late March, was now occurring in the first half of March: on March 10th, in fact.
• The summer solstice had also shifted from the second half of June to the first half of June,
• and the autumnal equinox, originally in late September, was now happening in the first half of September.

The culprit was the very small mismatch between the 365.25 days assigned to the “average” year by the Julian calendar, versus the actual length of the tropical year on Earth of 365.24219 days. This difference, although small, really adds up over timescales of more than 1000 years.

The Earth, moving in its orbit around the Sun and spinning on its axis, appears to make a closed, unchanging, elliptical orbit. If we look to a high-enough precision, however, we’ll find that our planet is actually spiraling away from the Sun, while the rotation period of our planet is slowing down over time. The same calendar that we use today cannot successfully be applied to either our distant past or future.

That tiny difference might only correspond to an average 10.8 minutes per year, but after some 1600 years had passed, that had accumulated to put the Julian calendar and the actual tropical year out of sync by around 12 full days. Either humanity was going to have to get used to a “drifting” calendar, where the seasons grew more and more out of sync with the calendar from their initial placement, or a new type of calendar system would have to be adopted, with some sort of “shift” imposed to put the calendar and the actual experiential year back into their original, intended configuration.

The next great leap in refining humanity’s calendar came in the 16th century: with the invention and promulgation of the Gregorian calendar. Pope Gregory XIII, in 1582, put forth a new calendar which swiftly overtook the Julian calendar in many parts of the world. In order to better align our calendar with the actual tropical year, the way we insert “leap days” was changed. Instead of occurring every 4 years, as in the Julian calendar, leap days would only be inserted every 4 years except for years that ended in “00,” marking a turn-of-the-century. Only if those years were divisible by 400 would they get a leap day; the year 2000 did, but 1900 didn’t and 2100 won’t.

The presence or absence of a February 29 on the calendar determines with great significance whether the equinox shifts forward or backward in time from the prior year’s equinox. 2020 marked the first year since 1896 where the entire United States experienced a March 19 equinox, and 2024 will mark another year with a leap day. Leap days occurred every 4 years under the Julian calendar, but the Gregorian calendar revised that to remove some of the leap days (on years ending with “00” but not divisible by 400) to better keep track of time.

That reform marked a huge improvement: instead of the 365.25 days-per-year that the Julian calendar brought, on average, the Gregorian calendar delivered a more accurate 365.2425 days-per-year, averaged over the centuries. Compared to the actual length of a tropical year, 365.24219 days, the Gregorian calendar is only off by about 27 seconds per year. This means, whereas the Julian calendar was pulled out-of-sync from the tropical year by one day roughly every 128 years, it would take some 3200 years for the Gregorian calendar to drift out-of-sync from the tropical year by even one day.

(Remarkably, if we added one small additional modification — that the years 3200, 6400, 9600, and all other years divisible by exactly “3200” wouldn’t get leap days — it would then take somewhere around 700,000 years for this new calendar to drift a single day out-of-sync with the tropical year.)

But in 1582, when Pope Gregory XIII put out his calendrical reform, there was another issue to deal with: the fact that the “old” calendar and the actual solar/tropical year were already misaligned by a significant amount. Although it was controversial, the decree was that, since “leap days” had been incorrectly added to 12 calendar years by this point under the Julian calendar:

• 100,
• 200,
• 300,
• 500,
• 600,
• 700,
• 900,
• 1000,
• 1100,
• 1300,
• 1400,
• and 1500,

the “fix” would be to remove some of them.

Starting in 1582, the Gregorian calendar began to displace the less-accurate Julian calendar all across the world. In order to accommodate this change and account for the mismatch induced by Julian calendar drift, dates needed to be removed from the calendar to allow humanity to “catch up” to the Earth. In 1752, the UK and its territories (including the now-USA) made the switch, having a September with 11 days missing in that particular year.

For many countries across the world (particularly in western Europe), including modern-day Italy, Spain, Poland, and Portugal, the resolution was that the “old” Julian calendar would continue to be used until October 4, 1582 (which was a Thursday), and then the next day (which would still be assigned as a “Friday”), would fall under the “new” Gregorian calendar system, but would be October 15, 1582. In other words, the 10 days from October 5, 1582 to October 14, 1582, never existed; they were omitted in order to allow the calendar year and the actual seasons to sync back up again. (Historically, these dates were chosen for religious reasons: they were the days that had the fewest feasts of saints to them.)

This “new” system of the Gregorian calendar quickly gained worldwide acceptance and came into use. Later in 1582, France and the Netherlands adopted the new calendrical system. In 1583, Austria, Switzerland, and Germany joined them. But many countries, particularly countries with large non-Catholic populations, resisted adopting the new system despite its superiority as a calendar.

The protestant parts of Germany, the Netherlands, and Switzerland, along with all of Denmark, Norway, and Iceland, didn’t adopt the Gregorian calendar until 1700. England (and its colonies, such as the modern-day USA) didn’t follow suit until 1752. Sweden and Finland adopted the Gregorian calendar in 1753, Japan in 1873, Egypt in 1875, and Russia adopted it in 1918. (Which is why the “October revolution” is so named, despite beginning on November 7, 1917, under the Gregorian calendar.) Greece became the final European country to make the change, back in 1923.

Although a great many countries first adopted the Gregorian calendar in the year 1582, it wasn’t until the 18th century that it was adopted in England, with many countries making the transition even later. As a result, the same date, as recorded in different countries, often corresponds to a different point in time.

The countries that switched later on, in order to get back into sync with the rest of the world, had to remove more than 10 days from their calendar, dependent on the number of Julian leap days that were inserted where the Gregorian calendar had none.

• In the United Kingdom and its then-colonies, because the year 1700 had passed, 11 days needed to be removed: September 3-13 of 1752.
• In Egypt and Japan, the year 1800 had also passed, and so 12 days had to be removed from their calendars when they switched.
• In Russia, because the year 1900 had passed prior to their adoption, 13 days had to be removed, and February 1-13, 1918 never existed there.

And for the places in the world (including a number of Orthodox churches and other religious calendars) that still use the Julian calendar for anything, the Julian dates remain 13 days out-of-sync with the modern (Gregorian) ones, but will drift to becoming 14 days out-of-sync as February 28 transitions to March 1 in 2100, as that will be a leap day on the Julian but not Gregorian calendars.

If we use modern, Gregorian dates for all parts of the world equally, we discover that many of the “facts” we learned about history didn’t occur when we were taught. Cervantes and Shakespeare actually died 11 days apart, not 1. Isaac Newton wasn’t born on Christmas in 1642 (the year Galileo died) but rather on January 4 of 1643. And the Plymouth Rock landing took place on January 5, 1621, rather than in 1620.

The Moon exerts a tidal force on the Earth, which not only causes our tides, but causes braking of the Earth’s rotation, and a subsequent lengthening of the day. The asymmetrical nature of Earth, compounded by the effects of the Moon’s and Sun’s gravitational pulls, causes the Earth to spin more slowly. To compensate and conserve angular momentum, the Moon must spiral outward. It is for this reason that Earth will no longer have total solar eclipses after another 600 million years, and that the length of each day is getting longer as time progresses.

Today, given the structure of the Solar System and our knowledge of gravity, it turns out there’s even more to the story than just picking a single calendar scheme that best matches up with the actual tropical year. Over time, because the Earth has a large moon and spins on its axis, there are tidal, frictional forces at play between the Sun-Earth-Moon system, which causes Earth’s rotation to slow, its day to lengthen, and also for the Moon to spiral out away from the Earth over time. The difference from one year to the next might be small — compared to precisely one year ago today, our planet takes an extra 14 microseconds to complete a full rotation — but those tiny differences inevitably add up over time.

While there are 365.24219 Earth days in a tropical year right now, that number is changing over time as Earth’s rotation continues to slow. In another 4 million years, we’ll have to remove leap years entirely, as a “longer” day will mean there will be precisely 365.0000 Earth days in a tropical year. Beyond that point, we’ll have to introduce negative leap days to keep our calendar in sync with the seasons. Some 21 million years from now, that number will drop to 364.0000 Earth days in a tropical year, and around 200 million years from now, Earth’s rotational period will lengthen so severely that it will overtake Mars for being the planet with the third-longest rotational period in the Solar System.

The Julian calendar held sway for some 1600 years, and while the Gregorian calendar is an improvement, it won’t be good forever. If we can survive, as a species, for thousands of years to come, we may want to consider a calendar that evolves along with our planet. It’s the only way to keep the tropical year and our way of marking time in sync, even as the physical properties of the Solar System change with time.

This article How humanity’s most enduring calendar failed us all is featured on Big Think.

If you dare to look out at the wide variety of galaxies found across the Universe, you’ll see that they tell a vastly different set of stories from one another. The largest, most massive variety of galaxies are the giant ellipticals, many of which haven’t formed any new stars over the latter half of our entire cosmic history. The next largest are spiral galaxies are like our own Milky Way, with a small number of regions forming new stars, but where the overall galaxy is largely quiet. And quite a few galaxies, particularly the smaller ones, are irregular: undergoing rapid, intense periods of star-formation. These include, among them, the interacting spiral galaxies, littered with millions of new stars along their dense spiral arms, as well as irregular starburst galaxies, where the entire galaxy transforms into a star-forming region.

Although all of these galaxy types are common today, the overall star-formation rate we see, at present, is the lowest it’s been in cosmic history for more than 13 billion years. Not since the extreme early stages of the Universe have we formed stars at such a low rate. The majority of stars formed in the Universe formed only in the first few billion years, with the star-formation rate having plummeted ever since. Here’s the cosmic story behind the cosmic formation of stars, and why our star-forming heyday is well in the distant past.

The very first stars to form in the Universe were different than the stars today: metal-free, extremely massive, and destined for a supernova surrounded by a cocoon of gas. There was a time, prior to the formation of stars where only clumps of matter, unable to cool and collapse, remained in large, diffuse clouds. It is possible that clouds that grow slowly enough may even persist to very late cosmic times.

Back at the very beginning, there were no stars, just the raw ingredients that make them: the subatomic particles that will wind up forming together to make atoms, clouds of gas, and eventually, stars and stellar systems. In the early days of the Universe, the matter density was far greater than it is today. There’s a very simple reason for this: there’s a fixed amount of material in the observable Universe, but the fabric of space itself is expanding over time. So you’d expect, when the Universe was younger, because the matter was denser, that there would have been more star formation back then, since more matter would be closer together to clump and form stars.

But there’s another effect that works against that. You have to remember that also, back in the early days, the Universe was more uniform than it is today. At the moment of the hot Big Bang, the densest regions of all were only about 0.01% denser than a typical, average-density region, and so it takes a long time for those overdense regions to grow and collect enough matter to form stars, galaxies, and even larger structures. Early on, you have factors working both for you and against you: the denser Universe makes star-formation easier, but the small nature of the overdensities means they require time to sufficiently gravitate and collapse.

The near-infrared view of the Tarantula Nebula taken with JWST is higher in resolution and broader in wavelength coverage than any previous view. It heavily expands on what Hubble taught us, and this wide-field view of our neighbor galaxy, the LMC, still showcases just 0.003778 square degrees in the sky. It would take 10.9 million images of this size to cover the entire sky. The super star cluster to the right of center, R136, is the largest, most massive new star cluster found within our entire Local Group of galaxies.

The way you form stars is pretty straightforward: get a large amount of mass together in the same spot, let it cool and collapse, and you get a new star-forming region. Often, a large, external trigger, like tidal forces from a large, nearby mass or rapidly-ejected material from a supernova or gamma-ray burst, can cause this type of collapse and new star-formation as well.

Both phenomena are easily visible just in the nearby Universe, including the Tarantula Nebula in the Large Magellanic Cloud, which is a collapsing cloud of gas with recent supernovae inside that trigger the collapse of different parts of the cloud, and in Messier 82 (the Cigar galaxy), which is being transformed into a galaxy-wide starbursting region under the severe gravitational influence of its larger neighbor, Messier 81.

However, neither of these phenomena form the greatest numbers of stars. Instead, the greatest trigger of all for star-formation is during what astronomers call a major merger. When two comparably-massed galaxies collide and merge together, a huge wave of star-formation can envelop the entire galaxy, causing what we call a starburst. These are the largest instances of star-formation in the Universe, and some of them are occurring even today.

This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds.

But that absolutely does not mean that star-formation has continued to occur at the same rates, or even at nearly the same rates, all throughout the Universe’s history. Most of the major mergers that will ever occur are already far in the rear-view mirror of the Universe’s history. The expansion of the Universe is a relentless phenomenon, just like gravitation. The problem is that there’s a competition going on between cosmic expansion and the attractive force of gravitation, and believe it or not, gravitation lost a long time ago.

If the Universe were made 100% of matter, and the initial expansion rate and the matter density balanced one another perfectly, we’d live in a Universe that would always have major mergers in its future. There would be no limit to the size of the large-scale structure that formed:

• star clusters would merge into proto-galaxies,
• proto-galaxies would merge into young, small galaxies,
• those galaxies would merge into the large spirals we have today,
• spirals would merge together to form giant ellipticals,
• spirals and ellipticals would fall into clusters,
• clusters would collide and form superclusters,
• and superclusters themselves would form together, leading to megaclusters,

and so on. As time continued to pass, there would be no limit to the scale at which the cosmic web grew and grew. We would live in a Universe that exhibited what we know as “self-similarity,” where, like a fractal, as we go to larger and larger distance scales, we just continue to repeat similar structures on and on, ad infinitum.

In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy. On scales larger than a few billion light-years, no structures will ever form, owing to the presence and late-time domination of dark energy.

Unfortunately, for everyone who’s a fan of all the new stars that could yet be formed, that scenario doesn’t describe our Universe. Our Universe has far less matter than needed for that to occur, and most of the matter we do have isn’t star-forming material at all, but rather some form of dark matter. In addition, most of the Universe’s energy isn’t matter at all, but rather comes in the form of dark energy, which only serves to drive the unbound cosmic structures on the largest scales of all farther and farther apart.

As a result, we don’t get any large-scale structures that are bound beyond the scales of galaxy clusters. Sure, some galaxy clusters will merge together, but there’s no such thing as a supercluster; those apparent structures are mere phantasms, to be inevitably destroyed as the Universe continues to expand. The new stars that our Universe will form will come from:

• major mergers from already-bound structures that haven’t yet smashed together,
• the steady, quiescent star-formation ongoing within spiral arms, dusty disks, and from the infall of molecular gas,
• and from the recycled, gas-rich reservoirs of material that are kept within galaxies, even as episodes of star-formation heat and energize them.

If we can model how, when, and how much these various physical phenomena contribute to star-formation, we can then model the star-forming history of our Universe, from inception to the present and even beyond.

This tiny region of the JADES survey shows a mix of galaxies: some that are relatively nearby, large, highly evolved, and massive; others that are at intermediate distances and have a mix of old-and-young stars in them, and a great number of very distant or even ultra-distant galaxies that are faint, heavily reddened, and potentially from the first 5% of our cosmic history. In this one little region, the power of JWST, and the evolution of the angular scale and star-formation rate of the Universe, is on full display. Views like this, of the Universe, were unfathomable just a few short decades ago.

Assuming that we understand our Universe, we can then ask what our star-formation history looks like. What we find is that the first stars should form early: after perhaps only 50-100 million years, when the small-scale molecular clouds can accrue enough matter to collapse. By the time the Universe is around 200-250 million years old, the first star clusters have merged together, triggering new, larger waves of star-formation and forming the earliest galaxies. By the time the Universe is 400-500 million years old, the largest galaxies have already grown to a few billion solar masses: around 1% the mass of the modern Milky Way.

A little bit later than this, the first galaxy clusters start to form after only a few more hundreds of millions of years. As they do, comparably-sized large galaxies begin to influence one another. It’s at this point that major mergers become common, and the cosmic web starts to get more and more dense. All of these features cause the star-formation rate to grow and grow as time goes on, at an ever-increasing rate. For the first 2-to-3 billion years of the Universe, the star formation rate only continues to rise. But then, it’s like something prevents it from rising farther. After about 3 billion years of age, the star-formation rate then stays stead, and slowly thereafter, begins to drop.

This four-panel animation shows the individual galaxies present within Abell 2744, Pandora’s Cluster, alongside the X-ray data from Chandra (red) and the lensing map constructed from gravitational lensing data (blue). The mismatch between the X-rays and the lensing map, as shown across a wide variety of X-ray emitting galaxy clusters, is one of the strongest indicators favoring the presence of dark matter.

Even though the star-formation rate remains relatively high, staying at about 80% of its maximum value until the Universe is around 5-to-6 billion years old, the decline from its peak around 3 billion years after the Big Bang is noticeable. It’s enough to make one wonder what the dominant factor at play is: why is the star-formation rate steadily decreasing over time?

It turns out it isn’t just due to one dominant factor, but a number of them, all working in tandem. Stars form out of (mostly) hydrogen and helium gas, which collapse and ignite nuclear fusion. This fusion increases the internal pressure within molecular clouds, working to expel much of the potentially star-forming material. As galaxies clump together to form groups and clusters, the gravitational potential gets greater, but the intergalactic medium also gathers more material inside it.

This means, as galaxies speed through denser regions of space (i.e., as galaxies fall into rich clusters), much of this potentially star-forming material gets stripped away, where it winds up in the intracluster medium, or the space between galaxies, stripped out of their host galaxies, where they would have formed many new subsequent generations of stars had they remained.

Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, eliminating its ability to form new stars. Phenomena such as this allow us to conclude that the galaxy, the cluster, and the gas within it are all made of matter, not antimatter.

Additionally, more and more of the material found in these galaxies becomes more heavily processed as time goes on: enriched with heavier and heavier elements. In a recent study by UC Riverside scientists, they found that forming stars today is not the same as forming stars was yesterday. In fact, the older (and more modern) a star-forming galaxy is, the longer the amount of time it takes for them to undergo and complete their star-forming periods.

Using some of their own newly discovered SpARCS (Spitzer Adaptation of the Red-sequence Cluster Survey) clusters, discovered over an area spanning more than 40 square degrees across the sky, the new UCR-led study discovered that it takes a galaxy longer to stop forming stars as the universe gets older:

• only 1.1 billion years when the Universe was young (4 billion years old),
• 1.3 billion years when the Universe was middle-aged (6 billion years old),
• and 5 billion years in the present-day (13.8 billion year old) Universe.

In other words, new stars form at a faster rate early on, and at a slower rate today. Add in dark energy, which restricts additional structure from forming, and you’ve got a recipe for a very quiet Universe.

The star-formation rate in the Universe is a function of redshift, which is itself a function of cosmic time. The overall rate, (left) is derived from both ultraviolet and infrared observations, and is remarkably consistent across time and space. Note that star formation, today, is only a few percent of what it was at its peak (between 3-5%), and that the majority of stars were formed in the first ~5 billion years of our cosmic history. Only about ~15% of all stars, at maximum, have formed over the past 4.6 billion years.

When we put it all together, we actually come up with a fascinating quantitative answer for the star-formation history of our Universe. We can state that, overall, a total of 2.21 sextillion (or 2.21 × 10²¹) stars have formed over the history of our Universe, at least, within the part presently observable to us. And of course, that number is for today: 13.8 billion years after the Big Bang. But those stars didn’t form uniformly throughout cosmic time. If you instead looked at the Universe when it was younger, you’d find that we had:

• 98% of the current number of stars had formed by the time we were 12.9 billion years old,
• 75% by the time we were 7.3 billion years old,
• 50% by the time we were 4.9 billion years old,
• 25% by the time we were 3.3 billion years old,
• 10% by the time we were 2.2 billion years old,
• 5% at 1.7 billion years,
• 1% at 1.0 billion years,
• 0.1% at about 500 million years,
• and only 0.01% at about ~200 million years.

Today, the star-formation rate is a shadow of what it once was. According to the most comprehensive studies ever undertaken, the star-formation rate has declined by a whopping 97% since it reached its maximum, 10-to-11 billion years ago.

This deep-field region of the GOODS-South field contains 18 galaxies forming stars so quickly that the number of stars inside will double in just 10 million years: just 0.1% the lifetime of the Universe. The deepest views of the Universe, as revealed by Hubble, take us back into the early history of the Universe, where star formation was much greater, and to times where most of the Universe’s stars hadn’t even formed.

What’s fascinating about our star-formation history is that the greatest uncertainties about it are found at the earliest times: within the first 1.0 billion years. But only around ~1% of all stars formed within that first 1.0 billion year epoch of our cosmic past, meaning that our uncertainty in the total number of stars that have ever formed is actually very small. The greatest numbers of stars formed when the Universe was about 1.5-to-8 billion years old, and while the star-formation rate has been declining for more than 10 billion years, it’s really only over the most recent ~5 billion years that the decline has accelerated so severely. It’s possible, in fact, that more than 95% of the total stars that will ever form have already been created.

As long as there is gas remaining in the Universe and gravitation is still a thing, there will be opportunities to form new stars. When you take a cloud of gas and allow it to collapse, only about 10% of that material winds up in stars; the remainder goes back into the interstellar medium where it will get another chance in the distant future. Although the star-formation rate has plummeted since the Universe’s early days, it’s not expected to drop off to zero until the Universe is many thousands of times its present age. We will continue to form new stars for trillions upon trillions of years. But even with all that said, new stars are much more of a rarity now than they have been at any point in our past since the Universe was in its infancy. With ever-increasing sets of data from JWST, ALMA, and other far-reaching telescopes, the last uncertainties are finally being pinned down in the cosmic story of stars.

This article What was it like when the Universe formed the most stars? is featured on Big Think.

The cosmic story that unfolded following the Big Bang is ubiquitous no matter where you are. The formation of atomic nuclei, atoms, stars, galaxies, planets, complex molecules, and eventually life is a part of the shared history of everyone and everything in the Universe. Even though all of these things likely arise at somewhat different times at different locations in the Universe, largely dependent on the initial conditions such as temperature and density, once enough time goes by, they’re found literally everywhere. At least once, here on Earth, life began at some point in the Universe. At the absolute latest, it appeared only a few hundred million years after our planet was first formed.

That puts life as we know it arising, at the absolute latest, nearly 10 billion years after the Big Bang. When the Big Bang first occurred, life was impossible. In fact, the Universe couldn’t have formed life from the very first moments; both the conditions and the ingredients were all wrong. But that doesn’t mean it took all those billions and billions of years of cosmic evolution to make life possible. Based on when the raw ingredients that we believe are necessary for the most primitive forms of life to arise from non-life, it’s reasonable to think that “first life” might have come around back when the Universe was just a few percent of its current age. Here’s the best scientifically-motivated story for how life might have first arisen in our Universe.

The existence of complex, carbon-based molecules in star forming regions is interesting, but isn’t anthropically demanded. Here, glycolaldehydes, an example of simple sugars, are illustrated in a location corresponding to where they were detected in an interstellar gas cloud: offset from the region presently forming new stars the fastest. Interstellar molecules are common, with many of them being complex and long-chained.

At the earliest moments of the hot Big Bang, the raw ingredients for life could in no way stably exist. Particles, antiparticles, and radiation all zipped around at relativistic speeds, blasting apart any bound structures that might have formed by chance. As the Universe aged, though, it also expanded and cooled, reducing the kinetic energy of everything in it. Over time, antimatter annihilated away, stable atomic nuclei formed, and electrons finally bound to them, forming the first neutral atoms in the Universe.

Yet these earliest atoms were only hydrogen and helium: insufficient for life. Heavier elements, such as carbon, nitrogen, oxygen and more, are required to build the molecules that all life processes rely on. For that, we need to form stars in great abundance, have them go through their life-and-death cycle, and return the products of their nuclear fusion to the interstellar medium.

It takes 50-to-100 million years to form the first stars, sure, which form in relatively large clusters. But in the densest regions of space, these star clusters will gravitationally pull in other matter, including material for additional stars and other star clusters, paving the way for the first galaxies. By time only ~200-to-250 million years have passed, not only will multiple generations of stars have lived-and-died, but the earliest star clusters will have grown into galaxies.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2-10, located 30 million light years away. This galaxy will likely evolve, post-merger, into another disk galaxy if copious amounts of gas remains within it, or into an elliptical if all or nearly all of the gas is expelled by the current starburst.

This is important, because we don’t just need to create the heavy elements like carbon, nitrogen, and oxygen; we need to create enough of them — and all of the life-essential elements — to produce a wide diversity of organic molecules.

We need those molecules to stably exist in a location where they can experience an energy gradient, such as on a rocky moon or planet in the vicinity of a star, or with enough undersea hydrothermal activity to support certain chemical reactions. And we need for those locations to be stable enough that whatever counts as a life process can self-sustain.

In astronomy, all of these conditions get lumped together under the umbrella of a single term: metals. A “metal,” to an astronomer, is any element heavier than hydrogen or helium, from lithium (element #3) all the way up as high as the periodic table can theoretically go. Whenever we look at a star, we can measure the strength of the different absorption lines coming from it, which tells us — in combination with the star’s temperature and ionization — what the abundances of the different elements are that went into creating it. Add them all up, and that gives you the star’s metallicity, or the fraction of the elements within it that are heavier than either plain hydrogen or helium.

What do planets outside our solar system, or exoplanets, look like? A variety of possibilities are shown in this illustration. Scientists discovered the first exoplanets in the 1990s. As of 2023, the tally stands at just over 5,000 confirmed exoplanets. None are known to be inhabited, but a few raise tantalizing possibilities: largely among the Earth-sized planets, not the super-Earth-sized ones.

Our Sun’s metallicity is somewhere between 1-and-2%, but that appears to be too excessive for a requirement for life. Stars possessing just a fraction of the heavy elements (metals) found in the Sun and the rest of the Solar System, might still have enough of the necessary ingredients, across-the-board, to make life possible.

Remarkably, we’ve detected more than 5000 exoplanets over the past ~20 years, and there are tremendous lessons to learn from the stellar systems that we do and don’t find the “rocky” ones in. In particular:

• Only 10 exoplanets orbit stars with 10% or fewer of the heavy elements found in the Sun.
• Only 32 exoplanets orbit stars with between 10% and 16% of the Sun’s heavy elements.
• And only 50 exoplanets orbit stars with between 16% and 25% of the Sun’s heavy elements.

That means, all told, that only 92 out of the 5069 exoplanets found as of early 2023 — just 1.8% — exist around stars with a quarter or fewer of the heavy elements found in the Sun. In other words, if you want to make rocky planets, the ones that we think support life, you need to enrich the interstellar medium sufficiently, and that takes time.

Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. For a long time, we assumed this configuration was typical and common. Today, we know better.

However, remember what’s happening in the Universe as far as stars are concerned: they’re forming from very early times onward, and the star-formation rate, even though it starts small, continually increases over the first ~3 billion years of cosmic history. As more stars form from the ashes of older stars that have lived-and-died, the heavy element content, as well as the probability of forming stellar systems that will possess rocky planets, increases as time goes on. While most stars won’t form with rocky planets around them until several billion years have passed since the Big Bang, the first ones to get there might take only around one billion years: the first truly hospitable locations for life to arise in the cosmos.

The big question, then, becomes “how?” How did life arise? What are the conditions that support its creation from non-life, what were the specific mechanism(s) that allowed it to happen, and in the places where life managed to sustain itself, i.e., to survive and reproduce and thrive for generation after generation, what were the conditions that arose that enabled a long-term unbroken chain of biological activity? Even though we haven’t figured out the answer to those questions as far as Earth’s own history is concerned, we’ve made tremendous advances in recent years, particularly on the “mechanism” side for life arising from non-life.

Early on, shortly after the Earth first formed, life likely arose in the waters of our planet. The evidence we have that all life that’s extant today can be traced back to a universal common ancestor is very strong, but the early stages of our planet, for perhaps the first 1-to-1.5 billion years, remains largely obscure. While life arose early on, there is no evidence that Earth came into existence with life already on it.

The best proxy for understanding where the ingredients for life came from is to simply look at the composition of the asteroids and comets we find in space, as well as the remnants of meteorites that have survived their journey down to Earth today. When we look inside these primitive objects, many of which we can use atomic technique to date back to ~4.56 billion years ago, we find:

• there are over 80 unique amino acids inside of them (despite the fact that only 22 participate in life processes on Earth),
• many of them are both left-handed and right-handed, even though all the ones that participate in life processes on Earth are exclusively left-handed,
• carbon-based organic molecules are also present, from the simple (like sugars) to the intermediate (like hexamethylenetetramine) to the complex (like polycyclic aromatic hydrocarbons),
• and, quite recently, we’ve discovered that all five of the nucleobases, which are the “bases” at the heart of each nucleotide found in molecules such as DNA and RNA that encode genetic information on Earth, are present in meteorites as well.

While there are some who claim that these ingredients, if you smash them all together in a primordial soup (i.e., an aqueous environment with an energy gradient), may have given rise to self-replicating life spontaneously, that’s by far a minority opinion. Instead, a vastly preferred pathway by almost all working biologists is the idea that the ability to metabolize something of nutritional value is what came first.

Deep under the sea, around hydrothermal vents, where no sunlight reaches, life still thrives on Earth. How to create life from non-life is one of the great open questions in science today, but hydrothermal vents are one of the leading locations where the first metabolic processes, the precursor to living organisms, may have first arisen. If life can exist down there on Earth, perhaps undersea on Europa or Enceladus, there’s life down there, too.

Let’s imagine what this might have looked like. On any world with enough liquid water, there will be plenty of naturally occurring aqueous environments:

• the salty oceans and tidepools,
• freshwater sources like lakes and rivers,
• or even subsurface oceans that persist underneath rocky or icy crusts.

There will also be sources of external energy in the form of sunlight and geothermal heat, including in deep sea vents and along hydrothermal fields. There will be minerals and ions dissolved in that water, as well as all sorts of molecules, including a wide variety of amino acids that can bind together. And, perhaps most importantly from a thermodynamic perspective, you have chemical non-equilibrium states at a wide variety of interfaces: solid earth/liquid water, liquid water/volcanic magma, and liquid water/atmospheric gas.

As amino acids smack into each other, they spontaneously form and break bonds, with chains of amino acids forming peptides. As ions come along and bind to these primitive peptides, they enable the creation of enzymes. These molecules are fragile and easy to destroy or denature, but they’re also very large in number and the possibilities — set by the so-large-it’s-barely-fathomable mathematics of combinatorics — boggle the mind. Some of the proteins that form, merely by chance, will gain the ability to perform specific functions. These functions might have included:

• the gathering or even the hoarding of resources, including specific peptides,
• the ability to split/recombine other molecules in a way that liberates usable energy in the process,
• and the ability to “bite” other useful molecules, while remaining intact themselves.

Whatever the case, the spontaneous creation of these metabolic peptides are all but inevitable. What comes next, fascinatingly, is a brand new but startling area of research.

If life began with a random peptide that could metabolize nutrients/energy from its environment, replication could then ensue from peptide-nucleic acid coevolution. Here, DNA-peptide coevolution is illustrated, but it could work with RNA or even PNA as the nucleic acid instead. Asserting that a “divine spark” is needed for life to arise is a classic “God-of-the-gaps” argument.

It has recently been shown that if you have nucleobases in an aqueous environment — things like RNA, DNA, or even PNA (peptide nucleic acids) — that these nucleotides will line up along the various amino acids in a peptide chain. If they can pair up with their conjugate base, or “peel off” and draw additional amino acids onto them, they can effectively reproduce, to a high degree of accuracy, the original peptide chain.

This scenario, known as RNA-peptide coevolution, is how most working scientists investigating the origin of life now believe that self-replication, built on the backbone of metabolic processes, first came about.

Although not every biologist agrees that:

• a free-floating molecule,
• that can metabolize resources,
• and replicate itself,

rises to the threshold of being “life” rather than “non-life,” this likely represents the first concrete steps that led from simple chemical processes to biological ones. These primitive “metabolizing replicators” likely came into existence alongside one another, possessing a great diversity among them, with many — if not most — of them certainly going extinct along the way. This predates a universal common ancestor on Earth, and even our notion of what a cell is, by many hundreds of millions (and maybe over a billion) years. Nonetheless, this is where current scientific thought takes us as to how life first originated on Earth.

The raw ingredients that we believe are necessary for life, including a wide variety of carbon-based molecules, are found not only on Earth and other rocky bodies in our Solar System, but in interstellar space, such as in the Orion Nebula: the nearest large star-forming region to Earth.

Because we have every reason to believe that the laws and ingredients we have on Earth are found all over the Universe, it makes sense to look for those same “fingerprints” wherever we’re capable of looking. In space, whether around the centers of galaxies or around massive, newly forming stars, or even in the environments where metal-rich gas is going to form future stars, we find a whole host of complex, organic molecules. These range from sugars to amino acids to ethyl formate (the molecule that gives raspberries their scent) to intricate aromatic hydrocarbons; i.e., molecules that are thought to be precursors to life.

So far, we’ve only found these molecular “bio-hints” nearby, of course, but that’s because we don’t know how to look for individual molecular signatures in environments that lie well beyond our own galaxy. However, as we look to greater and greater distances, we do indeed find that there are galaxies and portions of even very early galaxies that have the right populations of stars and the right metallicities to them to be excellent candidates for life to arise within them. In the most extreme cases, we find locations from within the first 1-2 billion years after the Big Bang that might potentially be home to life already.

This infrared portrait of the Small Magellanic Cloud, located just 199,000 light-years away, highlights a variety of features, including new stars, cool gas, and quite spectacularly (in green) the presence of polycyclic aromatic hydrocarbons: the most complex organic molecules ever found in the natural environment of interstellar space. The way that atoms link up to form molecules, including organic molecules and biological processes, is only possible because of the Pauli exclusion rule that governs electrons, and happens everywhere across the Universe where enough heavy elements are present.

It has to be said, however, that we still don’t know how life in the Universe (or even on Earth) got its start, including whether life as we know it is common, rare, or a once-in-a-Universe proposition. But we can be certain that life came about in our cosmos at least once, and that it was built out of the heavy elements made from previous generations of stars. If we look at how stars theoretically form in young star clusters and early galaxies, we could reach that abundance threshold after several hundred million years; all that remains is putting those atoms together in a favorable-to-life arrangement.

If the Universe forms the molecules necessary for life and then puts them in an environment conducive to life arising from non-life, like on a water-rich rocky planet, suddenly the emergence of biology could have come when the Universe was just a few percent of its current age. The earliest life in the Universe, we must conclude, could have been possible during even the first one or two billion years after the hot Big Bang began. Once enough stars live-and-die, the material from their corpses gets incorporated into new stars, new molecules, and even new planets. Get enough of this enriched material together under the right conditions, and that’s perhaps all it takes to result in life’s all-but-guaranteed arrival.

This article What was it like when life first became possible? is featured on Big Think.

One of the biggest challenges for modern astrophysics is to describe how the Universe went from a uniform place without planets, stars, or galaxies to the rich, structured, diverse cosmos we see today. Not just with a general story, mind you, but in gory detail, going not only as far back as we can see, but even farther: to what must have existed at an epoch where even our most distant observations are insufficient to take us there. Going back to the limits of what’s observable, to when the Universe was just a few hundred million years old, we find a slew of fascinating objects.

• Stars and star clusters exist in abundance.
• Galaxies with perhaps up to a billion stars light up the Universe.
• Even quasars with very large black holes formed early on: well before the Universe was even one billion years old.

It’s the old chicken-and-egg problem made new: if there’s a maximum rate at which black holes can grow, and the Universe wasn’t born with them, how did we make the ones that we see? In other words, how did the Universe make such ultra-massive black holes in such short periods of time? After decades of conflicting stories, scientists finally think we know what happened.

With millions of seconds (corresponding to hundreds of hours) of observations on this one region of the sky, just 0.11 square degrees in area, Chandra revealed hundreds of active supermassive black holes, as well as many other cosmic objects. X-ray observatories are very sensitive to the presence of active black holes.

Just 50-to-100 million years after the Big Bang, the very first stars of all began to form. Massive gas clouds started to collapse, but because they were made up of hydrogen and helium alone, they struggled to radiate heat away and dissipate their energy. As a result, these clumps that gravitationally form and grow need to get much more massive than clumps that form stars today, and that has repercussions for what kinds of stars form, as well as what kinds of astrophysical processes occur alongside their formation.

While today, typically, we form stars that are about 40% the mass of the Sun, the very first stars were about 25 times more massive, on average. Because the matter that leads to these stars, i.e., streams of gas, need to cool in order to collapse, it’s only the largest, most massive clumps that form early on that will lead to stars. The average “first star” might be ten times as massive as our Sun, with many individual stars reaching hundreds or even thousands of solar masses. In addition to these effects, there are cold, flowing streams of gas that collide and intersect, creating regions of dense, cold, neutral matter that rise up to tens of thousands (or perhaps even hundreds of thousands) of solar masses.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the growth of galactic structures.

There’s long been a tension in cosmology between what’s expected and what’s observed: are the black holes that form at the centers of galaxies made from the mergers and growth of stellar remnants alone, or was there a different mechanism at play, such as either the formation of primordial black holes (black holes that the Universe was born with) or from the formation of large seed black holes that arise independently of stars and stellar cataclysms?

Let’s first consider the “from stars and stellar remnants, alone” rout. Most of the first-generation stars that form will end their life in a supernova, leading to either a neutron star or a small, low-mass black hole. But without any heavy elements at all, the most massive stars will reach such high temperatures in their cores that photons, the individual particles of light, can become so energetic that they will spontaneously begin to produce matter and antimatter pairs from pure energy alone.

You may have heard of Einstein’s E = mc², and this is perhaps its most powerful application: a pure form of energy, like photons, can create massive particles so long as the fundamental quantum rules governing nature are obeyed. The easiest way to make matter and antimatter is to have photons produce an electron/positron pair, which will happen all on its own if temperatures are high enough, and happens in some of the most massive stars even today.

This diagram illustrates the pair production process that astronomers once thought triggered the hypernova event known as SN 2006gy. When high-enough-energy photons are produced, they will create electron/positron pairs, causing a pressure drop and a runaway reaction that destroys the star. This event is known as a pair-instability supernova. Peak luminosities of a hypernova, also known as a superluminous supernova, are many times greater than that of any other, ‘normal’ supernova.

In these ultra-massive stars, as in all stars, gravitation is attempting to pull all of that matter in toward the center. But photons, and all of the radiation produced in the cores of these stars, pushes back, and holds the star up, preventing its collapse. This happens independent of the star’s mass, as that “balance point” must be achieved inside any object that isn’t going to vary, oscillate, collapse, or explode. However, in these most massive stars, the radiation pressure is higher, the temperatures are higher, and the high-energy processes that occur are more numerous, including this phenomenon, known as “pair production.”

When you start producing electron-positron pairs from these photons, however, some of that radiation pressure drops, as photons, which move at the speed of light, exert a greater amount of outward pressure than massive particles that move at speeds slower than light. Rising up to a temperature great enough to produce these electron/positrons pairs also depletes your star’s ability to hold itself up against gravitational collapse. And while it’s true that there are a few, narrow mass ranges that lead to the star destroying itself entirely, a large fraction of cases will result in the entire star directly collapsing to form a black hole: the direct collapse scenario that has been observed in a few incidents even in the late-time Universe.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time.

This is a remarkable step! It means that the most massive stars of all, with many hundreds or even thousands of solar masses, can form when the Universe is just 100 million years old or so: less than 1% of its current age. These stars will burn through their nuclear fuel the fastest, in 1 or 2 million years, tops. And then, their cores will get so hot they’ll start turning photons into particles and antiparticles, which causes the star to collapse and heat up even faster.

Once you cross a certain threshold, all you can do is collapse. And this isn’t just theory, either; we’ve actually seen stars directly collapse without a supernova, leading directly to what could only be a black hole.

But that’s only the beginning. Whenever you have a large cluster of massive objects acting primarily under the force of gravity, different objects get kicked around from these interactions. The least massive objects are the ones that are the easiest to eject, while the most massive objects are the toughest to eject. As these stars, gas clouds, clumps, and black holes dance around, they undergo what’s known as mass segregation: the heaviest objects fall to the gravitational center, where they interact and can even merge.

By combining data of Pandora’s Cluster, Abell 2744, from the infrared JWST and from the X-ray sensitive Chandra space observatories, scientists were able to identify a number of lensed galaxies, including one that emits copious amounts of X-ray light from very early on in the Universe’s history, despite having extremely little ultraviolet/optical/infrared light. This “overmassive” black hole holds key information about the formation and growth of black holes.

All of a sudden, instead of a few hundred black holes of a few hundred or a few thousand solar masses, you can wind up with one single black hole of approximately 100,000 solar masses or even more.

Remarkably, that might not even be the fastest way to grow a massive “seed” for a supermassive black hole to form out of in our cosmic history. Just as massive-enough stars can directly collapse to a black hole without a stellar cataclysm, it’s thought that a dense, massive, and cold enough population of gas can directly collapse to a black hole, perhaps even without a single star at all as an intermediary.

This scenario recently got bolstered by an unexpected observation: made by combining both NASA Chandra X-ray data and JWST data of galaxies from the ultra-distant Universe. In JWST data, a distant, faint, low-surface-brightness galaxy was found from around 500 million years after the Big Bang: a galaxy that contains only around 10 million (maybe up to 100 million, but not more than that) solar masses worth of stars within it. Yet in this galaxy, an active, feeding, X-ray emitting black hole of around 9 million solar masses exists. This black hole couldn’t have formed from stars with so few other stars in this galaxy, but the direct collapse of a gas cloud is eminently possible.

If you begin with an initial, seed black hole when the Universe was only 100 million years old, there’s a limit to the rate at which it can grow: the Eddington limit. If seeds of several tens-of-thousands of solar masses arise early on and these SMBH seeds grow rapidly thereafter, there may be no conflict with what’s observed, after all.

The big issue is that we’ve already seen large, relatively grown-up black holes even in the early Universe: of hundreds of millions or even over a billion solar masses after just several hundred million years of cosmic evolution. These black holes can grow in bursts that typically exceed the maximum rate of growth (the Eddington rate) that’s sustainably allowed over long time periods, but if we trace that growth back to when we need a “seed” to grow from, those seeds must have been tens of thousands or even hundreds of thousands of solar masses to begin from; starting from a “stellar mass” seed won’t work!

You might think to take the black holes from a single star cluster, merge them together, bring other star clusters with their own black holes in, and have them merge as well. But even that can’t grow what we call an “overmassive” black hole, or a black hole whose mass is comparable to the mass of all stars within a galaxy combined. The fact that such a galaxy exists tells us that no, merging stellar mass black holes together, even en masse, can’t be the sole way that we made supermassive black holes in the Universe. Some sort of massive seed, more massive than standard star-formation allows for, is required.

This set of illustrations explains how a large black hole can form from the direct collapse of a massive cloud of gas a few hundred million years after the Big Bang. Cold streams of gas can lead to the direct collapse of a “seed” black hole of several tens of thousands (at least) solar masses, which can form even prior to any stars forming in the surrounding young galaxy. As the galaxy and black hole grow, eventually the stellar mass content will outweigh the more slowly-growing black hole.

By the time the Universe is no more than 250 million years old, however, star clusters, streams of gas, and these “seed” black hole regions can indeed merge together, as the relentless pull of gravity draws nearby regions into one another. Gravity is a force that truly favors the overdog, and as time goes on, tens, hundreds, and even thousands of these initial, early clusters can come together to grow into larger and larger galaxies. The cosmic web causes structures to merge together into ever-larger ones, especially at early times.

This can easily take us up to black hole masses that are several to many tens of millions of solar masses by the time we get to the first observed galaxies. Additional effects are at play as well; it isn’t just black holes that merge together to build supermassive ones in the center; it’s any matter that falls into them! These early galaxies are compact objects, and are full of stars, gas, dust, star clusters, planets and more. Whenever anything gets too close to a black hole, it’s at risk of getting devoured, as the black hole’s tidal forces will tear the object apart, forcing it into a stream, flow, and even an accretion disk surrounding the black hole.

This illustration of a tidal disruption event shows the fate of a massive, large astronomical body that has the misfortune of coming too close to a black hole. It will get stretched and compressed in one dimension, shredding it, accelerating its matter, and alternately devouring and ejecting the debris that arises from it. Black holes with accretion disks are often highly asymmetrical in their properties, but far more luminous than inactive black holes that lack them.

Remember that gravity is a runaway force: the more mass you have, the more mass you attract. And if something gets too close to a black hole, its matter gets stretched and heated, where it will become part of the black hole’s accretion disk. Some of that matter will get heated and accelerated, where it can emit quasar jets. But some of it, a lot of that mass, will fall in, causing the black hole’s mass to grow even further.

If there were one vocabulary word that astrophysicists who study the growth of objects via gravity wish that the general public knew, it would be this oddball: nonlinear. When you have a region of space that’s denser than average, it preferentially attracts matter. If it’s just a few percent denser than average, the gravitational attraction is just a few percent more effective than average. Double the amount that you’re overdense, and you double the amount you’re more effective at attracting stuff.

But when you reach a certain threshold of being about double the average, you become much more than twice as effective at attracting other matter. When you start “winning” the gravitational war, you win harder and more severely as time goes on. The most massive regions, therefore, not only grow the fastest, they eat everything around them. By the time half a billion years have elapsed, the amount of mass you’ve drawn into you can be enormous.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and with multiple instruments, including Hubble and ALMA. Based on measurements of the stellar populations found inside, this object, whose light comes from when the Universe was just 530 million years old, contains stars that are at least 280 million years old within it. Like many early galaxies, it contains a supermassive black hole that struggles to be explained by supernovae and steady growth alone.

The earliest galaxies and quasars we’ve ever found are among the brightest, most massive ones we expect to exist. They are the great winners in the gravitational wars of the early Universe: the ultimate cosmic overdogs. By time our telescopes reveal them, 300-to-700 million years after the Big Bang (with quite a few quasars joining the most distant galaxies in this regime), the most massive among them already have billions of stars and supermassive black holes of many hundreds of millions of solar masses within them. We haven’t yet found the earliest galaxies, stars, black holes, or quasars of all, but even the ultra-early ones we’ve found already exhibit substantial evidence of evolution to them.

However, don’t be fooled by those who point to these pieces of evidence and proclaim, “cosmology is broken!” This is not a cosmic catastrophe; this is a piece of evidence that showcases the runaway power of gravitation in our Universe. Seeded by the first generation of stars, cold streams of gas, and the relatively large black holes they produce, these objects merge and grow within a cluster, and then grow even larger as clusters merge to form galaxies and galaxies merge to form larger galaxies. By today, we have black holes tens of billions as massive as the Sun. But even in the earliest stages we can observe, billion-solar-mass black holes are well within reach. As we peel back the cosmic veil, improved data should teach us exactly how these cosmic behemoths grew up.

This article What was it like when supermassive black holes arose? is featured on Big Think.

Forming stars sounds like the easiest thing in the Universe to do, given enough time. However, making stars that are actually visible to an observer is, perhaps surprisingly, a lot more challenging. Once you get a sufficiently large amount of mass together, so long as you give it enough time to gravitate, you’ll be able to watch it collapse down into small, dense clumps. If enough mass comes together in those clumps under the right conditions, stars will no doubt ensue. This is how you form stars today, and it’s how we’ve formed stars all throughout our cosmic history, going back to the very first ones some 50-100 million years after the Big Bang.

But even with the first stars burning, as they go about fusing hydrogen into heavier elements and converting that energy into a form that results in the emission of tremendous amounts of light, those stars aren’t necessarily visible to anyone around to observe them. The Universe is simply too good at absorbing and blocking that light. The reason? All of the atoms in the Universe, during the time that the first stars exist, are neutral, and there are simply too many of them for the starlight to penetrate. It took hundreds of millions of years for the Universe to allow that light to freely pass through it: a time known (from the perspective of light) as the cosmic dark ages, but known (from the perspective of atoms) as the epoch of reionization. It’s a vital part of the cosmic story of us whose importance is greatly underappreciated.

More than 13 billion years ago, during the Era of Reionization, the Universe was a very different place. The gas between galaxies was largely opaque to energetic light, making it difficult to observe young galaxies. The James Webb Space Telescope (JWST) is peering deep into space to gather more information about objects that existed during the Era of Reionization to help us understand this major transition in the history of the Universe.

If you want to know what lights up the Universe, the answer always depends on which wavelengths of light you’re looking at it in. The Universe is always illuminated by the cosmic microwave background: the leftover radiation from the Big Bang itself. Early on, this radiation was coupled to the plasma of ionized nuclei and free electrons, prior to the formation of stable, neutral atoms. Less than half-a-million years after the Big Bang, neutral atoms formed and this radiation simply streamed, freely, amidst the sea of atoms.

But why wasn’t this radiation blocked by the now-intervening neutral atoms that populate the abyss of empty space? The reason is wavelength: it only streams freely through the atoms due to the fact that the cosmic radiation was much lower in energy than neutral (mostly hydrogen) atoms are capable of absorbing: at wavelengths too long for absorption to take place. If the radiation were higher in energy, atoms would not only absorb it, they would re-scatter it in all directions, where it would be further absorbed by additional atoms. It’s only because the radiation is so low in energy — it’s primarily infrared light — that it can freely pass through the space that neutral atoms occupy.

Dark, dusty molecular clouds, like this image of Barnard 59, part of the Pipe Nebula, found within our Milky Way, will collapse over time and give rise to new stars, with the densest regions within forming the most massive stars. However, even though there are a great many stars behind it, the starlight cannot break through the dust; it gets absorbed until more of the nebula itself becomes ionized. Only with longer-wavelength light, like in the mid-infrared, will this dust appear luminous when heated, rather than dark.

And the light-blocking effects of neutral atoms, so good at visible and ultraviolet wavelengths but so poor at inherently infrared wavelengths, plays out not only in the very early Universe, but in modern times as well. We see this even in our own galaxy: the stars and objects that persist in the Milky Way’s galactic center cannot be seen in visible light. The dust and gas that are present blocks it, just as they efficiently block visible light all throughout the galactic plane. However, at longer wavelengths, infrared light goes clear through those neutral atoms. This explains why the cosmic microwave background doesn’t get absorbed, but starlight does.

Thankfully, the stars that form in the Universe, particularly early on in cosmic history, can be massive and hot, where the most massive ones are much more luminous and hotter than even our Sun. Early stars can be tens, hundreds, or even (for Population III stars) thousands of times as massive as our own Sun, meaning they can reach surface temperatures of tens or even hundreds of thousands of degrees and brightnesses that are millions of times as luminous as our Sun. These behemoths are the biggest threat to the continued existence of any neutral atoms that happen to be spread throughout the Universe.

An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. But the conversion of matter into energy does something else: it causes an increase in radiation pressure, which fights against gravitation. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.

What determines whether atoms remain neutral, and capable of absorbing (visible) light, or whether they become ionized, and transparent to the light that our eyes typically perceive? It’s all about the energy of the radiation that strikes them. The key property of whether an atom becomes ionized by the surrounding/nearby stars is that, for stars above a certain temperature, they’ll emit some fraction of their light in the ultraviolet portion of the spectrum: energetic enough to ionize a neutral atom. And remember, at all moments in cosmic history, even today, the most common atom in the Universe by number is hydrogen: more than 90% of the atoms present are still simply hydrogen.

For a hydrogen atom in its lowest-energy state, it takes a photon of 13.6 eV (or more) to ionize it, which very few photons emitted from most stars possess. But that’s because most stars have their energy peak in either the visible or infrared portion of the spectrum: with fewer high-energy photons above a certain ultraviolet threshold. However, the hotter and more massive your star is, the more ionizing photons they produce. Because these are the shortest-lived stars, it’s only within a few million years of forming a new burst of stars that you get an excessive amount of ionizing photons, which coincides with large numbers of hydrogen atoms being ionized.

The Large Magellanic Cloud is home to the closest supernova of the last century, having occurred in 1987. The pink regions here are not artificial, but are signals of ionized hydrogen and active star formation, likely triggered by gravitational interactions and tidal forces. The pink regions specifically arise when electrons fall back onto ionized hydrogen nuclei, and transition from the n=3 to the n=2 energy level, producing photons of precisely 656.3 nm.

If you were to imagine a scenario in which all atoms inhabiting the Universe became ionized, the depths of star-free space would be clear for light to travel through, meaning we could see the distant Universe without a problem. All the starlight that was emitted would freely propagate through space, and none of it would be extinguished unless and until it arrived at the proverbial eyes of an observer. But even so long as a small percentage of the atoms remains neutral, that emitted starlight could be effectively absorbed, making it extraordinarily challenging to detect anything from the era of the first stars and galaxies.

It’s true that a smaller percentage of neutral atoms means that starlight must travel through larger numbers of them to be fully absorbed, as the “extinction” of light in astronomy is cumulative. What we need to happen, if we want the Universe to become truly transparent to starlight, is for enough star formation to occur that it floods the Universe with a sufficient number of ultraviolet photons to ionize the neutral matter in the intergalactic medium so that starlight can travel unimpeded. This requires a large amount of star formation, and requires it to occur quickly enough that the ionized protons and electrons don’t find one another and recombine again.

The open star cluster NGC 290, imaged by Hubble. When new stars form, they form with a variety of masses, colors, luminosities, and other properties. The heaviest stars will be the most luminous and emit the greatest number of ionizing, ultraviolet photons, but will live the shortest; the lightest stars will be the least luminous but can persist for many trillions of years.

The very first stars in the Universe make a small dent in the population of neutral atoms surrounding them, but the earliest of the star clusters are small and short lived. The Universe will remain largely neutral with them alone, especially once the most massive of those stars die and neutral atoms re-form. The second generation of stars, formed in the aftermath of the first generation’s death, fare little better.

The big problem is that these newly formed stars form in clumps and clusters of perhaps a few million solar masses at most. These early star clusters only have about 0.001% of the masses (and numbers) of stars found in the Milky Way, meaning that for the first few hundred million years of our Universe, the stars within it are barely enough to make a difference in the neutral matter permeating all of space.

But that begins to change when star clusters merge together, forming the first galaxies. As large clumps of gas, stars, and other matter merge together, they trigger a tremendous burst of star formation, lighting up the Universe as never before. As time goes on, a slew of phenomena take place all at once:

• the regions with the largest collections of matter attract even more early stars and star clusters toward them,
• the regions that haven’t yet formed stars can begin to,
• and the regions where the first galaxies are made attract other young galaxies,

all of which serves to increase the overall star formation rate.

In other words, there are three things going on all at once that interplay with one another.

1. There are neutral atoms, populating all of space everywhere, clumping and clustering together across the cosmic web.
2. There are stars and star-forming regions, found forming within the densest of clumps, and which produce radiation that ionizes these neutral atoms.
3. And there are the free electrons and bare atomic nuclei, created by those ionizing photons, but that are capable of finding one another and re-forming neutral atoms once again.

If we were to map out these various features within the Universe at this time, what we’d see is that the star formation rate increases at a relatively steady rate for the first few billion years of the Universe’s existence. In some favorable regions, enough of the matter gets ionized early enough that we can see through the Universe before most regions are reionized; in other, more unfavorable regions, it may take as long as two or three billion years for the last populations of pristine, neutral matter to be blown away.

If you were to map out the Universe’s neutral matter from the start of the Big Bang, you would find that it starts to transition to ionized matter in clumps, but you’d also find that it took hundreds of millions of years to mostly disappear. It does so unevenly, and preferentially along the locations of the densest parts of the cosmic web.

Past a certain distance, or a redshift (z) of 6, the Universe still has neutral gas in it, which blocks-and-absorbs light. These galactic spectra show the effect as a drop-to-zero in flux to the left of the big (Lyman-series) bump for all the galaxies past a certain redshift, but not for any of the ones at lower redshift. This physical effect is known as the Gunn-Peterson trough, and will block the brightest light produced by the earliest stars and galaxies.

Some line-of-sights become transparent to visible light relatively quickly: in just a few hundred million years, where star-formation is the most active and energetic, and where the greatest number of ultraviolet photons are produced at early times. Other lines-of-sight will still have pristine, neutral matter found within them as many as a couple of billion years after the Big Bang. The process of reionization is uneven, and doesn’t “complete” at the same rate in all locations and directions.

On average, it takes 550 million years from the inception of the Big Bang for the Universe to become reionized and transparent to starlight. We see this from observing ultra-distant quasars, which continue to display the absorption features that only neutral, intervening matter causes. We can also see this from ultra-distant galaxies, by looking at which features they display and which ones are effectively absorbed by the neutral matter within the intergalactic medium.

Because there are a few directions where the matter is reionized much earlier than average, it indicates to us that structure formation is uneven, and gives us hope of finding early galaxies even before that 550 million year limit. Hubble uncovered one such galaxy, GN-z11, whose light comes from an earlier time than the completion of reionization: just 407 million years after the Big Bang.

Only because the most distant galaxy spotted by Hubble, GN-z11, is located in a region where the intergalactic medium is mostly reionized, was Hubble able to reveal it to us at the present time. Other galaxies that are at this same distance but aren’t along a serendipitously greater-than-average line of sight as far as reionization goes can only be revealed at longer wavelengths, and by observatories such as JWST. At present, GN-z11 is only the 9th most distant galaxy known, with all others discovered by JWST.

However, other, more distant galaxies have since been discovered by JWST that are invisible to Hubble’s capabilities, as JWST’s sensitivity to longer wavelength, infrared light allows it to see features that are more difficult for neutral atoms to absorb; the Universe becomes effectively transparent earlier at longer wavelengths, when less reionization has occurred. Infrared eyes are what it takes to probe the epoch of reionization itself.

At the most extreme cosmic distances ever probed, there are not yet galaxy clusters in the Universe, and the first galaxies, which are largely to have taken shape between 200 and 250 million years after the Big Bang, cannot be revealed by visible light observations alone. But through the eyes of an infrared observatory, where the light is long-enough in wavelength to not be absorbed by these neutral atoms, this starlight might shine through after all.

It’s no coincidence, then, that the James Webb Space Telescope was designed to look in the near-and-mid-infrared portion of the spectrum, all the way out to wavelengths of 30 microns: some 50 times as long as the longest-wavelength light that human eyes can see. In fact, of the top 10 galaxies known at the end of 2023, 9 of the top 10 spots are JWST galaxies, with GN-z11 the sole confirmed galaxy not requiring either discovery or confirmation with JWST.

The four most distant galaxies identified as part of JADES, thus far, include three that surpass the threshold for “most distant galaxy” previously set by Hubble. With no more than a quarter of the total JADES data taken thus far, this record will likely fall again, perhaps multiple times, over the coming months and years, but the unambiguous feature of the Lyman break can clearly be seen. The most distant, JADES-GS-z13-0, took the record from Hubble in December of 2022, and still holds it today. Although these are among the youngest galaxies ever discovered, their stellar populations are not pristine.

The light created in the earliest era of stars and galaxies plays a cosmic role whose importance cannot be overstated: the role of making the Universe transparent to all wavelengths of starlight. The ultraviolet light works to ionize the matter around it, enabling visible light to progressively farther and farther as the ionization fraction increases. The visible light gets scattered in all directions until reionization has gotten far enough to enable our best telescopes today to see it. But the infrared light, also created by the stars, passes through even the neutral matter, giving our 2020s-era telescopes a chance to find them, even where no visible light is available.

Once starlight breaks through the sea of neutral atoms, even before reionization completes, it gives us a chance to detect the earliest objects we’ll ever have seen. It’s no surprise that even in just its first ~18 months of science operations (to date), that JWST has broken a slew of cosmic records, including records for:

• most distant galaxy,
• most distant proto-cluster of galaxies,
• most distant black hole,
• most distant red supergiant star,
• most distant gravitational lens,
• and most distant quasar.

The most distant reaches of the Universe are slowly succumbing to our improved instruments and observatories, bringing the previously unseen, at last, into view. We just have to keep looking, and eventually, we’ll find out what’s truly out there.

This article What was it like when the cosmic dark ages ended? is featured on Big Think.

Whenever you look out beyond the Milky Way today, as far as anyone’s ever been able to see, there’s no place you can look where you won’t eventually find a galaxy. There are galaxies absolutely everywhere, in all directions and locations, even at the greatest cosmic distances ever probed. Even if you were to take a dark patch of sky without any known stars, galaxies, or matter of any type within it, if you leave your telescope’s shutter long enough and you look in the proper wavelengths of light, thousands upon thousands of galaxies will be your reward. All told, there are estimated to be many trillions of galaxies found within the observable Universe, stretching for tens of billions of light years in all directions.

Yet, despite all the galaxies we’ve observed and measured their properties, never have we gone far enough back to encounter the very first ones ever made in the Universe. The current record-holder, despite its light arriving from when the Universe was only 320 million years old — 2.3% of its present age — is already evolved and full of old stars. The very first galaxies have not yet been discovered and must come from a time earlier than the epochs humanity has ever probed. But if we get lucky, we’ll get there soon. Here’s what those galaxies should be like.

The featured image shows galaxy NGC 7331 along with other members of its galactic group, including the prominent galaxies NGC 7335, 7336, 7337, and 7340. We now know that a large fraction of galaxies beyond the Milky Way are spiral-shaped in nature, and that all of the spiral nebulae we were considering in ~1920 are indeed galaxies beyond our own. The farther away we look, the smaller, younger, bluer, and less-evolved the galaxies we find. If we could look back far enough, eventually we’d come to an epoch where no galaxies yet exist.

The galaxies we see today, even the earliest, most distant ones, are already old. They’re massive, they’re huge, and they’re full of a variety of stars. For the most part, there are lots of heavy elements in there: approximately 1-2% of all the atoms present in most galaxies (by mass/weight) are composed of materials other than hydrogen or helium. That’s a big deal, considering that the Universe was born without carbon, nitrogen, oxygen, silicon, sulfur, iron, or practically any of the elements we find in stars and galaxies today. It was 99.999999% hydrogen and helium to start, but that’s down to 98-99% by today.

But it’s had billions of years and many, perhaps an innumerable amount, of generations of stars that lived-and-died previously to bring the types of galaxies we’re observing, complete with the variety of stars and stellar populations we find within them. If we look back to the distant Universe, we also look back in time, and find that galaxies were vastly different back then from how they appear today. They were smaller, bluer, more numerous, and poorer in these heavy elements than the galaxies we have today. Over the history of the Universe, galaxies have evolved substantially.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this effect goes to the extreme. As far back as we’ve ever seen, galaxies obey these rules.

But how did the very first galaxies form? And what was the Universe like when they first arose?

The cosmic story that brought them to us saw a number of important steps happen first. Matter won out over antimatter; atomic nuclei and then neutral atoms formed; the first generation of stars were born, died, and gave rise to the second generation of stars. But even after all these steps, there were still no galaxies around. The simple reason? The smallest-volume cosmic scales gravitationally collapse first, while the larger scales take longer.

Think about two important factors at play here: gravity and the speed of light. Gravity is the only mechanism that can bring ever larger and larger clumps of matter together. It’s limited, however, by the speed at which things can gravitationally grow, and it only propagates at the speed of light. Given that the Universe as-we-know-it began a finite amount of time ago with the hot Big Bang, and that the speed of gravity propagates at a finite speed, it makes sense that the overdensities on small cosmic scales would collapse first, making stars the first things that form, while larger scales (for galaxies, galaxy clusters, and the grand cosmic web) take longer amounts of time for gravity’s influence to dominate.

The interstellar medium, normally invisible except for the light it absorbs, can become illuminated by either reflected starlight or from having its atoms excited, causing it to emit its own light. While star clusters normally form in isolated regions of galaxies today, in the early Universe, they formed very close to one another, where they would then gravitationally attract one another, and eventually merge together to form the first proto-galaxies.

Imagine you start with a small mass: a clump of matter that rises by some amount over and above whatever the average density happens to be. If you have some additional mass for it to attract that’s a light-year away, it will take that matter an entire year to feel the force from the mass, since the gravitational force only travels at the speed of light. But if there’s an additional mass a hundred, or a million, or a billion light-years away, you have to wait for all that additional time to pass. Gravity isn’t instantaneous; it only travels at the speed of light.

So what happens, then, when you finally get a large amount of mass together in one place, from the gravitational collapse of your first stars and star clusters? They attract one another, and can finally do so effectively.

But the timescale for one massive star cluster attracting another is going to be much longer than the timescale for individual star clusters to form. Instead of looking at volumes of space that might be a few tens, hundreds, or even thousands of light-years on a side — the scale of what might collapse to form a star cluster — you need to look on scales tens-to-hundreds of times as large to bring together enough matter to start to make the first galaxies.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2-10, located 30 million light years away. This galaxy will likely evolve, post-merger, into another disk galaxy if copious amounts of gas remains within it, or into an elliptical if all or nearly all of the gas is expelled by the current starburst.

Remember this fact, as well: that the original overdensities that lead to both star clusters and galaxies are only one-part-in-about-30,000, meaning that these overdensities need to grow over large amounts of time. If it takes gravity tens-to-hundreds of times as long to propagate between star clusters (and hence, for those star clusters to gravitationally influence one another) than it does for gravity to propagate within an individual cluster, you might worry that it would wind up taking tens-to-hundreds of times as much time to make galaxies as compared to how long it took to make the first stars.

Luckily, this isn’t true! That’s an overestimate; the amount of time it takes to create galaxies is indeed longer than the amount of time it takes to make stars and star clusters, but not by nearly that severe of an amount. The power of an attractive gravitational force is cumulative, so it’s basically like starting a clock on a delay. The “star cluster” clock starts a few million years after the Big Bang; the “galaxy” clock begins perhaps ten million years after that, and starts with a handicap: it has farther to go to collapse.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Even after galaxy clusters form, surrounding galaxies and galaxy groups, including initially Milky Way-like galaxies, get drawn in. Over time, they will lose their gas and eventually cease forming new stars.

But this is okay! This is simply how structure formation works in the Universe as we look to progressively larger cosmic scales. We have, at the start, density imperfections existing on all scales, and they grow as soon as enough time has passed for gravity to begin attracting matter that’s located a certain distance away. We form the first star clusters quickly, after perhaps 50-to-100 million years. We form the second generation of stars almost immediately after, in another 5 million years or less, because the first generation of stars lives-and-dies so fast, and the material ejected back into the interstellar medium triggers a new generation of stars to begin forming shortly thereafter.

However, we then have to wait several tens of millions of years after those “polluted” stars form for the first galaxies to begin to take shape. The reason is that, in order to make galaxies out of these star clusters, those early star clusters must attract one another across the abyss of empty space, until they finally draw one another in so that they can merge. And it requires even longer timescales for large galaxies and then galaxy groups and galaxy clusters to arise. In this sense, structure formation in the Universe is a process that we call hierarchical.

While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.

The hardest challenge, observationally, when it comes to finding these first galaxies is that there haven’t yet been enough stars formed throughout the Universe to ionize all the neutral atoms in intergalactic space. So long as protons and electrons remain bound to one another, they behave as neutral atoms, which block and absorb light, particularly visible and ultraviolet light, which is a property that astronomers call “extinction.” The more neutral matter you have, the more efficiently your starlight gets extincted, which makes it all the more difficult to observe the light that’s being emitted behind this thick curtain of light-blocking material.

These conditions will remain until the Universe is flooded with enough sustained ultraviolet light to permanently kick those electrons off of their atoms. This means that so long as the light from the first stars (and first galaxies) gets absorbed by those atoms; the Universe remains in an opaque state, rather than a transparent one. The earliest galaxies we’ve ever seen date back to as little as 320 million years after the Big Bang, and these most distant galaxies were only discovered because they are both:

• located along a serendipitously more-ionized-than-average line of sight,
• and also because we’re observing them at long wavelengths, where the emitted ultraviolet light gets stretched and redshifted well into the infrared portion of the spectrum.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and with multiple instruments, including Hubble and ALMA. Based on measurements of the stellar populations found inside, this object, whose light comes from when the Universe was just 530 million years old, contains stars that are at least 280 million years old within it. Like many early galaxies, it contains a supermassive black hole that struggles to be explained by supernovae and steady growth alone.

However, there’s a method for attempting to “date” the age of the first galaxies that doesn’t necessarily involve finding these first galaxies directly. It’s clever, powerful, and educational to explore. Take, for example, the galaxy MACS1149-JD1. Before we entered the JWST era, it was the second-most-distant galaxy ever found, whose light arrives from 530 million years after the Big Bang. (Now it’s down somewhere around 12th place, showcasing the unprecedented power of JWST.) This is not a pristine galaxy, but it is a galaxy with a remarkable property: we can measure the populations of stars inside, and determine an age for those stars.

When we make those critical observations, we find that the stars inside of it are approximately 280 million years old, meaning that they first formed in a massive burst that occurred no later than 250 million years after the Big Bang. These massive bursts of star formation don’t simply occur because you had a star cluster; they occur when large mergers happen, giving rise to what astronomers call a starburst. Colliding gas causes material to collapse, which can trigger massive amounts of new star formation. Much larger and more powerful than a monolithically collapsing star cluster, these post-merger objects should signify the true first galaxies.

When a hydrogen atom forms, it has equal probability of having the electron’s and proton’s spins be aligned and anti-aligned. If they’re anti-aligned, no further transitions will occur, but if they’re aligned, they can quantum tunnel into that lower energy state, emitting a photon of a very specific wavelength (21 cm) on very specific, and rather long, timescales. The precision of this transition has been measured to better than 1-part-in-a-trillion, and has not varied over the many decades it’s been known. It is the first light emitted in the Universe after the formation of neutral atoms: even before the formation of the first stars, but also thereafter: whenever new stars are formed, ultraviolet emission ionizes hydrogen atoms, creating this signature once again when those atoms spontaneously re-form.

These first galaxies will be larger, contain more stars, be more massive, more luminous, and will leave an unmistakable signature as compared to the individual star clusters that came before. In fact, the first galaxies should create an observable imprint on the Universe, one that can be detected with a method known to astronomers today: 21 cm mapping. Not only will the stars that form within these early galaxies begin contributing to the reionization of the Universe, but wherever new stars are formed, we will find electrons recombining with their ionized nuclei. That act, when it occurs for hydrogen atoms, has a 50% chance of forming a configuration where the spins are aligned (up-up or down-down) and a 50% chance where the spins will be anti-aligned (up-down or down-up).

The up-down or down-up configurations are more stable, by a tiny amount. If you form the aligned configuration, it will transition down to the anti-aligned configuration on timescales of around 10 million years. And when it transitions, it emits a photon of a very specific wavelength: 21 centimeters. That photon then travels throughout the Universe, arriving at our eyes, redshifted by the expansion of the Universe. In 2018, there was a paper that came out, albeit very controversially, that claimed to detect this signature for the first time. Impressively, the timescale for when these first galaxies ought to have formed coincides very nicely with these observations. Although the observations were not confirmed, superior instrumentation should enable us to detect the actual signatures from spin-flipping hydrogen atoms arising due to these first galaxies in the coming years.

Backlit by the cosmic microwave background, a cloud of neutral gas can imprint a signal on that radiation at a specific wavelength and redshift. If we can measure this light with great enough sensitivity, we can actually hope to someday map out the locations and densities of gas clouds in the Universe thanks to the science of 21 cm astronomy. A dip in brightness temperature at redshifts of 15-20, observed in 2018, may be due to exactly the effect of 21-cm emission, although better instrumentation and better observational examples will be required to confirm such a claimed detection.

Whenever “cosmic dawn” occurred, whenever these first galaxies arrived, every piece of evidence points to a timetable of 200-250 million years as the main origin of the first galaxies. This may yet be observable by JWST, although no candidate galaxy from earlier than 320 million years has yet been verified by spectroscopic observations. The first galaxies required a large number of steps to happen first: they needed stars and star clusters to form, and they needed for gravity to bring these star clusters together into larger clumps. But once you make these first galaxies, they then become the largest cosmic structures to exist for their time, and they will continue to grow, attracting not only star clusters and gas, but additional small galaxies.

The cosmic web has taken its first major step up, and will continue to grow further, and more complex, over the hundreds of millions and billions of years to follow. Early on, galaxies grow primarily via mergers of small, low-mass clumps and clusters of matter, while later on, it will be by gradual accretion and gravitational infall. Meanwhile, the regions that were born with smaller initial overdensities will continue to grow, forming stars for the first (or second) time in places where no stars formed earlier. The great cosmic story of forming structures doesn’t happen all at once, but in bits-and-pieces throughout the cosmos. But once the first galaxies take shape, the race to form galaxies like our own has officially begun.

This article What was it like when the first galaxies began to form? is featured on Big Think.

When you look out at the Universe today, and see the vast, dark, backdrop littered with points of light that correspond to stars and galaxies, it’s difficult to imagine that it used to be almost identical everywhere. The Universe, back at its inception, was almost perfectly uniform on all cosmic scales. It was the same high temperature everywhere, the same large density everywhere, and was made up of the same quanta of matter, antimatter, dark matter, and radiation in all locations. At the earliest times, the only differences that existed were minuscule, at the 0.003% level, seeded by the quantum fluctuations imprinted during inflation.

But gravity and time have a way of changing everything. Over time, the excess antimatter annihilates away; first atomic nuclei and then neutral atoms form; over millions of years, gravity pulls matter into overdense regions, causing them to grow. Because overdensities differ by such great amounts on all scales, there are regions where stars form rapidly, within 100 million years or fewer, while other regions won’t begin forming stars for billions of years. But wherever the earliest stars form, that’s where the most interesting things happen first, including the existence of the second generation of stars: the first polluted stars in all of cosmic history.

An illustration of the first stars turning on in the Universe. Without metals to cool down the clumps of gas that lead to the formation of the first stars, only the largest clumps within a large-mass cloud will wind up becoming stars: fewer in number but greater in mass than today’s stars.

The very, very first stars are born in the most initially overdense regions of all, which grow by attracting surrounding matter the fastest. The gravitational growth of matter leads to the first stars forming somewhere between 50 and 100 million years after the Big Bang, with those stars being much more massive than the stars we see today. Because there’s so much mass inside them, undergoing the rapid, high-temperature reactions of nuclear fusion, they live fast. Within just a few million years, they’ve burned through all of their core’s fuel, leading to their dying in either a supernova or by directly collapsing to form a black hole.

And wherever this happens, that’s the end of the line for this population of “first stars,” which are the stars made exclusively of the pristine material (hydrogen and helium) forged in the crucible of the hot Big Bang. The outer layers of the stars that went supernovae, making up the majority of the former star’s mass, get blown off back into interstellar space, further enriched by the rapid absorption of neutrons. The supernova remnants that leave behind neutron stars, many of which are in binary systems, have a chance to collide with other neutron stars, giving rise to gamma-ray bursts, kilonova events, and the heaviest of the elements. All of a sudden, the Universe, in these regions, is not solely composed of hydrogen and helium anymore.

Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Mass, in an event like this, gets converted into two types of radiation: electromagnetic and gravitational. About 5% of the total mass gets expelled in the form of heavy elements.

After all the time required for the first stars to form — perhaps as little as 50 million in the densest places, typically between 200 and 550 million in most locations, but not for 2 or 3 billion years in the rarest, longest-lived pristine regions — they run out of fuel and die in as little as 1-5 million years. These very first stars, made out of the pristine elements formed just 3-4 minutes after the Big Bang, cannot survive for very long, as they’re all quite massive compared to the types of stars that primarily exist today. Once you make a sample of the “first stars,” you only have a short window before something other than hydrogen and helium exists in those regions.

Once the most massive stars among the “first stars” die, the interstellar medium becomes enriched with those heavy chemical elements. The atoms that exist are no longer solely hydrogen and helium (plus one-in-a-billion parts lithium) with nothing heavier. Suddenly there are abundant levels of carbon and oxygen, with copious amounts of silicon, sulfur, iron, nickel, and cobalt, plus all the elements made in supernovae and kilonovae, rising all the way up the periodic table. It’s not just from hydrogen and helium, but also with these enriched materials, which now flood the interstellar medium, that the next generation of stars will form.

This full-scale view of the Crab Nebula, from upper-right to lower-left, spans about 11-12 light-years in extent at the nebula’s distance of ~6,500 light-years. The outer shells of gas are expanding at around ~1500 km/s, or about 0.5% the speed of light. This is perhaps the best studied supernova remnant of all-time.

From examining the closest supernova remnants to us, including the Crab Nebula, we can infer that every supernova explosion pushes material outward at approximately the rate we observe there: creating a nebula 10 light-years across after approximately only 1,000 years, indicating that it’s expanding at around 1% the speed of light. Wherever the debris from the deceased first generation of stars cannot yet reach, the stars that eventually form there will still be pristine, since there’s no way for that processed material to make it into those pre-stellar nebulae. The existence of stars in one region of space doesn’t preclude pristine stars from forming nearby, just not in an overlapping region.

But wherever the debris does reach, all of a sudden the material that’s available to form stars is now full of atoms that contain heavier atomic nuclei. It might seem silly to you, under most circumstances, that astronomers classify “hydrogen” and “helium” separately, and then throw every element heavier than helium into its own class — and that we call them “metals” regardless of their other physical or chemical properties — but the existence of these heavier-than-helium elements really plays a tremendous role in the formation and evolution of stars.

The elements of the periodic table, and where they originate, are detailed in this image above. While most elements originate primarily in supernovae or merging neutron stars, many vitally important elements are created, in part or even mostly, in planetary nebulae, which do not arise from the first generation of stars.

You see, whenever stars are formed out of hydrogen and helium alone (i.e., in what astronomers would call a metal-free environment), there’s no efficient way for them to radiate away the heat generated by gravitational collapse. As mass from the outskirts falls in toward the center, potential energy gets converted to kinetic energy, and that kinetic energy, or the energy of motion, causes matter to heat up. If you can radiate that heat away, you can form stars from that material. If you can’t, you need to gather comparatively enormous clumps of matter in order to overcome that kinetic energy and trigger gravitational collapse, which means that the pristine first stars must have been extremely massive stars, even on average.

But once you have metals present, even if they’re just 0.001% (one part in 100,000) of the total fraction of atoms, they can serve as the excellent energy-radiators that the first stars lacked. As a gas cloud containing these heavy elements collapses, those heavy elements efficiently radiate that excess heat away much more efficiently than before, allowing the clumps of matter that exist to collapse into proto-stars not only much more quickly, but also possessing much lower masses. The first “polluted” stars are much more like the stars that form today than the very first stars of all.

Star-forming regions, like the ones found here inside the Carina Nebula, can form a huge variety of stellar masses if they can collapse quickly enough. With heavy elements in the mix, this is possible; without them, it really isn’t, and your stars are forced to be much heavier than the average star we form today.

Additionally, wherever you have supernovae, kilonovae, and other violent, cataclysmic events occurring nearby, they don’t just enrich the interstellar medium surrounding them. The shockwaves and propagating waves of matter, as they travel through space, will smack into surrounding gas clouds, where they can serve as a trigger for the gravitational collapse of those clouds, leading to new star formation events. The first stars don’t just provide the materials for a second generation of stars to form, but also the impetus, especially in a gas-rich environment, for their formation to commence almost immediately.

What does this result in? The biggest implications are that, shortly after the first stars form, live, and die, another new generation of stars will soon crop up, wildly different in character than the first generation that was formed by pristine materials alone. These second-generation stars will no longer weigh in at around 10 solar masses, on average, but rather will run the full gamut of star sizes and masses that we know today. Perhaps, if our understanding of star formation is correct, they’re similar to the stars that form locally: with only about 0.4 solar masses, on average, with many being higher in mass but the majority being even lower in mass.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star. However, in the early Universe, almost all of the stars were O or B-class stars, with an average mass 25 times greater than average stars today.

Yes, there will still be a few large, massive stars, extending up into the hundreds of solar masses, but even the most massive among them won’t be comparable to the biggest and most massive examples found among the first stars. There will soon be additional supernovae, neutron stars, and kilonovae that result as well, which again enrich the interstellar medium further and eliminate adjacent “pristine” populations of matter. In very short order, the earliest, first stars that form will wipe themselves out wherever they exist, only to be replaced by this second generation of stars, which are rife with smaller, redder, and less massive members that weren’t present among the first stars.

As a result, in the very young Universe, we expect to see populations of first stars, which are exclusively hot and blue (and short-lived), living alongside regions that contain more evolved populations of stars. Those “polluted” regions, which already have black holes, second-generation stars, and low-mass, low-luminosity stars among them, will swiftly replace the pristine populations that preceded them. The quest to find the truly first stars faces the challenge that one must disentangle them, wherever they exist, from the subsequent polluted generations of stars that will swiftly arise from the ashes of the shortest-lived first stars.

This is a Digitized Sky Survey image of the oldest star with a well-determined age in our galaxy. The aging star, cataloged as HD 140283, lies over 190 light-years away. The NASA/ESA Hubble Space Telescope was used to narrow the measurement uncertainty on the star’s distance, and this helped to refine the calculation of a more precise age of 14.5 billion years (with a substantial uncertainty of plus or minus 800 million years).

No one, to date, has ever found definitive evidence for even a single first-generation star, as every environment we’ve found contains a mix of red-and-blue stars, indicating that a significant amount of pollution has already infiltrated these stellar populations. The very first stars, perhaps surprisingly, are counterintuitively known among astronomers as Population III stars. Why? Because stellar populations were named in the order they were discovered.

1. The Sun is a Population I star, but it’s highly processed and made out of metal-rich material that has gone through many generations of stellar life-and-death.
2. The second population ever discovered, Population II stars, are these metal-poor stars that form as early as the second generation of all stars. They can live an extremely long time, and a few of them, like the famed Methuselah star, are still around in our galaxy today, despite being over 13 billion years in age.
3. But Population III stars have yet to be discovered; they ought to exist, but are only theoretical at this point. The observational evidence for them has not yet arrived.

On the left, the infrared light from the end of the Universe’s dark ages is shown, with the (foreground) stars subtracted out. 21 cm astronomy will be able to probe epochs in the Universe’s history even farther back than the formation of the first stars, but sufficiently advanced observatories have not come around to rise to this challenge.

In addition, there’s one more fundamental difference between Population II stars and Population III stars: the possibility of having planets around them. The very first stars, composed of hydrogen and helium alone, could only conceivably create tenuous, massive, puffy gas giant worlds. Without a massive, dense core, they’re easily evaporated and dissociated by the radiation emitted by stars, particularly if those stars are massive, ultra-luminous, and rich in ionizing, ultraviolet radiation. It’s speculated by many that planet-formation around Population III stars may be unsustainable and pragmatically impossible.

But once you have the presence of metals, all of a sudden you can form dense, rocky clumps in your protoplanetary disk, which leads to a mix of rocky and gaseous planets. Once you make the second generation of stars, you can make planets too, complete with complex and even organic molecules. Although it has been demonstrated, observationally, that fewer than 2% of all exoplanets that exist have been found around stars with less than a quarter of the heavy elements found in the Sun, there are some low-metallicity examples of stars with planets. Among the pristine ones, however, it’s likely not even possible.

The four most distant galaxies identified as part of JADES, thus far, include three that surpass the threshold for “most distant galaxy” previously set by Hubble. With no more than a quarter of the total JADES data taken thus far, this record will likely fall again, perhaps multiple times, over the coming months and years, but the unambiguous feature of the Lyman break can clearly be seen. The most distant, JADES-GS-z13-0, took the record from Hubble in December of 2022, and still holds it today. Although these are among the youngest galaxies ever discovered, their stellar populations are not pristine.

The very first stars live for only an extremely short time, owing to their high masses and large luminosities and rates-of-fusion. Once they die, ejecting enormous amounts of now-processed material back into the interstellar medium, the space around them becomes polluted with the fruits of their lives: heavy elements. These heavy elements enable the second generation of stars to form, but they now cause stars to form in a different manner from before. The heavy elements efficiently radiate heat away, giving rise to a less massive, more diverse generation of stars, some of which can survive even to the present day.

Now that we have observatories like JWST performing their science operations, many astronomers hold out hope that this and other novel observatories (such as ALMA) may yet reveal a population of these first stars. However, because they’re likely to be found alongside polluted, second-generation stars, disentangling those two populations and “proving” that we are seeing Population III stars is a daunting challenge. But once these second-generation stars begin to form, they make something else possible: the first galaxies. And that, as we’re finding out already, is where JWST is truly pushing back the cosmic frontiers.

This article What was it like when the first “polluted” stars formed? is featured on Big Think.

The cosmic story that gave rise to us is a story rife with creation and destruction. At the start of the hot Big Bang, energetic particles, antiparticles, and quanta of radiation were created. Fractions-of-a-second later, most of the particle-antiparticle pairs had annihilated away. Protons and neutrons formed within the first second, and then over the subsequent minutes, atomic nuclei fused together, creating the first elements. Over the next several hundred thousand years, neutral atoms finally formed, and gravitation pulled matter together into clumps. Eventually, some of the largest clumps gravitationally collapsed, creating the first stars.

But these stars, made up of the pristine material forged in the hot Big Bang, would not remain the only luminous objects in the Universe for very long. As these stars were overwhelmingly massive, 25 times the typical mass of stars created during modern times, they burned through their fuel rapidly, causing them to evolve through their life cycles extremely quickly. The more massive a star is, the shorter its lifespan, meaning that these very first stars didn’t live for long at all. The death of the first stars was absolutely necessary to give rise to the Universe as we know it today. Here’s the cosmic story you haven’t heard.

An artist’s conception of what the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. The neutral atoms surrounding it get ionized, and get blown off, quenching (or ending) star formation and growth in that region. These stars will be short lived, with fascinating and important consequences.

In order to form stars, the gas you’re going to make it out of needs to collapse. But gravitationally collapsing means you have to radiate energy away; the very act of collapsing is one of energy transfer, where gravitational potential energy gets turned into kinetic energy, and where the kinetic energy of matter (i.e., the energy of motion) causes that material to heat up. Today, heavy elements are the best and most efficient energy-radiators that exist, which means that clouds of gas can collapse efficiently, and form all sorts of stars, from the rare ones that come in at hundreds of solar masses down to very small, faint ones at the low-mass end: right at the lower limits of what defines a star.

Early on, however, there were no heavy elements, since those only arise from stars in some way. The first stars, therefore, can only be made out of large clumps of matter that have enough mass to overcome the effects that come from being unable to efficiently radiate your heat away. This is the primary reason behind why the first stars are very large and massive: 10 solar masses on average, with many stars reaching up to hundreds of solar masses and likely with a few that crest the 1000 solar mass threshold, which is unheard of at modern times.

The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1-10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars.

This leads to a seemingly paradoxical statement that I simply call the Blade Runner conundrum. In Blade Runner, one of the main characters is told, “The flame that burns twice as bright lives but half as long,” and while that might be true for flames, the situation is even worse for stars. It’s true that the more massive a star is, the brighter it burns, and the shorter it lives, but “twice as bright” for “half as long” is a gross understatement. While a star like our Sun might live around 10-12 billion years before reaching the end of the fuel in its core, these early stars, with hundreds or even thousands of times the mass of our Sun, are expected to have a lifetime of ~0.01% our Sun’s: just 1-2 million years, before they meet their demise.

As the cores of these stars fuse hydrogen into helium at an incredibly rapid pace, they give off thousands-to-millions (or more) the luminosity of our Sun continuously. For a star ten times the mass of our Sun, that process might last only around 10 million years before running out of hydrogen fuel, whereas more massive stars have even shorter lifetimes. At that point:

• the core contracts and heats up, fusing helium into carbon,
• when it runs out of helium, it heats up and fuses carbon into neon and oxygen,
• and then burns oxygen up to form magnesium, silicon, and sulfur,
• eventually reaching iron, nickel, and cobalt,
• and then ending in a spectacular supernova explosion.

An animation sequence of the 17th century supernova in the constellation of Cassiopeia. Surrounding material plus continued emission of EM radiation both play a role in the remnant’s continued illumination. A supernova is the typical fate for a star greater than about 10 solar masses, although there are some exceptions, as stars can either have enough mass stolen from them to avoid that fate, or can experience core instabilities that lead to their direct collapse.

The cycle of nuclear fusion in massive stars creates a large amount of heavy elements in the periodic table, building elements up to around iron (element 26), and then a supernova occurs. Those elements then experience a rapid bombardment by neutrons as they get blasted off back into the interstellar medium at the moment of the supernova’s detonation, where they typically rise up the periodic table to reach zirconium (element 40), with heavier elements capable of being formed out of those now-enriched, heavier-than-ever-before elements.

What that supernova leaves behind, which is the former core of the progenitor star, is typically a neutron star: a collapsed mass that’s greater than our Sun, but no bigger than perhaps a dozen miles from end-to-end. The most massive stars that go supernova are likely to form black holes rather than neutron stars, but the majority of stars that undergo a core-collapse supernova will be between 8-and-40 solar masses, and are much more likely to leave a neutron star behind.

That’s not even the end of the story, as within these dense, early environments, neutron star-neutron star collisions should be relatively common, giving rise to kilonova events.

Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Mass, in an event like this, gets converted into two types of radiation: electromagnetic and gravitational. About 5% of the total mass gets expelled in the form of heavy elements.

When these neutron star collisions occur, they give rise to either a larger neutron star or a black hole with about 95% of their combined mass, which you might anticipate. But these neutron star collisions also result in runaway, explosive reactions, causing the emission of gravitational waves, neutrinos, electromagnetic radiation of all types, and the expulsion of large quantities of heavy nuclei. These nuclei come in both stable and unstable varieties, and typically climb all the way up the periodic table, producing the majority of the heaviest elements that are ever created in the Universe.

While merging neutron stars are responsible for a large fraction of the elements niobium, molybdenum, tin, tellurium, barium, some of the lanthanides, as well as a little bit of mercury and lead, they produce the overwhelming majority of all the other elements heavier than zirconium, including elements that are far heavier than even uranium and plutonium. In combination with supernovae, neutron star-neutron star mergers help give rise to the full suite of elements that make up the periodic table, including the absolute heaviest elements that naturally occur.

The most current, up-to-date image showing the primary origin of each of the elements that occur naturally in the periodic table. Neutron star mergers, white dwarf collisions, and core-collapse supernovae may allow us to climb even higher than this table shows. The Big Bang gives us almost all of the hydrogen and helium in the Universe, and almost none of everything else combined.

The “average” star, among the first stars, might be around 10 times the Sun’s mass, and might have just 0.1% of the Sun’s lifetime, dying in a supernova after around 10 million years or so. But as is frequently the case with “averages,” a great number of stars will be of above-average mass, and those stars will live for even shorter periods of time. There are stars hundreds or even a thousand times as massive as our Sun that get created here, and they burn through their fuel even more quickly. Shining as bright as millions or even tens of millions of Suns, each one has a unique fate.

In general, there are three mass-dependent possibilities for what will happen to such a star.

One is just a higher-mass analogue of what you’d expect from the earlier supernovae: a massive supernova that leaves only a black hole, rather than a neutron star, behind. The core of a supernova collapses, and in most cases, that’s going to lead to a neutron star. But there’s a limit, somewhere between 250% and 300% the mass of the Sun, to what a neutron star can achieve before it collapses under its own gravity. When it crosses that threshold, the neutron star collapses all the way into a black hole: the second most common fate for the first stars.

The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.

At even higher masses, however, the temperatures inside the star reach such great levels that a special process begins to take place. There’s enough free energy that, for the photons flying around inside the star’s core, there’s the possibility that they can spontaneously form particle-antiparticle pairs. Two photons can spontaneously transform, under these conditions, into an electron and positron, if energies are high enough.

This carries with it some new physics: whereas the radiation pressure from the photons was what held the star up against gravitational collapse, the loss of photons means a loss of pressure, and the star begins to collapse further. As it does, the temperature goes up, making it more likely that photons will convert into electron-positron pairs. This becomes a runaway process, and the star’s core collapses entirely.

This process is then known as a pair-instability supernova, or, if you prefer colorful language, a hypernova explosion. These are extremely rare in the modern Universe, but the first stars should have had many instances of this type of cataclysm. The less massive pair-instability supernovae will lead to a black hole at the core, while blowing off their outer layers, while the more massive ones will destroy the star entirely, giving rise to a much more severely-enriched portion of the interstellar medium in the vicinity where they occurred.

This diagram illustrates the pair production process that astronomers once thought triggered the hypernova event known as SN 2006gy. When high-enough-energy photons are produced, they will create electron/positron pairs, causing a pressure drop and a runaway reaction that destroys the star. This event is known as a pair-instability supernova. Peak luminosities of a hypernova, also known as a superluminous supernova, are many times greater than that of any other, ‘normal’ supernova.

It’s theorized that stars of different masses will reach the pair-instability threshold at different times in their life cycles, making the elements they expel and enrich the Universe with a variable that is not yet well-understood. In general, though, we have a pretty good theory of how, assuming that no major mass-transfer events occur to either add mass to the star or siphon mass off of it, a star will evolve, live, and die based solely on two factors: its initial mass and the fraction of heavy elements, or metallicity, that it’s born with.

The biggest uncertainties in this calculation, perhaps unsurprisingly, are to be found at the extreme low-metallicity end: for the stars made out of the most pristine material. Because all of the stars we observe at present are no longer pristine, but have been thoroughly enriched with material that’s passed through multiple generations of stars in the past before forming the stars that are now present today, it’s very difficult to find a star with less than a certain fraction (about 0.1%) of the heavy elements found in our Sun. Although we think that pair instability supernovae are common, as are normal supernovae that create black holes or neutron stars, those aren’t the only options.

Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. We must also consider that effects other than mass and metallicity may indeed play major roles in determining the fate of massive stars, including in whether they can contribute to enriching the interstellar medium.

Finally, stars of either severely extreme masses or that undergo just the right set of processes could experience a fate that doesn’t result in a cataclysmic explosion at all: no supernova of any type. Instead, these massive stars could directly collapse into a black hole. There doesn’t need to be a runaway fusion reaction for this to occur; there might not be an explosion or any ejecta; the mass could, all at once, simply overcome the radiation coming from its central region, leading to gravitational collapse. And once an event horizon forms, collapse down to a black hole is all but inevitable.

Although this might seem like a science-fiction scenario, we actually have some remarkable evidence that this occurs in nature, not only for “pristine” stars but even at late times. All across the Universe, there have been stars that were imaged at one or more points in the past, but when we look back today, at where they once were, they appear to simply have vanished. There’s no radiation in any wavelength of light, nor is there any remnant that we can see despite all the ways we know of to look for them. It appears, somehow, as though these massive stars have simply directly collapsed: likely into a black hole.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time.

It’s theorized that the process of direct collapse, either from the most massive stars or from clouds of gas that never passed through a stellar phase, is responsible for the origin of the seeds of the supermassive black holes that occupy the centers of galaxies today. It may be the deaths of the most massive stars, which create black holes hundreds or thousands of times the mass of the Sun, that contribute, but it could also be clumps of matter that reached tens or hundreds of thousands of times the Sun’s mass that directly collapsed to create these seed black holes, which could then grow to the enormous masses they’re observed to have at late times.

Over time, mergers and gravitational growth will lead to the most massive black holes known in the Universe, black holes that are millions or even billions of times the mass of the Sun by today. It took perhaps 50-to-100 million years to form the very first stars in the Universe with many pristine stars still forming several hundred million years after the Big Bang. However, once you form stars, it takes only another million or two years after that for the most massive among them to die, creating black holes and spreading heavy, processed elements across the interstellar medium. As time goes on, the Universe, at long last, now contains heavy elements, and begins to resemble the Universe we know and inhabit today.

This article What was it like when the very first stars died? is featured on Big Think.

For perhaps as long as the first 100 million years after the start of the hot Big Bang, the Universe was devoid of stars. The matter in the Universe required just half-a-million years to finish forming neutral atoms, but stars would take much longer to form for a variety of reasons. For one, gravitation on cosmic scales is a slow process, made even more difficult by the high energies of the radiation the Universe was born with. For another, the initial gravitational imperfections were small: just 1-part-in-30,000, on average. And for yet another, gravity only propagates at the speed of light, meaning that when the Universe is very young, there’s only a very small distance range over which other masses can “feel” the gravitational force from any particular initial mass.

As the Universe cooled, gravitation began to pull matter together into clumps and eventually clusters, growing faster and faster as more matter was attracted together. Eventually, we reached the point where dense gas clouds could collapse, forming objects that were dense and massive enough to ignite nuclear fusion in their cores. When those first hydrogen-into-helium chain reactions began taking place, we could finally claim that the first stars had been born: a process taking at least 50 million years and maybe as much as 100 million years or more for even the very first ones to ignite. Here’s what the Universe was like back then.

The overdense regions that the Universe was born with grow and grow over time, but are limited in their growth by the initial small magnitudes of the overdensities, the cosmic scale on which the overdensities are found (and the time it takes the gravitational force to traverse them), and also by the presence of radiation that’s still energetic, which prevents structure from growing any faster. It takes tens-to-hundreds of millions of years to form the first stars; small-scale clumps of matter exist long before that, however.

By the time 50-to-100 million years have passed since the onset of the hot Big Bang, the Universe is no longer completely uniform, but has begun to form a structure resembling a great cosmic web, all under the cosmic influence of gravity. The initially overdense regions have grown and grown, attracting more and more matter to them over time. Meanwhile, the regions that began with a lower density of matter than average have been less able to hold onto it, giving it up to the denser regions that surround them.

In this sense, gravity is what we know as a “runaway” force, where the (matter) rich regions get richer, and the initially matter-poor regions get poorer over time. As overdense regions grow, they draw more and more matter into them, dominated by neutral atoms and streams of gas. These very dense regions accumulate more and more mass, but there’s a problem: as they gravitationally collapse, the atoms collide and generate heat. That heat must be radiated away for stars to form, but hydrogen and helium are terrible at radiating heat away. As a result, the streams of gas grow more and more massive, increasing the mass, temperature, and pressures within them in the densest locations.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the growth of galactic structures.

Driven by the first molecule in the Universe, an ion known as helium hydride, cooling does indeed take place, but only slowly. The very densest regions take one of two paths: they can either directly collapse to form black holes, potentially with tens of thousands or even a hundred thousand solar masses at the outset, or those dense regions can fragment to begin forming stars. The initial mass of these massive clumps has to be enormous: hundreds or even thousands of times the mass that star-forming regions typically have today. When cooling is inefficient, gas remains diffuse, and cannot collapse to form stars unless tremendous amounts of mass are gathered together in one place.

The slightly less initially dense regions will get to those same places eventually, but tens-to-hundreds of millions of years later, as gravitational growth depends both on the initial size of your overdense seeds as well as the mass distribution of the surrounding regions. Regions that begin with only a modest overdensity will take perhaps half-a-billion years or more to form stars for the first times, while regions of merely average density might not begin forming stars until much later: until as many as a couple of billion years have passed.

The first stars and galaxies in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. Without metals to cool them down or radiate energy away, only large-mass clumps in the heaviest-mass regions can form stars. The very first star will likely form at 50-to-100 million years of age, based on our best theories of structure formation.

The very first stars, when they ignite, do so deep inside molecular clouds. They’re made almost exclusively of hydrogen and helium; with the exception of the approximately 1-part-in-a-billion of the Universe that’s lithium, there are no heavier elements at all. As gravitational collapse occurs, the energy gets trapped inside this gas, causing the interior temperatures deep inside the still-forming proto-star to heat up. It’s only when, under high-density conditions, the temperature crosses a critical threshold of around 4 million K that nuclear fusion of hydrogen into helium, occurring in a chain reaction, begins. When that occurs, things start to get interesting.

For one, the great cosmic race that will take place in all future star-forming regions begins in earnest for the first time in the Universe. As fusion begins in the first proto-star’s core, the gravitational collapse that continues to grow the mass of the star is suddenly counteracted by the radiation pressure emanating from the inside. At a subatomic level, protons are fusing in a chain reaction to form deuterium, then either tritium or helium-3, and then helium-4, emitting energy at every step. As the temperature rises in the core, the energy emitted increases, eventually fighting back against the infalling of mass due to gravity.

An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. But the conversion of matter into energy does something else: it causes an increase in radiation pressure, which fights against gravitation. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.

These earliest stars, much like modern stars, grow quickly due to gravitation. But unlike modern stars, they don’t have heavy elements in them, so they cannot cool as quickly; it’s more difficult to radiate energy away without heavy elements. It’s molecular hydrogen (H2) and the helium hydride ion that are left as the most efficient “cooling” mechanisms, but they’re all far less efficient than particles containing elements that become common later on, such as oxygen and carbon, which will swiftly (but have not yet) become the 3rd and 4th most common elements in the Universe. Because you need to cool in order to collapse, this means it’s only the largest, most massive clumps that will lead to stars.

Because of how massive these clumps need to grow in order for stars to form, the first stars that are forming in the young Universe wind up having a mass that is, on average, about 10 times more massive than our Sun. The most massive stars that formed early on, whereas they cap out at around 200-300 solar masses today, could have reached up to many hundreds or even several thousands of solar masses. For comparison, the average star that forms today, 13.8 billion years after the Big Bang, is merely about 40% the mass of our Sun, or 1/25th of what it was for the first stars.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star. However, in the early Universe, almost all of the stars were O or B-class stars, with an average mass 25 times greater than average stars today.

The radiation emitted by these very massive stars is peaked, in terms of the various wavelengths of light that get emitted, differently than our Sun. While our Sun emits mostly visible light, these more massive, early stars emit predominantly ultraviolet light: higher energy photons than we typically have today. Ultraviolet photons don’t just give humans sunburns; they have enough energy to knock electrons clean off of the atoms they encounter: they ionize matter.

Since most of the Universe is made out of neutral atoms, with these first stars showing up in these clumpy clouds of gas, the first thing the light does is smash into the neutral atoms surrounding them. And the first thing those atoms do is ionize: breaking apart into nuclei and free electrons, populating the Universe with these entities for the first time since before the initial formation of neutral atoms just 380,000 years after the Big Bang. This process is known as “reionization,” since it’s the second time in the Universe’s history (after the initial plasma phase of the hot Big Bang) that atoms became ionized.

An artist’s conception of what the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. The neutral atoms surrounding it get ionized, and get blown off, quenching (or ending) star formation and growth in that region. These stars will be short lived, with fascinating and important consequences.

However, because it takes so long for most of the Universe to form stars, there aren’t enough ultraviolet photons to ionize most of the matter just yet. For hundreds of millions of years, neutral atoms will dominate over the reionized ones, and what’s more, some of the ionized electrons will fall back onto ionized atomic nuclei, re-neutralizing these reionized atoms once again. The starlight from the very first stars cannot propagate very far; it gets absorbed by the intervening neutral atoms almost everywhere. Some of these neutral atoms will scatter that light, while others will become ionized once again.

The ionization and the intense radiation pressure from the first stars forces star formation to cease shortly after it begins; most of the gas clouds that give rise to stars will have those dense clumps of matter blown apart, evaporated away by this radiation. The matter that does remain around the star collapses into a protoplanetary disk, just like it does today, but because there are not yet any heavy elements, only diffuse, giant planets can form. The first stars of all couldn’t have hung onto small, rocky-size planets at all, as the radiation pressure would destroy them entirely.

Electron transitions in the hydrogen atom, along with the wavelengths of the resultant photons, showcase the effect of binding energy and the relationship between the electron and the proton in quantum physics. The Bohr model of the atom provides the coarse (or rough, or gross) structure of these energy levels. Hydrogen’s strongest transition is Lyman-alpha (n=2 to n=1), but its second strongest is visible: Balmer-alpha (n=3 to n=2), which emits visible (red) light at a wavelength of 656 nanometers.

The radiation doesn’t just destroy aspiring planets, it destroys atoms as well, by kicking electrons energetically off of the nuclei and sending them into the interstellar medium. But even that leads to another interesting part of the story: a part that creates light whose signatures will someday be observable.

Whenever an atom becomes ionized, there’s a chance it will run into a free electron that was kicked off of another atom, leading to a new neutral atom. When neutral atoms form, their electrons cascade down in energy levels, emitting photons of different wavelengths as they do. The last of these lines is the strongest: the Lyman-alpha line, which contains the most energy. Some of the first light in the Universe that’s visible is this Lyman-alpha line, allowing astronomers to look for this signature wherever light exists.

The second-strongest line is the one that transitions from the third-lowest to the second-lowest energy level: the Balmer-alpha line. This line is interesting to us because it’s red in terms of the color we see, rendering it visible to the human eye.

This Hubble composite of the Orion Nebula includes objects Messier 42 and Messier 43, spans about 24 light-years across, and shines with both emitted and reflected light from thousands of new stars. The enormously bright “pink” features are a combination of the white light emitted from stars and reflected off of the neutral matter, with the red transition of hydrogen atoms, the Balmer-alpha transition, superimposed atop that white light.

If a human were somehow magically transported to this early time, we’d see the diffuse glow of starlight, as seen through the fog of neutral atoms. But wherever the atoms become ionized in the environs surrounding these young star clusters, there would be a pinkish glow coming from them: a mix of the white light from the stars and the red glow from the Balmer-alpha line. This signal is so strong that it’s visible even today, in environments like the Orion Nebula in the Milky Way.

After the Big Bang, the Universe was dark for millions upon millions of years; after the glow of the Big Bang fades away, there’s nothing that human eyes could see. But when the first wave of star formation happens, growing in a cosmic crescendo across the visible Universe, starlight struggles to get out. The fog of neutral atoms permeating all of space absorbs most of it, but gets ionized in the process. Some of this reionized matter will become neutral again, emitting light when it does, including the 21-cm line over timescales of ~10 million years.

But it takes far more than the very first stars to truly turn on the lights in the Universe. For that, we need more than just the first stars; we need them to live, burn through their fuel, die, and give rise to so much more. The first stars aren’t the end; they simply mark the dawn of a new chapter in the cosmic story that gives rise to us.

This article What was it like when the first stars began to shine? is featured on Big Think.

Although it might seem that the world changed long ago from the Hubble era to the JWST era, the reality is that humanity’s greatest space-based observatory of all-time is less than two years old. It launched on Christmas Day, 2021, and required six months of deployment, commissioning, and calibration operations before it was ready to begin the primary phase of its life: full-time science operations. Since those milestones were achieved in July of 2022, JWST has been our cosmic workhorse, revealing the Universe in a whole new light, with unprecedented resolution and wavelength coverage to view the cosmos.

While its first sets of spectacular images were released during 2022, this past year, 2023, represents the very first year that we had this remarkable observatory operating full-time, surveying the Universe near and far to reveal some of the most incredible views, plus many unexpected scientific discoveries, that pretty much no one could have anticipated. Here, without further ado, are my favorite JWST science images released in 2023.

A very distant galaxy, found in the background of JWST’s image of galaxy cluster Abell 2744 (Pandora’s cluster), emits copious amounts of X-rays, consistent with a black hole of between 10 and 100 million solar masses. The galaxy itself has only about that much mass in stars, making this the first “missing link” in discovering the connection between black hole and galaxy growth in the early Universe.

1.) Our most distant black hole ever. It was only last month, while combining Chandra X-ray data with JWST’s deep, infrared views of galaxy cluster Abell 2744, that scientists revealed a tiny, distant, early galaxy with only around 10-to-100 million solar masses worth of material in it, but that was incredibly X-ray luminous, indicating an active black hole of around 9 million solar masses. Not only is this the most distant black hole ever discovered, it’s also our first example of such an extreme mass ratio, where the central black hole is right around as massive as all the stars in the host galaxy combined. Our understanding of black hole-galaxy formation and coevolution will never be the same.

This full-scale view of the Crab Nebula, from upper-right to lower-left, spans about 11-12 light-years in extent at the nebula’s distance of ~6,500 light-years. The outer shells of gas are expanding at around ~1500 km/s, or about 0.5% the speed of light. This is perhaps the best studied supernova remnant of all-time.

2.) JWST’s view of the Crab Nebula. In the year 1054, a supernova went off in the Milky Way galaxy: so brilliant and enduring it was visible from Earth for a long period of time. Now, nearly 1000 years later, we can look in that same region of sky and find the Crab Nebula: a supernova remnant more than 10 light-years across, with a young, energetic, spinning neutron star at its core, the Crab Pulsar. Whereas Hubble’s visible light views highlight various elements and knots of gas that reflect light, JWST’s infrared views showcase the presence of dust, accelerated electrons, and even the carved-out features by the central pulsar’s winds and magnetism. The question of the mass mystery, or of where all the supposedly “missing material” that would have been needed for the progenitor star to explode, may yet find its solution in the still-being-analyzed JWST data.

This ultra-distant view of the Universe comes from a portion of the JADES survey, leveraging JWST’s capabilities. Although there are trillions upon trillions of stars producing the light powering these galaxies, they extend back for tens of billions of light-years in space. In reality, the density of stars in space is incredibly low.

3.) JWST’s deepest-ever view: the JADES view. The JWST Advanced Deep Extragalactic Survey, or JADES, collaboration has released a fully zoomable, explorable view of their field, with various NIRCam filters and NIRSpec spectra capable of being overlaid atop an enormous set of the objects imaged. Although this represents a relatively narrow field-of-view in the sky, it contains the most distant galaxy ever discovered so far, as well as a slew of candidate objects that may yet prove to be even farther away. It showcases the incredible reach and variety of what’s possible with JWST.

Here, evaporating gaseous globules are seen at the edge of a star-forming region within the Orion Nebula, with newborn stars, Herbig-Haro objects, and many fainter sources of light, including protostars, brown dwarfs, and even planetary-mass objects found inside. As the gas continues to boil away, more and more of these lower-mass objects should be revealed.

4.) JWST peers inside the Orion Nebula. When you look inside the nearest large star-forming region to us in the Milky Way, the Orion Nebula, what are you going to find? Under JWST’s eyes, there are an enormous number of brilliant, glittering new stars still in the process of forming. Some of them, shown here, are Herbig-Haro objects: massive young stars that are highlighted by stellar outflows. In other cases, there are proto-stars, still in the process of formation, young singlet and binary stars that have already finished forming, and nebulous regions that even JWST cannot penetrate. Lastly, there were some surprises: Jupiter-mass objects that are members of no stellar system, including a surprisingly large fraction of them that are binary objects. The images are as beautiful as the science is profound.

This gravitationally lensed system from the COSMOS-Web field consists of a compact, massive galaxy located ~17 billion light-years away, and a more distant galaxy 21 billion light-years away whose light is stretched into a ring-like shape. The decomposition of the two components is shown at bottom.

5.) The most distant gravitational lens ever. At the center of this image, a massive compact galaxy is found, located about 17 billion light-years away within this expanding Universe. The ring around it, with two red spots, is actually a single, more distant galaxy that’s located along the same line-of-sight as the closer galaxy, but gravity has distorted it into a ring: an example of gravitational lensing. While more distant background lenses have been spotted, this represents the most distant foreground lens — the object actually doing the lensing — ever discovered.

Many unusual features can be seen in this JWST image of the El Gordo galaxy cluster, as this massive cluster’s gravity distorts the shape, brightness, and many other properties of the background galaxies behind it.

6.) The most massive galaxy cluster for its time. Although galaxy clusters are found all across the Universe, they’re expected to grow larger and larger in mass over cosmic time. For the time at which it was discovered in the Universe, the El Gordo galaxy cluster, imaged here by JWST, is the most massive one known: with over two quadrillion solar masses of material inside it, despite its light coming from more than 5 billion years ago. Within this cluster, marked A and B, are the gravitationally lensed galaxies known as “La Flaca,” which is Spanish for “the skinny one” (a fitting counterpart to El Gordo, or “the fat one”) and the Fishhook. In reality, both of these lensed galaxies are completely normal; their light is stretched into these unusual shapes by the foreground gravity of the galaxy cluster in front of them.

One of the most exciting features found in the El Gordo field, as seen with JWST’s eyes, is the most distant red giant star ever discovered: Quyllur, which is the Quechua term for star. It is the first red giant star found more than 1 billion light-years away, and it’s actually over 10 billion light-years away. It was only visible due to JWST’s unique capabilities coupled with El Gordo’s gravitational lensing magnification.

7.) The most distant red supergiant star ever. Located in the same field as El Gordo, and hence in the same field as the Fishhook and the “La Flaca” lensed galaxies, is a single red supergiant star known as Quyllur: the most distant red supergiant ever discovered. Although the previously-discovered star Earendel, also imaged by JWST but discovered first by Hubble, is even farther, this shows that finding individual stars in the early Universe isn’t a one-off proposition, but rather that the combination of JWST’s incredible capabilities plus the enhancement of gravitational lensing can reveal individual stars farther back in cosmic time than via any other method.

This image shows not only the central dual cores of galaxy cluster G165, but also the labeled lensed features. All told, there are at least 21 independent multiply-imaged background light sources found in this field of view.

8.) Triply-lensed “Supernova H0pe” discovered. Sometimes, very distant galaxies have their light stretched out into multiple images by the effects of gravitational lensing. When we’re very lucky, a transient event, such as a supernova, will occur in that lensed galaxy, allowing humanity to observe the supernova event on replay in each of the multiple images. The reason this provides such hope, or H0pe in this case, is because the quest to measure the expansion rate of the Universe today, also known as H0 or the Hubble constant, gives two different answers dependent on which set of methods are used.

The discovery of Supernova H0pe provides a potential way to resolve this conundrum over the expanding Universe, and perhaps future observations of multiply lensed supernovae, which JWST should be outstanding at finding, will be just what we need to resolve the so-called “Hubble tension.”

This mid-infrared view from JWST’s MIRI instrument reveals the dusty features located within the barred spiral galaxy NGC 7496. The bright spikes seen in the mid-infrared indicate the presence of an active, central black hole in this galaxy: one of the key features that JWST can reveal in nearby galaxies that no other observatory is sensitive to.

9.) Dusty secrets within spiral galaxies. Most of the images we see of spiral galaxies are taken in visible light, where the stars shine brightly but where neutral matter, particularly dust grains, appear dark, blocking that light. Not so with JWST’s MIRI instrument, which highlights and illuminates the dust inside these galaxies, showing the locations of future and current new star-formation. In this view of galaxy NGC 7496, not only are the dust lanes prominently revealed, along with the pinkish-white regions showcasing regions where new stars are already forming, but the center of the galaxy exhibits brilliant diffraction spikes: evidence for an actively feeding supermassive black hole at the galaxy’s center.

This animated view that transitions between JWST’s NIRCam and JWST’s MIRI views of the Ring Nebula reveals the difference in structure and detail that different wavelengths of light can reveal. The nebula appears bigger in mid-infrared light because outermore, cooler components radiate at wavelengths that are invisible at short wavelengths, but that emit detectable signatures at longer wavelengths.

10.) The Ring Nebula. Viewed with both the NIRCam and MIRI instruments independently, this nebula is among the most famous planetary nebulae known: what’s left behind when a dying, Sun-like star blows off its outer layers in its death throes, while its core contracts down to form a white dwarf. You can find, in both views, intricate details of the inner filaments, which are actively being evaporated away by radiation, as well as roughly 10 concentric arcs outside of the main ring feature that are hydrocarbon-rich in the MIRI image. No other observatory has ever revealed this level of detail inside the Ring Nebula.

This annotated JWST image of Saturn shows its three imaged moons, the main disk of the planet, and many features in the main rings of Saturn, including the Cassini division and the Encke gap.

11.) JWST’s stunning view of Saturn’s rings. What shines brighter than Saturn, according to JWST’s eyes? Why, Saturn’s rings of course. Whereas Saturn itself is a relatively cool planet with a cloud-and-haze rich atmosphere separated into bands by latitude, it’s mostly very faint in infrared light. However, its rings are 99.9% composed of water-ice, which is even more reflective in infrared light than in visible light, leading to this unique and stunning view of Saturn’s rings. In this image from JWST, the A, B, C, and F rings are all visible, as are the Cassini division and the Encke gap. Saturn was the final gas giant planet in our Solar System imaged by JWST, completing our Solar System’s family portrait.

The most recent wide-field view of Uranus as seen with JWST reveals not only the planet and its rings and innermost moons, but its five outer moons, two nearby stars in the Milky Way, and hundreds of galaxies much farther away. The four-filter view of this field is taken with JWST’s NIRCam imager, and represents humanity’s best view of Uranus since Voyager 2’s flyby in 1986.

12.) Uranus, new and improved. Although JWST caught its first view of Uranus in February of 2023, the data it acquired on September 4, 2023 shows a far more spectacular view. 9 of its 13 inner moons, plus all five of its main large moons, are all revealed, as are at least five of its rings along with several features on the planet itself: a dense polar cap that fades away toward equatorial latitudes, punctuated by a dark band and with Uranian storms ranging closer to the equator. As Uranus approaches its solstice for the first time since 1986, these JWST views teach us information that no other observatory can reveal.

This NIRCam view of a selection of the gravitationally lensed region surrounding galaxy cluster SMACS 0723 contains multiple lensed galaxies, including the thrice-appearing Sparkler galaxy, highlighted here. The “sparkles” have been identified as star-forming knots of gas appearing atop already-existing globular clusters. Below the left-center of the second image of the Sparkler galaxy, a foreground star within the Milky way shows the characteristic diffraction spike pattern for JWST.

13.) A cosmic sparkler. Although this shows a portion of the very first science image released by JWST, it wasn’t until January of 2023 that this remarkable feature, known as the Sparkler galaxy, was discovered in JWST data. In the yellow boxes, shown above, are three images of the same distant galaxy, lensed, stretched, and magnified by the gravity of foreground cluster, SMACS 0723. The “sparkles” that are most easily visible in the largest, central image are actually globular clusters that are brightly undergoing new episodes of star-formation. When JWST examined these clusters in detail, it found that they already had older populations of stars inside, shedding new light on how “second bursts” of star-formation can occur inside globular clusters: a feature that only a fraction of all known globular clusters possess.

The structure of the Fomalhaut stellar system is revealed for the first time in this annotated JWST image. A central inner disk, followed by a (likely planet-caused) gap, an intermediate belt, more planets (and another gap), and finally a Kuiper belt analog, complete with what’s been dubbed the “great dust cloud” newly forming inside, are all revealed.

14.) An intermediate belt surprise. We’ve often looked at our Solar System as the prototype for what we expect to find elsewhere in the Universe. While planets can exist both close to and far from a star, we expect there to be a series of frost lines, with the innermost one corresponding to an asteroid belt and the outermost one corresponding to a Kuiper belt. Yet, when JWST examined the young stellar system Fomalhaut, it found something our Solar System doesn’t possess: an intermediate belt, found exterior to the inner disk where the asteroid belt should be, but interior to the Kuiper belt analogue. Is this feature typical of stellar systems, meaning we’re the outlier, or is it unusual, meaning it’s the outlier? More data is needed, but this is a puzzle we didn’t even know would need to be solved prior to 2023.

The galaxies that are members of the identified proto-cluster A2744z7p9OD are shown here, outlined atop their positions in the JWST view of galaxy cluster Abell 2744. At just 650 million years after the Big Bang, it’s the oldest proto-cluster of galaxies ever identified. This is early, but is consistent with simulations of when the earliest proto-clusters should emerge from the most initially overdense regions.

15.) The most distant galaxy cluster ever. Earlier in 2023, scientists spectroscopically analyzed a series of distant, very red, faint, galaxies found in the field-of-view behind Pandora’s cluster: Abell 2744. They found that at least seven of these galaxies are at precisely the same redshift, indicating the presence of a proto-galaxy cluster, the earliest one ever found at just 650 million years after the Big Bang. While Hubble had found the earliest proto-galaxy cluster previously known, at 800 million years after the Big Bang, and the CEERS collaboration found one just 1.2 billion years after the Big Bang, this cluster, with a mouthful of a name of A2744z7p9OD, was discovered by the GLASS collaboration, showcasing the importance of viewing many different areas of the sky in the quest for the most distant classes of objects of all.

These 15 images represent just a tiny fraction of the views and science that have come out of JWST, and the best part is we likely have another 20 years of excellent JWST science to look forward to. The great cosmic story, and our understanding of it, is in many ways only beginning to be unfolded.

This article The top 15 JWST images of 2023 is featured on Big Think.

If you examine a planet like Earth over the course of a year, you’ll notice a great many changes. Day to day, the most significant changes will come from cloud cover and weather patterns, as the motion of storms, fronts, and water throughout Earth’s atmosphere are all variable. On longer timescales, the changing of the seasons will lead to the greening and browning of continents, the advance and retreat of glaciers, ice sheets, and polar caps. And these changes will be punctuated by singular events: geomagnetic storms, blackouts, and severe weather events at various times. All of these changes affect our planet’s appearance, dependent on when we take our particular snapshot.

But for Uranus, the story is far more dramatic. Unlike Earth, with its ~23° axial tilt, Uranus rotates almost perfectly on its side, with a ~98° axial tilt: just 8° off from perfect sideways rotation. Instead of a single calendar year, Uranus takes 84 Earth years to complete a revolution around the Sun. And this means that every 21 years, it transitions from Uranian solstice, where one pole points directly at the Sun and the other point directly away, to Uranian equinox, where each part of that world receives equal night and daylight, and then back again in the next 21 years. With its second view of Uranus, the true power of JWST for investigating this outer Solar System world has come into focus, and what we’re finding is already blowing scientists away.

The comparison of the central features of Uranus as seen previously by JWST in February of 2023,(top) versus its latest views in September of 2023 (bottom). The number of additional features seen in the later image is striking.

Let’s work our way from the inside out. First, in this ultra-close-up view of our 7th planet, you can clearly see the planet itself has a bright, highly reflective feature on the right side of this image. It appears to be densest in one small, roughly circular region: that is the polar cap on Uranus’s south pole. Whereas in visible light, Uranus appears as just a monochrome bluish ball at this point in time, the large amount of high-altitude ice and clouds in its atmosphere still persist, as the southern hemisphere is only now approaching its next solstice, which will arrive in 2028.

Surrounding the dense polar cap is a less dense region around it, where the polar cap still persists but is far less dense. As we look farther away from the pole and move toward equatorial latitudes, not only does the density of the cap decrease, as one might expect to find warmer regions down toward more equatorial latitudes, but dark lanes appearing toward the edge of that polar cap: evidence that the cap is evaporating as the seasons change. Finally, below the southern border of the polar cap, additional bright features — storms, likely due to a combination of seasonal and meteorological effects — can be seen at still closer-to-equatorial latitudes.

This up-close view of Uranus shows several of its rings, including the inner Zeta ring and several of the outermore rings. Beyond the final bright, dense rings, a series of more diffuse, outer features can be seen, as Uranus continues to have rings even beyond where JWST can probe.

There appears to be a bright, reflective aura at the edge of Uranus, as seen by JWST’s eyes. Many have wondered at seeing this phenomenon: what is it?

Is there a ring that encircles the planet right at the top of its atmosphere, rendering invisible except where seen edge-on? That’s not quite right; observations with other instruments and up-close from Voyager 2 discount that notion.

Is there a ringed system orbiting it that’s simply located right at the upper edge of Uranus’s atmosphere, interior to the known, identified rings but identifiable to JWST’s eyes? Unlikely as well, as both Voyager 2 observations and space-based observations with Hubble, which have found previously unidentified rings around Uranus, show no evidence for such a feature.

Instead, it’s most likely to be due to an upper layer of haze: above the three layers of clouds (water-ice, ammonia, and hydrogen sulfide clouds) found at high pressures, and still above the methane cloud layers lying at higher altitudes. Instead, above the tropopause, there are likely layers of hydrocarbon haze, and where the planetary atmosphere becomes thin, those hazes are more heavily reflective, resulting in Uranus’s bright appearance to JWST’s infrared views.

This schematic view, to-scale, shows the Uranian ring system along with the known moons that orbit and shepherd the rings around Uranus along with it. The dense inner rings, from epsilon on inward, are the ones clearly imaged by JWST.

Moving farther outward, the Uranian rings shine brilliantly. The innermost ring is Uranus’s Zeta (ζ) ring: elusive to most instruments but thoroughly revealed by JWST’s NIRCam imager. Exterior to the Zeta ring are a series of additional, relatively bright rings:

• the α and β (Alpha and Beta) rings, which are closely spaced together and are both relatively broad and deep, located about 3-4000 km outside of the Zeta ring,
• the η (Eta) ring, which has a bright outer component, and is about 6000 km greater in radius (about one planet Earth radius) than the inner Zeta ring,
• the δ (Delta) ring, which has a bright inner component and is a little more than 1000 kilometers farther outward than the Eta ring,
• and the thick ε (Epsilon) ring, shepherded by Uranus’s moons Cordelia and Ophelia (not captured by JWST), which represents the thick, bright, outermost of the five clearly visible Uranian rings captured by JWST.

There are several other rings of Uranus, but beyond the Epsilon rings are what appear to be a faint series of concentric rings: these are the wider and more distant ν (Nu) and μ (Mu) rings, which are the wide, outermost, but thin and tenuous rings of Uranus, with many moons found in their vicinities.

Under the incredibly sharp views of JWST’s eyes, a whopping nine of Uranus’s innermost 13 known moons are revealed. All except the smallest and innermost moons can be identified, with the inner rings and planetary features all illuminated in infrared light within them.

Beyond the rings, which have a few small moons that are not-quite-visible to JWST’s eyes, lie the prominent innermost moons of Uranus. These include:

• Bianca, the third-most-inner moon,
• Cressida, the fourth,
• Desdemona, the fifth,
• Juliet, the sixth,
• Portia, the seventh,
• Rosalind, the eighth,
• Belinda, the tenth (sorry, Cupid fans, it’s too small to show up here),
• Perdita, the eleventh,
• and Puck, the twelfth and largest of the inner moons of Uranus.

There’s another moon known outside of Puck, Mab, which is also too faint to be seen by JWST.

This is an incredibly impressive feat; we have known about all but three of the inner moons of Uranus since the time of Voyager 2, and JWST was able to reveal all of those except for Cordelia and Ophelia (the innermost two, likely lost in the Uranian rings). Additionally, while it was unable to find Cupid and Mab, the smallest known Uranian moons, it was able to find Perdita, the next smallest and one that was not found in Voyager 2 data. It turns out that JWST is outstanding for finding Uranus’s moons, and that’s before we even move beyond Mab: to where Uranus’s five largest and most prominent moons can be found.

The five largest moons of Uranus, in order from the innermost to the outermost, are Miranda, Ariel, Umbriel, Titania, and Oberon, with the latter two being the largest and first-discovered among Uranus’s moons. All of these moons and the innermost one rotate within a single degree of Uranus’s orbital plane except for Miranda, which is inclined by 4.3 degrees.

However, as we move farther outward, they spectacularly appear. The innermost of Uranus’s large moons is Miranda, which was discovered only in 1948, by a very famous astronomer whom you may better know for the belt of cometary material named after him: Gerard Kuiper. Whereas all of Uranus’s inner and the other four large moons are inclined at less than 1° to the planet’s orbital plane, Miranda is inclined at more than 4°, making it unique.

Beyond Miranda, the two larger moons Ariel and Umbriel can be located: more than 1000 km in diameter apiece. These moons were known for much longer, as both were discovered in 1851 by England’s William Lassell, who also discovered Saturn’s moon: Hyperion and Neptune’s largest moon: Triton.

And lastly, the final Uranian moons imaged by JWST are also its two largest: Titania (at 1577 km in diameter) and Oberon (at 1523 km in diameter), both discovered by William Herschel, the discoverer of Uranus itself, just 6 years after finding the Solar System’s 7th planet at all. Unlike the innermore moons that only appear as points or blobs, all five of these Uranian moons are so bright and reflective that they possess their own diffraction spikes.

The most recent wide-field view of Uranus as seen with JWST reveals not only the planet and its rings and innermost moons, but its five outer moons, two nearby stars in the Milky Way, and hundreds of galaxies much farther away. The four-filter view of this field is taken with JWST’s NIRCam imager, and represents humanity’s best view of Uranus since Voyager 2’s flyby in 1986.

But that’s not all. In this same field of view, even though it was viewed at just a single “snapshot” in time on the date of September 4, 2023, a huge amount of additional features can be found. At the left of the image, a brighter object than Uranus or any of its moons, at least in infrared light, can be seen: that is a relatively bright star that just happens to be close by Uranus: too faint to be seen with the naked eye. To the top right of the image, a fainter star also within the Milky Way, identifiable by its diffraction spikes as well, represents the only other star in the Milky Way visible in this field.

Beyond the Solar System and the stars in our Milky Way, an enormous number of other faint points and smudges of light can be seen: these are galaxies located tens, hundreds, or even thousands of millions of light-years away. These galaxies can be found everywhere: where Uranus and its rings and moons both are and are not; the only reasons some of them are obscured is:

• because there are closer, bright, foreground objects (like Uranus, its rings, moons, or Milky Way stars) in front of them,
• or because they are too faint to be seen in this limited-time exposure, as the Uranian system’s features are bright enough to all be taken in a relatively short period of time.

This first view of Uranus, its rings, and its moons with JWST was groundbreaking, as it revealed the planet’s polar caps, many of its rings, and several of its moons as well, plus a large number of galaxies in the background atop that. However, it pales in terms of detail to the image taken just 7 months later with the same telescope and instrument. Looking at this image next to the later one taken in September, the biggest difference is the smaller number of filters used to create this image.

Compare that JWST view to the one above: of the same system, but taken earlier this year: on February 6, 2023, just about 7 months prior to the more recent JWST photo. While some of the features appear very similar, it’s obvious that there are:

• greater amounts of detail,
• more moons,
• fainter rings,
• and a far greater number of background galaxies,

revealed in the more recent image. Why is this?

Sure, there’s a little bit more observing time, and that definitely helps. But it’s the same instrument, on the same telescope, with the same hardware and software, viewing the same set of celestial phenomena. The big difference, however, is the addition of two new observing filters. Whereas the earlier (February) image only viewed with the 1.4 micron and 3.0 micron medium-band NIRCam filters, the later (September) image also added in data from 2.1 microns and 4.6 microns, exposing details that are either faint or invisible in those other wavelengths of light.

Just as humans have far superior color vision to dogs, because we have three (or four) types of cones compared to just two, viewing the Universe in additional bands of infrared light can significantly improve the types of details you’re sensitive to.

Preliminary total system throughput for each NIRCam filter, including contributions from the JWST Optical Telescope Element (OTE), NIRCam optical train, dichroics, filters, and detector quantum efficiency (QE). Throughput refers to photon-to-electron conversion efficiency. By using a series of JWST filters extending to much longer wavelengths than Hubble’s limit (between 1.6 and 2.0 microns), JWST can reveal details that are completely invisible to Hubble. The more filters that are leveraged in a single image, the greater the amount of details and features that can be revealed.

While Uranus is interesting in its own right, and certainly deserves a second visit now that nearly four full decades have passed since our first-and-only visit to it, there’s another important reason that JWST would want to turn its infrared eyes onto this ice giant world in our outer Solar System: exoplanets. These Uranus-size worlds are very common in the Universe, and while many of the ones we know best are relatively close to their parent stars and are therefore warm, Uranus actually possesses the coldest temperatures of any planet in our Solar System during most times of the Uranian year.

If we’re going to study exoplanets, we would be fools to not study, in great detail and with the same instruments, the “exoplanet analogues” right here in our own Solar System. How do planets of this size work? What is their meteorology like, and what types of weather phenomena appear on these planets under a variety of different conditions? By studying Uranus, especially as it makes that critical transition from equinox-to-solstice and then, afterward, back again toward the next equinox, we may learn a great amount about this planet’s atmospheric processes. And, because of that, it may help us better understand what’s going on with similarly sized (and similarly cold) planets found all throughout the Milky Way.

This animation shows the four super-Jupiter planets directly imaged in orbit around the star HR 8799, whose light is blocked by a coronagraph. The four exoplanets shown here are among the easiest to directly image owing to their large size and brightness, as well as their huge separation from their parent star. Our ability to directly image exoplanets is constrained to giant exoplanets at great distances from bright stars, but improvements in coronagraph technology will dramatically change that story.

It also helps us prepare for the next great era in astronomy: the era of exoplanet direct imaging. In the coming years and decades, improvements in coronagraph technology, which blocks the light from a parent star but enables us to see the light coming from its orbiting planets, is expected to improve to contrasts of between one and ten billion. This means that a planet that is just one-billionth, or even one-ten billionth as bright as its parent star can be observed if the light from the parent star can be blocked, and won’t be lost in its glare. Even if the planet shows up as just a single pixel, we can learn a great deal about it, including its wind speeds, atmospheric contents, and cloud properties and variability.

What would the properties of a planet be if it possessed an extremely severe axial tilt? How does heat flow work on a planet with such extremes, and what does Uranus’s “night” side look like? Without a mission to the outer Solar System, these questions will not be answered, and it seems like these questions are of paramount importance, knowing full well the sheer extent of the variety of planets found around stars in this Universe. If we want to know more about Uranus, a mission to the outer Solar System is necessary. Until then, we can all marvel at what we’re learning just from observations with JWST!

This article JWST’s new and improved exam of Uranus shines is featured on Big Think.