Fate of the First Stars (Script) (Patreon)

Soon after the Big Bang, the first generation of monstrously large stars ignited, lit up the universe, and then died. The resulting swarms of supernova explosions enriched the universe with the first heavy elements and LOTs of black holes. They shaped everything that came after. These were the stars of Population III, and they are one of the most enduring mysteries in astrophysics.

The Sun is a latecomer to our universe. In its light we see telltale signs of the generations of stars that came before it. See, the Sun and all stars are made of the raw material forged in the heat of the Big Bang itself – hydrogen and helium. Mostly. When the Sun’s light is broken into a spectrum, it reveals traces of many of the heavier elements of the periodic table. These elements were forged in the cores of earlier generations of stars – stars that exploded as supernovae and spread their element-enriched guts through the galaxy long before the Sun was even a twinkle in the eye of a giant molecular cloud.

Astronomers categorize stars according to the relative quantity of heavy elements that they possess. By the way, astronomers call any element heavier than helium a metal, and the relative quantity of metals versus hydrogen and helium is a star’s metallicity. Stars that formed more recently tend to have the highest metallicities because they contain the dust of more stellar generations past. We divide stars up into three populations. The Sun is a Population I star, meaning 2-3% of its mass is metals. That’s a lot. “Pop- I” stars formed the most recently and are still forming today – typically in the disks of spiral galaxies.

Population II stars are metal-poor, with metallicities at around 0.1%. These are the oldest stars that we see in the Milky Way. They were born long ago, when galaxies like the Milky Way were still forming in the early universe. Today they’re found in the galactic bulge or in globular clusters – ancient, dense islands of stars that orbit far out in the galactic halo.

Population III stars have no heavier elements whatsoever. They were the first-ever stars shining in the first-ever protogalaxies, born of the pristine hydrogen-helium gas that filled the universe soon after the Big Bang. I’d tell you where to find them today, but it’s not clear that we’ve ever seen one. And that’s not for lack of trying. Astronomers have been searching for the mythical Pop-III generation for decades. Yet they MUST have once existed. We’re starting to think they may all be long dead.

OK, so these things formed right at the beginning of the universe. Makes sense they’d be gone by now, right? Except that the longest-lived stars – red dwarfs - have lifespans of trillions of years. No red dwarf has ever burned out. Even stars a little smaller than our Sun – the orange-ish K-type stars – live for longer than the current age of the Universe. Stellar life span gets shorter the more massive the star – and I’ll get back to why. Stars of the Sun’s mass and higher that formed over 13 billion years ago  - near the beginning of the Universe - WOULD now be long gone. And this brings us to the leading theory as to the mysterious disappearance of Population III. They were gigantic. All of them. And every single one has long since burned out.

Before we get to WHY pop-III stars were so large, let’s unravel this whole lifespan thing. Massive stars live fast, die young, and leave invisible, spacetime warping corpses. One might think that having more mass – more hydrogen to fuse in their cores - would allow a star to burn longer. However, the light that burns twice as bright burns half as long, and these stars burned so very, very brightly.

Physics time. The cores of stars are under extreme pressure due to the gravitational crush of their great mass. The more mass, the greater the pressure. By the ideal gas law, temperature increases with pressure, and so the cores of very massive stars are much hotter than our Sun’s – up to a couple hundred million Kelvin versus the Sun’s 15 million K. The rate of nuclear fusion reactions is incredibly sensitive to temperature. A small increase in mass means a small increase in core temperature, but that results in a dramatic increase in fusion rate and therefore energy output. A star 10 times the mass of the Sun shines around 10,000 times brighter. Burning through 10 times the fuel at 10,000 times the rate compared to the Sun means its life is a thousand times shorter – only 10 million years. Even replicants had a better tradeoff.

The smallest Population III stars may have had masses of several times the Sun’s, while the largest could have been as much as 1000 or more times the Sun’s mass. By comparison, the most massive later stars are at most a couple hundred solar masses. With masses that high, all Pop-III stars would have gone supernova while the universe was still in its infancy.

So why do we think the first stars were so massive? Based on our understanding of how stars form, they must have been. This is where we get back to this metallicity thing. Stars form when vast clouds of mostly molecular hydrogen collapse under their own gravity. For that collapse to proceed, the pull of gravity needs to overcome the cloud’s own thermal pressure. Warmer clouds have more internal energy, helping them to stay puffed up against their own gravity. To collapse into stars, clouds have to cool. It turns out that even a sprinkling of heavier elements produces a powerful cooling effect. As these “metals” get jostled in a warm cloud, their electrons absorb energy, jumping up in energy levels. Those electrons then lose that energy by emitting light at specific wavelengths – “signature” photons that are different for every element or molecule. Those photons quickly escape the cloud, taking energy with them and helping to cool things down.

So as a metal-rich giant molecular cloud begins to contract under gravity, it can shed its thermal energy quickly – and that includes the extra heat that builds up due to its increasing density. Unimpeded by pesky thermal pressure, the cloud collapses quickly. In fact any over-dense lump within the cloud will itself collapse, causing the cloud to fragment. This occurs until whatever weak thermal pressure remains can halt the freefalling gas. After that point the contraction is much slower and those cloud fragments become stars. But without metals to help cooling, a giant cloud of pristine hydrogen-helium gas can’t shed its heat quickly enough. Thermal pressure kicks in much earlier to slow the collapse, before much of the fragmentation happens. Pressure and temperature have time to equalize across the cloud before it breaks apart. The result is much larger cloud chunks that evolve into gigantic stars. By the way, this sort of cloud fragmekntation is described by the Jeans instability.

Even generous estimates give these gigantic Population III stars only a few million years to live. In the gas-rich environment of the early universe, we expect there were violent waves of star formation followed by cascades of supernova explosions ripping through the first proto-galaxies. Those first stars changed the face of the Universe. They produced the first heavy elements that would someday become dust and new stars and planets and, well, us. They pumped out ultraviolet radiation which began the work of energizing – ionizing - the atomic and molecular hydrogen that filled the universe. This began the Epoch of Reionization, which saw the universe shift from being a hazy, nearly opaque fog of hydrogen gas to a crystal clear hydrogen plasma.

These enormous stars are also thought to have left behind enormous black holes when they died. In fact it may be that stars greater than 250 solar masses can collapse directly into a black hole without exploding. Clusters of giant stars become clusters of giant black holes, which in turn would merge into monsters of thousands or tens of thousands of solar masses. These were probably the seeds of the so-called supermassive black holes with millions to billions of times the mass of the Sun that we find lurking in the centers of galaxies. Such black holes power quasars, which themselves had a huge influence on the evolution of our Universe.

For purely theoretical objects, Population III stars sure were important. That’s why we keep trying to find them – or at least find evidence of what they were really like.

We have never seen a star that had zero metal content. Now it may be that there were some smaller Pop-III stars which still live, and lurk among the younger populations in disguise . In their long wanderings through the galaxy they may have collected enough dust in their atmospheres to disguise themselves as the younger generations. They may also have churned up the heavy elements that they produce in their own cores to enhance their metallicity. But the smart money seems to be on Pop-III stars being long gone.

When we look out into the universe as far as our telescopes can see, we do see primitive-looking galaxies shining out from the earliest of times. They radiate intense light at a signature ultraviolet wavelength of hydrogen. It’s hard to make sense of this light unless there are a ton of Population-III stars in those galaxies. But the evidence is still circumstantial. The hunt continues for the first stars in the universe. They may have raged for only a cosmic instant in the beginning of time, but their influence is still felt across the reaches of space time.