[MUSIC PLAYING] 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 three.

And they are one of the most enduring mysteries in astrophysics.

[MUSIC PLAYING] 16 00:00:36,420 --> 00:00:39,690 The sun is a late-comer 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 metalicity.

Stars that formed will recently tend to have the highest metalicities, because they contain the dust of more stellar generations past.

We divide stars up into three populations.

The sun is a population one star, meaning 2% to 3% of its mass is metals.

And that's a lot.

Pop one stars formed the most recently, and are still forming today, typically in the disks of spiral galaxies.

Population two stars are metal pore, with metalicities around 0.1% or even lower.

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, which are ancient, dense islands of stars that orbit far out in the galactic halo.

Population three stars have no heavier elements whatsoever.

They were the first ever stars, shining in the first ever proto galaxies, born of the pristine hydrogen and helium gas that filled the universe soon after the Big Bang.

I'd like to tell you where they are 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 three generation for decades.

Yet they must have once existed.

We're starting to think they may be all long dead.

OK, so these things formed at the beginning of the universe.

Makes sense they'd all be gone 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 orangish K-type stars-- live for longer than the current age of the universe.

Star lifespan gets shorter the more massive the spar.

And I'll get back to why.

But 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 three.

They were gigantic-- all of them.

And every single one has long since burned out.

Before we get to why pop three stars were so large, let's unravel this whole lifespan thing.

Massive stars live fast, die young, and leave beautiful space-time 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.

OK, 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.

And by the ideal gas law, temperature increases with pressure.

So the cores of very massive stars are much hotter than our suns-- up to a couple hundred million Kelvin, versus the sun's 15 million K. Now, 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.

Now, burning through 10 times the fuel at 10,000 times the rate, compared to the sun, means its life is 1,000 times shorter-- only 10 million years.

Even the smallest population three stars would have had masses of at least several times that of the sun, while the largest would have been as much as 1,000 or more times the Sun's mass.

By comparison, the most massive lighter stars are, at most, a couple of hundred solar masses.

With masses that high, all population three stars would have gone supernova while the universe was still in its infancy.

So why do we think the first stars were so massive?

Well, based on our understanding of how stars formed, they must have been.

This is where we get back to that metalicity thing.

Stars form when vast clouds of mostly molecular hydrogen collapse under their own gravity.

Now, for that collapse to proceed, the pull of gravity needs to overcome the cloud's own internal thermal pressure.

Warm 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 when there's a metal-rich giant molecular cloud that begins to contract under its own 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 free falling gas.

At that point, the contraction is much slower, and those cloud fragments become stars.

But without materials 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 fragmentation is described by the Jeans instability.

Even generous estimates give these gigantic population three stars only a few million years to live.

And in the gas-rich environment of the old universe, we expect that 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, of ionizing, the atomic and molecular hydrogen that filled the universe.

This began epoch of re-ionization, which saw the universe shift from being a hazy, nearly opaque fog of hydrogen gas to the crystal clear and extremely diffuse hydrogen plasma that we see today.

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 around 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.

Now, 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 later evolution of our universe.

For purely theoretical objects, population three stars sure were important.

That's why we keep trying to find them, or at least find evidence of what they were really like.

But we have never seen a star that has zero metal content.

Now, it may be that there were some smaller pop three stars that still live.

In there 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 metalicity.

But the smart money seems to be on pop three 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, with a signature ultraviolet wavelength of hydrogen.

It's hard to make sense of this light, unless there are a ton of population three 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 at the beginning of time.

But their influence is still felt across the reaches