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MATT O'DOWD: This episode is sponsored by Audible.
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.
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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
of space-time.
A big thank you to Audible for sponsoring today's episode,
and also for making it possible for me to research space-time
while riding crowded New York subways.
I'm currently reading "Sapiens," by Yuval Harari.
It talks about how homo sapien's domination
over their other human cousins, like Neanderthals, etc.,
may have been due to our unique ability to invent and believe
fictitious ideas, like religion, and nations, and money.
Go us.
Check it out for free if you like.
Audible.com/spacetime gets you a free 30-day trial.
In a recent episode, we talked about how humans might
evolve if we migrate to Mars.
You guys had a lot of opinions on this one.
Better & Better points out that martian humans
may evolve their own microbes that would be deadly to humans.
Yeah, and that's without question .
They definitely would.
This would only increase the isolation,
and perhaps speed up the evolutionary divergence.
A few of you suggested that humans
are essentially immune to evolution
due to our powerful control over our environments
and our modern medicine.
And this is a pretty common misunderstanding.
Firstly, a trait doesn't have to kill you or save your life
to be subject to natural selection.
All it has to do is change your odds of having children.
It doesn't even have to be a huge effect.
Over many generations we see that there
are shifts in populations that select traits
that are either slightly more advantageous or even just
fashionable.
This leads to slow divergence in separate populations.
Regardless of the quality of our medical care,
mutations keep happening.
It may be that physical environment
is no longer as strong a force in driving natural selection.
But that won't allow us to maintain a steady genome
without direct manipulation.
At the very least, with the removal
of environmental pressures, the passive evolution
that has maintained certain traits is impacted.
In fact, as SalemSays, and several of you,
point out that direct genetic manipulation probably
will happen.
We may take evolution into our own hands,
with some very unnatural selection.
That's not necessarily a bad thing, as long
as we get it right.
But I didn't want to get too far into it,
because it really is a whole can of worms.
And it's also pretty hard to predict
what people will choose to do with that sort of technology.
David Webster asks whether there's 4G coverage on Mars.
Sadly, no.
It's still only 0.4 g.