[MUSIC PLAYING] 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. [MUSIC PLAYING] 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.
I appreciated the explanation of why metalicity affects stellar size. I've seen it said often but hadn't seen the reason explained.
From about 6mins to 8mins, there is a lot of really cool galactic cloud visualizations - I was hoping some people in here might know how they're made and may be able to point me towards some good papers/github profiles.