So the other day I was shopping for some headlamps
for my car and I thought it would be really
funny to put some of those super expensive
xenon headlamps into my eleven year-old car.
I did a little bit of research and it actually
turns out that those headlamps are not compatible
with my car. But that's okay because they're
really expensive. But why are they so expensive?
I must say I think that's a perfectly reasonable
question. Of course it's a reasonable question,
it's an excellent question. How does a person
answer why something happens? Okay, well,
one of the reasons it might be so expensive
is there's not a lot of it in the Earth's
atmosphere. But why is there not a lot of
it in the Earth's atmosphere? Well, in order
to answer that question, we have to look at
a bigger picture and see what happens when
stars die, and that's the story that I want
to tell you today on Once Upon A Spacetime.
Once Upon A Spacetime, there were two gas
clouds, stuck in an epic battle between gravity,
trying to pull them in and make them collapse
in on themselves, and radiation pressure,
trying to push them out. Eventually, one day,
gravity advanced enough that hydrogen atoms
in the center were able to fuse to form helium.
And at this point, our two stars were born,
each about 15 times the mass of the Sun. As
the stars grew up, they went through their
hydrogen, turning it into helium, each time
releasing a little mini-explosion. Throughout
their lives, they were stuck in this battle
between gravity and radiation pressure from
the explosions. But so long as they were able
to generate enough outward pressure, they
were able to keep from collapsing in on themselves,
and were in hydrostatic equilibrium. But as
the hydrogen gets used up, gravity starts
to take over, causing the cores to contract
and when this happens, the temperature increases,
and even heavier things can form: carbon,
neon, silicon, heavier and heavier each time,
and at each step, whatever gets made gets
used up as fuel, faster and faster each time
until it reaches iron. And at this point,
fusion can no longer occur. And this marks
the death of the star. With the end of fusion,
there's no longer enough support against gravity,
and the stars start to collapse from the inside
out. The cores get so hot and dense that atoms
get stripped away, and neutrinos are produced,
carrying energy away. The cores become as
dense as the nuclei of atoms. Material from
further out in the star falls in and bounces
off of the solid core, pushing what still
remained further out in the star out in one
of the largest and most energetic explosions
in the universe. The stars have gone supernova,
and all in a fraction of a second.
What remained were stellar corpses, made up
of material tightly packed together at nuclear
density, held prisoner by the force of gravity.
Imagine, if you will, the mass of more than
300,000 Earths, wrapped up together in a tight
little ball about as wide as a city. Billions
of years later, these two neutron stars would
meet each other again, brought together by
the same force that created them: gravity.
They danced around each other, spinning faster
and faster each time, until eventually, they
crashed together.
Now I want to take you even deeper into these
neutron star mergers and follow the story
of characters tinier even than the atomic
scale. When neutrons and protons are together,
they form an atomic nucleus. And just like
the hydrogen nuclei that formed out stars
in the beginning, these nuclei interact with their
worlds in different ways. They can add neutrons
and protons and get bigger, or they can decay
and become smaller. But one of their main
goals is to be stable and live in the Valley
of Stability. On one side of the Valley, nuclei
have too many protons, and are proton-rich.
On the other side of the Valley, nuclei have
too many neutrons, and are neutron-rich. For
light nuclei, living in the Valley usually
means having about the same number of neutrons
and protons. But in the upper regions of the
Valley, this tends to shift, and stable nuclei
tend to have more neutrons. A single neutron
is only stable for about 15 minutes, and at
that time, it decays into a proton, electron,
and anti-electron neutrino. And in a similar
way, a nucleus that has too many neutrons
can get to the Valley by turning some of its
neutrons into protons. And the further away
from the Valley this nucleus is, the faster
it is going to do this. But there once was
a proton that found itself surrounded by neutrons,
caught in the outflows of a neutron star merger.
The neutrons latched onto the proton, forming
a heavier and heavier nucleus, and doing this
so fast that it forced the proton to stray
far from the Valley of Stability. But for
every couple of neutrons that latched onto
the proton, it was able to turn some of those
neutrons into protons. Thus the nucleus, now
no longer a single proton, but still very
neutron-rich, was able to grow into heavier
and heavier elements. But every step closer
to stability meant more steps further
away, keeping it from the Valley as the nucleus
grew to be more and more neutron-rich.
As the seconds went on, turning into minutes,
the onslaught of neutrons lessened, but by
this time, the nucleus was already quite heavy:
it had become an actinide. And at this point,
a new path opened up. The nucleus could fission,
undoing what its ancestors had done in the
stars billions of years earlier. It could
split into two parts. Then from there, since
the free attacking neutrons from earlier were
now locked up in other nuclei or were spread
out enough, the daughter products from this
fission could find their way back to the Valley,
easier than they could before.
Thus our actinide nucleus was able to split
into two, and those two daughter products
were able to decay their way back into the
Valley, and they were able to form the region
where xenon lived.
So currently scientists are trying to understand
all of the different complicated and messy
parts that go into actually making this process
happen. But for the purposes of our question,
it seems like we actually don't need that
much. We need a couple of stars to live out
their lives and then die in some of the most
explosive, energetic events in the universe;
we need a couple billion years, and we need
a whole lot of neutrons. That's only like
three things. So I know before I said that
those xenon headlamps seemed kind of expensive,
but in retrospect, they don't actually seem
that bad!
