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PART 4:
The next one that some of you saw is about Stellar Evolution, Stellar Collapse. What
happens to very very massive stars. The idea is to get across several key points, the first
being that our Sun is a star, which many young kids don’t appreciate. Then the fact that
stars have a finite amount of fuel in them. They last for a long time. Our Sun has gone
for 5 billion years. It’ll go for another 5 billion years (ish). But there is a finite
amount of fuel, and something happens when they run out of fuel. And for stars that are
much, much more massive than the Sun, when the fuel runs out there’s a very spectacular
endpoint to the entire evolution, and what happens is that in stars, at the very center
to begin with, you have primarily Helium. The gravity of the star, the weight of the
outer layers of the Sun or any star pushing down on its center makes the center incredibly
hot. Hot enough so that these Helium atoms smash into one another and fuse to make, um,
sorry, the Hydrogen atoms smash into each other and fuse to make Helium and other elements.
And, eventually that Helium runs out. The Hydrogen runs out. And when it does, there’s
no furnace at the center of the star to provide pressure to hold the gravity up any more and
the star starts to collapse again until the center gets hot enough to fuse Helium to heavier
elements. For massive enough stars, that process keeps going on. You keep depleting the fuel
at the center, collapsing more, making the center really, really hot and dense, and making
heavier elements, all the way up to the point where the center of the star is pure Iron
gas. And beyond that point, when you fuse Iro together, you don’t get energy out of
it, and so it can’t support itself, and the whole star collapses very, very, very
quickly. And when it collapses quickly (We’ll see if I can make this work. I’ve done it
twice today. Once it went beautiful, once it just went random.) But imagine this being
the top layers of a star collapsing down because gravity is pulling toward the center of the
star, and there’s no pressure pushing back, cause the furnace at the center of the star
has gone out. So it’s going to free-fall down toward the center. The layers before
that are going to free-fall as well, until they squish themselves to the point where
they’re, you know, think of it as being all of a sudden as hard as a rock. So this
channel here, here’s the lightest layers of the star on the outside. You eventually
get to the core where it has to bounce. And what happens is this. The outer layers of
the star get flung off dramatically, and bounce all around the room in what’s called a supernova
explosion. Supernova explosions are incredibly bright. For a couple of days, the light from
a supernova explosion can outshine the light of the rest of the galaxy that the star is
in. And several really important things happen during that process, one of which is that
those materials in the center of the star, the Oxygen and the Carbon and the Helium and
the Iron and Silicon, all of these things that were built up in the center of the star
get flung out into space. And this is crucial because in the early Universe, the Universe
consisted mostly of Hydrogen and Helium. And that’s boring. Those are the simplest atoms
to make, which is why there were so many of them. But you don’t get the Iron in your
blood or the Carbon that makes up cotton that makes up t-shirts, which is why this says
that this t-shirt is made of recycled atoms. You don’t get those atoms until they’ve
been fused in stars. Okay? So supernova explosions are incredibly important for the evolution
of the Universe as a whole. And this goes on to explain that with telescopes, we can
actually see these explosions happen, and we see the aftermath of them, the big explosions
sweeping up material out into space. And it gives a couple of examples of explosions that
have happened recently enough that they’re in the historical records.