Welcome back, everyone.
So, in our last lecture we talked about the fate
of stars like the sun, what their corpses look like.
Now we're going to talk about the corpses of
massive stars, not all massive stars, and we're going to
have to talk about this, but certainly the lower
mass of the, the lower-mass kind of massive stars.
so, if you recall when our discussion of how stars, massive stars evolve, what
we figured out was that, that they go through a sequence of nuclear burning.
They're able to burn hydrogen to helium,
helium to carbon, carbon to neon, et cetera.
All the way up until iron.
So once you have produced a significant iron core, the jig is up.
And what happens now?
Well, remember that a star is always at war with itself.
There's always a battle between the energy flowing outward via nuclear fusion and the
gravity which is pushing inward, or pulling
inward that wants to crush the star.
And once the iron core is there, the jig is up,
and there's not much more that can happen, and the core, or
the entire star, has to collapse on itself.
So now, instead of millions of years, the star is going to be evolving on the
order of milliseconds as this, all of this
outer material comes raining down onto the core.
And the core itself is going to collapse.
Now as the core collapses, the core squeezes ever tighter.
Interesting quantum-mechanical transmutations occur, where
basically the protons and electrons
in the core, in the, in the iron nuclei, will actually,
com, combine, will transform into neutrons.
And, you need to do this because neutrons have
no electric charge, and the only way to squeeze
material closer together, as, as has to happen because
of the collapse, is by getting rid of electric charge.
So, when protons and electrons combine to
form neutrons, they also have to form neutrinos.
They also, there's another particle that comes out.
Neutrinos we've talked about when we talked about the sun.
They are these ghostly particles that
basically have to, can travel through enormous quantities
of material and not even notice it's there.
So enormous amounts of neutrinos are produced during
the, this transmutation as the core is shrinking.
And what's amazing, is that the, this, this shrinking core of the star
is, in fact, so dense that now
even neutrinos are, start interacting with matter.
And it is, in fact, the pressure of
the neutrinos that actually will cause this collapsing
star, to blow off its outer layers.
So what you end up with is what's
called a supernova, an enormously bright, enormously powerful explosion.
It is the most energetic phenomenon in the universe after the big bang itself.
And what happens is, is the, neutrino pressure
is driving a shock wave through the material
which is taking the material that was falling
inward and turning it around and blowing it outward.
The speeds of material that, that, the speeds that can be reached here are
tens of thousands of kilometers per second.
And all this material, and all of the highly processed elements that have
been created by nuclear fusion inside the massive star are now being blown outwards.
So one of the important roles of supernovae, is to
actually take things like neon, and sulfur, and silicon, et cetera.
You know?
Enriched elements, and to seed them throughout the, the universe.
These explosions are very powerful.
Because without these supernovae, we wouldn't have many of the chemical
elements that we need in order to to have life for example.
So supernovae are not only these immensely powerful explosions, but they
are also the means by which the galaxy is seeded with
heavy elements, so it's important to understand that every element larger
than iron in the universe today was formed in a supernova.