An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.
Supernovas and Neutron Stars
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Confronting The Big Questions: Highlights of Modern Astronomy
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University of Rochester
272 classificações
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How do Stars Evolve and Die?
The Life of Stars - Star formation, fusion and stellar middle age, black holes and stellar death-
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Adam FrankProfessor
Physics and Astronomy
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.
Okay, so what happens after the supernova?
What's left over? So,
we've blown off the outer layers of a star,
but is anything left, inside or at the core?
And, for in some cases, for the
lower-mass massive stars, there is something left over.
And it, what's, what's called a neutron star.
It is a star, now, where just like with
white dwarfs, we saw that electron quantum mechanical pressure.
The movement of electrons due to quantum mechanics squeezing everything in
Is what kept the star supported.
In a neutron star, it'll be the quantum mechanical motion of
the neutrons that keep the, neutron star supported against its own weight.
But this star is incredibly, this core, this corpse is incredibly dense.
In fact a, solar-mass neutron star would be about the size of Manhattan.
So think about that.
You've squeezed all of the mass in the
sun into a, something the size of Manhattan Island.
So a teaspoon of a neutron star would weigh as much as a skyscraper.
So these, this is matter under unbelievably extraordinary conditions,
conditions which we just don't see under, in everyday life.
The gravity is so powerful on a neutron star that if
you were to drop a marshmallow from the height of, you know,
from a few meters high, it would generate the same energy when
it hit the neutron star as, you know, many, many atomic bombs.
so, you know, this is a really,
this is matter in, in, under extreme conditions.
Now, what's interesting about, neutron stars is that since most
stars have some spin to them, and we've already talked
about the idea of conservation of angular momentum, where if
something is spinning and it shrinks it has to spin faster.
What we expect with neutron stars is that they are born spinning very rapidly, and
we have good evidence of this because
we have found these extraordinary objects called pulsars.
And these are,
essentially neutron stars that have very strong magnetic fields, and
once again we see magnetic fields playing a strong role.
And the spin axis of the neutron star and the magnetic axis are misaligned.
And charged particles, which are forced to move along the magnetic axis,
actually create radiation which then gets beamed around as the, neutron star orbits.
And so we call these pulsars because, from a distance,
if that beam of radiation is actually pointed at the Earth,
we see very rapid, very regular pulses of radio energy hitting us.
The rapid ones are moving at milliseconds.
The more, the ones that have slowed down, because
they slow down over time, are, have pulses every
second or so, and these are, you know, these,
neutron pulsars are really remarkable, examples of extreme physics.
And we've been able to use them to,
to, do everything from, understand, matter at very extreme conditions or
also learn a lot about general relativity, Einstein's most important theory.
So pulsars are a really, fascinating examples of,
physics at the edge of, of our, you know,
physics that's really telling us something, about the edge
of our understanding of matter and space and time.
But now, our next step to go is to ask
our questions.
What happens if, a star is so massive that even a neutron star doesn't form?
That the mass, the gravity of what's
left overwhelms even neutron quantum mechanical pressure.
What happens then?
The answer is a black hole, and that leads to
some of the most interesting physics that we can talk about.
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