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Welcome back, everyone, well, now we're able to answer the question that we
started off with in this section of our class, do stars last forever?
And as we've seen, the answer is a definitive no, they don't last forever.
Stars have life stories, and what we've seen is that the fate
of a star depends very much on the mass that it starts with.
So let's just go from low mass stars to high mass stars, and
remind ourselves of what that fate is like.
So for very, very low mass stars, what we have seen is that, well, they're kind
of hard to find, and they're difficult to study because they're hard to find.
But we do know that their nuclear burning is going to go very slowly, and
that they were predicted to live trillions of years,
at least 100 times longer than the current age of the universe.
So these stars will be burning, though not very brightly, for
many, many trillions of years to come.
So they will die, but none of these stars have yet died.
For low and medium mass stars, stars like the Sun,
what we've seen is that after expending their hydrogen gas and
turning into helium, eventually, they swell to become red giants.
The cores contract, the cores eventually are able to start burning helium
into carbon, fusing helium into carbon, allowing them to live a little bit longer.
But eventually, even this fuel runs out.
The stars swell up one more time, to what's called an asymptotic
giant branch star, blow off their outer layers in very slow but
powerful winds, exposing, eventually, the core, what's left over,
the inert carbon core, with just a little bit of nuclear burning on the top.
That material drives very fast, though lighter, winds,
which create the beautiful planetary nebula that we've seen.
And then eventually, the core just dies as a white dwarf,
just fades away as the inert carbon core is
forced to use quantum mechanical pressure, degeneracy pressure, to support itself.
For massive enough stars, above eight solar masses on the main sequence,
we've seen that they actually will be able to start generating energy by burning,
by fusing carbon.
And there's a whole sequence of elements that we burn,
all the way up until we create a iron core.
And once iron is in the core, that's pretty much the end of the story.
And these stars then, once there's iron in the core,
you can no longer create energy via fusion.
The support for the star runs out, very quickly, the star collapses on itself.
The neutrons that are produced in the squeezing of the core, or the neutrinos,
excuse me, produced in the squeezing of the core contribute to
a massive explosion, called a supernova.
And what's left over, depending on the mass of the star that we started off with,
whether it's 10 solar masses, or 50 solar masses, or 100 solar masses,
you'll either end up with a neutron star, which is very much like a white dwarf,
a star that is supported by quantum mechanical degeneracy pressure.
For low enough mass of stars, that neutron star will be stable and
will be able to exist for, essentially, forever.
And when they are spinning, when they are born, usually, rapidly spinning,
these things are called pulsars.
But for very massive stars, those cores are going to have too much mass, and
even neutron degeneracy pressure isn't going to be able to support them.
And they will collapse to form a black hole.
So for the most massive stars that form and that exist on the main sequence,
we expect all of those to turn into black holes.
The lower-mass massive stars will produce neutron stars.
Stars like the Sun produce white dwarfs, and then the very low mass stars will also
eventually produce white dwarfs, but none of them have even reached that point yet.
So there you go, now we've answered two very important questions in our study
of the universe, and now we're ready to move on to the next one.
Adam FrankProfessor
