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Welcome back, everybody.
So in our last lecture, we talked about how stories evolved off the main sequence.
What happens when they stop burning hydrogen?
But now for all of the, whether it's intermediate or low mass star or high mass
star, we always reach a point where pretty much there's nothing left to burn.
You can't do any more burning.
So for low mass stars, again, and we define that as being,
actually we call these low and intermediate mass stars,
anything that has less than eight solar masses on the main sequence.
The final state of the star is called a white dwarf, and
these are basically the corpse of stars like the sun.
This is what the eventual fate of the sun is going to be.
So what a white dwarf basically is, is it's the carbon core
of the star of the low intermediate mass star with some gas around on its edges.
Now, there's no nuclear burning going on anymore.
You would say, well, why doesn't gravity just completely crush this thing in?
How can it hold itself up?
So in order to understand this, what we need is quantum mechanics.
And one of the most important principles in quantum mechanics is what's called
the Heisenberg Uncertainty Principle.
And what comes out of this is, the idea that you can never know certain
parameters, certain quantities in physics exactly at the same time.
So for example, if you were able to somehow exactly pin down
a electrons location, then we'd be completely uncertain about its velocity.
Likewise, if you could know its velocity completely,
we'd be completely uncertain about its position.
Now this may seem very weird to you,
but these are just the laws that the universe has been built with.
This is what makes quantum mechanics weird.
Now why is this important for a white dwarf.
Well, what we've done is we've taken all of these carbon atoms, and
we've squeezed them down to just enormous densities.
Densities so much that a thimble-full of a white dwarf,
would weight as much as a Volkswagen Bug.
So, what we've done, we've squeezed this material so strongly,
that basically its location has been very firmly pinned down.
But that means that, its velocity is very uncertain.
Meaning, that it has a range of velocities.
And the quantum mechanical motion of the material inside the white
dwarfs that actually provides support against its own gravity.
Now a star like the sun, when it becomes a white dwarf,
will basically be the size of the Earth, which is pretty compressed for a star.
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So there are fundamentally quantum mechanical and
they are, the surfaces are very hot, 100,000 degrees or so.
But because there's no nuclear burning going on,
that surface will eventually cool down.
So once a star becomes a white dwarf, it's on a long, slow track to becoming a black
dwarf, to becoming something that's so faint, that it'll essentially become
invisible to telescopes, or very difficult to detect, very faint and very cool.
This takes billions of years, however, so
this is the eventual fate of a star like the sun.
Adam FrankProfessor
