BRIAN: So Paul, you've shown us that if you
take the idea behind quantum mechanics, the Heisenberg Uncertainty Principle,
and you combine that with the ideas behind gravity,
that you can push things together with gravity,
and the Heisenberg Uncertainty Principle ultimately ends up pushing things
apart, you can explain something like these white dwarfs that we see.
These really massive stars.
PAUL: Yeah, we've solved one of the problems, the problem of how
you could make something that dense not collapse.
How it can actually support itself against intense gravity.
But there's still another problem, a nasty one.
Which is, we need this lump to be not made of hydrogen.
If it's hydrogen, it's got a nuclear fuse.
Most everything in the universe is hydrogen.
What isn't hydrogen is helium, which is almost as bad.
How could we get something that isn't hydrogen and helium, a dense lump
of the stuff, when that's not what most of the universe is made out of.
BRIAN: Well, I have an idea.
If we look at our sun, our sun is made out of primarily hydrogen and helium.
But of course it's being powered by the conversion of hydrogen to helium.
And then we believe, eventually, helium to things like carbon and oxygen.
So one could imagine that if we looked at a star like our sun,
not now, but later on, after it's burned through most of its nuclear fuel,
in the center you should have a reservoir of a bunch of stuff
which is like carbon and oxygen, or maybe helium,
depending on exactly how the nuclear processes work.
So that's a good place to look.
PAUL: OK.
So in the middle of a star, you're going to accumulate just what we want,
a large, dense pile of heavier elements.
But the trouble is, that's only ever going to be the core of the star.
In our own sun, it's only about the central half
a percent that's actually doing nuclear fusion.
Most of the rest is just this is incredibly thick blanket of hydrogen,
which is just there to press down on that central half
a percent to make it dense enough to undergo nuclear fusion.
Even when a star like our sun comes into its life.
It's still only going to be a little bit in the middle of those
turning to this type of stuff.
You've still got this huge shell of hydrogen around the outside.
How are you going to get rid of that?
BRIAN: Well, I think the best thing to do
is to go out and have a look around the universe.
When we look out at the universe, we see that all stars are not
exactly like our sun.
Some of the objects are much, much bigger than our sun.
Not in terms of mass, but in terms of size.
And we call these red giant stars.
And indeed, we believe our sun, in the future,
is going to turn into a red giant.
Because it turns out that the nuclear reactor in the center of our sun
changes over time.
Right now it's burning hydrogen into helium.
But in the future, that's going to exhaust that supply of hydrogen,
and it's going to start wanting to burn the hydrogen
in a shell outside of the core into helium.
And when it does that, it's going to become much more energetic.
And that's going to cause the outer part of the star to puff up to a size much,
much bigger than today, almost all the way out to where the earth is.
And so that red giant star, of course, doesn't solve our problem.
But that big puffy star, that gas is only barely attached now to the core.
Because the gravity is so much weaker.
PAUL: Yes.
If you look at images of red giants, or red super giants,
you see they're actually rather fuzzy edged.
It's very hard to get a picture of these things.
But all pictures you can get using clever optics
and similar techniques show that, in fact, it's not like our own sun,
with a nice sharp edge.
It's a very fuzzy edge.
A lot of stuff is actually being blown out of these things.
BRIAN: That's right.
Because they're not very stable.
And as I said, the thing that makes our sun round
is the fact that there's just so much gravity.
I mean, it's much-- the gravity's really pulling things together and making
that giant sphere.
Here things get extended, and this is one of the few stars
where you can actually see the star and how big it is.
And this leads us naturally to a time when, for example, the star
itself runs completely out of its ability
to fuse its hydrogen and helium into heavier elements.
And then what's going to happen?
The center of the star is going to be overcome by gravity.
And it's going to want to come together until it is-- reaches
that magical point where that quantum mechanical pressure pushes
back and stops it.
But there's going to be a lot of energy in that process.
You have this weakly attached envelope of stuff,
and it's likely to get blown off a little bit into something
you might see like this.
A planetary nebula.
Possibly my favorite object in the cosmos.
And you can see right in the center, you've
got a white dwarf, a really tiny star that's
mass that we've been talking about.
And then you've got all this junk that has sort have been blown off.
Not really violently, but kind of gently,
to form this amazing neon sign in the sky.
PAUL: Yes.
And there's a lot of beautiful pictures of these things.
Here are some taken by the Hubble Space Telescope.
BRIAN: That's one of my favorites.
PAUL: This is lovely, yes.
What's lighting this all up is the ultraviolet light
from the white dwarf in the middle.
The core in the middle is, as you said, very small, very dense, and very hot.
Because it's just come out of the middle of a star.
And so it's emitting ultraviolet light, which
is coming out and zapping the gas around it, knocking
the electrons up the energy levels.
As they fall back down again, they emit all these lines we see.
So they're absolutely beautiful things.
But we're seeing that, in fact, most of the mass of what was in the middle
here, the star may be much like our own sun, and it's lost most of its mass.
Most of the mass has been blown out in these cataclysmic last few days, weeks,
months, years, leaving just behind the core.
so maybe we can, in this method, get rid of all this huge blanket of hydrogen,
and give us a core.
BRIAN: So yes, these things litter the sky.
But they don't last very long, so it's quite interesting.
They're not rare in the sky.
And for astronomical objects, these planetary nebula really only
have lifetimes of order 100,000 years or so.
So they are short-lived.
And that tells you that it's not an unusual phenomenon.
Almost every star that's born is going to end up
producing one of these at some point, to explain how many
that we see across the sky.
PAUL: But then after their 100,000 years or so is over,
the nebula has faded away, you're still going to be left with a white dwarf.
It's not got any power source anymore, it's
not actually generating any energy.
But it's got a lot of heat inside it, because it started off
at a very, very high temperature.
And it will just slowly cool down as it radiates the heat.
So it might start off at a very high temperature.
As time goes on, it'll get a little bit cooler,
a little bit cooler, a little bit cooler.
So it might start off as a very blue colored dwarf,
and then become a bit more white, and eventually might end up sort of yellow,
or even red as it cools down.
Though probably the universe isn't old enough
for them to have reached that stage yet.
BRIAN: It's sort of a cooling ember, a rock that you heat up in a fire
and then you know that if you grab that rock, very soon afterwards,
you're going to burn your hand, because it's still hot.
And so-- but it cools over time.
And because things that are hot glow, you can see these things.
And they literally litter the sky, these white dwarfs,
and it's a beautiful process that makes them.
PAUL: So, pretty amazing things.
This white dwarf, the whole mass of a sun
in the size-- it's only the size of a planet.
The incredible density, the incredible gravity,
the combination of quantum mechanics and gravity.
They're amazing things.
But they're not really all that violent.
And this is a course on the violent universe.
Or are they?
Well, that's what we're going to talk about next time.
