The internal structure of stars is set up by hydrostatic equilibrium,
the balance between gravity trying to crush the star down small and
pressure trying to make the star blow away.
When the two are in balance, hydrostatic equilibrium,
the star is structurally stable.
It's hard to demonstrate that in the laboratory because I can't get a big self
gravitating ball of gas easily into this room.
You have to be a star or a very large planet like Jupiter.
But we can use a simple stand in, a simple toy balloon.
This is a demonstration of a kind of hydrostatic equilibrium.
But now instead of gravity, we're gonna use the elastic of the latex that
the balloon is made out of for the force that wants to crush it in.
The analogy works as follows.
You take a balloon and you begin to inflate it.
[NOISE] By blowing into it.
As I increase the pressure and the amount of gas inside, [SOUND] the latex expands,
but it stops expanding when the force of the latex which wants to make
it collapse exactly balances the force of the air pressure inside.
If I let out some of the air, [SOUND] the latex can relax to a smaller size and
achieve a new equilibrium.
When I put a lot more air in, [SOUND] the latex can
expand a great deal, achieving a new equilibrium.
This is pretty much the same way that gravity and
pressure equilibrium would work in a large star.
But of course in the latex balloon, I'm cheating a little bit.
Because I'm changing the amount of air inside the balloon.
What if I leave the amount of air inside the balloon fixed, by tying it off.
At this point the equilibrium is maintained even if I press on it from
the outside, the balloon deforms to maintain a new equilibrium.
Stars can do the same thing.
If a star is in hydrostatic equilibrium and I set it rotating,
it will deform until it achieves a new equilibrium.
But that's not the only way to change the pressure, putting air in or
taking air out.
I can be radical about it and
puncture the balloon, or we can change the pressure of the air inside the balloon.
If I inflate this balloon inside of a nice warm room like this, and
were to walk outside on a cold winter day, the balloon would begin to shrivel,
because the gas pressure inside the balloon would drop.
And the elastic can cause the balloon to collapse achieving a new equilibrium,
it also works the other way.
If I inflate a balloon outside when it's well below zero, wait for
it to equilibrate with the outside air temperature and bring it in the house.
If you inflated it to just below bursting outside it might actually pop
when it comes into micro equilibrium with the warm house.
Unfortunately it's a nice spring day outside, it's about as hot outside as it
is in, so we're going to have to do something a little bit more radical.
And what we're gonna do for radical, is I brought some liquid air.
This is liquid nitrogen,
liquid nitrogen has a temperature of nearly 200 degrees below zero.
Minus 196 degrees Celsius for those who want to keep track of things.
And I'm gonna fill up this pie plate with some of this liquid nitrogen.
You can see it bubbling away, liquid nitrogen is a very clear liquid.
It evaporates almost immediately back into nitrogen gas in the room.
We're gonna let it bubble and burble here a bit.
Now you can see the pie pan coming into equilibrium,
as the wave of boiling ceases.
So I have,
in one hand a balloon fully inflated with air at around room temperature.
It's probably about 30 degrees Celsius in here under these hot lights.
In the pie pan, liquid nitrogen at a temperature of minus 200 degrees Celsius.
If I place the balloon in the nitrogen and begin to cool it,
I'm gonna roll it around to more equally even out the effect.
We begin to see the air temperature drop inside the balloon.
I'm handling this very gently because nitrogen is very cold.
And the balloon deflates.
When I take it out and hold it up in front of the hot lights,
you can see the balloon will slowly start to inflate back,
eventually coming up to the fully inflated size we saw before.
I can just set it in nitrogen and leave it to set for a while.
This is the same way it works inside of a star.
If the energy generation inside of a star runs faster,
it causes the gas to be hotter.
As the gas is hotter, the internal pressure of the star begins to heat up.
And it begins to expand.
If the internal energy of the star makes less energy than is needed, the envelope
with the star is less hot, the pressure drops, and the star collapses in response.
I can make this balloon by getting it colder and
colder, eventually collapsed almost all the way down.
And so if I drop the temperature down dramatically.
This is dramatically as I'm going to get.
I can see pretty soon the balloon becomes really floppy and loose.
Because the air pressure inside is now dropping a great deal.
And if I then take it out of the nitrogen.
Move the nitrogen to one side.
Sat down the balloon and
if I wait just a little while, the balloon will begin to inflate.
There's a cheat here of course, the elasticity of latex goes down as it gets
colder but this is one of the ways we make this demonstration really effective.
So, hot air, expand.
Cold air, deflate.
The same thing happens in a balloon as happens inside of a star.