Variable stars

[Outbackmanyep]

Forgive me for asking such a stupid question....

Why does a star expand when it runs out of stuff to burn?
And what causes a variable star to expand and contract?

I can understand a massive star going supernova from the lack of radiation pressure when the reactions stop at the core when it produces iron, and gravitational pressure wins the day....but what are the processes involved when a variable star expands and contracts?

I see in one book i have that says this "As the central core of the star contracts, it releases gravitational potential energy, which is absorbed in the surrounding envelope, thereby forcing the outer part of the star to distend greatly"

Where does the gravitational potential energy come from?
How does the absorption cause the outer envelope to expand?

Intriguing!

Any thoughts?

[robagar]

the collapsing core must heat up as its volume decreases and pressure increases by the rules of gas thermodynamics. I wonder if the radiation emitted in the process is enough to heat up and expand the outer layers?

[robagar]

btw about potential energy - imagine two things stuck together by gravity. If you pulled them apart you would use up some energy. But when you release them you get the stored "potential" energy back.

[Doug]

can't say I know that much about variable stars, been looking at rr lyrae types lately and they are the senior citizens or the celestial set. population 2 stars meaning old, they are running on low hydrogen reserves. This causes them to cool down, and in so doing, they contract, which causes an increase in internal temp. They must then expand in order to radiate the incresed heat energy or else explode. When they expand, they loose the excess heat because there is insufficient fuel to maintain that higher level of radiation. This in turn cuases them to again shrink, repeating the cycle. When the star expands and contracts its luminosity also cycles, but I think it is not what would seem logical at first. The larger size is also the brightest, not the coolest.
for a better explanation google 'rr lyrae variables'

Doug

[Astrolabe]

Hi Bob

I could not resist you signature and I envy the fact that you are lost in Calabi - Yau space as not many us can enter this space intellectually. If your position is taken literally, it will make Alice in Wonderland a mere walk in the park. I am still struggling with the tensors. If you were in Sydney I would like to buy you lunch and discuss rigorous geometries. In any case if you are in Sydney lunch is on me.

This hobby of ours is great as you can go from griding mirrors, to indulge yourself in a visual feast of astronomical objects, to spacetime geometries to cosmology and to exotic superstring theories.

Best regards

George (astrolabe)