[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?