Formation/Expansion of Red Giant

[Guni22]

Hello Physics Forums, complete astronomy noobie here. My question is about how a main sequence star turns into a red giant and starts expanding.

Reading from here http://en.wikipedia.org/wiki/Red_giant

"When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remains, initiating fusion of hydrogen to helium in the shell."

Why does a contracting core lead to heating of the outer shell of the star?

Edit: I see now that a contracting core is accompanied with a "release of gravitational potential energy". How does this manifest exactly? How does the energy released by contracting/collapsing the core get to the outer shell?

 

[Naty1]

I suspect what is being described is that things like electron confinement which occurs during collapse...when gravity overcomes nuclear generated heat...causes additional heating just outside the core where fusion/fission cane now begin. In other words, things just hot hotter so the shell gets hotter.

Such energy release due to gravitational forces is some 30 times greater than nuclear energy as a heat source says Kip Thorne.

In BLACK HOLES AND TIME WARPS Thorne describes the formation of various entities..black hole, white dwarfs, etc, and each has some unique characteristics...but he doesn't happen to describe red giants in detail.

 

[Drakkith]

When you compress a gas it heats up. I think that's all that's happening is that the gas is getting compressed and heating up as the star runs out of fuel. I would assume it's getting compressed and heating up faster than it can get rid of that built up heat, eventually leading to temperatures high enough for helium fusion.

 

[twofish-quant]

Why does a contracting core lead to heating of the outer shell of the star?

This is an excellent question and like a lot of excellent questions, there is no easy answer. When I took stellar evolution, my instructor said that one of the things that he has been looking for since he was a graduate student is an intuitive explanation for why red giants form, and that's also been something that I've been looking for.

Edit: I see now that a contracting core is accompanied with a "release of gravitational potential energy". How does this manifest exactly? How does the energy released by contracting/collapsing the core get to the outer shell?

The basic idea is that the star is supposed to be in pressure-gravity balance so if you make one part of the star contract, another must expand so that the system stays in balance.

If that seems like an insufficient explanation to you, then that's fine because I'm not terribly satisfied by it either.