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.

 

[pmacias]

Hello and welcome to the world of astronomy! The formation and expansion of a red giant is a fascinating process that occurs in the later stages of a star's life. Let me try to explain the process in more detail.

A main sequence star, like our Sun, is powered by the fusion of hydrogen into helium in its core. This fusion process releases a tremendous amount of energy, which creates an outward pressure that balances the inward pull of gravity. This balance between the two forces is what keeps the star stable and shining for billions of years.

However, as the star uses up its hydrogen fuel, the balance between gravity and the fusion process is disrupted. The core of the star starts to contract due to the pull of gravity, which increases the pressure and temperature in the core. This increase in temperature causes the remaining hydrogen in the outer layers of the core to start fusing into helium, creating a shell of fusion around the core.

This fusion process in the outer shell releases a lot of energy, which heats up the outer layers of the star. This is why a contracting core leads to heating of the outer shell of the star. As the core continues to contract, more and more energy is released, causing the outer layers of the star to expand and cool, leading to the formation of a red giant.

Now, as for how the energy released by the contracting core gets to the outer shell, it is through a process called convection. Convection is the transfer of heat through the movement of hot material. In the case of a star, as the core contracts and heats up, the hot material rises to the surface and carries the energy with it, heating up the outer layers of the star.

I hope this explanation helps to clarify the process of how a main sequence star turns into a red giant. It's a complex and fascinating process, and there is still much we are learning about it. Happy stargazing!