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?