I'm reading about the sub giant branch (SGB) and the evolution to the red giant branch (RGB).
On the SGB stars have burned all hydrogen into helium, as a result, they have an inert helium core. The core begins to contract and the pressure increases in order to re-establish hydrostatic equilibrium. my question is, how? Nowhere have I found an explanation as to the reason behind the pressure rise in the core, so, I have tried to come up with my own; I just want to know if I am right or wrong.
Due to the inert core no energy is produced to counter gravity, so gravity begins to take over and the core contracts. For some reason or another the pressure of the core increases to counter gravity and regain hydrostatic equilibrium. This also causes the temperature of the core to rise and move hydrogen out into a thin shell surrounding the core, where it continues to burn.
I understand that degenerecy sets in when the star reaches the RGB and not quite at the SGB. Its difficult for me to understand where the energy comes from to counter gravity in the core.