# How do inert helium cores in sub giant stars create a pressure force?

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.

• The shell hydrogen burning should be enough to counteract the force of gravity, although there may still be come contraction. However, degeneracy pressure can play a big role. Commented Aug 6, 2015 at 14:52
• Why does the pressure of the core increase after it ceases to burn hydrogen though? Commented Aug 6, 2015 at 15:03
• There is still some shrinking, just not catastrophic shrinking. This can increase pressure (I believe, though I'm not sure). Commented Aug 6, 2015 at 15:03
• The answer is that pressure increases with density. Not sure what you're struggling with here? This is true for perfect gases (Boyle's law) or for degenerate gases. The phrase "no energy is produced to counter gravity" is meaningless, as energy and gravity are two completely different things. Commented Aug 6, 2015 at 15:08
• Ah, so as the helium concentration increases in the core the density and pressure increase. Also, apologizes for my sloppy writing - energy and gravity are indeed two different things. I meant to say that if the core is inert no force can counter gravity, but it can. What I think I have done here is missed out the fact that the pressure comes from the momentum of the helium atoms and inert means chemically inactive, not lacking momentum - in the case of chemicals. Commented Aug 6, 2015 at 15:25