# What makes a gas supported by electron degeneracy pressure isothermal?

I am trying to understand why the cores of red giants are isothermal. They become supported by degenerate electrons due to extreme pressures relative to the thermal energy in the core which forces the electrons to occupy the lowest energy states up to close to ionization energy. I just don't understand what makes the cores nearly isothermal.

Does the lack of temperature dependence allow the thermal energy to conduct throughout the core?

• You have the right answer for why the gas is isothermal, but beware of one thing-- your wording sounded like you believe that degeneracy is making the pressure high, but actually it is the released kinetic energy that makes the pressure high. All degeneracy does is make the temperature low, so it drives down the thermal pressure. So you're right the thermal pressure is low compared to the total gas pressure, but degeneracy only acts on the former, the latter is just from kinetic energy. Apr 7, 2017 at 1:15
• There is no such thing as pressure = degeneracy pressure + thermal pressure. It is just pressure caused by the momentum distribution of the particles, which is a function of (number) density and temperature. The statement about ratio of pressure to thermal energy (density) is correct: Roughly 2/3 for a non-degenerate ideal gas, and of order $(E_F/kT)^2$ for a degenerate gas. Apr 7, 2017 at 7:49