Why don’t stars collapse?
Limiting case:  Assume no nuclear fusion, only energy source is gravity.
Star is “almost” in hydrostatic equilibrium
Star radiates energy:  If nothing else happened T would drop, P
would drop, star would shrink.
Star does shrink, but in doing so gravitational energy is converted to
heat, preventing T from continuing to drop.
In fact, since star is now more compact, gravity is stronger and it
actually needs higher P (so higher T) to prevent catastrophic collapse
As star shrinks, ˝ of gravitational energy goes into heating
up star, ˝ gets radiated away
Rate at which it radiates energy, so rate at which it shrinks,
is limited by how “insulating” intermediate layers are