PX282 - E5a - post-main sequence evolution

a helium core and a hydrogen-burning shell
core exhausted hydrogen first as it has a higher temperature
the shell feeds the core with helium
core grows until thermal pressure cannot support it
the core is inert and isothermal with no luminosity coming out of it
from here, it is known that $L \propto \frac{d T}{d r} ⟹ \frac{d T}{d r} = 0 :$ isothermal
eventually, the core collapses and becomes degenerate, triggering the red giant branch
note: if $M > 2 M_{⊙}$ , $H e$ can fuse before degeneracy