Main Sequence Lifetime Recall that a lifetime can be represented as The luminosity is fueled by fusion of hydrogen, which comprises most of the mass of the star. The evolution of a star depends on the mass of the star. After H is exhausted in the core, fusion occurs in spherical shell around the now He-rich core.
Luminosity is up, temperature is down, and radius is up.
The core contracts during this time. The star becomes slightly smaller and hotter in such a way that the luminosity does not change much. The stars become Horizontal Branch Stars. He core changes to C. Not hot enough to burn the C, but there is fusion of He in a shell, and there is still fusion of H in another shell. The star is simply not massive enough to begin C-fusion, so it is basically dead.