Fusing 1 kilogram of helium into carbon by the triple alpha process yields only 60 trillion joules, less than 10 percent as much. Thus, as the star's core becomes hotter, and the fusion reactions powering it become less efficient, each new fusion fuel is used up in a shorter time. Thus, giants are supergiants are rare compared to main sequence stars of the same mass. Eventually, every star runs out of fuel for fusion. The energy content of the star drops as it continues to radiate photons into space.
Pressure drops in the core.
The star collapses into a black hole.