What mechanism immediately takes dominance in a star after core hydrogen fusion ceases?

Answer

Gravity asserts its dominance over the star’s massive outer layers

Once the fusion furnace in the core sputters due to lack of hydrogen fuel, the outward pressure ceases, allowing gravity to begin contracting the now inactive helium core.

What mechanism immediately takes dominance in a star after core hydrogen fusion ceases?
starstellar evolutionexpansionhydrogen fusionhelium fusion