What reactions occur within a star and its stellar equilibrium

Fusion reactions.

Stellar equilibrium is also known as hydrostatic equilibrium. A star is in equilibrium when the pressures inside the star balance out the force of gravity. The pressure is thermal pressure resulting from the fusion reactions taking place in the star's core.

A star stays in equilibrium until the supply of hydrogen in the core is depleted. After that the star starts to collapse and heat up until further fusion reactions commence.