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.
In a stable star, the gas pressure pushing out from the center is equal with the gravity pulling atoms inward to the center – when these forces are equal, the star is at equilibrium. Once a star reaches equilibrium for the first time, it will start burning (fusing) hydrogen into helium.
