In the context of stars, what defines the equilibrium state?

The equilibrium state of a star is defined by the balance between the inward force of gravity and the outward pressure generated by nuclear fusion occurring in its core. In hydrogen-burning stars, such as our Sun, fusion in the core creates enormous amounts of energy, which produces an outward pressure. This pressure counteracts the gravitational pull trying to collapse the star inward.

When these two forces are perfectly balanced, the star is said to be in hydrostatic equilibrium. This state ensures the star remains stable, neither expanding uncontrollably nor shrinking. If fusion pressure exceeds gravitational pull, the star could expand, and if gravity overrides fusion pressure, the star would begin to contract.

This fundamental balance is key to the life of a star and plays a crucial role in the various stages of stellar evolution.