Final answer:
Equilibrium in stars refers to the balance of Gravity vs Pressure, known as hydrostatic equilibrium. This balance occurs during the main sequence stage of a star's life and is exemplified by our Sun's stable state.
Step-by-step explanation:
Equilibrium in stars refers to the balance of a) Gravity vs Pressure. Gravity always tries to collapse the mass of a star towards its center, and the mechanism that opposes this gravitational collapse is internal pressure, primarily due to nuclear fusion, which produces energy and thereby exerts an outward force. This state is known as hydrostatic equilibrium, where the inward force of gravity is exactly balanced at each point by the outward force of gas pressure.
During the main sequence stage of a star's life, there is a balance between gravitational forces and internal pressure, maintaining the star in a stable state. The Sun is a prime example of a star in hydrostatic equilibrium. It is neither expanding nor contracting significantly because the internal pressure balances the weight of its outer layers. The balance between these forces ensures the stability of such stars throughout much of their lifetime.