Final answer:
Hydrostatic equilibrium is the balance between the star's internal pressure and gravity, allowing it to maintain stability against collapse or indefinite expansion. This equilibrium is fundamental to main-sequence stars like the Sun, where it is facilitated by energy production through the fusion of hydrogen into helium.
Step-by-step explanation:
Hydrostatic equilibrium in a star is the balance between the internal pressure of the star and gravity. In the interior of a star, these forces are in a constant tug of war, with gravity pulling inward and the star's internal pressure pushing outward. When these forces are perfectly balanced, the star is stable, preventing it from either collapsing under its own weight or expanding indefinitely.
In the case of main-sequence stars like our Sun, this balance is achieved through energy production via nuclear fusion at the core, specifically the fusion of hydrogen into helium. During this process, a considerable amount of energy is released, contributing to the outward thermal pressure that counteracts the force of gravity. The expression of this balance can be simplified as 4H → He + 2e + 2y + 2v+Q, where Q represents the energy that facilitates this hydrostatic equilibrium.