What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?

Gravitational equilibrium in stars is achieved through the balance of two opposing physical processes: gravity and pressure. Specifically, these are:

1. **Gravitational Force**: This force attempts to contract the star and is due to the mass of the star. Gravity pulls all of the star's mass inward, towards the center, causing it to collapse.

2. **Thermal Pressure**: This is the force that opposes gravity and tries to expand the star. It is generated by the energy produced in the star's core, primarily through nuclear fusion reactions that convert hydrogen into helium (in main-sequence stars). This reaction releases a tremendous amount of energy which increases the temperature and, consequently, the pressure within the star.

These two forces are in continuous opposition. When they are balanced, the star is said to be in a state of hydrostatic equilibrium, which is another term for gravitational equilibrium. At this point, the inward pull of gravity is exactly balanced by the outward push of thermal pressure, and as a result, the star remains stable in size.