Final answer:
A star in hydrostatic equilibrium is perfectly balancing its internal pressure with gravitational forces, ensuring stability. This balance is similar in binary systems but with two stars gravitating around each other. Despite evolution, stable stars maintain equilibrium for extended periods.
Step-by-step explanation:
If a star is in hydrostatic equilibrium, its radiation pressure pushing outwards is perfectly balanced by the inward pull of gravitational forces at every point within the star. This state of balance ensures that the star neither collapses due to gravity nor expands uncontrolled due to excess pressure. The concept of hydrostatic equilibrium applies not only to stars but also to planetary atmospheres and oceans, where it describes the balance between the weight of overlying layers and the pressure that supports them.
In a stable binary orbit, two stars influence each other, their mutual gravitational forces causing them to orbit a common center of mass, rather than one star simply orbiting the other. This can have a profound effect on the evolution and characteristics of the stars involved.
The stability of a star, like the Sun, is indicative of its condition of equilibrium, where all internal forces are balanced and conditions such as temperature, pressure, and density remain constant over time. While stars do evolve, changes during the stable phase of a star's life cycle are usually so gradual that the star can be considered in equilibrium.