Hydrostatic Equilibrium

Balance between gravity and gas pressure. In the case of a star, gravity originates in mutual gravitational attraction of the entire mass of the star. Gas pressure is produced by nuclear reactions that heat the star’s gas. Any change in the internal temperature of a star results in a change in its diameter: if a star cools, it will contract; if it grows hotter, it will expand. The same concepts apply to the gaseous atmospheres of planets.

Image Source: cse.ssl.berkeley.edu

Some or all content above used with permission from J. H. Wittke.