Hydrostatic equilibrium
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Hydrostatic equilibrium occurs when compression due to gravity is balanced by outward pressure.
Fluids
The Hydrostatic equilibrium pertains to hydrostatics and the principles of equilibrium of fluids. A hydrostatic balance is a particular balance for weighing substances in water. Hydrostatic balance allow the discovery of their specific gravities.
Physics
In astrophysics, in any given layer of a star, there is a balance between the thermal pressure (outward) and the weight of the material above pressing downward (inward). This balance is called hydrostatic equilibrium. A star is like a balloon. In a balloon, the gas inside the balloon pushes outward and the elastic material supplies just enough inward compression to balance the gas pressure. In a star the star's internal gravity supplies the inward compression. Gravity compresses the star into the most compact shape possible; a sphere. Stars are round because gravity attracts everything in an object to the center.
In physics, Hydrostatic equilibrium also explains why Earth's atmosphere does not collapse to a very thin layer on the ground and how the tires on your car or bicycle are able to support the weight of your vehicle.
See also: Statics
Material is taken from Nick Strobel's Astronomy Notes. Stroble, Nick. (May, 2001). Nick Stroble's Astronomy Notes. (http://www.astronomynotes.com/starsun/s7.htm)