Hydrostatic Equilibrium
From Slackerpedia Galactica
(Or Why Stars Are Round)
Hydrostatic equilibrium is a term describing a state of balance between gravity and pressure. In astronomy, it usually refers to the balance reached in stars where the pressure created by nuclear fusion balances the force of gravity, causing the star to form a relatively stable spheroid shape.
Sometimes the pressure wins and the star expands, which leads to larger than normal stars such as red giants. In such stars the pressure creates winds that blows outward through the outer layers of the star. This expands the star and creates a tenuous atmosphere, in fact there is much debate among astronomers about exactly where a red giant's atmosphere ends and interstellar space begins. Proving, yet again, that astronomers have way too much time on their hands.
Some other times, gravity wins - such as in supernovae explosions.
Michael has written a blog post explaining the forces at work.
