The mass of gas in a star is held in place by gravity. A star is said to be self-gravitating, because it contains sufficient mass (typically about a million times that of the Earth) for its own gravitational force-field to be sufficiently strong to keep itself together. A star is also gaseous because, even despite large pressures, it is very ho throughout.

The material of a star is acted on by two main forces: the gravitational force, which tends to draw the stellar material inwards, and a pressure force which pushes outwards. In most stars these two forces are exactly balanced at every point of the star. An in-balance of the two forces leads to a net force which, by Newton’s second law of motion, leads to a net accleration or motion. The out-sides of some variable stars, such as Cepheid variable stars, for example, are believed to be pulsating, inwards and outwards, as a consequence of a small imbalance between the gravitational and pressure forces. A large imbalance between the gravitational and pressure forces acting on the material of a star would have catastrophic consequences. Indeed this may be what happens in super-novae explosions. If the pressure support were removed from a star such as the Sun it would completely collapse under its own gravity in about half an hour! This has not been observed, and we believe that in most stars there is an extremely fine balance between the pressure and gravitational forces. Such stars are said to be in hydro¬static equilibrium which we write, at each point of the star-Pressure Force = Gravitational Force.

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