Stars are spherical balls of hot gas.
Despite the complexity of the gas motions and the nuclear reactions, the internal structure of a star is governed by a simple balance between pressure and gravity. A star is held together by the gravitational forces among all the gas particles. The gravity force is directed toward the center, where it compresses the gas and causes it to heat up. Greater heat means more rapid motion of the gas particles, which causes them to resist being close to each other. This internal pressure is the force that opposes gravity and stops the star from collapsing to a tiny size.