Properties of a star remain unchanged for quite a long time. The gravitational force and the force due to the pressure of the hot gas act together on a star. The gravitational force acts towards the centre of the star and tries to bring the gas particles close together. Hot gas shows the tendency to spread and its force acts away from the centre of the star. This force tries to disperse the gas particles. A balance between the gravitational force and the force due to the hot gas keeps the star stable.