In a star, two types of forces are acting simultaneously. These two forces are: the force of gravity and the internal pressure developed in the star due to the tremendous amount of energy released during the nuclear fusion reactions taking place inside it. These two forces act in the opposite directions. The gravitational force tends to compress the star further whereas the internal pressure tends to stop the gaseous matter from collapsing further.
The star is now in delicate equilibrium under these two opposite forces – The gravitational attraction trying to compress the gaseous matter and ignite the fusion reaction, and the internal pressure generated by the energy released by nuclear fusion trying to stop the gaseous matter from collapsing. This balance can continue for millions of years. In this stage the temperature of the star is just right to sustain the nuclear fusion reactions and the rate of the fusion reactions is just right for preventing the star from collapsing further. In this stage the star continues to liberate energy due to fusion reactions taking place inside it.
Our sun is now in this balanced stage of development. It was formed about 4600 millions years ago and will continue to radiate energy for about an equal period of time in the future. If there were no internal pressure due to the energy of fusion the sun would have contracted drastically within about half an hour, under the very strong gravitational forces.