Evolution of a Star

Even stars have birth, life and death!! The raw material for the formation of a star is mainly hydrogen gas and some helium gas. The life cycle of a star begins with the gathering of hydrogen gas and helium gas present in the galaxies to form dense clouds of these gases. The stars are then formed by the gravitational collapse of these over-dense clouds of gases in the galaxy. The first step in the formation of a star from gases is the protostar.

Stars Do Not Collapse-

Why? Well, a star emitting heat and light energy, has two types of forces acting continuously: The gravitational attraction of the enormous gaseous matter which wants to compress the star further and the internal pressure developed in the star because of energy released during nuclear fusion reactions going inside it, tending to stop the gaseous matter from collapsing further.

As a consequence, the star is now in equilibrium under the action of two opposing forces-the gravitational force trying to compress the gases, and the internal pressure due to nuclear energy trying to stop the gaseous matter from collapsing. If internal pressures were not built up within the star, a star would have collapsed within less than half an hour.
In a mature star, the pressure forces from within the star balance the gravitational force from outside. This balance can continue for millions of years. Our sun is now in this balanced state of its development. At this stage, the temperature in the interior of the sun is just right to sustain the fusion reaction, and the rate of this fusion reaction produces just the right pressure to balance the gravitational compression.

During all this time, the nuclear fusion reactions continue to liberate energy and make the star shine. Our sun (which is also a star) was formed about 4600 million years ago and will continue to give out energy for an equal period of time. If, however, there were no internal pressure in the sun produced by the energy of nuclear fusion, then our sun would have contracted drastically (shrunk drastically) within half an hour, under the action of tremendous gravitational forces. Well, that could not have happened to the Sun!