We see innumerable stars in the sky every night. Some of the stars are very bright while others are dim. Some are small, others are big. You must be curious to know how stars came into existence.
All these stars, in fact, have evolved from the gases and clouds of particles spread in space. Whenever such clouds or gases due to their own gravitational pull, contract, they get transformed into a spherical ball. In order to evaluate it to convert into a star, the weight of such a cloud should be thousand times more than that of the sun. When such a cloud starts contracting, the pressure so generated produces heat. A portion of this heat is radiated out in various directions. This helps the cloud to further contract. A stage is reached when the cloud is broken into several pieces and every piece goes on contracting. When these pieces become very hot, they start radiating light and then each shining piece becomes a self-luminous star.
These stars continue their contraction further till the temperature, at their centres reaches millions of degree centigrade. At this temperature thermo-nuclear reactions start. These reactions are similar to those which take place in a hydrogen bomb. In these reactions four nucleus of hydrogen combine to form a helium nucleus. The emission of energy from the stars is caused by these reactions.
These stars exist as long as their hydrogen content does not go below ten percent. Their life span extends to billions of years. Our Sun is also a star in which thermonuclear reactions are going on. As a result of these reactions, we have been receiving heat and light energy from the Sun constantly. It is estimated that our Sun will live for 10 billion years. Half of its life is, of course, over.