How are the stars formed?

We see innumerable stars in the sky every night. Some of the stars are very bright while others are dim. Some are small, while others are big. Stars have puzzled mankind right from the very beginning. Do you know how stars came into existence? All stars have evolved from gases, mainly hydrogen and clouds of matter spread in space. Whenever such clouds or gases contract, due to their own gravitational force, they get transformed into a spherical ball. In order to enable it to convert into a star, the weight of such a cloud should be a 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 contract further. 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 selfluminous star. These stars continue contracting indefinitely till the temperature at their centres reaches millions of degrees centigrade. At this temperature, thermo-nuclear reactions (fusion) start. These reactions are similar to those which take place in a hydrogen bomb. In these fusion reactions, four nucleus of hydrogen combine to form a helium nucleus. The emission of heat and light energy from the stars is caused by fusion 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 constantly receiving heat and light energy from the Sun. It is estimated that our Sun will live for 10 billion years. Half of its life is over