Life of a Star:

A young star is thought  to be composed largely of  hydrogen gas. Hence, the most likely place for a star to be born is in one of  the numerous clouds of hydrogen gas that exist in the interstellar space. Stars are now believed to form inside large dense  interstallar clouds of  gas. It may happen that for some reasons,  not fully known so far, a gas  'cloud starts contracting. Under the influence of  gravitational pull of  the gas, its contraction may continue further. Once such a process begins, a very large volume of gas clouds is affected. As gravity pulls in the clouds, the pressure in the cloud increases. Also, as  the cloud contracts, the temperature at its.  centre increases. At this stage, it is called a protostar. 

When the temperature becomes sufficiently high (about 4 million degrees centigrade), a nuclear reaction starts in  the protostar, in which the hydrogen nuclei fuse together to make helium nuclei. In this process a large amount of  energy is released. The energy travels to  the surface of  the star and is radiated in the form of light, heat and other electromagnetic radiation. This energy creates an outward pressure and force. The contraction of  the star stops oqly when the inward pull of gravity is balanced by  the outward force of  this radiant energy. At such a time the star becomes stable in size and temperature. The Sun has been in such a stable situation for the past 5 billion years. Nuclear reactions in  the Sun convert about four hundred million tons (4X10¹⁴ grns) of hydrogen into helium every second. It is expected that the Sun will remain in  thk state for another 5 billion years. As the star consumes a significant percentage of  the hydrogen fuel in its core, the nuclear reaction decreases and the outforce of  the radiant enem weakens. The core of the star further contracts because its gravitational pull becomes more than the out-force of  radiant energy. But this raises the temperature of  the core. Meanwhile, the hydrogen nuclei 'burn'  in the outer layer or shell surrounding the core. The extra heat from the core as well as the heat generated  in the outer layers cause the star's outer region to 'boil'  and expand. The star becomes big and its brightness increases. But, as the outer layer expands