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