Write the life story of a star.

Write the life story of a star.                       

A.  A young star is largely composed 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 also formed inside large dense interstellar clouds of gas. Under the influence of the gravitational pull of the gas, a gas cloud starts contracting. As gravity pulls in the clouds, the pressure, and temperature in the cloud increases. This stage 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 only 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.
        As the star consumes a significant percentage of the hydrogen fuel in its core, the nuclear reaction decreases and the out force of the radiant energy 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 causes the star's outer region to 'boil' and expand. The star becomes big and its brightness increases. But, as the outer layer expands farther away from the nuclear furnace, its temperature falls. The puffed-up star looks red and cool. If it is many times more massive than the Sun, it becomes a red supergiant. If it is sun-sized or only slightly more massive than the Sun, it becomes a slightly swollen red giant.
        The red giant stage of a star is a relatively short stage. In this stage, the star consumes its hydrogen at a very fast rate, piling up helium in its core. As the fuel burns, the core contracts further, producing temperatures as high as 100 million C. At this point, the helium nuclei in the core fuse together in another nuclear reaction to form carbon nuclei. This is a critically unstable moment in a star's life with two layers of the star burning at the same time-an outer layer where hydrogen is being turned into helium and inner core where helium is being turned into carbon. Hereafter the fate of the star depends on the mass of its core.
            If the mass of the core is less than 1.4@ where M, is the Sun's mass, the contraction of the core halts when it is about the size of the Earth. This limit of 1.4M is known as the Chandrasekhar limit. Such a star is known as a white dwarf.
           If the core mass of the star is in the range 1.4M,--3Mo, or the star mass is between 8M, to 15 M,, the core shrinks to a radius of about 10 km and a neutron star is formed.
           If a star starts with a mass of more than 20 M,, its contraction continues. Then the core of the star collapses to become a black hole. Its gravity is now so strong that nothing, not even light, can leave it.
          Sometimes, massive stars (with the core mass between 3 M, and 15 M,) explode, releasing a tremendous amount of energy. Such explosions are called 'supernova'.