Life history of stars

Stars are formed from massive clouds of dust and gas (mainly hydrogen)  that are found in space.
The particles in a cloud are attracted to one another by the force of gravity. As millions of particles come together, the cloud shrinks, its density increases and heat is generated. As the temperature at the core increases to over 1000000°C,nuclear fusion reactions begin. Vast amounts of energy are released and a glowing ball of gas appears, marking the birth of a star.

    Most stars are formed in this way but their life histories vary according to their sizes at birth. The small stars live the longest ;the medium - sized stars live for about 10000million years;while large stars live for about a few million years.

A star that is small at birth does not produce much energy. Its size or brightness does not change appreciably during its lifetime and it fades away when its store of hydrogen is used up.
A medium sized star is very bright when young.  The hydrogen atoms at its core fuse to form helium thereby liberating a large amount of energy. It cools slightly and enters its middle age, which is its most active period. As the hydrogen at its core is used up, the hydrogen in the outer layer (shell) of the star to expand into a luminous red giant. Meanwhile, the core contracts, becomes denser and the temperature starts to increase. At about 100000000°C,the helium nuclei begin to fuse to form the nuclei of heavier elements like carbon-12,Oxygen-16,Neon-20 and Magnesium - 24 in reactions similar to the following :
He + He -->Be
He + Be -->C
He + C --->O
He + O --->Ne

   These reactions stop when the store of helium in the core is used up. The outer layer  or shell of the star usually drifts away to leave a small star, the white dwarf(the core), which cools and fades away eventually.

    A star that is large at birth produces much larger amounts of energy than the medium sized star. Such a star often becomes a supergiant. When the hydrogen and helium in its core are used up in fusion reactions, the nuclei of heavier elements like carbon-12 undergo fusion to form much heavier elements such as sodium. This happens at temperatures of about 700000000°C. At higher  temperatures than this, the nuclei of the transition group of elements (such as Nickel, Chromium, Manganese and Cobalt)  are formed. If the temperature continues to rise, the nuclei of the transition group of elements tend to break down to helium nuclei in processes which use up enormous amounts of energy. The star collapses suddenly and explodes as a supernova. During this explosion, many types of nuclear reactions occur. A supernova is a rare phenomenon. If sufficient mass is left behind after the explosion, a white dwarf may form