The staring material for the formation of a star is mainly hydrogen gas and helium gas. A star is conceived when a condensed pocket of hydrogen and helium gas forms in outer space. Now let us understand how a condensed pocket of gas forms in space.
We know that all objects attract each other by a force called the force of gravity. Gravity, however, is not a very strong force, it is the weakest natural force known to man. Therefore, if the cloud of hydrogen gas contains a small number of atoms, the force of gravity between them would not be strong enough to tie them permanently together.
On the other hand, suppose the hydrogen cloud contains a very large number of atoms. Since the force of gravity extends over long distances, each atom feels the gravitational pull of all other atoms in the hydrogen cloud. If the number of atoms is sufficiently large, the combined effect of all these small pulls of gravity will be strong enough to prevent any of the atoms in the cloud gas from leaving the cloud and flying into space again. The gas cloud becomes a permanent entity, held together by the mutual attraction of all the atoms present in it. Then the cloud begins to contract under its own gravity setting off the process which will convert this huge condensed gas cloud into a star.
Such a tight contracting cluster of atoms held in the grip of its own gravity, is called a PROTOSTAR.
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The protostar is not yet a star and does not emit light.
The temperature of the gas cloud in the beginning is quite low (can be as low as – 1730 C). Now let us understand how does a protostar, which is a collection of atoms, drawn together out of the cold gas of space, become a flaming sphere of gas called a star?
Formation of Stars out of Protostars
The force of gravity acting on different atoms in the protostar, draws every atom towards the centre of the cloud. As a result, the protostar shrinks in size and its density increases. As the atoms in the protostar fall towards the centre, they pick up speed like any falling object. Because of higher speed and greater density of the atoms, the atoms in the gas cloud collide with one another more frequently, thereby raising its temperature. This means that the temperature of contracting protrostar rises. The process f contraction and rise in temperature can continue for about a million years. During this period, the inner temperature of the protostar increases from a mere – 1730 C (which is the temperature of the interstellar gas out of which protostar had formed0 to about 107` C.
The temperature of about 107` C marks a critical threshold in the life of the collapsing sphere of gas. At these extremely high temperatures, the protons (hydrogen nuclei) at the centre of the protostar collide together and undergo a nuclear fusion reaction to from helium nuclei. In this reaction a tremendous amount of energy is released. This further raises the temperature and pressure.
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The release of nuclear energy marks the birth of the star. The protostar now begins to glow and becomes a star.