Twentieth century astronomers have predicted dark areas in space. The gravitational attraction of these areas is so high that anything which goes into it cannot come out. Even light cannot escape their gravitational pull. Hence they do not emit light. These areas are called Black Holes or collapsars.
A German astronomer, Karl Schwarzschild predicted the existence of black holes in 1907. He theoretically proved that black holes are the end results of all stars whose mass is much greater than that of the sun. The existence of black holes was first theoretically proved in 1939 by J. Robert Oppenheimer and Hartland S. Synder as a consequence of General Theory of Relativity.
Let us consider a star whose mass is greater than that of the sun. Its size remains normal due to the balance between the two forces — one being the expansion force caused by the enormously high temperature which tends to expand the star’s material, and the other being the enormous gravitational pull which tends to contract the star’s substance.
At some stage in the star’s life, after thousands of millions of years, its nuclear fuel decreases causing a fall in its core temperature. As a result, the gravitational pull becomes stronger than the expansion force. Gradually the star begins to collapse.
In this process, the atoms present in the star break into electrons, protons and neutrons. The mutual repulsion between the electrons prevents further contraction. The star, at this stage, is known as ‘White Dwarf’. In this process, the star is reduced to one-hundredth of its original size; thereby the gravitational pull in the White Dwarf becomes about 10,000 times more than the original value.
Under certain conditions the gravitational pull becomes strong enough to overcome the electron repulsion. The star begins to contract further and in this process of contraction, electrons and protons combine to form neutrons. The star at this stage is called “Neutron Star”. Its size is now reduced to five hundredth part of the dwarf star and the gravitational attraction becomes about 100,000,000,00 times that of the original star.
The light emitted from the neutron star reduces its energy and as a result its size further decreases. At some stage, no radiations come out from this star. It is then called a Black Hole which is the smallest and most dense object in the universe.
Scientists are still looking for evidence of the actual existence of black holes in the universe. They have detected Cygnus X-1 as a black hole in 1974. In 1983, US astronomers detected another X-ray source in the large Magellanic cloud.