There are different stages in the life of the stars. Stars form initially as a small cloud of gas and dust in the outer space. Then they undergo many changes during their lifetime. In brief, they start their life in the form of a gas cloud called ‘nebula’. Eventually they become big and luminous producing heat and light by nuclear reactions. This stage is called ‘star’. At the end of this stage, a star becomes a ‘red giant’ which is a large bright star with a cool surface.
At this stage it runs out of fuel at its centre. The nuclear reaction stops because of lack of fuel. The star’s gravity pulls it inward, compressing the core. They start becoming smaller and smaller entering the ‘dwarf’ stages. Now, the star is said to be in its final phase. With the core getting heated up, supernova explosion occurs and the stars of less mass become ‘neutron star’ and more dense ones form a ‘black hole’. A star’s end is a black hole’s beginning. Einstein’s theory predicted the existence of black holes.
The core is the central part of the black hole called singularity. Black holes are so very cold and dense that nothing can escape their gravitational pull. They could be either so small or massive. They could be rotating if the star from which they got formed was a rotating star or stationary one.
The nearest black hole is 1,600 light years away. Due to their strong gravitational field black holes continuously pull objects into it until they finally vanish. Though one cannot see a black hole, its presence can be detected by astronomers by studying its effect on the objects around it. Black holes also emit radiations. This is because when something falls into the black hole, they get heated up to very high temperature that they start to emit X rays and powerful gamma rays.