A black hole is an object with such a strong gravitational field that even light cannot escape from its surface. A black hole may be formed when a massive object (very big object) undergoes uncontrolled contraction (a collapse) because of the inward pull of its own gravity. We shall now describe how the black holes are formed from neutron stars after the supernova explosions of big stars. When a supernova explosion of a very massive star takes place, then the gaseous matter present in the outer shell (or envelope) of the star is scattered into space but the core of the star survives during supernova explosion.
This heavy core of the supernova star continues to contract (shrink) and becomes a neutron star. The fate of this neutron star depends on its mass. If the neutron star is very heavy, then due to enormous gravitational attraction, it would continue to contract indefinitely (or continue to shrink in size indefinitely). And the vast amount of matter present in a neutron star would be ultimately packed into a mere point object. Such an infinitely dense object is called a black hole. Thus, black holes are formed by the indefinite contraction of heavy neutron stars under the action of their own gravity.
The neutron stars shrink so much and become so dense that the resulting black holes do not allow anything to escape, not even light, from their surface. This is because the black holes have tremendous gravitational force. Since even light cannot escape from black holes, therefore, black holes are invisible, they cannot be seen. The presence of a black hole can be felt only by the effect of its gravitational field on its neighboring objects in the sky.
For example, if we see a star moving in a circle with no other visible stars in the centre, then we can conclude that there is a black hole at the centre. And it is the gravitational pull exerted by this black hole which is making the star go in a circle around it. Thus, if the sun were to become a black hole, then the earth would continue to go round it, though we would not ‘see’ any object where the sun is located at present. One possible location for a black hole is in the Cygnus constellation.