A black hole is defined as an object  whose matter has become  so.condensed that its gravitational field is strong enough to prevent light escaping from it . the radius of the black hole resulting from the collapse of an object of mass M is called the schwarchild radius Rs and is given by Rs=2gm/c where G is the universal constt of gravitation and C is the  speed of light 

                         Stars after undergoing evoluionary stages  or after running short of their fuel their outer layer explodes in the form of a very  bright supernova while the core collapses to  to form a very hifh density black hole.It is so dense that millions of tons of matter can be dumped  into very small core escape.Even light can not  come out of it. The masses  in a black hole continues to collapse and it becomes too small to be seen until it  reaches zero size . A black hole has an intense gravitational pull but it is not visible,