I think Subrahmanyan was most famous for all his research on dying stars. Whenever a star dies one of three things could happen, if it's small it will become a white dwarf, If average sized it will become a neutron star, if it's large it could become a black hole. The Chandrasekhar limit says that if a star has more mass than 1.44 times our sun it can't become a white dwarf. Black holes are small objects in space with a very high gravity. White dwarves are a small very dense star that is typically the size of a planet.