The phenomenon in which a star collapses into an object whose gravity prevents anything from escaping is the formation of a black hole.
A massive star can reach this fate after exhausting the nuclear fuel that supported it against gravity. Once fusion can no longer provide enough outward pressure, the core collapses. A supernova may accompany the collapse, but it is an explosive event, not the compact object itself. If the remaining core is sufficiently massive, even neutron degeneracy pressure cannot halt the implosion.
The boundary around the resulting object is called the event horizon. It is not a solid surface; it marks the point beyond which every possible future path leads inward. Light cannot escape from inside it, which is why the object appears black to a distant observer. General relativity predicts the black hole, although the ultimate physics at its central singularity is not yet understood.
Smaller stellar remnants become white dwarfs or neutron stars instead. Black holes can also grow by accreting matter and merging with other black holes, and many galaxies contain supermassive black holes at their centers.