What defines a black hole?

A black hole is defined as a region in space where gravity is so strong that nothing, not even light, can escape from it. This intense gravitational force results from a significant amount of mass being compressed into a very small volume, which occurs at the end of a massive star's lifecycle.

When a star exhausts its nuclear fuel, it can no longer support itself against gravitational collapse, leading to the formation of a black hole if the remaining mass is sufficient. The boundary around a black hole is known as the event horizon, which marks the point beyond which no information or matter can escape. The characteristics and behavior of black holes are derived from the principles of general relativity, which describe how mass distorts spacetime and the extreme nature of gravitational fields in these regions.

This fundamental nature of black holes distinguishes them from other astronomical objects, such as supernova remnants, which are explosive outcomes of star death, rather than the end states like black holes themselves.