Moreover, the intense gravitational field of the black hole prevents any light or other electromagnetic radiation from escaping. But where lies the "point of no return" at which any matter or energy is doomed to disappear from the visible universe? How does such a thing come into existence? Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. The singularity is where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. Here it's no longer meaningful to speak of space and time, much less spacetime. Jumbled up at the singularity, space and time cease to exist as we know them. As a stars lifetime goes on, nuclear fusion in the core generates electromagnetic radiation, including the particles of light known as photons. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass. As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward. The contraction of the core causes its temperature to rise and allows remaining nuclear material to be used as fuel. The star is saved from further collapse -- but only for .
a while. Eventually, all possible nuclear fuel is used up and the core collapses. How far it collapses, into what kind of object, and at what rate, is determined by the star's final mass and the remaining outward pressure that the burnt-up nuclear residue (largely iron) can muster. If the star is sufficiently massive or compressible, it may collapse to a black hole. .
The first person to discuss black holes was a Cambridge man by the name of John Mitchell. Mitchell argued that it was possible to have a star that was sufficiently massive and sufficiently small in size that its escape velocity would be greater than the velocity of light (Hawking 117).