National Aeronautics and Space Administration

Only in the last few decades as astronomers started looking at the Universe in radio, infrared, ultraviolet, X-ray, and gamma-ray light have we learned very much about black holes. However, the concept of a black hole has been around for over 200 years. English clergyman John Michell suggested in 1784 that some stars might be so big that light could never escape from them. A few years later, French mathematician Pierre Simon de Laplace reached the same conclusion. Michell and Laplace both based their work on the ideas about gravity put forth by Isaac Newton in 1687. Newton had said that objects on Earth fall to the ground as a result of an attraction called gravity. The more massive (heavier) an object is, the greater its pull of gravity. Thus, an apple would fall to Earth. His theory of gravity ruled unchallenged until 1915 when Einstein's general theory of relativity appeared. Instead of regarding gravity as a force, Einstein looked at it as a distortion of space itself.

Shortly after the announcement of Einstein's theory, German physicist Karl Schwarzschild discovered that the relativity equations led to the predicted existence of a dense object into which other objects could fall, but out of which no objects could ever come. (Today, thanks to American physicist John Wheeler, we call such an object a "black hole".) Schwarzschild predicted a "magic sphere" around such an object where gravity is so powerful that nothing can move outward. This distance has been named the Schwarzschild radius. It is also often referred to as the event horizon, because no information about events occurring inside this distance can ever reach us. The event horizon can be said to mark the surface of the black hole, although in truth the black hole is the singularity in the center of the event horizon sphere. Unable to withstand the pull of gravity, all material is crushed until it becomes a point of infinite density occupying virtually no space. This point is known as the singularity. Every black hole has a singularity at its center.

Ignoring the differences introduced by rotation, we can say that to be inevitably drawn into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light; therefore, once light passes through, even it cannot escape. Wonderfully, the Schwarzschild radius can be calculated using the Newtonian equation for escape speed

For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be

This equation implies to us that any object with mass M can become a black hole if it can achieve a radius of R!