The bizarre spatial anomaly known as a black hole was born in the mind of man as a mathematical concept, an extrapolation of what ought logically to occur in a space where the force of gravity would be such as to overcome all other types of forces maintaining the integrity of matter itself. Assuming that mass could continue to exist under such conditions, it would eventually be compressed into a singularity, distorting all spacetime around it such that within its area of immediate influence nothing could escape its gravitational pull, not even light: becoming a literal ‘black hole’ in space.
The concept of a body in space so gravitationally massive that its escape velocity would be greater than the speed of light originates independently with two 18th century scientists: the astronomer John Mitchell (1783) and the physicist Pierre-Simon Laplace (1795). However, nothing in the Newtonian physics of the time suggests why gravity should have any effect on a massless phenomenon such as light, nor why a massed object should not be able to achieve sufficient acceleration to exceed the speed of light. Although the name ‘black hole’ is generally credited to John Archibald Wheeler due to its use in a 1967 lecture, the term was cited as early as 1964 in a letter by Anne Ewing, and it may already have been in common use ever since this extrapolation of relativity was conceived. Certainly by the 1970s the ‘black hole’ had entered the public consciousness to the point that it had become the subject of many a grade school science fair display.
Both relativistic and quantum physics are required to understand the nature of the black hole. Relativity describes how all positive mass warps the structure of its surrounding spacetime such that smaller masses and energies tend to move toward the primary mass, a sensation manifested as the force of gravity. Should a mass become great enough to warp its surrounding spacetime such that its escape velocity would exceed the speed of light, neither energy nor mass (two expressions of the same reality, per E=MC^2) would ever be capable of escaping the event horizon, the vertical distance from the singularity outside which gravity would finally decrease to below the speed of light. Apart from the warping of spacetime, no event inside the event horizon could ever affect the universe outside, because no non-gravitational information could ever escape. Although outside its event horizon the black hole warps space only insomuch as would any other object of the same mass, as mass/energy continued to be absorbed the mass of the black hole would increase, and so proportionately would the radius of its event horizon.
Thus relativity creates an image of a singularity in spacetime which steadily exerts an attractive force on its surroundings, and which in time would devour them all, never to escape.
What defines a singularity in space is its Schwarzschild radius, a distance proportional to its mass. For example, the Schwarzschild radius for the sun would be three kilometres, while the Schwarzschild radius for the earth would be only nine millimeters. Any object smaller than its Schwarzschild radius is a black hole. Except for rotating objects, the Schwarzschild radius will be the same as the event horizon.
The most likely known-universe source for black holes are stars over the Tolman-Oppenheimer-Volkoff (TOV) limit in mass (estimated to be between 1.5 and 3 solar masses). In the late stages of their lives, such stars become supernovas and may blow off much of their mass in the explosion. If, after the supernova event, their mass is still over the TOV limit, the force of gravitational collapse will be greater than the mutually repulsive neutron-neutron interaction, and then such a star is speculated to become a black hole.
Until quite recently, the black hole remained a purely theoretical concept. Then, in 1964, a stellar object was discovered in the constellation Cygnus which emitted X-rays and other hard radiation in bursts and otherwise acted exactly as a black hole would be expected to. Cygnus X-1 (for X-ray emitting object) is believed to be approximately eight thousand lightyears away, based on red-shift measurements and pulsar norms. Since then, thirteen other objects with similar properties have been observed.