General Astronomy/History of the Black Hole
The Black Hole Concept edit
The idea of a star that is too massive to fully counteract the force of its own gravity is not new, dating at least as far back as the latter part of the 1700s. English scientist John Mitchell and French mathematician Pierre-Simon LaPlace both proposed "dark stars" with the general characteristics of black holes. However, such concepts were in the complete absence of any realistic understanding of how even "normal" stars produce energy, so the concept was little more than a curiosity, and was not seriously considered at the time.
A more realistic picture of such bizarre stellar end-products had to wait until Einstein's General Theory of Relativity, which is actually a new way of looking at gravity. Hot on the heels of Einstein's theory was Schwarzschild's work on the effective theoretical size of a black hole, confining it to what is today called a "Schwarzschild Sphere."
However, there was not even any consensus on what to call them -- they were variously known as "dark stars," or even "frozen stars" -- until physicist John Wheeler in 1967 used the term "black hole" in a lecture.
Additional considerations include whether or not the black hole is rotating, or emitting particles via the as-yet-undetected "Hawking Radiation," or whether any "information" (in the sense used in physics) could be extracted from a black hole, and others.
Still, the basic concept of a black hole remains that of a body whose gravitational force is so intense as to raise the escape velocity at its surface to the speed of light or greater. In that case, no amount of energy would be able to pry a single particle away from a black hole, including the essentially mass-less particles called "photons". Although quantum physics may conceded that subatomic particles can seep through the "event horizon" (the effective surface of a black hole) over exceedingly long periods of time, the general definition still holds.
Due to the nature of a black hole, precluding any exact visual detection (except up closer than we as humans want to get), we may never actually see one. However, as discussed in the previous section, there is abundant evidence that black holes exist. There are black holes of "solar" mass, meaning that their mass is on the same order as that of our Sun (although most natural black holes must be somewhat more massive than the Sun), up to black holes millions to billions of times as massive as the Sun, many of which appear to reside in the cores of galaxies. http://www.nasa.gov/multimedia/videogallery/index.html?media_id=142531021
The first observational (if somewhat indirect) evidence came in 1964 with the discovery of a strong source of X-rays emanating from the constellation Cygnus. The nature of this X-radiation was the same as that predicted to be produced from charged particles spiraling down through the gravitational field of an extremely compact and massive body - a black hole.
Since the 1964 discovery of Cygnus X-1, hundreds of other black hole candidates have been discovered by the same method.
Today, no serious astronomer or physicist doubts the existence of black holes. The details may be lacking, but their reality, for all intents and purposes, is abundantly confirmed.