Background image: The image shows the dark region from which no light can escape, surrounded by stars.
The 'dark star', an object so massive that its escape velocity exceeded the speed of light, was first proposed by John Michell in 1783 within the paradigm of Newtonian mechanics. Within the context of General Relativity, in which the speed of light is absolute and frame independent, whilst the singularity and event horizon were early identified as mathematical properties of the Schwarzchild solution, their physical meaning – and particularly the meaning of the event horizon, where in spherically symmetric coordinates there is an apparent singularity – was unclear. David Finkelstein was the first to show that the latter singularity was only a coordinate singularity and that the physics in its vicinity was, if unusual, perfectly well-defined. He correctly interpreted them (within classical theory) as surfaces that can only be crossed in single direction. Stephen Hawking and Jacob Bekenstein were later to show that in quantum theory black holes should in fact radiate a thermal spectrum. The one-directional nature of event horizons, and the 'no hair' theorems (stating that little information about the interior of a black hole can be determined from its exterior) lead to the information loss paradox.
In the present epoch, Hawking radiation from black holes formed by stellar collapse, assuming it exists, would be infinitesimal. Since no other information can escape a black hole, observations of their presence are of necessity indirect, and come through their action in gravitational lensing, and in the formation of accretion disks from which X-rays are emitted. The first observational evidence of black holes was found in the form of an X-ray binary star system, Cygnus X-1, in the constellation Cygnus, discovered in 1964.
Current models predict the presence of a super-massive black hole at the centre of most galaxies, with observations of orbits of stars in the Milky Way pointing to a central black hole in our galaxy of around four million solar masses.