John Michell had suggested in 1783 that, if stars were heavy enough, light would be prevented by gravity from leaving the surface. Pierre Laplace made a similar suggestion 13 years later. These ideas of ‘dark stars’ were mostly ignored in the nineteenth century, as light was then thought to be a massless wave not influenced by gravity.
In 1915 Albert Einstein published his general theory of relativity, and in the following year Karl Schwarzschild solved Einstein's field equations for a point mass. He showed that a black hole could theoretically exist by calculating the Schwarzschild radius or event horizon around an extremely dense object. In 1939 J. Robert Oppenheimer, George Volkoff and Hartland Snyder developed the theory of neutron stars and black holes, showing that stars greater than a certain mass would continue to collapse indefinitely to form black holes. In the 1960s Roy Kerr developed the theory of rotating black holes, and in the following decade Stephen Hawking suggested that black holes can emit particles and radiation, and so gradually erode over time.
A number of black hole candidates have been discovered in recent decades. The most convincing cases include those of Cyg X-1, A0620-00 and V404 Cygni.
Black holes are not restricted to stellar objects. In fact astronomers had speculated for some time that supermassive black holes may exist at the centre of active galaxies powering their radio jets. But supermassive black holes may also exist at the centre of ordinary galaxies.
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