The radius of the event horizon is called the schwarzschild radius after the German astronomer Karl Schwarzschild, who, in 1916, discovered the correct solution of Einstein's equations. This solution describes an isolated non-rotating black hole. In 1963 the corresponding solution for a rotating black hole was discovered by the New Zealand physicist Roy Kerr. The Schwarzschild radius of a black hole can be very small. A star three times the Sun's mass will probably form a black hole with a Schwarzschild radius of only 9 km.
Astronomers believe that many black holes are to be found in the Milky Way.
Gas drawn from the surface of the ordinary visible star in the system is swept around into an accretion disc surrounding the apparently invisible member of the system, that is, the black hole. This swirling disc of gas is believed to be heated by friction to temperatures of tens of millions of degrees. Such a hot gas would emit X-rays which are probably those seen during satellite observations of this system. Some astronomers have suggested that very large black holes may also be found at the centres of some galaxies including, perhaps, our own. It has also been suggested that very small black holes may have been formed during the highly compressed initial phase of the Universe.