Black hole candidate. X-ray image of the binary star system known as Cygnus X-1. Cygnus X-1 probably consists of a black hole and a supergiant star with a mass larger than 30 solar masses, orbiting together. Gas from the outer layers of the supergiant star is pulled outwards by the strong gravitational field of the black hole. This material does not fall straight onto the black hole but forms an accretion disc around it. The X- ray emission seen here comes from the gas in the disc which is heated up to millions of degrees by friction and dissipational processes. The image was taken by the ROSAT X-ray astronomy satellite.