binary star system that is a strong source of X-rays and that provided the first major evidence for the existence of black holes. Cygnus X-1 is located about 8,000 light-years from Earth in the constellation Cygnus. The primary star, HDE 226868, is a hot supergiant (supergiant star) revolving about an unseen companion with a period of 5.6 days. Analysis of the binary orbit led to the finding that the companion has a mass greater than seven solar masses. A star of that mass should have a detectable spectrum, but the companion does not; from this and other evidence astronomers have argued that it must be a black hole. The X-ray emission is understood as being due to matter torn from the primary star that is being heated as it is drawn to the black hole.