Where is Cygnus X-1?
Cygnus X-1 is an X-ray binary in the constellation Cygnus, the swan, that was one of the first X-ray sources discovered when it was detected in 1962. It is called Cygnus X-1 because it was the first X-ray source discovered in the constellation Cygnus. The visible object HDE226868 is a 9th magnitude blue supergiant star whose radial velocity curve shows an orbital period of a little less than a week. The fact that the object is a strong X-ray emitter and that the optical and X-ray emission varies on very short time scales (as short as one one-thousandth of a second) suggest that the companion might be a black hole.
