Cygnus X-1 — Cygnus X-1 (often abbreviated to Cyg X-1) is an X-ray source in the Cygnus constellation considered to be one of the most likely black hole candidates.
The optical counterpart (HDE 226868) is a variable 8.9 magnitude star (visible with good binoculars in good observing conditions.) at right ascension 19 h 56.5 min and declination of 35 deg 4 min (for 1950 epoch).
Cyg X-1 is a binary star that contains a O9-B0 supergiant (with a surface temperature of 31000 Kelvin) and a compact object.
The mass of the supergiant is approximately 20-30 solar masses. The compact object has a mass of 7-13 solar masses; as the largest possible mass of a neutron star can not exceed three solar masses, it is believed to be a black hole.
The X-rays are produced in an accretion disk that is formed by matter flowing from the supergiant into the black hole. Cygnus X-1 is the brightest persistent source of hard X-rays (E > 20 keV) on the sky. The distance to Cygnus X-1 is about 2500 parsecs.