An Etymological Dictionary of Astronomy and Astrophysics

A → binary system containing one of the best candidates for a → black hole. Cygnus X-1 was discovered as an → X-ray source in 1965. It is one of the brightest X-ray sources on the sky, so that it was detected by the earliest → X-ray observation attempts. This binary system, distant of 2.5 kpc, consists of the O9.7 Iab type → supergiant HDE 226868 and a → compact object orbiting around with a period of 5.6 days. The mass of the unseen companion, significantly larger than 5 → solar masses, suggests that it is a black hole. Focused → wind accretion from a → primary star being extremely close to filling the → Roche lobe drives the powerful source of the X-ray radiation.

See also: → Cygnus.