A strong source of X-rays 8000 l.y. away in Cygnus. The X-rays originate from the inner region of an accretion disk, which is formed as material flows from the 9th-magnitude blue supergiant HDE 226868 towards a compact companion. This unseen companion is calculated to have a mass of 8–16 solar masses, considerably greater than the mass limit for a neutron star, leading to the conclusion that it must be a black hole. Gas is drawn from the supergiant star by the strong gravitational pull of the black hole. As the gas falls through the accretion disk, the gas heats up to millions of degrees, hot enough to emit X-ray radiation, which can be observed by space-borne X-ray observatories.
Subjects: Astronomy and Astrophysics.