Cygnus X-1

An intense source of X-rays in the constellation Cygnus, discovered in 1966. It has since been the subject of much study and speculation, being regarded as a candidate black hole. The source has been identified as a binary star system, the primary of which, designated HDE 226868, is a hot supergiant of spectral type O or B. The best evidence from the binary orbit of HDE 226868 around its unseen companion suggests that the invisible star has a mass significantly higher than the upper limit for neutron stars. The conclusion is that the invisible star can only be a black hole. The X-ray energy is generated by matter from the primary streaming on to the compact companion.