Cygnus X-1 is a famous binary star system, consisting of a black hole (bottom left) in orbit around a blue supergiant called HDE 226868. The blue star is 30 to 40 times the mass of the Sun and 25 times its radius. The proximity of the blue star to its ravenous black hole companion means that HDE 226868 is tidally distorted. However, it does not overflow its Roche lobe. This means that there is probably no bridge of matter between the two objects. Nevertheless, the black hole is surrounded by an accretion disc, which is probably material it has captured from the bluegiant's stellar wind.