Cygnus X-1

Cygnus X-1 (often abbreviated to Cyg X-1) is an X-ray source in the constellation Cygnus, and considered to be a black hole. It is a high-mass X-ray binary, with the optical counterpart (HDE 226868) being 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).
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A source of intense X-rays which is widely thought to be the site of a stellar sized black hole. The X-ray source is a binary star system in which one component is a blue supergiant star, the other is very dense and compact. The X-rays are thought to be coming from hot gas in an accretion disc around the companion. The compactness of the companion suggests it is even more dense than a neutron star and thus a black hole fits the bill.