Cygnus X-1 is the first widely recognized directly observed black hole candidate, discovered as a powerful X-ray source in a binary system.
The system contains a blue supergiant star and a compact unseen companion. Matter pulled from the star forms a hot accretion disk around the companion, producing intense X-rays. The companion’s mass is too large for a white dwarf or neutron star, making a black hole the accepted explanation.
Cygnus X-1 became especially important after observations in the early 1970s established its strong case. The system is about 6,000 light-years from Earth in the constellation Cygnus. Its black hole has been estimated at roughly 20 solar masses, though measurements have been refined over time.
The phrase “first observed black hole” can be misleading. Astronomers do not see the black hole’s surface; they infer it from its gravitational effects and radiation from surrounding matter. Cygnus X-1 was an early landmark, while later gravitational-wave detections and horizon-scale images supplied different kinds of evidence.