Cygnus X-1 became the first widely accepted stellar-mass black-hole candidate.
Located in the constellation Cygnus, Cygnus X-1 is a binary system containing a massive blue supergiant and a compact companion. The companion cannot be seen directly, but its mass is too large to be a neutron star, making a black hole the best explanation. Gas pulled from the visible star forms an extremely hot accretion flow that emits X-rays.
The source was discovered during a 1964 rocket flight designed to detect cosmic X-rays. Its identification as a black-hole system became influential because astronomers could measure the visible star’s motion and infer the unseen companion’s mass.
Cygnus X-1 is not the closest black hole to Earth, and it was not the first black hole ever theoretically proposed. It was important because it provided unusually strong observational evidence for a black hole in a stellar binary system.