Cygnus X-1

The first strong case

Cygnus X-1 was discovered as a powerful X-ray source in 1964. Its X-rays vary rapidly and come from a region far too compact to be an ordinary star. By the early 1970s, observations of the companion star's orbital motion showed that the unseen object was far too massive to be a neutron star.

The case became strong enough that many astronomers accepted it as a black hole. It remains one of the best-studied stellar-mass black holes and a classic testbed for accretion physics.

An X-ray binary

The black hole orbits the blue supergiant HDE 226868 once every 5.6 days. Material drained from the star forms an accretion disc around the black hole. As the gas spirals inward, it heats to millions of kelvin and radiates strongly in X-rays.

The system also produces a compact radio jet, showing that even stellar-mass black holes can launch outflows. Its orientation and distance make it a useful laboratory for studying how accretion and jets vary together.

Why it is famous

Cygnus X-1 featured in a famous 1974 bet between Stephen Hawking and Kip Thorne over whether it contained a black hole. Hawking later conceded, a sign of how persuasive the observational evidence had become.