What was the name of the black hole that Stephen Hawking bet on?

The black hole Stephen Hawking bet on was Cygnus X-1, an X-ray source in the constellation Cygnus. In the wager, Hawking argued that Cygnus X-1 was not a black hole, while his colleague Kip Thorne argued that it was.

The friendly bet was made in the 1970s, usually dated to 1975, although the written document bears witness signatures dated 10 December 1974. Hawking later described his position as a kind of insurance policy: if black holes did not exist, much of his research would be wrong, but he would at least win the wager.

Cygnus X-1 became one of the strongest early candidates for a stellar-mass black hole. It is a binary system containing a compact object orbiting a blue supergiant star, with matter from the star producing powerful X-rays as it falls toward the compact object.

Hawking conceded the bet in 1990 after mounting observational evidence supported Thorne’s view. The prize included a subscription to Penthouse for Thorne, while Hawking received four years of Private Eye.