Stephen Hawking bet against the first black hole, hoping he would lose
In 1975 Stephen Hawking wagered Kip Thorne that a bright X-ray source in the constellation Cygnus was not a black hole. It was an insurance policy: if black holes turned out not to exist, he would at least win the bet. By 1990 the evidence was strong enough that Hawking happily conceded.
The object first showed up in 1964, when two Aerobee rockets launched from White Sands in New Mexico swept Geiger counters across the sky. X-rays cannot get through the atmosphere, so instruments have to be lofted high to catch them, and the survey turned up eight new sources. One was labelled Cyg XR-1, later Cygnus X-1, and nothing striking sat at its position in radio or visible light.
Longer looks came from orbit. Riccardo Giacconi and Herb Gursky had proposed an X-ray satellite in 1963, and NASA launched it as Uhuru in 1970. It caught Cygnus X-1 flickering several times a second, which meant the glow had to come from a region no bigger than about Jupiter, since light itself limits how fast distant parts can coordinate. In 1971 radio astronomers at Leiden and in the United States pinned the source to a blue supergiant catalogued as HDE 226868.
The clincher was motion. In 1972 Louise Webster and Paul Murdin in England, and Charles Thomas Bolton in Toronto, working separately, measured the supergiant's wobble and deduced a heavy unseen partner. It outweighed the largest possible neutron star, and by the end of 1973 most astronomers accepted it as a black hole, the first X-ray source to win that status. Its mass is now put at about 21.2 Suns.
The pair lies around 7,000 light-years away and circles every 5.6 days, separated by only about a fifth of the Earth-Sun distance. Wind from the big star feeds a disk around the black hole, heated to millions of degrees, while twin jets carry energy outward. That arrangement mirrors active galactic nuclei in miniature, earning it the label microquasar. The black hole may have formed without a supernova, and the two bodies are expected to merge in some five billion years.
Source: Cygnus X-1
