Cygnus X-1

node · earth · Constellation Cygnus, as seen from Earth

The X-ray binary that was for decades the best case that stellar-mass black holes exist. It was found as one of the brightest X-ray sources in the sky in 1964, during the sounding-rocket surveys that opened X-ray astronomy, and became identifiable only when radio observations in 1971 pinned the position precisely enough to match it to the supergiant star HDE 226868. Spectroscopy of that star gave the orbit, the orbit gave a mass function, and the mass function gave a compact companion too heavy to be a neutron star. It is the subject of the most famous wager in the field: Stephen Hawking bet Kip Thorne in 1974 that it was not a black hole, and conceded in 1990. A 2021 analysis combining a new radio parallax distance of about 2.2 kiloparsecs revised the compact object upward to roughly 21 solar masses and inferred a spin close to the maximum the Kerr solution allows. Not a place on Earth.