Stellar-mass black hole
Cygnus X-1
The first object ever widely accepted as a black hole — discovered in 1964, and famously the subject of a physics wager between Stephen Hawking and Kip Thorne.
About Cygnus X-1
Cygnus X-1 is a binary system: a black hole locked in a close orbit with a massive blue supergiant star, HDE 226868, completing one orbit every 5.6 days. The black hole pulls gas from its companion star, heating it to millions of degrees as it spirals in — hot enough to blaze in X-rays, which is how the system was first detected in 1964. A precise 2021 re-measurement of the system's distance revised the black hole's estimated mass upward significantly, from an earlier estimate of about 14.8 solar masses to roughly 21.2 — making it the most massive stellar-mass black hole known from direct observation (as opposed to gravitational-wave detections, which have found somewhat larger examples).
Interesting facts
- Stephen Hawking famously bet Kip Thorne in 1974 that Cygnus X-1 would turn out not to be a black hole — he conceded the bet in 1990 as the evidence became overwhelming.
- Its companion star, HDE 226868, is more than 40 times the mass of the Sun and about 300,000 times brighter.
- Cygnus X-1's 2021 mass revision came from more precise distance measurements, not a change in the object itself — a reminder that even well-studied black holes still get more accurately measured over time.