Black hole
Cygnus X-1
The first black hole discovered — an X-ray binary with HDE 226868
An X-ray source discovered by rocket in 1964; in 1971 it became the first black-hole candidate, identified as the invisible 21-solar-mass companion of the blue supergiant HDE 226868. Gas pulled from its companion heats to millions of degrees in the accretion disc and emits X-rays. It was the subject of the famous Hawking–Thorne bet.
| Mass | 21.2 solar masses |
|---|---|
| Event horizon radius | 63 km: the size of a city |
| Time for light to circle the event horizon once | 0.0013 seconds |
| Mass | 21,2 M☉ |
| Companion | HDE 226868 — O9.7 Iab blue supergiant, ~40 M☉ |
| Orbital period | 5.6 days |
| Distance | 2,220 pc (7,240 light-years) |
| Rotation | a* > 0.99 (near maximal) |
| Discovery | 1964 (Aerobee roketi) · 1971 (kara delik tanımı) |
The event horizon is the boundary where the escape velocity equals the speed of light: nothing inside, not even light, can get out. Its radius grows in direct proportion to the mass (the Schwarzschild radius, for a non-rotating black hole).