Black hole
V404 Cygni
X-ray nova famous for its 2015 outburst
A low-mass X-ray binary in Cygnus. During its 1989 and 2015 outbursts it became the brightest X-ray source in the sky within days; in 2015 its jets were seen to change direction within seconds (precession). The first black hole whose distance was measured directly by radio parallax.
| Mass | 9 solar masses |
|---|---|
| Event horizon radius | 27 km: the size of a city |
| Time for light to circle the event horizon once | 5.6 × 10⁻⁴ seconds |
| Mass | 9,0 M☉ |
| Distance | 2,390 pc (7,800 light-years) |
| Orbital period | 6.47 days |
| Outbursts | 1938, 1956, 1989, 2015 |
| Discovery | 1989 (Ginga uydusu patlaması) |
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).