Cosmic Scale
Cygnus X-1
Stellar-mass black hole · 7,200 light-years
How big is Cygnus X-1?
Cygnus X-1 is about 125 km across.
See Cygnus X-1 in 3D, at true scale
Cygnus X-1 is a black hole of about 21 Suns locked in a 5-day orbit with a blue supergiant star. Gas streaming off the companion spirals in, heats to millions of degrees and glows in X-rays, which is how it was found in 1964 by a detector carried above the atmosphere on a sounding rocket.
At a glance
| Diameter | 125 km |
|---|---|
| Mass | 4.22 × 10³¹ kg |
| Diameter compared with Earth | 0.010 × Earth |
| Distance | 7,200 light-years |
| Companion | Blue supergiant, 40 Suns |
| Orbit | 5.6 days |
Good to know
- Stephen Hawking bet Kip Thorne that it was not a black hole, as insurance against his own life's work being wrong. He conceded the bet in 1990.
- Its event horizon is only about 125 km across, but you would need to look from within a few thousand kilometres to see it bend light.
- The gas reaches roughly 10 million degrees before it crosses the horizon.