Cygnus X-1 is a galactic X-ray source in the constellation Cygnus, and the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3×10−23 Wm−2 Hz−1.

Distance to Earth: 6,197 light years

B−V color index: +0.81

U−B color index: −0.30

Absolute magnitude (M_V): −6.5±0.2

Radius: 20–22 R_☉