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Cygnus X-1

A notable blue star with its even more notable companion. The B star is the first black hole whose existence was proofed.
Because both objects are very close to each other the black hole slowly swallows its much bigger companion. Thereby the inflowing gas heats up to several million kelvin and emits X-rays. That was how the black hole was discovered because we can't see it directly.
Probably the black hole once was an ordinary giant star, bigger than its partner, which was disrupted by a supernova.

Constellation: Cygnus
Distance: 7200 light-years
Radial velocity: -13 km/sec
Orbit period of Cygnus X-1 A and B: 5.59982 days

Cygnus X-1 A

Spectral class: O9
Visual magnitude: 8.92
Luminosity: 400 000 * Sun
Mass: 30 * Sun
Diameter: 20 * Sun

Cygnus X-1 B

Mass: 12 * Sun
Diameter: circa 15 kilometers

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