Cygnus X-1
It was discovered in 1964 during a rocket flight, and is one of the strongest X-ray sources seen from Earth. Cygnus X-1 was the first X-ray source accepted as a black hole. It is among the most studied astronomical objects in its class. The compact object is estimated to have a mass about 14.8 times the mass of the Sun. It is too small to be any known kind of normal star, or other likely object besides a black hole. If so, the radius of its event horizon is about .