What do astronomers know about the black hole in Cygnus?
Cygnus X-1 is the brightest X-ray source in the constellation Cygnus and appears to coincide with a faint, ninth magnitude, supergiant star identified as HD 226868, located 8,100 light years away. It is also known to be a binary star with an orbital period of 5.6 days. A careful study of the X-ray emission detected by the UHURU satellite revealed that Cygnus X-1 is highly variable. In addition to the regular 5.6-day period that is seen optically, variations as short as 0.001 seconds have been recorded in the optically visible star HD 226868 is orbited by a black hole every 5.6 days, which draws in some of the matter from the visible star, converting it into X-rays. The mass of the unseen companion is estimated to be at least 3.4 solar masses, a number much more than the mass of either stable white dwarfs or neutron stars, both of which would be optically invisible at this distance. In addition, the X-rays that such objects produce would be much less energetic than those found in the Cy
