Archive of Astronomy Questions and Answers

Could you say more 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 9th 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 system's X-ray emission. The available data suggest that the optically visible star HD 226868 is orbited by a black hole 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. In addition, the X-rays that such objects produce would be much less energetic than found in the Cygnus X-1 system. Considering this, together with the rapid, millisecond variability which suggests an X-ray source only 300 kilometers across, the evidence for a black hole orbiting HD 226868 is currently thought to be very convincing. Due to the mass of the optically visible companion star estimated to be about 6-8 times the sun's, most stellar evolution models predict that within a few million years such a star will end its life as a supernova leaving behind either a neutron star or a second black hole.