A binary star is a pair of stars that orbit each other. In a binary system, a black hole and a normal, visible star could orbit one another so closely that the black hole would strip gas from the normal star. The gas would then fall violently toward the black hole and be heated by friction among the gas atoms. Just outside the black hole, the temperature would rise to tens of millions of degrees. As a result, energy would radiate from the gas as X rays visible to X-ray telescopes.
Astronomers believe that seven binary star systems they have observed probably contain black holes.
Each system is a source of intense X rays, proof that it contains either a neutron star or a black hole. In addition, the visible star orbits the collapsed star at such a high velocity that the collapsed star must be more massive than 3 solar masses. The binary system most likely to contain a black hole is V404 Cygni, which is in the Milky Way. This system is in the constellation Cygnus, and is about 11,000 light-years from Earth. The mass of the collapsed star in V404 Cygni is at least 6 solar masses.