About a decade after its discovery in 1964, a combination of X-ray and optical observations led to the conclusion that Cygnus X-1 was a black hole - the first such identification. The Cygnus X-1 system consists of a black hole with a mass about 10 times that of the Sun in a close orbit with a blue supergiant star with a mass of about 20 Suns. Because of its brightness and close proximity, Cygnus X-1 continues to attract the interest of scientists seeking to understand the nature of black holes and how they affect their environment.