Cygnus X-1 is a binary star system located in the constellation Cygnus. It consists of a massive, visible star and an unseen companion with extremely strong gravitational effects. In the early 1970s, observations by astronomers led to the conclusion that the unseen companion in Cygnus X-1 was likely a black hole, making it the first black hole candidate discovered. This discovery played a significant role in confirming the existence of black holes and furthering our understanding of their properties.