Black holes are celestial objects that do not emit any electromagnetic wave and therefore we cannot observe them with the naked eye alone. Astronomers usually identify the existence of black holes by detecting X-rays emitted by the accretion disk near them. Not long ago, during a study of a binary star system designated as HR 6819, astronomers discovered that one of the stars in the binary system orbits an unknown celestial object at a period of 40 days, and calculated that the mass of this unknown celestial object is at least 5 times that of the Sun. Hence, it is concluded that this celestial object should be a black hole and HR 6819 should be a triple star system.

HR 6819 is 1000 light years away from the Earth. Located in the constellation Telescopium at the southern celestial sphere, it has an apparent magnitude of 5.3 which is visible to the naked eye. Before this study, the closest known black hole to Earth was about 3000 light years away. This discovery has rewritten the record and marks the first black hole that can be located by naked-eye observation.

More information: https://www.eso.org/public/news/eso2007/