How to find a black hole?

Black holes are space objects formed as a result of the relativistic gravitational collapse of massive bodies. Black holes have such a strong gravitational field that even quanta of light cannot leave the object. As a result, the black holes themselves are invisible.
The presence of a black hole in a binary system can be judged by the hard X-rays emitted by the matter of a nearby star, falling at high speed onto the black hole. Radiation occurs as a result of friction between gas layers.
A black hole can be indirectly detected by the motion of a companion star, visible in the optical range, around a common center of mass.
A method for detecting a massive black hole by the effect of a gravitational lens has also been proposed. Each star behind the black hole will be seen as a pair of stars with exactly the same characteristics.

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