Black Holes can only be identified indirectly because they do not emit electromagnetic radiation.

Quite often a Black Hole is part of a binary system with a visible star as its partner. The motion of the visible star then allows to estimate the mass of the Black Hole.

A Black Hole can also be identified if its attracts and swallows gas from a nearby star. The gas particles undergo a heavy acceleration and become so hot that typical X-rays are emitted.

The image on the left shows the center of our galaxy. It was taken by X-ray satellite Chandra in September 1999. Orange and yellow tones are interpreted as emissions from remnants of a supernova that exploded about 10,000 years ago. In the central cloud, a brighter spot can be observed. This is a black hole candidate.