X-ray astronomy is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects. X-ray radiation is absorbed by the Earth's atmosphere, so instruments to observe X-rays must be taken to high altitude, in the past with balloons and sounding rockets. Nowadays, X-ray astronomy is part of space research and X-ray detectors are placed in satellites.

X-ray emission is expected in sources which contain an extremely hot gas at temperatures from a million to hundred million kelvins, in general in objects in which the atoms and/or electrons have a very high energy. The discovery of the first cosmic X-ray source in 1962 came as a surprise. This source is called Scorpius X-1, the first X-ray source found in the constellation of Scorpius, located in the direction of the center of the Milky Way. Based on this discovery, Riccardo Giacconi received the Nobel Prize in Physics in 2002. Later it was found that the X-ray emission of this source is 10,000 times greater than its optical emission. In addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. It is now known that such X-ray sources are compact stars, such as neutron stars and black holes. The energy source is gravitational energy, which comes from gas heated by the fall in the strong gravitational field of such objects.

Nowadays, many thousands of X-ray sources are known. In addition, it appears that the space between galaxies in a cluster of galaxies is filled with a very hot, but very dilute gas at a temperature of between 10 and 100 megakelvins. The total amount of hot gas is five to ten times the total mass in the visible galaxies.