X-ray astronomy
X-ray astronomy is a branch of astronomy that studies the emission of X-rays from celestial objects. X-ray radiation is absorbed by the atmosphere, so instruments to collect x-rays must be at high altitude, in the past they were used on balloons and sounding rockets. At present x-ray astronomy is part of space research and x-ray observatories are installed on satellites.
X-ray emission is believed to come from sources containing very hot gas at several million Kelvin, usually from objects whose atoms or electrons have high energy. The discovery of the first X-ray source from space in 1962 came as a surprise. That source was called Scorpio X-1, in the constellation Scorpio toward the center of the Milky Way. For this discovery, Riccardo Giacconi was awarded the Nobel Prize in Physics in 2002. It was later discovered that the x-ray emission from this object is 10,000 times greater than that captured in the optical emission. That is, the total energy emitted by this x-ray source is 100,000 times greater than that emitted by the Sun at all wavelengths. These x-ray sources are known to be stellar remnants, such as neutron stars or black holes. The source of the energy is in the gravitational energy, which comes from the gas heated by the fall in the gravitational field of those objects.
Thousands of x-ray sources are known today. Furthermore, the space between the galaxies in the galaxy clusters appears to be filled with very hot but thin gas at a temperature of 100 million kelvins. The total amount of gas is five to ten times the total mass of the visible galaxies.