X-rays are a form of light. They are in fact more energetic than the visible light our eyes are sensitive to. X-radiation is part of the electromagnetic spectrum, just like visible light, radio waves, microwaves, etc. X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (left), a German physics professor working in a laboratory in the Physical Institute of the University of Wurzburg. For this discovery, he won the first Nobel prize in physics (1901).

Since cosmic X-ray radiation is absorbed by the Earth's atmosphere, space-based telescopes are needed for X-ray astronomy. X-ray astronomy started by using rocket-borne detectors in the late 1950s and early 1960s, to Uhuru, the first orbiting X-ray astronomy satellite, in the 1970s. Paving the way for more advanced Space observatories such as Einstein Observatory (1978), the first fully imaging X-ray telescope put into space, and later with the Chandra X-ray Observatory, which was launched in 1999 and is still in operation. Riccardo Giacconi played a key role in the development of X-ray Astronomy and hence was awarded a share of the Nobel Prize in Physics in 2002 "for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources".

X-ray binaries are a class of binary stars that are luminous in X-rays. The system contains two stars, one typically compact (see above) and another one similar to our Sun. The X-rays are produced by matter falling from one component, called the donor to the other component, called the accretor, which is very compact.

Scorpius X-1, (cataloged as Sco X-1), is the brightest X-ray source in the sky, the first such object discovered in the direction of the constellation Scorpius. Detected in 1962, its X-radiation is not only strong but, like other X-ray sources, quite variable as well.

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess of non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray, and gamma-ray wavebands. A galaxy hosting an AGN is called an "active galaxy". The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.