• Electromagnetic radiation in the 10-9-10-13m wavelength region, overlapping longer wavelength gamma rays and shorter wavelength ultraviolet radiation. X-ray astronomy is a new science because X-rays are unable to penetrate the Earth’s atmosphere, necessitating satellites or at least balloons or sounding rockets to allow them to be observed. X-ray astronomy has been one of the most fruitful branches of the science for some years, because it allows previously invisible effects of great fundamental importance to be observed. The X-ray sky contains many familiar objects, like the Sun, whose corona is an intense X-ray source. But most of the objects which dominate the X-ray sky are faint in visible light, so that important new science can be done in X-rays. The centres of galaxies, including our own, are powerful X-ray emitters, and so are the regions surrounding neutron stars, black holes and other highly condensed objects, so that pulsars are observed in X-rays as well as in other wavelengths. The first satellite for detailed, long-term X-ray astronomy observations was launched in 1970 (Small Astronomical Satellite), and the total observational base of X-ray astronomy is still far less than that for optical or radio astronomy, so that the field is still ripe for major discoveries. X-rays emitted by material orbiting black holes are the main way in which black holes can be detected and indirectly observed.


  • electromagnetic radiation whose wavelengths range between approximately 10-8 and 1011 meter, which corresponds to frequencies of 1016 to 1019 Hz respectively, although the defined interval varies. X-rays are located between ultraviolet and gamma radiation. Such rays consist of high-energy photons, and penetrate living tissue and other matter to varying degrees, with less penetrating rays called soft X-rays, and more penetrating rays, whose energy and frequency are higher, known as hard X-rays. X-rays are produced, for instance, when matter is bombarded by electrons with sufficient energy, as occurs in an X-ray tube. X-rays are used as a medical diagnostic technique, for therapy, and in the study of the structure and quantitative analysis of materials. Also called X-radiation, X-ray radiation, or roentgen rays.
  • synonymX-radiation
  • synonymX-ray radiation