X-ray radiation lies on the opposite side of the electromagnetic spectrum from Infrared radiation. Thus, X-rays have much shorter wavelengths and much higher energies than visible light. A typical X-ray photon has an energy 1000 times that of a visible photon. Astronomers classify X-rays by their energy. ‘Soft’ X-rays have energies between 0.1 and 1.0 keV, while ‘Hard’ X-rays have energies between 1.0 and 10.0 keV. By definition, 1 eV (electronvolt) is the amount of energy gained (or lost) by the charge of an electron moved across an electric potential difference of one volt.
X-ray astronomy is a fairly new field of study, because it can only be done from satellites. Water absorbs radiation at these wavelengths, so our atmosphere is opaque to X-rays. The diagram below gives a quick overview of what wavelengths of electromagnetic radiation are absorbed by our atmosphere.
Many exotic astrophysical objects emit X-ray radiation including black holes, active galactic nuclei (AGN), pulsars, galaxy clusters, and binary systems containing a white dwarf, neutron star, or black hole. In supernova remnants, strong X-ray emission traces the strong shock wave and hot shocked gas. After a supernova, material from the explosion races outwards and plows into the surrounding gas. The hot gas and high-energy particles created at this shock front glow at a range of wavelengths (radio through X-ray) for thousands of years. An image of Kepler’s supernova remnant at several wavelengths is shown below. The green coloring indicates low energy X-rays while the blue indicates high energy X-rays.
At the moment, there are two major X-ray satellites that are currently operating and taking data. XMM-Newton was launched by the European Space Agency (ESA) in 1999. It has better spectral resolution and a larger field of view than Chandra. The Chandra X-Ray Observatory was also launched in 1999 by NASA and is currently operated by the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. Chandra has much better spatial resolution than XMM-Newton.
If you are interested in learning more about X-ray astronomy, check out this overview from the University of Cambridge: