High-energy electromagnetic radiation (light), with short wavelength (less than, roughly, 10 nanometers) and high frequency (greater than about 1016 Hertz). X-rays would be produced by blackbody radiation at temperatures in excess of a million degrees. Sources of astrophysical X-rays include accretion disks, gas impacting on neutron stars, X-ray Bursters, and hot gas located in the centers of galaxy clusters.