The study of X-rays, gamma rays, and cosmic rays coming from the Universe. The energy of the radiation is usually expressed in terms of electronvolts (eV). X-rays and gamma rays range in energy from 100 eV to over 1010 eV. There is no sharp cutoff between the X-ray and gamma-ray regions of the spectrum. Cosmic rays are atomic and subatomic particles which move through space at velocities approaching that of light. Their energies range from about 108 to 1020 eV. See also cosmic rays; gamma-ray astronomy; x-ray astronomy.
Subjects: Astronomy and Astrophysics.