definition gamma ray
gamma rays, n an electromagnetic radiation of short wavelength emitted by the nucleus of an atom during a nuclear reaction. Composed of high-energy photons, gamma rays lack mass and an electric charge and travel at the speed of light.
High-energy telescopes study the X- and gamma-ray regions of the electromagnetic spectrum. X-ray telescopes can form images of their field of view, although the mirror design is more complex than for lower-energy instruments. Gamma-ray telescopes simply point in a certain direction and count photons received.