Gamma-rays are photons at the extreme end of the electromagnetic spectrum. They are very energetic but, like X-rays, they are absorbed by a few metres of ordinary air so gamma-ray detectors are placed on rockets, balloons or satellites.

Gamma-rays tend to be produced in enormous explosions, so ordinary stars and galaxies are not very bright in images made using gamma-ray telescopes. The brightest sources in the gamma-ray sky are quasars, supernovae, gamma-ray bursters and neutron stars.

Famous gamma-ray observatories include the SWIFT satellite, the Compton Gamma-Ray Observatory, and INTEGRAL.