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.