gamma-ray astronomy

Study of celestial objects that emit gamma rays (energetic photons with very short wavelengths). Much of the radiation detected comes from collisions between hydrogen gas and cosmic rays in our Galaxy. Some sources have been identified, including the Crab Nebula and the Vela pulsar (the most powerful gamma-ray source detected).

Gamma rays from space cannot be directly detected at ground level, but they can be studied indirectly by observing the secondary particles arriving at the ground after gamma-ray photons have collided with atoms of atmospheric gases. Gamma rays can be studied directly by use of balloon-borne detectors and artificial satellites. The first gamma-ray satellites were SAS II (1972) and COS-B (1975), although gamma-ray detectors were carried on the Apollo 15 and Apollo 16 missions, in 1971 and 1972, respectively. SAS II failed after only a few months, but COS-B continued working until 1982, carrying out a complete survey of the galactic disc.