The study of gamma rays produced within our
Galaxy, the Milky Way. They may be due to
collisions between hydrogen gas and cosmic
rays. Some sources have been identified,
including the Crab nebula and the Vela pulsar
(the most powerful gamma-ray source
detected). Gamma rays are not plentiful and
only about a million gamma-ray photons have
been collected. This is equivalent to the
number of photons of visible light received
from a star such as Sirius in about a second.
Gamma rays are difficult to detect and are
generally studied 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
16 missions. SAS II failed after only a few
months, but COS B, carrying a single
gamma-ray experiment, and intended to be
operational for only two years, was finally
switched off 1982 after a mission in which it
carried out a complete survey of the galactic
disc.