Cosmic γ rays

The first positive detection of gamma rays from the Galaxy was made by the OSO-3 satellite in 1968 but very variable results emerged from the early balloon experiments. Only recently has the position become clear as the result of observations made from two satellites devoted entirely to gamma-ray astronomy, which have given an excellent map of much of the sky seen in the light of 100-MeV gamma rays. The first was the Small Astronomy Satellite 2 (SAS-2), developed by NASA's Goddard Space Flight Center, and second the European COS-B satellite, which is currently operating successfully. Gamma-ray astronomy should contribute substantially to answering questions on the origin of cosmic rays; on the large-scale structure of our Galaxy (much of which is hidden from optical view by dust clouds); on the existence of a very extensive 'halo' around our Galaxy; on the mechanism by which pulsars pulse; and perhaps may give another view of the early evolution of the Universe. Work on these topics is discussed. (U.K.)