Galactic Gamma-Ray Sources

In Volume 5 of the Annual Review of Astronomy and Astrophysics (1967) Giovanni Fazio (76a) wrote that ... up until now, no photons of energy greater than 100 keV originating from beyond the solar system have definitely been detected. Fourteen years later, gamma-ray astronomy is an accepted branch of observational astrophysics, with something like 105 photons (E > 100 MeV) originating from beyond our solar system having been collected. The motivations for doing gamma-ray astronomy today are the same as those that were so attractively depicted in the fifties to spur on the experimental efforts and repeated in the sixties, even tho.ugh the first results showed that the original estimates had been optimistic, often by orders of magnitude. They are that gamma-ray photons open a new window on energetic nonthermal processes in the Universe, with the added bonus that they travel unscattered and unabsorbed through great lengths of galactic and intergalactic space. It was never foreseen, however, that the observations would disclose objects whose main astronomical property is to emit