Front Cover: Radio map of the supernova remnant Cassiopeia A made with the National Radio Astronomy Observatorys (NRAO) Very Large Array. The bright ring of radio emission marks the interface between the rapidly expanding sphere of ejecta from the supernova and the shock-heated interstellar medium outside. This supernova occurred about 300 years ago, and the sphere is now about 10 light-years across. Acceleration of cosmic rays is thought to occur at shocks driven by expanding supernova remnants. (Courtesy of NRAO.)

Back Cover: Interactions of cosmic-ray protons and nuclei with the gas in the interstellar medium create all kinds of secondary particles. Among these are neutral pi-mesons, which in turn produce high-energy gamma rays from the decay pi-0 to two gammas . The back cover shows a map in galactic coordinates of the diffuse gamma-ray flux seen by the EGRET instrument on the Compton Gamma-Ray Observatory. Interpretation of these observations confirms that the cosmic-ray sources must supply about 3x10^40 ergs/s. (Courtesy of NASA.)