GAMMA-RAY LINE EMISSION FROM RADIOACTIVE ISOTOPES IN STARS AND GALAXIES

Our modern laboratory of nuclear physics has expanded to encompass par ts of the universe, or at least our Galaxy. Gamma rays emitted by the decays of radioactive nuclei testify to the production of isotopes thr ough nuclear processes in astrophysical events. We collect measurement s of the Galactic gamma-ray sky in spectral lines attributed to the de cay of radioactive Be-7, Na-22, Al-26, Ti-44, Ni-56, Ni-57, and Fe-60. We organize and collate these measurements with models for the produc tion sites in novae, supernovae, stellar interiors, and interstellar c osmic-ray interactions. We discuss the physical processes and the spat ial distribution of these production sites, along with models of the c hemical evolution of the Galaxy. Highlights of measurements made in th e last decade include detailed images of the Galaxy in Al-26 radioacti vity and detection of Co-56 and Co-57 from SN 1987A, Ti-44 from Cas A, and possibly Ni-56 from SN 1991T. The Al-26 mapping of recent Galacti c nucleosynthesis may be considered as a new view on the entire ensemb le of massive stars in the Galaxy. The local Cygnus region shows promi nent radioactive emission from well-known stellar clusters, but the ab sence of gamma-rays from the closest Wolf-Rayet star, WR 11, in the Ve la region is puzzling. SN 1987A studies in gamma-rays measure the radi oactive powering of the supernova light curve directly, which will be particularly important for the dim late phase powered by Ti-44. The Ni -57/Ni-56 isotopic ratio determinations from gamma-rays provide additi onal guidance for understanding SN 1987A's complex light curve and now appear to be uniformly settling to about twice the solar ratio. Cas A Ti-44 production as measured through gamma-rays presents the interest ing puzzle of hiding the expected, coproduced, and large Ni-56 radioac tivity. Core-collapse supernova models need to parameterize the inner boundary conditions of the supernova in one way or another, and now en joy another measurement of the ejecta that is definitely originating f rom very close to the difficult regime of the mass cut between ejecta and compact remnant. Other relevant measurements of cosmic element abu ndances, such as observations of atomic lines from the outer shells of the production sites or meteoritic analysis of interstellar grains, c omplement the rather direct measurements of penetrating gamma-rays, th us enhancing the observational constraints of nuclear astrophysics mod els.