Gamma-Ray Line Emission from Radioactive Isotopes in Stars and Galaxies

Our modern laboratory of nuclear physics has expanded to encompass parts of the universe, or at least our Galaxy. Gamma rays emitted by the decays of radioactive nuclei testify to the production of isotopes through nuclear processes in astrophysical events. We collect measurements of the Galactic gamma-ray sky in spectral lines attributed to the decay of radioactive ^7Be, ^22Na, ^26Al, ^44Ti, ^56Ni, ^57Ni, and ^60Fe. We organize and collate these measurements with models for the production sites in novae, supernovae, stellar interiors, and interstellar cosmic-ray interactions. We discuss the physical processes and the spatial distribution of these production sites, along with models of the chemical evolution of the Galaxy. Highlights of measurements made in the last decade include detailed images of the Galaxy in ^26Al radioactivity and detection of ^56Co and ^57Co from SN 1987A, ^44Ti from Cas A, and possibly ^56Ni from SN 1991T. The ^26Al mapping of recent Galactic nucleosynthesis may be considered as a new view on the entire ensemble of massive stars in the Galaxy. The local Cygnus region shows prominent radioactive emission from well-known stellar clusters, but the absence of gamma-rays from the closest Wolf-Rayet star, WR 11, in the Vela region is puzzling. SN 1987A studies in gamma-rays measure the radioactive powering of the supernova light curve directly, which will be particularly important for the dim late phase powered by ^44Ti. The ^57Ni/^56Ni isotopic ratio determinations from gamma-rays provide additional guidance for understanding SN 1987A's complex light curve and now appear to be uniformly settling to about twice the solar ratio. Cas A ^44Ti production as measured through gamma-rays presents the interesting puzzle of hiding the expected, coproduced, and large ^56Ni radioactivity. Core-collapse supernova models need to parameterize the inner boundary conditions of the supernova in one way or another, and now enjoy another measurement of the ejecta that is definitely originating from very close to the difficult regime of the mass cut between ejecta and compact remnant. Other relevant measurements of cosmic element abundances, such as observations of atomic lines from the outer shells of the production sites or meteoritic analysis of interstellar grains, complement the rather direct measurements of penetrating gamma-rays, thus enhancing the observational constraints of nuclear astrophysics models.