Abstract
Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV γ-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nu-clear de-excitation, and positron annihilation. The substantial information carried byγ-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted atγ-ray energies, and radioactivity provides a natural physical clock that adds unique information.