  The measurement of gamma rays from cosmic sources at MeV energies is one of
the key tools for nuclear astrophysics, in its study of nuclear reactions and
their impacts on objects and phenomena throughout the universe. Gamma rays
trace nuclear processes most directly, as they originate from nuclear
transitions following radioactive decays or high-energy collisions with
excitation of nuclei. Additionally, the unique gamma-ray signature from the
annihilation of positrons falls into this astronomical window and is discussed
here: Cosmic positrons are often produced from beta-decays, thus also of
nuclear physics origins. The nuclear reactions leading to radioactive isotopes
occur inside stars and stellar explosions, which therefore constitute the main
objects of such studies. In recent years, both thermonuclear and core-collapse
supernova radioactivities have been measured, and complement conventional
supernova observations with measurements of their prime energy sources. The
diffuse radioactive afterglow of massive-star nucleosynthesis in gamma rays is
now being analysed towards how massive stars feed back their energy and ejecta
into interstellar gas. Large interstellar cavities and superbubbles have been
recognised to be the dominating structures where new massive-star ejecta are
injected, from 26Al gamma-ray spectroscopy. Finally, the puzzling
bulge-dominated intensity distribution of positron annihilation gamma rays is
measured in greater detail, but still not understood; a recent microquasar
flare provided evidence that such objects may be prime sources for positrons in
interstellar space, rather than distributed nucleosynthesis. We also briefly
discuss the status and prospects for astronomy with telescopes for the
nuclear-radiation energy window.
