  The measurement of gamma rays at MeV energies from cosmic radioactivities is
one of the key tools for nuclear astrophysics, in its study of nuclear
reactions and how they shape objects such as massive stars and supernova
explosions. Additionally, the unique gamma-ray signature from the annihilation
of positrons falls into this same astronomical window, and positrons are often
produced from radioactive beta decays. Nuclear gamma-ray telescopes face
instrumental challenges from penetrating gamma rays and cosmic-ray induced
backgrounds. But the astrophysical benefits of such efforts are underlined by
the discoveries of nuclear gamma~rays from the brightest of the expected
sources. In recent years, both thermonuclear and core-collapse supernova
radioactivity gamma~rays have been measured in spectral detail, and complement
conventional supernova observations with measurements of origins in deep
supernova interiors, from the decay of $^{56}$Ni, $^{56}$Co, and $^{44}$Ti. The
diffuse afterglow in gamma rays of radioactivity from massive-star
nucleosynthesis is analysed on the large (galactic) scale, with findings
important for recycling of matter between successive stellar generations: From
$^{26}$Al gamma-ray line spectroscopy, interstellar cavities and superbubbles
have been recognised in their importance for ejecta transport and recycling.
Diffuse galactic emissions from radioactivity and positron-annihilation
$\gamma$~rays should be connected to nucleosynthesis sources: Recently new
light has been shed on this connection, among others though different
measurements of radioactive $^{60}$Fe, and through spectroscopy of positron
annihilation gamma~rays from a flaring microquasar and from different parts of
our Galaxy.
