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Gamma-ray lines from cosmic sources display the action of nuclear reactions in cosmic sites. Nuclear
transitions following radioactive decays or high-energy collisions with excitation of nuclei result
in emission of gamma rays at characteristic energies, and thus provide a direct link to the isotopes
providing radioactive energy inputs (as, for example, in SN Ia or kilonovae). The gamma-ray line
from the annihilation of positrons at 511~keV and its associated special continuum falls into the
same energy window, although of different origin. We briefly review astronomical gamma-ray telescope
types and cosmic gamma ray spectrometry, with the status of corresponding instruments and missions,
including future perspectives. We then present a discussion of recent results from such measurements,
and the challenges and open issues for the future. This includes, specifically, the diffuse radioactive
afterglow of massive-star nucleosynthesis in $^{26}$Al and $^{60}$Fe gamma rays, which is now being
exploited towards the cycle of matter driven by massive stars and their supernovae, and the morphology
and dynamics of interstellar medium. Then, stellar explosions such as thermonuclear or core-collapse
supernovae, novae, and kilonovae are subject to studies through gamma-ray lines. Here shortlived
radioactivities play key roles, such as $^{56}$Ni and $^{44}$Ti decays for the case of supernovae.
Explosion non-sphericities that have recently been recognised will be highlighted. We will also
discuss how we should relate to the above the distribution of positron annihilation gamma ray emission
with its puzzling bulge-dominated intensity distribution, which is measured through spatially-resolved
spectra. These indicate that annihilation conditions may differ in different parts of our Galaxy, and
helps to reveal the complex paths recycling matter from nucleosynthesis sources to next-generation stars.
Contact :Toru Tamagawa <tamagawa a.t. riken.jp>