Learning from gamma-ray lines of cosmic radionuclei.

Gamma ray lines from cosmic sources reveal the results of nuclear reactions in cosmic sites. Following radioactive decays, characteristic lines from excited daughter nuclei can be measured with space-borne gamma ray telescopes which address the MeV energy range of the electromagnetic spectrum. High-energy collisions with excitation of nuclei, and the 511 keV line and its associated special continuum from the annihilation of positrons falls into the same energy window, although of different origin. We review astronomical gamma-ray telescopes and cosmic gamma ray spectrometry, with their concepts and corresponding instruments and missions. Recent results and the challenges and open issues for the future are summarised. Among these are the diffuse\ radioactive afterglow of massive-star nucleosynthesis in ²⁶Al and ⁶⁰Fe gamma rays, addressing the cycle of matter driven by massive stars and their supernovae. Then, the complex processes making stars explode as either thermonuclear or core-collapse supernovae are enlightened through gamma-ray lines from shortlived radioactivities, ⁵⁶Ni and ⁴⁴Ti. Positron annihilation gamma ray emission throughout our Galaxy is an interesting side note here.