  Gamma-ray instrumentation for astronomical spectroscopy consists of
multiple-interaction detectors in space combined with sophisticated
post-processing of detector events on ground. Spectral signatures in the MeV
regime originate from transitions in the nuclei of atoms (rather than in their
electron shell). Nuclear transitions are stimulated by either radioactive
decays or high-energy nuclear collisions such as with cosmic rays. Gamma-ray
lines have been detected from radioactive isotopes produced in nuclear burning
inside stars and supernovae, and from energetic-particle interactions in solar
flares. 56Ni directly reflects the source of supernova light. The paucity of
corresponding 44Ti gamma-ray line sources reflects the variety of dynamical
conditions herein. 26Al and 60Fe are dispersed in interstellar space from
massive-star nucleosynthesis over millions of years. Gamma-rays from their
decay are measured in detail by gamma-ray telescopes, astrophysical
interpretations reach from massive-star interiors to dynamical processes in the
interstellar medium. Nuclear de-excitation gamma-ray lines have been found in
solar-flare events, and convey information about energetic-particle production
in these events, and their interaction in the solar atmosphere. The
annihilation of positrons leads to another type of cosmic gamma-ray source. The
characteristic annihilation gamma-rays at 511 keV have been measured long ago
in solar flares, and now throughout the interstellar medium of our Milky Way
galaxy. But now a puzzle has appeared, as a surprising predominance of the
central bulge region was determined. This requires either new positron sources
or transport processes not yet known to us. In this paper we discuss
instrumentation and data processing for cosmic gamma-ray spectroscopy, and the
astrophysical issues and insights from these measurements.
