  We propose to advance investigations of electromagnetic radiation originating
in atomic nuclei beyond its current infancy to a true astronomy. This nuclear
emission is independent from conditions of gas, thus complements more
traditional stronomical methods used to probe the nearby universe. Radioactive
gamma-rays arise from isotopes which are made in specific locations inside
massive stars, their decay in interstellar space traces an otherwise not
directly observable hot and tenuous phase of the ISM, which is crucial for
feedback from massive stars. Its intrinsic clocks can measure characteristic
times of processes within the ISM. Frontier questions that can be addressed
with studies in this field are the complex interiors of massive stars and
supernovae which are key agents in galactic dynamics and chemical evolution,
the history of star-forming and supernova activity affecting our solar-system
environment, and explorations of occulted and inaccessible regions of young
stellar nurseries in our Galaxy.
