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We propose to advance investigations of electromagnetic radiation originating in atomic nuclei beyond its current infancy to a true astronomy. Such nuclear emission is independent from conditions of gas, thus complements more traditional astronomical 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 teneous 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. This White paper addresses Science Areas “Stars and Stellar Evolution (SSE)” and “The Galactic Neighbourhood (GAN)” of the US National Academy’s Decadal Survey Outline Structure.