Radioactive isotopes in the interstellar medium
  Radioactive components of the interstellar medium provide an
entirely-different and new aspect to the studies of the interstellar medium.
Injected from sources of nucleosynthesis, unstable nuclei decay along their
trajectories. Measurements can occur through characteristic gamma rays that are
emitted with the decay, or in cosmic material samples through abundances of
parent and daughter isotopes as they change with decay. The dynamics and
material flows within interstellar medium are thus accessible to measurement,
making use of the intrinsic clock that radioactive decay provides. We describe
how measurements of radioactive decay have obtained a break-through in studies
of the interstellar medium, after first summarizing the characteristics of
radioactivity and the sources of unstable nuclei.
