As such it links the global galactic evolution with the localized stellar evolution. The complex structure of the ISM is a consequence of the fact that the ISM is an open system that tries to balance itself according to the physical and chemical conditions that prevail locally. Since most of the ISM is cold or, in the presence of a radiation field, warm, most of its continuum radiation and spectral line emission emerges in the MIR, FIR, and Submm. The molecular clouds absorb part of the radiation field, and produce a diversity of species that only exist in a well-defined local equilibrium. The diffuse interstellar radiation field plays the dominant role in the chemistry of more isolated MCs.
Massive stars dominate their environment by their strong and energetic radiation field.
These stars eject processed, i.e. metal enriched, material back into space. The study of the composition of the reprocessed material and its interaction with the ISM will strongly impact on our understanding of the chemistry and dynamics of the ISM as well as on the galactic chemical evolution.