interstellar astrophysics

Interstellar astrophysics (or interstellar astronomy) refers to the study of the interstellar medium (ISM), including the gas (nebulae, HI regions, molecular clouds, etc.) and dust. The term refers to study of the medium within a galaxy and by definition, this includes that of the galactic halo. The medium between galaxies has its own term, intergalactic medium (IGM), but undoubtedly for some types of study, both are relevant.