The space between the stars is by no means empty. It contains rarefied gas, dust particles, a magnetic field, and relativistically moving electrons, protons and other atomic nuclei. These various forms of interstellar energy density prove to be quite tightly coupled together, with the result that for many purposes they together form a single dynamical entity. The terms interstellar medium and interstellar matter, or ISM for short, are useful short-hands for this composite dynamical entity.
Although the ISM accounts for only a small fraction of the mass in galaxies, one of the most fundamental differences between galaxies of different Hubble types concerns the distribution of their interstellar media. Moreover, differences in the stellar contents and light distributions of early- and late-type galaxies can in large measure be traced to differences in their interstellar media. Hence a study of the ISM is a prerequisite for understanding why galaxies are as we observe them.
Until the early 1980s it was thought that only late-type galaxies had significant interstellar media. Advances in X-ray and microwave astronomy have now demonstrated that many early-type galaxies also have rich interstellar media. Hence the difference between early- and late-type galaxies lies not so much in the quantity of interstellar matter, as in how the ISM is structured.
Spitzer (1982) provides an exceptionally lucid and non-technical account of the theory and observations of the Milky Way’s ISM, while Spitzer (1978)provides a wealth of technical detail on the physics of the ISM. An outstanding collection of reviews of every aspect of the ISM will be found in Thronson & Shull (1992).
Figure 1 Absorption by interstellar Na atoms in spectrum of ε Ori. [After Hobbs (1968)]
