Introduction – a global view of the interstellar medium

Hendrik van de Hulst, the theorist who predicted the 21-cm line of neutral hydrogen, once remarked that, if you set out to detect an emission or absorption line from an atom, ion or molecule in astronomy, you are bound to discover it somewhere in the Universe. This statement is particularly true of the interstellar medium because it is now understood that it is far from equilibrium and that a very wide range of densities and temperatures are present – those found largely reflect the characteristics of the observing tools used by the astronomer. It is no surprise, therefore, that there is a great deal of physics to be studied. Astrophysically, the understanding of the nature and properties of the interstellar gas is of the first importance, since it is out of this medium that new stars are formed. It is continually replenished because of mass loss from stars, and so the medium plays a key role in the birth-to-death cycle of stars. The same astrophysics is applicable to the study of diffuse gas anywhere in the Universe, be it galaxies, the intergalactic gas or the gas clouds in the vicinity of active galactic nuclei. These diagnostic tools are essential for determining the physical conditions in which high energy astrophysical processes take place. Furthermore, interstellar gas will prove to be an essential ingredient of the fuelling mechanisms for active galactic nuclei.

The interstellar medium amounts to about 5% of the visible mass of our Galaxy.