ABSTRACT
We have seen in Chapter 2 how microscopic processes give information to astronomers about the bulk properties of the interstellar medium; for example, observations of the 2.6 mm line of the CO molecule may tell us about the temperatures and densities of the regions in which the line is formed. However, these microscopic processes do more than that: they also control the bulk properties of the medium, such as temperature and density. In determining its properties they also determine the evolution of the interstellar material and so—to a large extent—they determine, for example, sites of star formation and other large-scale features of the Galaxy. In this chapter we discuss microscopic processes, especially those which directly or indirectly determine the temperature of the interstellar gas, for temperature is a dominating parameter.
