With this chapter, we begin a broad discussion of molecular clouds from a physical perspective. Our treatment throughout Part II will be more theoretical and quantitative than previously, as the underlying goal is to provide a basis for understanding cloud structure and protostellar collapse, the subjects of Part III. We will not attempt an exhaustive coverage of molecular cloud physics, but concentrate instead on those areas that seem at present to be most relevant for star formation.
To explore conditions within the clouds that produce stars, astronomers rely mainly on observations of spectral lines emitted by various molecules. Hence, our initial goals in this chapter are to describe how such species form throughout interstellar space and how their abundances reflect local conditions. We next turn to the simplest and most common molecules that have been employed as tracers of cloud properties. The aim is to present, succinctly but accurately, the physical principles underlying the most readily observed transitions. Chapter 6 will then demonstrate how these transitions are used in practice to determine cloud properties.
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