Having surveyed the gas content of the Galaxy, we now take a closer look at the molecular component. Table 3.1 summarizes the physical properties of Galactic molecular clouds. For later reference, we have distinguished a number of cloud types, but any such classification scheme necessarily has a degree of arbitrariness. The diameter L, for example, is really a characteristic value within a range that blends into the adjacent type. Note that our listing is in order of increasing AV, the typical visual extinction along a line of sight through the cloud interior. At the low end are diffuse clouds. These are relatively isolated entities, with comparable amounts of atomic and molecular hydrogen. The fact that AV is near unity means that much of the light from background stars can actually traverse these objects. Absorption lines seen in such radiation, particularly in the ultraviolet, have proved to be of considerable value for studies of molecular abundances and chemical reaction networks. However, diffuse molecular clouds represent a minor fraction of interstellar gas and are never found to produce stars, so we will not be examining them in any detail.
We pass instead to the next category in Table 3.1, the giant molecular clouds. Here, the reader has already encountered one important example, the complex in Orion. We first discuss more systematically the properties of such structures. To aid in the analysis, we introduce and then utilize the virial theorem, a powerful tool for understanding the mechanical equilibrium of self-gravitating bodies. We then turn our attention to dense cores and Bok globules, the much tinier entities associated with the birth of individual stars.
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