The detection of infrared point sources within many dense cores shows unequivocally that these cloud structures form stars. In this chapter and the next, we confront the central issue of how this transition actually occurs. Here we study, again from a theoretical perspective, the collapse process itself. Chapter 11 will then examine the properties of the nascent star and its infalling envelope of gas and dust.
In our study of equilibria, we remarked that no single model has self-consistently incorporated the combined effects of gravity, thermal pressure, rotation, and magnetic fields. The same statement applies to the problem of cloud collapse, apart from calculations that assume a disklike geometry (see § 10.1 below). Even if more general results were available, they would be difficult to appreciate without the necessary background. Accordingly, we follow our previous approach of building up the reader's intuition through simplified models that include only a subset of the relevant effects. Our first order of business, however, is to see how an initially stable cloud, like the observed starless dense cores, can evolve to the point where collapse becomes inevitable.
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