Most Galactic star formation occurs in giant molecular clouds. These structures are clumpy and possess extended envelopes of HI gas. Both the random motion of clumps and the pressure associated with an internal magnetic field prevent immediate gravitational collapse. Thermal pressure is unimportant in this regard. After a period not greatly exceeding 107 yr, the entire cloud is dispersed by winds and radiative heating from massive stars it previously created.
A convenient tool for analyzing molecular clouds, or any other self-gravitating structure, is the virial theorem. Here we compare the magnitudes of global, integrated quantities such as the gravitational potential and bulk kinetic energies. In this way, one avoids constructing a detailed interior model, while still gaining understanding of the dominant forces at play.
Giant molecular clouds lie at one end of a hierarchy of morphological types, differing in both linear size and mass density. The internal motion of all clouds decreases systematically with size. Near the other end of the hierarchy are the dense cores and Bok globules that actually produce individual stars and binaries. Cores and globules are intrinsically elongated and rotate slowly in space. Prior to the start of protostellar collapse, they are supported against self-gravity by a combination of thermal and magnetic pressure.
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