Pressure Equilibrium

We first need to draw a distinction between HII regions and the others. The ionized gas here arises from the ultraviolet radiation of nearby, massive stars. Each individual region is thus a transient phenomenon that disappears over the 106 yr lifetime of its parent object. Neutral hydrogen, however, persists for much longer periods and over vast regions devoid of stars.

Consider first the atomic component. With regard to its origin and stability, an essential clue is provided by the gas pressure. For a typical HI cloud with number density n = 30 cm~3 and temperature T = 80 K, the product nT is 2 x 103 cm~3 K. This figure is matched, within

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