The physical state of the interstellar medium varies greatly from one region to the next because the gas temperature depends on the local energy input. There exist large, cool cloud complexes in which both dust grains and many different molecular species are abundant. Often new stars are born in these dense clouds, and since they are sources of thermal energy the stars will heat the gas surrounding them. If the stellar surface temperature is sufficiently high, most of the energy will be emitted as photons with λ < 912 Å. This radiation has sufficient energy to ionize hydrogen. Thus young, luminous stars embedded in gas clouds will be surrounded by emission regions in which the gas temperature and consequently the pressure will be much higher than in cooler clouds. The emission nebulae therefore will expand; this expansion is probably aided by strong stellar winds.
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