We now turn our attention to the mechanisms by which molecular clouds lose energy to interstellar space. Within a quiescent cloud, neither hydrogen nor helium can radiate away an appreciable fraction of the total thermal energy content. Hence, any cooling of the gas is necessarily indirect and relies on the presence of minor constituents that do act as effective radiators. Hydrogen and helium collide inelastically with ambient atoms, molecules, and dust grains, exciting internal degrees of freedom. These excitations decay through the emission of photons.
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