Observations of the Cooling of the Interstellar Gas

Cooling of the gas of the dense interstellar clouds takes place primarily at submillimeter wavelengths. At about 1mm cooling is due to the many rotational lines of heavy moleules, but at shorter wavelengths light molecules (hydrides) become the dominant species, with some assistance from the fine structure transitions of atomic carbon. Photodissociation regions at the surfaces of molecular clouds are cooled by fine structure transitions of atoms and atomic ions. A presentation is given of some aspects of the observed emission due to heavy molecules, light molecules, atoms and ions.