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Molecules present in the interstellar medium, especially in molecular clouds. They can survive only when shielded from the destructive effect of ultraviolet radiation from stars and so are found in dense interstellar or circumstellar clouds. Prior to 1963, CH (methylidyne), CH+ and CN (cyanogen) were the only interstellar molecules known, having spectra in the visible range. Radio emission at 18 centimetres wavelength was recognized as being from hydroxyl (OH) in 1963. Over 90 different molecules have been identified since 1968, primarily by means of their characteristic spectra at millimetre wavelengths. Most are simple organic molecules.
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