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An astronomical source emitting microwave radiation characteristic of the hydroxyl (OH) molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar space and in the cool envelopes of evolved stars. The first detection of OH was in 1963. Four microwave spectral lines, near a wavelength of 18 centimetres, were seen in absorption from the direction of the galactic centre against the radio source Sagittarius A. In 1965 emission was detected from a source in the Orion Nebula in which the relative strengths of the four lines were not as predicted by theory and as seen in normal sources. A line at a frequency of 1,665 MHz was found to be fifty times too strong; this was explained in terms of maser action. The OH molecules absorb infrared radiation then re-radiate the energy at the particular frequency of the 1,665-MHz spectral line.
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