OH, the hydroxyl radical, holds the distinction of being both the first molecule detected in interstellar space at radio frequencies (Weinreb et al. 1963) and the first astronomical maser ever discovered (Weaver et al. 1965). This molecule still provides the largest variety of astronomical sources displaying maser activity. Strong OH maser emission is observed in both star-forming regions and late-type stars. Weak maser effects are prevalent in large molecular clouds and in comets as they circle around the sun. OH maser emission has also been detected in external galaxies. It is appropriate that OH masers be the first ones discussed in detail.
KeywordsOptical Depth Main Line Column Density Maser Emission Satellite Line
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