An Etymological Dictionary of Astronomy and Astrophysics
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An interstellar → maser phenomenon in which water (H₂O) molecules undergo the processes of → population inversion and → stimulated emission. H₂O masers are detected toward star formation regions and the envelopes of evolved stars. The maser emission comes from regions that are typically quite small, not larger than the solar system. The main emission frequency is 22 GHz, which shows up in strong lines. There are, however, other H₂O maser transitions at 380 GHz and 183 GHz, which are much weaker than the 22 GHz line. The former transitions are sporadically detected since they are strongly absorbed in the Earth's atmosphere, because of its high water vapor content.

→ water; → maser