A maser source in which the water (H2O) molecule is excited to maser action. They are the most widely distributed of all the cosmic masers. There are many different H2O maser lines. The first to be discovered, in 1969, was the powerful line at 22.2 GHz (13.5 mm) in the Kleinmann–Low Nebula in Orion. Other H2 O lines at higher frequencies are difficult to observe with ground-based radio telescopes because of strong absorption by water vapour in the Earth's atmosphere. Water masers are found in star-forming regions, circumstellar envelopes, comets, and in the nuclei of some active galaxies in the form of megamasers.
Subjects: Astronomy and Astrophysics.