An astrophysical MASER (Microwave Amplification by Stimulated Emission of Radiation) is a source of stimulated spectral line emission. Maser emission is observed from the circumstellar envelopes of evolved stars, molecular clouds/star-forming regions, active galactic nuclei, supernova remnants, comets, and the Saturnian moons. It arises from molecules such as water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), silicon monoxide (SiO), ammonia (NH3), silicon sulphide (SiS), hydrogen cyanide (HCN), and from atomic hydrogen recombination lines. Masers are compact, of high brightness temperature, and often display narrow spectral line widths, polarized emission and variability. Free electron-cyclotron astrophysical masers additionally exist.
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