Masers
  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.
