An Etymological Dictionary of Astronomy and Astrophysics

1. A source of very intense, narrow-band, coherent microwave radiation involving → stimulated emission, as in the → laser.
   

2. A device that generates such radiation.
   

3. In astronomy, maser emission detected from a number of molecules and associated with several environments: the vicinity of newly forming stars and → H II regions (OH, water, SiO, and methanol masers); the circumstellar shells of evolved stars, i.e. red giants and supergiants (OH, water, and SiO masers); the shocked regions where supernova remnants are expanding into an adjacent molecular cloud (OH masers); and the nuclei and jets of active galaxies (OH and water masers). The hydroxyl radical (OH) was the first interstellar maser detected (Weinreb et al. 1963).

See also: Maser stands for Microwave Amplification by Stimulated Emission of Radiation; → laser.