maser source

A small celestial source of electromagnetic radiation, displaying narrow spectral lines and having an anomalously high brightness temperature, in which the emission is thought to be by maser action: radiation at the maser frequency is amplified when excited molecules in the source emit photons in phase with the passing radiation (maser stands for microwave amplification by stimulated emission of radiation). Several different types have been identified including OH (hydroxyl) masers radiating at 1.665 gigahertz, H₂ O (water) masers at 22.235, 321, 325, and 380 GHz, SiO (silicon oxide) masers at 43.424 and 86.243 GHz, and CH₃ OH (methanol) masers at frequencies up to 338 GHz.

Maser sources have been detected, for example, in the atmospheres of old variable stars and in molecular clouds associated with newly formed or forming stars. The first maser source – an OH maser – was discovered in the Orion nebula in 1965. Hundreds of sources containing hydroxyl masers have since been found, some of which also have water masers. Masers are usually very variable radio sources and produce highly polarized radio emission from regions that are typically about one parsec in diameter. Because of their small size, very precise position measurements are possible using VLBI. Proper motion measurements over a period of several years have allowed a much more precise measurement of the distance to several giant molecular clouds and to the galactic center, using the techniques of statistical parallax and moving-cluster parallax. See also Mira stars; OH/IR star.

单词	maser source
释义	maser source maser source A small celestial source of electromagnetic radiation, displaying narrow spectral lines and having an anomalously high brightness temperature, in which the emission is thought to be by maser action: radiation at the maser frequency is amplified when excited molecules in the source emit photons in phase with the passing radiation (maser stands for microwave amplification by stimulated emission of radiation). Several different types have been identified including OH (hydroxyl) masers radiating at 1.665 gigahertz, H₂ O (water) masers at 22.235, 321, 325, and 380 GHz, SiO (silicon oxide) masers at 43.424 and 86.243 GHz, and CH₃ OH (methanol) masers at frequencies up to 338 GHz. Maser sources have been detected, for example, in the atmospheres of old variable stars and in molecular clouds associated with newly formed or forming stars. The first maser source – an OH maser – was discovered in the Orion nebula in 1965. Hundreds of sources containing hydroxyl masers have since been found, some of which also have water masers. Masers are usually very variable radio sources and produce highly polarized radio emission from regions that are typically about one parsec in diameter. Because of their small size, very precise position measurements are possible using VLBI. Proper motion measurements over a period of several years have allowed a much more precise measurement of the distance to several giant molecular clouds and to the galactic center, using the techniques of statistical parallax and moving-cluster parallax. See also Mira stars; OH/IR star.
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