In 1965 a number of radio emission spectra were found very intense and narrow lines c P = 18, see Further investigation showed that the lines belong to the molecule OH, and their unusual properties - the result of maser radiation. In 1969 the opening of maser sources of water molecules at P = 1,35 cm, were later discovered masers working on other molecules. For the maser radiation is necessary population inversion levels (number of atoms in the upper resonant level higher than the bottom). Then passing through the substance with the resonant frequency of the light wave is rising, not diminishing (this is called a maser effect). In order to maintain the inverse of the population requires constant pumping power, so all the space masers in two types:
Masers associated with young (age 105 years), the CQ-hot stars (and possibly with protozvezdami) and located in the areas of star formation.
Masers associated with strong cold proevolyutsionirovavshimi high luminosity stars.