in 1969, methanol emission in 1970, and silicon monoxide emission in 1974, all emanating from within molecular clouds. These were termed masers, as from their narrow line widths and high effective temperatures it became clear that these sources were amplifying microwave radiation. Masers were then discovered around highly evolved late-type stars (named OH/IR stars). First was hydroxide emission in 1968, then water emission in 1969 and silicon monoxide emission in 1974. Masers were also discovered in external galaxies in 1973, and in the Solar System in comet halos. Another unexpected discovery was made in 1982 with the discovery of emission from an extra-galactic source with an unrivalled luminosity about 106 times larger than any previous source. This was termed a megamaser because of its great luminosity; many more megamasers have since been discovered. A weak disk maser was discovered in 1995 emanating from the star MWC 349A, using NASA's Kuiper Airborne Observatory. Evidence for an anti-pumped (dasar) sub-thermal population in the 4830 MHz transition of formaldehyde (H2CO) was observed in 1969 by Palmer et al. Detection The connections of maser activity with far infrared (FIR) emission has been used to conduct searches of the sky with optical telescopes (because optical telescopes are easier to use for searches of this kind), and likely objects are then checked in the radio spectrum. Particularly targeted are molecular clouds, OH-IR stars, and FIR active galaxies. Known interstellar species The following species have been observed in stimulated emission from astronomical environments: OH CH H2CO H2O NH3, 15NH3 CH3OH HNCNH SiS HC3N SiO, 29SiO, 30SiO HCN, H13CN H (in MWC 349) CS Characteristics of maser radiation The amplification or gain of radiation passing through a maser cloud is exponential. This has consequences for the radiation it produces: Beaming Small path differences across the irregularly shaped maser cloud become greatly distorted by exponential gain. Part of the cloud that has a slightly longer path length than the rest will appear much brighter (as it is the exponent of the path length that is relevant), and so maser spots are typically much smaller than their parent clouds. The majority of the radiation will emerge along this line of greatest path length in a "beam"; this is termed beaming. Rapid variability As the gain of a maser depends exponentially on the population inversion and the velocity-coherent path length, any variation of either will itself result in exponential change of