A radio source in which the spectral lines of an atom, ion, or molecule are greatly amplified by maser action to produce an intense source of radio emission. The first maser to be identified in space was the OH maser at 1665 MHz, in 1965. Maser emission is now known from many other molecules, including water (H₂O), silicon monoxide (SiO), methanol (CH₃OH), ammonia (NH₃), formaldehyde (H₂CO), hydrogen cyanide (HCN), and silicon sulphide (SiS), and from hydrogen ions. Sometimes many spectral lines from the same molecule can be observed as masers. For example, over 30 different maser lines of SiO have been observed in the circumstellar envelope of the star VY Canis Majoris. Masers occur in star-forming regions (interstellar masers), in the circumstellar envelopes of red giant stars (circumstellar masers), in comets, in some planetary atmospheres, and in some active galactic nuclei, where they may be so bright that they are classed as megamasers. See also Cyclotron Maser; H₂O Maser; Methanol Maser; Sio Maser.