Astrophysical maser
Naturally occurring masers amplify microwave radiation in space.
An astrophysical maser is a natural source of stimulated spectral line emission, usually found in the microwave part of the electromagnetic spectrum. It can occur in molecular clouds, comets, planetary atmospheres, stellar atmospheres, and other conditions in interstellar space.
Like a laser, a maser’s emission is seeded and monochromatic, matching the frequency of the energy gap between two quantum-mechanical energy levels of the species in the gain medium, which has been pumped into a non-thermal population distribution. However, natural masers do not have the resonant cavity engineered into laboratory masers. Instead, the emission comes from a single pass through the gain medium, so it generally lacks the spatial coherence and mode purity of a lab maser.
It is often said that astrophysical masers are not "true" masers because they lack oscillation cavities. But the laser community deliberately disregarded the distinction between oscillator-based lasers and single-pass lasers early on. This language mismatch has led to other paradoxical definitions. For instance, if the gain medium of a misaligned laser produces emission-seeded but non-oscillating radiation, it is called amplified spontaneous emission (ASE), which is considered unwanted or parasitic. Some researchers add that this requires insufficient feedback or an unmet lasing threshold—meaning the system is intended to behave as a laser. Astrophysical maser emission is actually ASE, but is sometimes called superradiant emission to distinguish it from the lab version, which only adds confusion since both are superradiant. In some lab lasers, like a single pass through a regeneratively amplified Ti:Sapph stage, the physics is directly analogous to an amplified ray in an astrophysical maser.
The practical limits of the "m" in maser, standing for microwave, are used in various ways. When lasers were first developed in the visible spectrum, they were called optical masers. Charles Townes argued that "m" should stand for molecule, since molecular energy states usually provide the masing transition. Along these lines, some use "laser" for systems exploiting electronic transitions and "maser" for those using rotational or vibrational transitions, regardless of output frequency. Some astrophysicists use "iraser" for a maser emitting at a few micrometres, even though the optics community calls similar sources
- field
- Astrophysics
- known_for
- Naturally occurring stimulated spectral line emission in space
- first_discovered_species
- OH (hydroxide) in 1965
- notable_types
- Megamasers, disk masers
- common_species
- OH, H2O, CH3OH, SiO, HCN, NH3, H2CO
Lore & Background
In 1965, Weaver et al. unexpectedly discovered emission lines in space at a frequency of 1665 MHz, initially attributed to a hypothetical form of interstellar matter named 'mysterium' before being identified as line emission from hydroxide molecules in compact sources within molecular clouds. Further discoveries followed: water emission in 1969, methanol emission in 1970, and silicon monoxide emission in 1974, all emanating from within molecular clouds. These were termed masers because their narrow line widths and high effective temperatures indicated they were amplifying microwave radiation.
Masers were subsequently discovered around highly evolved late-type stars (OH/IR stars), in external galaxies in 1973, and in comet halos within the Solar System. In 1982, an extra-galactic source with luminosity about 10^6 times larger than any previous source was discovered and termed a megamaser. 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.
Reader's Guide
Astrophysical masers provide valuable information on the conditions—temperature, density, magnetic field, and velocity—in environments of stellar birth and death and the centers of galaxies containing black holes, leading to refinements in existing theoretical models. Their study has been enabled by techniques such as very long baseline interferometry (VLBI), which helps overcome spectral absorption from unpumped molecules. The exponential gain of maser radiation produces characteristic features: beaming (where maser spots are much smaller than their parent clouds), rapid variability, line narrowing, saturation, and high brightness temperatures. The nomenclature surrounding astrophysical masers remains debated; some researchers argue they are not 'true' masers due to the lack of oscillation cavities, while others note that the distinction between oscillator-based and single-pass lasers was intentionally disregarded by the early laser community. The emission from astrophysical masers is amplified spontaneous emission (ASE), sometimes termed superradiant emission, adding to the confusion since both sources are superradiant.
Did You Know?
- The first astrophysical maser emission was discovered in 1965 at 1665 MHz and initially attributed to a hypothetical substance called 'mysterium'.
- Megamasers have luminosities about 10^6 times larger than any previously known maser source.
- Astrophysical masers lack the resonant cavity of laboratory masers and produce emission through a single pass through the gain medium.
- The term 'maser' was originally advocated by Charles Townes to stand for 'molecule' rather than 'microwave'.
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