Astrophysical maser

An astrophysical maser is a natural source of stimulated emission that appears as a spectral line, most often in the microwave range. Such emissions can originate in molecular clouds, comets, planetary and stellar atmospheres, and other interstellar environments.

Like a laser, a maser’s emission is seeded and monochromatic, matching the energy gap between two quantum levels of the emitting species, which has been pumped into a non-thermal population distribution. However, natural masers lack the engineered resonant cavity of laboratory masers. Their emission results from a single pass through the gain medium, so they generally lack the spatial coherence and pure mode structure of a lab-built maser.

It is often claimed that astrophysical masers are not “true” masers because they lack an oscillation cavity. Yet the laser community deliberately set aside the distinction between oscillator-based and single-pass lasers early on. This language mismatch has led to other paradoxical definitions. For instance, if a misaligned laser’s gain medium produces seeded but non-oscillating radiation, it is called amplified spontaneous emission (ASE), which is considered unwanted or parasitic. Some researchers add that such a system lacks sufficient feedback or has not met the lasing threshold—meaning the user intended it to behave as a laser. Astrophysical maser emission is actually ASE, but is sometimes called superradiant emission to distinguish it from the lab phenomenon. This only adds confusion, as both sources 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 also applied inconsistently. When lasers were first developed in the visible range, they were called optical masers. Charles Townes argued that the “m” should stand for molecule, since molecular energy states typically provide the masing transition. Along those lines, some use “laser” for any system exploiting an electronic transition and “maser” for one exploiting 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 lasers. The term “taser” has been used for lab masers in the terahertz regime, though astronomers might call these sub-millimeter masers and lab physicists often call them gas lasers or, specifically, alcohol lasers after the gain species. The electrical engineering community generally restricts “microwave” to frequencies between about 1 GHz and 300 GHz—wavelengths from 30 cm to 1 mm.

Simply having a pumped population inversion is not enough to observe a maser. Velocity coherence along the line of sight is required so that Doppler shifting does not prevent inverted states in different parts of the gain medium from radiatively coupling. While polarization in lab lasers and masers can be achieved by selectively oscillating desired modes, polarization in natural masers arises only from a polarization-state-dependent pump or a magnetic field in the gain medium.

Astrophysical maser radiation can be quite weak and may go undetected due to limited sensitivity, the remoteness of observatories, and spectral absorption from unpumped molecules of the same species in surrounding space. This last obstacle can be partly overcome by using spatial filtering inherent in interferometric techniques, especially very long baseline interferometry (VLBI). Studying masers provides valuable information about temperature, density, magnetic fields, and velocities in environments of stellar birth and death, as well as in galactic centers containing black holes, helping refine theoretical models.

In 1965, Weaver and colleagues made an unexpected discovery: emission lines in space at 1665 MHz of unknown origin. At the time, many researchers still thought molecules could not exist in space, despite McKellar’s discoveries in the 1940s. The emission was initially attributed to a hypothetical interstellar substance called “mysterium,” but was soon identified as line emission from hydroxide molecules in compact sources within molecular clouds. More discoveries followed: water emission in 1969, methanol in 1970, and silicon monoxide in 1974, all from within molecular clouds. These were called masers because their narrow line widths and high effective temperatures indicated they were amplifying microwave radiation.

Masers were then found around highly evolved late-type stars, called OH/IR stars: hydroxide emission in 1968, water in 1969, and silicon monoxide in 1974. Masers were discovered in external galaxies in 1973, and in the Solar System in comet halos. Another unexpected discovery came in 1982.