An interstellar maser is a maser formed through the interaction between high-energy starlight and a nearby region rich in molecules; the first was discovered in 1965. The conditions needed to produce an interstellar maser tend to be found especially with star-forming regions and with late-type stars that are losing mass.

 

The environs of stellar nurseries contain dense pockets of molecular material rich in hydroxyl (OH), water (H₂O), and methanol (CH₃OH) molecules. Nearby luminous infant O or B stars heat their dusty envelopes with energetic photons, causing the dust to re-emit in the infrared, which in turn excites the molecules into maser action. Within a cloud the maser-emitting region is no more than about 100 billion kilometers across (about 10 times the size of Pluto's orbit). Its power is given in terms of the number of maser photons emitted per second. For typical H₂O masers in star-forming regions, such as those in the Orion molecular cloud, this is about 10⁴⁶ per second ( s^–1); however, the H₂O maser associated with W 49, a Wolf-Rayet star, puts out 10⁴⁹ s^–1, making this the strongest maser source in the Galaxy. The intensity of maser emission can vary dramatically on a time scale of a year. One of the most spectacular examples is the H₂O outburst in Orion that happened in late 1979 and that lasted for 8 years. The intensity of this one maser component suddenly increased by a factor of 1,000, making it temporarily the brightest H₂O maser source in the sky.

 

Observational evidence indicates that once the young star begins to radiate through nuclear fusion, OH masers survive only until the ionized region expands to a diameter of about 0.3 light-year, while H₂O masers last for about 100,000 years after the star switches on. OH and H₂O masers have also been found in the nuclei of active galaxies, such as NGC 3079 and NGC 1068. The power of these extragalactic masers are stronger (by about a million for OH masers (megamasers) and by a thousand for H₂O masers) than known maser sources in our galaxy.

 

Masers are also found near long-period variables (LPVs), and arise when the turbulent upper photosphere of a luminous star undergoing mass loss is exposed to the radiation from below. These stars are typically class M (with 10,000 solar luminosities) and have cool surface temperatures, usually around 2,500K. Since they are evolved, their photospheres contain appreciable abundances of heavier elements, including silicon and oxygen, which supply an ideal environment in which silicon dioxide (SiO) masers can form. The LPV VY Canis Majoris is a prime example of such a star. VY CMa shows a triple peaked SiO maser line at 43 GHz. Physically, this has been interpreted as a spherically symmetric circumstellar envelope with an inner maser region at rest relative to the star, and an outer masing region expanding away from the star at around 10 kilometers per second; this gives rise to blue- and red-shifted peaks on either side of the one at rest. For more details, see circumstellar maser.

 

Because interstellar maser emission often is very strong and arises from extremely compact regions, it can be observed with interferometric methods, such as very long baseline interferometry, that yield high spatial resolution. Such observations provide detailed information on the physical conditions in the emission regions and their chemical composition, velocity, and magnetic field structure.

 

Molecules that exhibit maser action in celestial objects
molecule	name	frequency (GHz)	characteristics*
OH	hydroxyl	1.612	O, M
	"	1.667	O, M
	"	1.720	O
H2CO		4.829	O
CH3OH	methanol	12.178	O
SiS	silicon sulfide	18.155	C
H2O	water	22.235	O, M
NH3	ammonia	23.870	O
SiO	silicon oxide	43.122	M, S, O
	"	86.243	M, S
HCN	hydrogen cyanide	89.087	C

*O means that the maser emission is frequently found in star-forming regions; M, in M stars; S, in S stars; C, in carbon stars