Masers have been detected in a wide variety of astrophysical environments. Perhaps the most astounding feature is the range of scales: the smallest maser environments are objects familiar to us from our own Solar System – comets and planetary atmospheres – whilst the largest masers form in molecular tori around the nuclei of certain galaxies, and may be up to 1 kpc (∼3 × 1019 m) in size. Some of these environments are so violent that, in a naive view, its is difficult to see how the necessary molecules can survive. However, it is the extreme nature of the environments that aids the pumping of masers. Often, we can deduce that gas molecules have motions characteristic of one temperature (a local kinetic temperature) whilst the radiation which is present is characteristic of a different, and usually higher, temperature. Maser molecules cannot attain a distribution of population amongst their energy levels which represents an equilibrium at either temperature, and these nonlocal-thermodynamic-equilibrium (NLTE) conditions allow population inversions to form.

Galactic star-forming regions

The formation of stars from the gravitational collapse of clouds of interstellar gas remains, in its details, one of the great unsolved problems of astrophysics. Our Galaxy, the Milky Way, is a spiral type, which is still forming stars at a significant rate at the current epoch; not all galaxies do. Elliptical and lenticular galaxies have very little interstellar gas compared with spirals, and are forming very few new stars. Within the spiral category, ‘early’ types (Sa, SBa), with large nuclei and tightly wound arms, are comparatively gas-poor compared with ‘late’ types (Sc, SBc), with relatively smaller nuclei and more open spiral arms.