In Chapter 2, I recount a brief history of the discovery of the major maser species. This chapter is designed to be understood by a reader with little mathematical knowledge. For those more interested in the technical details relating to radio telescopes, I refer the reader forward to Chapter 4, and particularly to Section 4.1.

During the late 1950s, technological developments allowed radio telescopes to observe at frequencies above 1 GHz (the L-band in radar terminology). A key L-band transition is the ‘spin-flip’ line of atomic hydrogen at 1421 MHz, which allowed astronomers to map the neutral hydrogen content of the Galaxy. Clouds excited by the formation of massive stars are photoionized, rather than neutral, and ionized hydrogen emits a continuous spectrum of radiation through the thermal bremsstrahlung process (see Section 2.9.1). As this process is thermal, it has a spectrum which rises with frequency at radio wavelengths if optically thick (see Eq. (4.7)), making it stronger at L-band relative to non-thermal emission, when compared with earlier observations at lower frequencies. Non-thermal radiation processes typically have a spectrum which decays as a power-law with frequency in the radio region. A catalogue of sources containing ionized hydrogen, comprising mostly massive, star-forming, Galactic gas clouds, was constructed by Westerhout (1958) from continuum observations at 1390 MHz, and its sources were classified by a W-number (for example W49), a nomenclature which has continued to be used up to the present day, and in particular for the case study of W3(OH) in Section 2.9.