Introduction

Abstract

Astronomical masers occur naturally in the interstellar space, the site of many fascinating physical phenomena. Studies of the interstellar medium began at the turn of this century with the discovery of evidence for the existence of interstellar material. Various atomic absorption lines in stellar spectra displayed a correlation between depth of absorption features and distance to the background star, indicating that the absorbing material resides in the intervening space. In addition, certain absorption lines in the spectra of some binary stars did not partake in the periodic velocity variations exhibited by the majority of lines, again indicating an origin outside the stellar systems. Studies of the interstellar medium accelerated considerably after the discovery of radio emission at 21 cm, the wavelength of the spin-flip transition between the ground-state hyperfine levels of atomic hydrogen. Observations at 21 cm revealed the presence of cool (temperatures ≲150 K) and dense (densities ≳10 cm⁻³) clouds imbedded in a hot (T∼10⁴ K) and rarefied (n∼0.1 cm⁻³) intercloud medium.