Astrophysical masers

A brief introduction to the concept of a natural maser is followed by a history of the discovery of masers in space and a discussion of their basic properties. Several arguments are put forward that effectively exclude the possibility of such bright, compact radio sources arising from any other radiation process. The present state of observational knowledge is reviewed with reference to various source regions: massive star-forming regions; circumstellar envelopes; cores of external galaxies (megamasers); supernova remnants; comets; and planetary atmospheres. A complementary review of maser theory discusses progress on the understanding of the saturation, polarization and beaming of astrophysical masers, and the need for accurate molecular data, such as collisional rate coefficients. Likely future opportunities are discussed for both observations and theory. New instruments, some space based, and improved detector technology will revolutionize maser observations, while improved computing power will allow ever more sophisticated modelling. Various applications of masers as astrophysical tools are considered, with applications to distance measurement, cosmology and the SETI programme.