Astronomical masers are observed in star-for ming regions or stellar objects in out Galaxy, but also in the nuclear regions of other galaxies. Certain molecules can be 'pumped' into an excited state by their environment, such that they amplify their own line emissions or some background continuum radiation. This 'avalanche of photons' results in luminous maser emission lines that serve as valuable diagnostics of the physical environment. Strong interstellar OH (hydroxyl) masers in our Galaxy are mostly pumped by colli-sional processes and shocks, but OH 'mega-masers' in the nuclei of other galaxies are typically a million times, more luminous.
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