Megamaser

A megamaser is an exceptionally luminous natural microwave amplifier, usually located in another galaxy. Its emission arises when molecular gas amplifies microwave radiation through stimulated emission.

Maser: A maser amplifies microwave or radio-frequency radiation through stimulated emission. Megamasers are the exceptionally luminous extragalactic members of this broader class.
NGC 4258: A nearby spiral galaxy whose water megamaser traces a rapidly rotating disk around a massive central object. Its precise maser spots enabled a geometric measurement of the galaxy's central mass and distance.
Very Long Baseline Interferometry: A radio astronomy technique that combines widely separated antennas to produce extremely high angular resolution. It resolves individual maser spots and measures their positions and motions.
Astrophysical maser: A naturally occurring maser produced by stimulated emission in space. Megamasers are exceptionally luminous examples of this broader astronomical phenomenon.
Charles Townes: An American physicist who co-invented the maser and laser and shared the 1964 Nobel Prize in Physics. His maser work established the stimulated-emission principle later observed in space.
Stimulated emission: A process in which an incoming photon prompts an excited atom or molecule to emit a matching photon. This quantum process creates the coherent amplification underlying megamaser emission.
Water megamaser: A natural microwave amplifier produced by water molecules, commonly emitting near a wavelength of 1.35 centimeters. Water megamasers reveal dense, warm gas near active galactic nuclei.
Hubble constant: The present-day rate at which the universe's expansion increases distance between sufficiently separated galaxies. Geometric distances to water megamaser galaxies provide an independent route to measuring it.
Kilomaser: An extragalactic maser whose luminosity is intermediate between ordinary galactic masers and megamasers. Its lower luminosity separates it from the exceptionally bright systems classified as megamasers.
Maser astronomy: The study of natural microwave amplification by stimulated emission in astronomical environments. Megamasers became a luminous extragalactic branch of this field.

Linked from 1 page

Messier 106Narrower topic: Messier 106’s water emission is unusually powerful and belongs to the extragalactic megamaser phenomenon.