A maser, like a laser, is a source of bright electromagnetic radiation, with the difference being that maser radiation is not optical light but rather longer wavelength, microwave radiation. Small, dense molecular clouds in interstellar space sometimes produce natural masers; water vapor in clouds undergoing active star formation generates some of the most spectacular such masers. In the most dramatic cases, a water vapor maser can radiate more energy at a single wavelength than does the Sun over its entire visible spectrum.
Masers are interesting in their own right, but also because their bright emission provides a powerful diagnostic probe of regions where massive star formation is still underway. CfA astronomer Nimesh Patel and his colleagues have used a coordinated set of widely separated radio telescopes (an interferometer) to study a dramatic region of star formation about ten thousand light-years away, achieving a spatial resolution of only a few hundred astronomical units (one AU is the average distance of the Earth from the Sun). This spectacular precision is possible because the masers are so bright.
The star formation region was known to have several clumps of young, high mass stars accompanied by phenomena typically associated with such star birth like powerful outflows and shocks. The astronomers combined relatively recent and archival observations of masers in the region spanning a period of about ten years, starting in 1999; the precision of the data enabled them to detect the masers motion over this period. The maser clusters, which are distributed over about a thousand AU, had some clumps seen to move as much as fifty AU, corresponding to velocities of about twenty kilometers per second (forty-five thousand mph).
In the case of one bright region, the measurements over ten years found that the material is tracing the shell of an outward-moving shock, presumably propelled by radiation from the young star forming at the center. The results confirm and extend detailed models of how newly born massive stars affect their environment.
"Multi-epoch VLBA H2O Maser Observations Towards the Massive YSOs AFGL 2591 VLA 2 and VLA 3," J. M. Torrelles, M. A. Trinidad, S. Curiel, R. Estalella, N. A. Patel, J. F. Gomez, G. Anglada, C. Carrasco-Gonz´alez, J. Canto, A. Raga and L. F. Rodrıguez, MNRAS 437, 3803, 2014.