Today's Astronomy Picture of the Day discusses a method for determining the distance to a galaxy that I had not seen before. https://apod.nasa.gov/apod/ap170707.html For a far away galaxy, the distance to M106 is well-known in part because it can be directly measured by tracking this galaxy's remarkable maser, or microwave laser emission.I tried to find some references to this method by searching the Internet, but all I could find was https://en.wikipedia.org/wiki/Astrophysical_maser#Extragalactic_sources,and this was not useful. I would much appreciate some help in locating a reference that explains in detail how this distance method works. ADDED I found another source that gave a bit more information about the method but without the detail I am seeking. https://www.cfa.harvard.edu/research/rg/extragalactic-distance-scale Supermassive black holes are now thought to lie at the centers of many if not all galaxies. Under the right conditions, X-ray emission from hot gas very close to the black holes can stimulate water molecules further out to emit maser (i.e., microwave laser-like) emission. The emission lines from these maser regions are so sharp and strong, and the angles measurable by very long baseline interferometry so exquisitely small (milliarcseconds) that it is possible to measure maser orbital velocities to within a fraction of a parsec of the black hole, just a few tens of thousands of Schwardschild radii. Further, these masers are orbiting so rapidly (~1000 km/s) that after a few years it is possible to measure quite readily orbital accelerations. These measurements can be used to obtain very direct "geometric" distances for their host galaxies, free of the specific systematic and calibration errors that occur in the conventional "distance ladder" methods for determining extragalactic distances (e.g., measurement of Cepheid light curves). Combination of maser and traditional techniques has provided the best estimate of the distance scale to date.