Willman 1. This image from the Sloan Digital Sky Survey has been processed to highlight Willman 1 (white blob) amongst the foreground stars. Credit: Beth Willman/New York University/Sloan Digital Sky Survey).
Willman 1 (Wil 1) is a satellite galaxy of our own Milky Way, which is by far the smallest galaxy known. It was discovered in 2004 by Beth Willman of New York University, orbits 120,000 light-years from the center of the Milky Way Galaxy.
Measurements made by a team of astronomers led by Nicolas Martin of the Max Planck Institute for Astronomy in Heidelberg, Germany, and published in 2007, shown that the stars within Willman 1 are moving at an exceptionally low velocity implying that this dwarf system has a very small gravitational pull equivalent to a mass of only about 500,000 times that of the Sun.
Martin and his team caution that it is possible for a heavier galaxy to mimic a lighter one, for example by having much more dark matter than one would expect in its outskirts. However, if Willman 1 really is as lightweight as it seems, then it may be only the first of many such ultra-low-mass galaxies waiting to be found. Hundreds of such systems could be in orbit around the Milky Way alone.