Science: Estimating the total mass of a galaxy is not an exact science—partly because most of the mass is due to dark matter, which can’t be seen. An earlier estimate of the Milky Way’s mass, made in 2009 and based on measurements of its rotation speed, put the total at a few trillion times the mass of the Sun. New data from the European Southern Observatory in Chile and the William Herschel Observatory in Spain, however, show that the stars farthest from the galactic center are moving much more slowly than previously thought. Using that information, Alis Deason of the University of California, Santa Cruz, and colleagues have determined that the galaxy’s mass could be less than half the 2009 estimate. The researchers presented their results at this week’s annual meeting of the American Astronomical Society.