Boylan-Kolchin says there's a 90 percent chance that the Milky Way is between one trillion and 2.4 trillion times as massive as the sun. He hopes Hubble will provide precise proper motions of two other distant satellite galaxies—Leo II and Canes Venatici I—as well as Leo T, a galaxy even farther than Leo I that does not yet orbit the Milky Way but is falling toward it. Then he thinks he can cut the uncertainty in the Milky Way's mass drastically.
Our galaxy's exact mass has implications throughout the universe. "The Milky Way is the keystone in understanding more distant galaxies," Boylan-Kolchin says. "Therefore, getting a good mass for the Milky Way is very important for modeling the Milky Way and other galaxies."
Lest the new work fill Milky Way residents with excessive pride, Beers says, "We're still number two." Of the several dozen galaxies that populate the Local Group, the largest remains Andromeda, which Beers estimates is at least twice as massive as the Milky Way. Still, most new Local Group stars are made right here in the Milky Way rather than in its larger neighbor.