The vast Andromeda galaxy has been growing by gobbling up neighboring galaxies, reports the BBC. Astronomers mapping the galaxy—at 2.5 million light years away, the Milky Way's closest neighbor—discovered stars within that they believe to be the remnants of dwarf galaxies consumed by Andromeda. The galaxy is currently sucking millions of stars away from the Triangulum galaxy, and will eventually consume it completely.
The finding demonstrates the "hierarchical model" of galaxy formation, researchers say. "Because of the gravity of the bigger galaxies, smaller galaxies get pulled apart, and stars in them get pulled out into long stellar streams," one astronomer explained. The Milky Way—which is currently absorbing the Sagittarius dwarf galaxy—will someday become fodder for Andromeda itself, astronomers say, although not for another few billion years.