The Andromeda Galaxy (M31) is the largest member of the Milky Way’s gang of galactic neighbors, known as the Local Group. With around a trillion suns worth of mass, Andromeda’s gravitational influence is a force to be reckoned with. And according to new research, no galaxy in the Local Group knows this better than M32, an oddball satellite galaxy now orbiting Andromeda.In a study published today in Nature Astronomy, researchers showed that about 2 billion years ago, the Andromeda Galaxy cannibalized one of the largest galaxies in the Local Group, turning it into the strange compact galaxy known as M32 that we see bound to Andromeda today. This massive collision stripped M32’s progenitor galaxy (dubbed M32p) of most of its mass – taking it from a hefty 25 billion solar masses to just a few billion solar masses.“Astronomers have been studying the Local Group — the Milky Way, Andromeda, and their companions — for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said co-author Eric Bell, an astronomer from the University of Michigan, in a press release Andromeda’s formation history is a bit of a murky subject. Although some previous studies have suggested that Andromeda grew over the eons by steadily merging with many smaller galaxies, others indicate that the galactic colossus underwent a single large merger at some point in its past.To investigate how Andromeda accumulated its mass, the authors of the new study ran cosmological simulations of galaxy formation to show that Andromeda’s observed properties — including a massive yet nearly invisible halo of stars — can be well explained by a single major merger with what was once the third-largest galaxy in the Local Group, M32p. This long-dead galaxy was at least 20 times as massive as any galaxy that has ever merged with the Milky Way. And, based on the study, its corpse is still likely traveling around Andromeda in the form of M32.“M32 is a weirdo,” said Bell. “While it looks like a compact example of an old, elliptical galaxy, it actually has a lot of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.”