Andromeda has swallowed the Milky Way’s sister galaxy
In four billion years, the Milky Way will collide with our large neighbouring galaxy of Andromeda. Astronomers think that a similar collision took place 2 billion years ago, when Andromeda swallowed the Milky Way’s sister galaxy. The remains can be observed today as a small satellite galaxy known as M32.
Andromeda and the Milky Way are the two major galaxies of the Local Group, that includes a total of 54 galaxies. During its life span of about 10 billion years, Andromeda probably swallowed a series of smaller galaxies, but so far, it has been impossible to reconstruct their sizes and the distribution of their stars in the large galaxy. That has now been done with M32. US scientists from the University of Michigan discovered a ring of dim stars on the outskirts of Andromeda and suspected that they might have belonged to M32. The scientists made a computer simulation which turns back the clock, revealing a scenario that is perfectly consistent with M32 colliding with Andromeda 2 billion years ago. In the collision, M32’s exterior stars shook loose and entered into an orbit in Andromeda. What remains is the core of the original galaxy. The simulation also shows that M32 was the third largest galaxy of the Local Group before the collision.
M32 is a small galaxy with a diameter of no more than 6,500 light years. It only includes mass corresponding to 3 billion Suns.