A halo of stars surrounds the Milky Way, but researchers disagree how it got there. One theory proposes that it formed from the same cloud of gas as the Galaxy itself; the other says the halo is the remains of several 'dwarf galaxies' that were originally separate from but close to the Milky Way proper. A survey of about three million halo stars weighs heavily in favour of the latter hypothesis.
Eric Bell of the Max Planck Institute for Astronomy in Heidelberg, Germany, and his colleagues compared data from the Sloan Digital Sky Survey with several models. The halo's structure, they say, suggests that it is what remains of several smaller galaxies that were subsumed into the Milky Way after it formed.