Second Halo Found Around Milky Way

Daniela Carrolo and her team at the Turin Astronomical Observatory in Italy have found that there are two haloes of stars surrounding the Milky Way, rather than only one, as was previously thought.

The second halo was discovered during the Sloan Digital Sky Survey, when Carrolo and her team were examining the motion of 20,000 Milky Way stars. 

The inner halo is flattened, while the outer halo is shaped like a sphere.  While the inner halo rotates at 20 kilometres per second, the outer halo rotates at a much faster rate, 70 kilometres per second.

There are three times as many heavy atoms in the inner halo than in the outer.  Since heavy atoms did not exist in the early universe, this suggests that the haloes formed at different times, and that therefore the galaxy did not form in one stage.