WE CALL it home, but the Milky Way can still surprise us. It seems the halo of stars surrounding our galaxy had a more complex birth than we once thought.
Daniela Carollo at the Turin Astronomical Observatory in Italy and her colleagues examined the motion of 20,000 stars in the Milky Way, as part of the Sloan Digital Sky Survey, which is mapping millions of galaxies. By tracking so many stars, they spotted that there are actually two haloes, not one, and they rotate in opposite directions and at different speeds.
The inner halo is flattened and rotates at 20 kilometres per second, while the outer halo is spherical and spins in the opposite direction at 70 kilometres per second (Nature, vol 450, p 1020).
Further observations that heavy atoms are three times as plentiful in the inner halo's stars as in the outer confirmed suspicions that the ...