Andromeda Galaxy Halo Details - 4

Relying on the deepest visible-light images ever taken in space, astronomers using NASA's Hubble Space Telescope (HST) have reliably measured the age of the spherical halo of stars surrounding the neighboring Andromeda galaxy (M31). To their surprise, they have discovered that approximately one-third of the stars in Andromeda's halo formed only 6 to 8 billion years ago. That's a far cry from the 11-to-13 billion-year age of the stars in the Milky Way's halo.