Credit: Illustration: NASA, ESA, and A. Feild (STScI); Science: NASA, ESA, A. Deason and P. Guhathakurta (Univ. of California, Santa Cruz), and R. van der Marel, T. Sohn, and T. Brown (STScI)
This illustration shows the disk of our Milky Way galaxy, surrounded by a faint, extended halo of old stars. Astronomers using the Hubble Space
Telescope to observe the nearby Andromeda galaxy serendipitously
identified a dozen foreground stars in the Milky Way halo. They measured
the first sideways motions (represented by the arrows) for such distant halo stars. The motions indicate the possible presence of a shell in the halo, which may have formed from the accretion of a dwarf galaxy. This observation supports the view that the Milky Way has undergone
continuing growth and evolution over its lifetime by consuming smaller
galaxies.