Halo stars in Andromeda Galaxy (M31), Hubble Space Telescope (HST) image. Spiral galaxies, like M31, have a spherical halo of stars surrounding them. The stars in the halo are less densely packed than in the spiral arms or nucleus (neither seen). This view shows a small part of the halo, containing an estimated 300,000 stars. Also seen, are distant galaxies and a globular cluster (bottom centre). This is the first time M31's faint halo stars have been seen, allowing their ages to be estimated and providing evidence for past galactic collisions of the Andromeda Galaxy. Image created in May 2003, using the HST's Advanced Camera for Surveys (ACS).

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