May 7, 2003: 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.See the rest:
Astronomers think that the collision with another large galaxy or the ravaging of several smaller galaxies scattered the young stars into Andromeda's halo. The newly discovered younger stars in Andromeda's halo are richer in heavier elements than those in our Milky Way's halo, or in most of the small dwarf galaxies that surround the Milky Way. The stars' age spread and chemical make-up suggests three possibilities: (1) Collisions destroyed the young disk of Andromeda and dispersed many of its stars into the halo; (2) a single collision destroyed a relatively massive invading galaxy and dispersed its stars and some of Andromeda's disk stars into the halo; and/or (3) many stars formed during the collision itself. Astronomers say it will take more detailed observations to unravel the evidence of these early cataclysmic events.
Astronomers needed to make several hundred exposures to survey the bright and the faint stars in Andromeda's halo. The image shown here was made from 250 separate exposures. The observations were taken from Dec. 2, 2002 to Jan. 11, 2003 with the Advanced Camera for Surveys (ACS), but the total exposure time was 3.5 days. Astronomers needed the lengthy observations because the halo's ordinary Sun-like stars are very faint. In fact, before using the ACS to tackle the halo stars, astronomers had observed only the brightest ones. The sharp eyes of the ACS, however, uncovered about 300,000 stars that astronomers had never seen before.