The deepest visible-light image ever taken of the sky resolves approximately 300,000 stars in the halo of the nearest neighboring spiral galaxy, Andromeda (M31). The photo was taken with the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope. Because the image captures both faint dwarf stars and bright giant stars, astronomers can estimate the age of the halo population by analyzing its distribution of color and brightness. The halo is a spherical cloud of stars around Andromeda, located 2.5 million light-years from Earth.
The surprising result is that these stars span a wide range of ages, from 6 to 13 billion years old. The age range is much wider than that of the population of stars in the halo of our own Milky Way, where 11- to 13-billion-year-old stars reside. The presence of younger stars in the Andromeda halo is probably the result of a more violent history in the galaxy, due to mergers with smaller galaxies.
In addition to the field of halo stars, a globular cluster in Andromeda falls near the bottom of the image [the white, spherical object]. Due to the great depth of the image, thousands of background galaxies also can be seen through the veil of stars in Andromeda. One particularly interesting pair of galaxies can be seen on the right-hand edge of the image, about a third of the way from the top. They are a blue and a red galaxy crashing into each other. A pinwheel-looking spiral galaxy is nearby. A handful of very bright stars [including the one at upper right] are in the foreground, in our Milky Way.
The image was made from 250 separate exposures taken from Dec. 2, 2002 to Jan. 11, 2003. The total exposure time was 3.5 days.
The members of the M31 halo science team are: T.M. Brown, H.C. Ferguson, E. Smith (STScI); R.A. Kimble, A.V. Sweigart (NASA/GSFC); A. Renzini (ESO); R.M. Rich (UCLA); and D.A. VandenBerg (U. of Victoria)