By Ron Cowen
Often referred to as twins, the Milky Way and the nearby galaxy Andromeda have a similar spiral shape, size, and mass. What’s more, Andromeda appears as unperturbed as the Milky Way. But a new image unveiled this week belies that serene portrait. The most detailed visible-light picture ever taken of the heavens reveals that Andromeda was walloped by an intruder galaxy much more recently than the Milky Way was.
The image, the result of 3.5 days of observations with the recently installed Advanced Camera for Surveys on the Hubble Space Telescope, is several times as sensitive as the Hubble Deep Field images (SN: 3/1/03, p. 139: Mature Before Their Time). The new picture for the first time resolves faint, sunlike stars in a galaxy other than the Milky Way. The picture shows 300,000 individual stars in a portion of Andromeda’s halo, a spherical cloud of stars that extends from the galaxy’s center. The brightness and colors of these stars indicate that they range in age from 6 billion to 13 billion years. In contrast, the stars in the Milky Way’s halo are almost all between 11 and 13 billion years of age.