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 the NASA/ESA 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.

Credit: NASA, ESA and T.M. Brown (STScI)