A computer generated image released on September 14, 2011 by the science journal Nature shows the impact of the Sagittarius Dwarf elipitcal galaxy (blue stream of stars) with the Milky Way Galaxy (multicolored disk), against a background of galaxies seenin the Hubble Deep Field. A new theory aired today posits that the formation of the arms of the Milky Way, a galaxy of some 200 billion stars with spiral limbs that whirl around a thin disk was formed when it was hit by the dwarf galaxy sending cascades of stars flying to the galactic rim. The giant impact sent streams of stars from both galaxies. The stars were eventually tugged outward by the Milky Way's rotation into the limbs that we perceive today. In both episodes, what remained of Sgr Dwarf pierced through the Milky Way's disk. AFP PHOTO / NATURE / ERIK TOLLERUD (Photo credit should read Erik Tollerud/AFP/Getty Images)