Astronomers have been revising their views of how galaxies form. They hypothesize that a large galaxy, such as our Milky Way, formed (at least in part) by many smaller galaxies being drawn together gravitational and merging into a single larger system.

Most of that merging happpened so long ago that little trace of the individual galaxies has been lost. New observation, however, reveal "debris trails" in the halos of several galaxies. These trails are long, narrow, streams of stars...... the remnants of a dwarf galaxy that has fallen into the larger galaxy and has been pulled apart.