Near the constellation Coma Berenices, some 63 million light-years away, is NGC 4651, a spiral galaxy about the size of our Milky Way. Extending 100 thousand light-years past its galactic disk is an umbrella-shaped structure.
The giant cosmic umbrella is now known to be composed of tidal star streams – extensive trails of stars gravitationally stripped from a smaller satellite galaxy. The small galaxy was eventually torn apart in repeated encounters as it swept back and forth on eccentric orbits through NGC 4651. In fact, the picture insert zooms in on the smaller galaxy’s remnant core, identified in an extensive exploration of the system, using data from the large Subaru and Keck telescopes on Mauna Kea. Work begun by a remarkable collaboration of amateur and professional astronomers to image faint structures around bright galaxies suggests that even in nearby galaxies, tidal star streams are common markers of such galactic mergers. The result is explained by models of galaxy formation that also apply to our own Milky Way.