Scientists scanning a distant galaxy cluster with Spitzer spotted a collision between four massive ellipticals--the first observed merger of so many large galaxies.
Lead author Kenneth Rines (Harvard-Smithsonian Center for Astrophysics) compared the scene to "four sand trucks smashing together, flinging sand everywhere." (See the press release.) The sand, in this case, is a spray of billions of stars knocked out of position by the energy of the collision. Some of the stars will eventually be drawn back in to the new mega-galaxy; others will be orphaned in intergalactic space.
A notable property of the galactic quartet: it is gas-poor, unlike most observed mergers in which a rich supply of gas ignites to form new stars. According to Rines, this suggests that the biggest galaxies in the universe are new combinations of old stars.
Galactic collisions sound pretty violent in print--the popular language (the press release refers to the merger as a "Monster Galaxy Pileup" and a "cosmic smash-up") belies the slow-motion reality of these events, which play out over millions of years. Plus, it is extremely unlikely that any single star will collide with another--the scene most people intuitively picture when we talk about galaxies crashing in to one another.