July 2, 1995 The Cartwheel Galaxy Picture Credit: NASA, Hubble Space Telescope Explanation: The Cartwheel Galaxy shows a ring that is the result of a collision between a small and a large galaxy. After a small galaxy has moved through a big galaxy - in this case one that probably resembled our own Milky Way - a star formation wave moves out from the impact point like ripples across the surface of a pond. When galaxies collide it is rare that any two stars actually collide. Gravity, however, causes density waves to move out through the galaxy which in turn triggers the formation of hot, bright young stars, producing the ring that we see in this picture. For more information see NASA, Hubble Space Telescope Scientific Institute press release. We keep an archive of Astronomy Pictures of the Day. Astronomy Picture of the Day is brought to you by Robert Nemiroff and Jerry Bonnell . Original material on this page is copyrighted to Robert J. Nemiroff and Jerry T. Bonnell.