Cartwheel galaxy. Nucleus of the Cartwheel Galaxy, revealing comet-like knots of gas (white streaks). The Cartwheel, once a flat spiral galaxy, was hit by another galaxy about 200 million years ago. The heads of the gaseous knots are a few hundred light-years across and the tails are 1000-5000 light-years long. The gaseous knots, travelling at about 1. 1 million kilometres per hour, were probably formed from collisions between fast and slow-moving material. This collision created arrowhead-shaped patterns called a bow-shock, similar to the wake that a boat forms on a lake. Image taken by the Hubble Space Telescope.