The bright blue ring around the galaxy reveals billions of new stars born because of the collision. A galaxy is a giant assembly of gas, dust and millions or billions of stars. The Cartwheel Galaxy was once a normal galaxy similar to our home galaxy, the Milky Way, with pinwheel-like spiral arms winding outward from the galaxy's center. The Cartwheel's spiral structure is beginning to reemerge as seen in the faint arms extending out from the bull's eye core to the outer ring. Some galaxies are found in great clusters, with dozens or even thousands of members that gravitationnally jostle each other.
Galaxies travel through space at speeds approaching two million miles per hour.
In clusters, the member galaxies are relatively close together. Inevitably, collisions occur. In the Cartwheel Galaxy, a smaller intruder galaxy, possibly one of the two galaxies on the right side of the image, careened through the galaxy's core. Like a rock tossed into a lake, the collision sent a ripple of energy into space, plowing gas and dust in front of it at 200,000 miles per hour. The great energy wave heated and compressed the galaxy's dust and gas, producing an expanding ring of several billion new stars.