Hoag's Object
A ring galaxy is a galaxy with a ring-shaped appearance. The ring contains massive, relatively young and extremely bright blue stars, and the central region only has a small amount of relatively dim matter. Because most ring galaxies have hollow centers, some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a large galaxy. Because most of a galaxy consists of empty space, the "collision" rarely results in any actual collisions between stars. However, gravitational perturbations caused by traveling through large galaxies could cause fluctuations that contribute to star formation.
Hoag's Object is an example of such a galaxy. It was discovered by Art Hoag in 1950.