From NASA Recently-observed features include concentric rings rippling out from the impact area in a series of star formation waves, ending in the outermost ring. "It's like dropping a stone into a pond, only in this case, the pond is the galaxy, and the wave is the compression of gas," said Appleton. "Each wave represents a burst of star formation, with the youngest stars found in the outer ring." Previously, scientists believed the ring marked the outermost edge of the galaxy, but the latest Galaxy Evolution Explorer observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring. This means the Cartwheel is a monstrous 2.5 times the size of the Milky Way.