Figure 13-41 Galaxies with Rings (a) A composite image of the Cartwheel Galaxy. This ring-shaped assemblage 500 million light-years from Earth is likely the result of one galaxy, probably the blue-white one below it at the eight o’clock position, having passed through the middle of the larger one. Astronomers suspect that the passage created a circular density wave in the Cartwheel, analogous to the ripple created by a rock thrown in a pond. This wave stimulated a burst of star formation, creating many bright blue and white stars. Ultraviolet is in blue, visible light in green, infrared in red, and X-ray in violet. (b) Infrared image of the Andromeda Galaxy. The ring of hot dust indicates star formation, probably caused by the passage of another galaxy through Andromeda. The fact that the ring is disturbed suggests that yet another galaxy had a close interaction with Andromeda.