Ring galaxy AM 0644-741, Hubble Space Telescope (HST) image. The ring galaxy (upper right) is seen as a ring (blue) of star clusters surrounding the nucleus (yellow, left side of ring). The galaxy was formed when a smaller galaxy passed through a larger spiral galaxy. The gravitational effect of the collision triggered the formation of young, bright blue stars in the ring-shaped area. The ring is around 150,000 light years across. The stars in the nucleus are older and appear yellow. This galaxy lies 300 million light years from Earth, in the constellation Dorado.