The Double Ring Galaxies of Arp 147 from Hubble

How could a galaxy become shaped like a ring? Even more strange: how could two?
The rim of the blue galaxy pictured on the right shows an immense ring-like structure
30,000 light years in diameter composed of newly formed, extremely bright, massive
stars. This blue galaxy is part of the interacting galaxy system known as Arp 147, and
shows a ring because it has recently collided with the other galaxy in the frame, the red
galaxy on the left. Unusually, even this red galaxy shows a ring like band, although it is
seen nearly edge-on. When galaxies collide, they pass through each other -- their individual
stars rarely come into contact. Clouds of interstellar gas and dust become condensed,
causing a wave of star formation to move out from the impact point like a ripple across the
surface of a pond. The above image was taken last week by NASA's Hubble Space Telescope to
demonstrate the ability of its Wide Field Planetary Camera 2 after some recent technical difficulties.