Ring Galaxy AM 0644-741 from Hubble.

How could a galaxy become shaped like a ring? The rim of the blue galaxy pictured on the right is an immense ring-like structure 150,000 light years in diameter composed of newly formed, extremely bright, massive stars. That galaxy, AM 0644-741 , is known as a ring galaxy and was caused by an immense galaxy collision. When galaxies collide , they pass through each other -- their individual stars rarely come into contact. The ring -like shape is the result of the gravitational disruption caused by an entire small intruder galaxy passing through a large one. When this happens, 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 intruder galaxy is just outside of the frame taken by the Hubble Space Telescope . This featured image was taken to commemorate the anniversary of Hubble's launch in 1990. Ring galaxy AM 0644-741 lies about 300 million light years away.