The blue ring of young stars around the nucleus of AM 0644-741 is 150,000 light-years in diameter, making it larger than our entire galaxy, the Milky Way. Ring galaxies are one example of how collisions between galaxies can produce an significant change in one galaxies structure. One may result from a collision in which an intruder galaxy plunges directly through the disk of a target galaxy. In the case of AM 0644-741, the galaxy that pierced through the ring galaxy is out of the frame of this Hubble image but can be seen in larger-field images. The resulting gravitational shock caused by the collision drastically alters the orbits of stars and gas in the target galaxy’s disk, causing them to rush outward. As that wave spreads out, gas clouds collide and are compressed. The dense clouds contract under their own gravity, collapse, and new stars form.
Image Credit: NASA