AM 0644-741 (sometimes called The Southern Ellipse) is a ring galaxy, approximately 300 million light-years away in the direction of the southern constellation Volans. It is receding from us at about 6600 km/sec.
The yellowish off-center nucleus was once the center of a normal spiral galaxy and the overlapping/double ring of brilliant blue star clusters, which currently surrounds the center, is some 150,000 light-year in diameter, making it larger than our Milky Way Galaxy.
Ring galaxies are formed when 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 image but visible in larger-field images.
The collision creates a shock wave that causes the gas and dust to rush outward, somewhat like ripples in a pond after a large rock has been thrown in. As the shock ring plows outward, gas and dust clouds collide, are compressed and then collapse gravitationally to form an abundance of new stars in a ring around the outside.
The rampant star formation explains why the ring is so blue: it is continuously forming massive, young, hot stars, which are blue in color. Another sign of robust star formation is the pink regions along the ring. These are rarefied clouds of glowing hydrogen gas that is fluorescing as it is bombarded with strong ultraviolet light from the blue stars.
Anyone who lives on planets embedded in the ring would be treated to a view of a brilliant band of blue stars arching across the heavens. The view would be relatively short-lived because theoretical studies indicate that the blue ring will not continue to expand forever. After about 300 million years, it will reach a maximum radius, and then begin to disintegrate.
This image is taken by the Hubble Space Telescope.
Image Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)