Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2009 June 6



One-Armed Spiral Galaxy NGC 4725

Credit & Copyright: Mike Siniscalchi

Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 seems to have only one. In this sharp color image, the solo spira mirabilis is tightly wound, traced by bluish, newborn star clusters. The odd galaxy also sports obscuring dust lanes, a prominent ring, and a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation.