Starbirth region in Arp 220, Hubble Space Telescope image. Arp 220 is a galaxy 250 million light years from Earth in the constellation Serpens. At this distance, the bright blue-white dots within the galaxy are too bright to be stars. They are massive star clusters containing the mass of millions of suns. They formed when Arp 200 collided with another galaxy around 700 million years ago. Only the brightest star clusters are seen through the haze of dust that envelopes this galaxy. Arp 220 is an ULIRG, an ultra-luminous infrared galaxy, and glows brightly in infrared. This image, in visible light, was obtained in August 2002 by Hubble's Advanced Camera for Surveys instrument.