The sharp eye of the Hubble Space Telescope's Advanced Camera for Surveys has uncovered more than 200 mammoth star clusters in the heart of the galaxy Arp 220.
The clusters are the bluish-white dots scattered throughout the image. The heftiest Arp 220 cluster - about 10 million solar masses - is twice as massive as any comparable star cluster in the Milky Way Galaxy. Arp 220 collided with another galaxy about 700 million years ago, fueling the frenzy of star birth in a small region about 5,000 light-years across. The galaxy is a nearby example of the aftermath of two colliding galaxies.
The clusters are so compact, however, that even at their moderate distance they look to Hubble like brilliant single stars. Astronomers know the clusters are not stars because they are much brighter than a star would be at that distance, 250 million light-years away in the constellation Serpens.
The massive clusters seen in this visible-light image are just the tip of the iceberg. More star clusters are not visible because they are obscured by the dust that engulfs this galaxy. In fact, much of the visible light in this photo is reddened by dust, like Earth's sky at sunset. Because of this dust, Arp 220 shines brightest in infrared light and is called an ultra-luminous infrared galaxy (ULIRG). ULIRGs are the products of mergers between galaxies, which can create firestorms of star birth. Starlight from the new stars heats the surrounding dust, causing the galaxies to glow brilliantly in infrared light.
The Advanced Camera observations were taken in visible light in August 2002.