Messier 51

These Hubble images show off two dramatically different face-on views of the spiral galaxy M51, dubbed the Whirlpool Galaxy. The image at left, taken in optical/visible light, highlights the attributes of a typical spiral galaxy, including graceful, curving arms, pink star-forming regions, and brilliant blue strands of star clusters. In the image at right, most of the starlight was removed, revealing the Whirlpool's skeletal dust structure, as seen in near-infrared light. This image provides a sharp view of M51's dense dust. To map the galaxy's dust structure, researchers collected the galaxy's starlight by combining images taken in visible and near-infrared light. The visible-light image captures some of the light; the rest was obscured by dust. The near-infrared view, however, reveals more starlight because near-infrared wavelengths of light penetrate dust. The researchers then subtracted the total amount of starlight from both images to see the galaxy's dust structure. The red color in the near-infrared image traces the dust, which is punctuated by hundreds of tiny clumps of stars, each about 65 light-years wide. This is the first time astronomers have seen these stars. The star clusters are not visible in optical wavelengths of light because dense dust enshrouds them. The image reveals details as small as 35 light-years across. Astronomers expected to see large dust clouds, ranging from about 100 light-years to more than 300 light-years wide. Instead, most of the dust is tied up in smooth and diffuse dust lanes. An encounter with another galaxy may have prevented giant clouds from forming. Probing a galaxy's dust structure serves as an important diagnostic tool for astronomers, providing invaluable information on how the gas and dust collapse to form stars. Researchers constructed these images by combining visible-light exposures from Jan. 18 to 22, 2005, with the Advanced Camera for Surveys (ACS) and near-infrared–light pictures taken in December 2005 with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).