M51 galaxy. Satellite image of the M51, or Whirlpool, galaxy (red) and its companion galaxy (blue). This image shows regular thin filaments bridging the gaps between the dust-rich spiral arms of M51. The reason for these strange structures is not yet understood. The whirlpool and star formation in M51 are thought to be triggered by the ongoing interactions between the two galaxies, which are kept together by their mutual gravitational attraction. They lie 37 million light years away in the constellation Canes Venatici. Image taken by an infrared array camera aboard NASA's Spitzer satellite.