Whirlpool galaxy (M51), Hubble Space Telescope image. The large spiral galaxy M51 is interacting with its small companion (NGC 5195, right). The spiral arms of M51 are studded with pink starbirth regions, which glow as radiation from the newborn stars within them ionises the hydrogen they contain. The regions between the spiral arms look relatively empty, but they are actually criss crossed by dark dust lanes. This pair of galaxies is thought to lie about 37 million light years away in the constellation Canes Venatici. This image is a mosaic of images taken by the Advanced Camera for Surveys (ACS) on the HST in January 2005.