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Whirlpool galaxy (M51), optical image. The large spiral galaxy M51 is interacting with its small companion (NGC 5195, left). 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, which are best seen where one of the arms is silhouetted against NGC 5195. This pair of galaxies is thought to lie about 37 million light years away, in the constellation Canes Venatici. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT GENDLER)