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Pinwheel Galaxy (M101, NGC 5457), optical HST image. This is one of the largest spiral galaxies known, at over 170,000 light years across. The nucleus contains mainly stars that are older and yellower than the arms. The arms appear bluer as they contain younger stars, and numerous bright starbirth regions. The Pinwheel Galaxy lies around 27 million light years away in the constellation Ursa Major. This image was obtained by the Hubble Space Telescope (HST). (KEYSTONE/SCIENCE PHOTO LIBRARY/Hubble Legacy Archive/ROBERT GENDLER/SCIENCE PHOTO LIBRARY)