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Spiral galaxy M101, Hubble Space Telescope image. M101 is also known as the Pinwheel Galaxy. It lies around 25 million light years from Earth in the constellation Ursa Major. The disc of the galaxy is some 170,000 light years across, nearly twice as wide as our own Milky Way. The spiral arms appear blue as they contain hot young stars. The central nucleus is yellow as the stars there are older, and much of the light is obscured by dust. Released in 2006, this is the most detailed image of a galaxy ever taken by the HST. It is composed of 51 separate HST exposures taken between 1994 and 2003, which were superimposed on ground-based images. The full 562Mb image is available on request. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA/ESA/STSCI)