Hand-coloured optical photograph of the Pinwheel Galaxy (M101, NGC 5457), a large, bright spiral galaxy (type Sc) some 20 million light years away in the constellation of Ursa Major. M101 has a mass of three thousand million Suns and a diameter of approximately 200,000 light years, around twice that of our own galaxy, the Milky Way. Its extremely narrow, far-flung spiral arms are dominated by huge clouds of glowing hydrogen, some of which are 3000 light years across. See New Astronomy page 179.