English: Pinwheel Galaxy (M101) is a face-on spiral galaxy 21 million ly away in the constellation Ursa Major - Large galaxy (170,000 ly in diameter) with twice the number of stars in the Milky Way - High population of H II regions with molecular hydrogen gas clouds ionized by extremely bright and hot young stars creating superbubbles - It is asymmetrical due to the tidal forces from interactions with its six companion galaxies that form the M101 Group -
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