Messier 101, the Pinwheel Galaxy, is a large, face-on “grand design” spiral galaxy in Ursa Major, cataloged as NGC 5457.
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Messier 101, the Pinwheel Galaxy, is a large, face-on “grand design” spiral galaxy in Ursa Major, cataloged as NGC 5457. Distance estimates generally cluster around 21-25 million light-years, and at that distance its roughly 170,000-light-year diameter makes it noticeably larger than the Milky Way. Gravitational interactions with several smaller companion galaxies have given its spiral arms an asymmetric, slightly lopsided structure, and its arms are richly dotted with star-forming HII regions. M101 has hosted several observed supernovae, including the well-studied SN 2011fe. For astrophotographers it is a favorite luminance/RGB target, rewarding long integration time to resolve the delicate spiral structure and pink-tinged star-forming knots strung along its arms against a relatively faint overall surface brightness.
