People likely call M101 the name already taken by M33 because it looks like a pinwheel. There are also a lot of “whirlpool” and “needle” galaxy nicknames out there, but only one true Whirlpool Galaxy (M51) and Needle Galaxy (NGC 4565). — Michael E. Bakich, Senior Editor
Actually, M101 in Ursa Major has no common name (a real shame), although recently observers have begun to label it the Pinwheel Galaxy. M33 is the one true Pinwheel Galaxy, however. The name originated long before many of today’s astroimagers were able to capture M101 so well. M33’s name comes from a time when most observers were visual only.