The Pinwheel galaxy, also known as M101, is 170,000 light-years across — nearly twice the diameter of our galaxy, the Milky Way. It is located in the constellation Ursa Major, near the end of the handle of the Big Dipper at a distance of nearly 21 million light years. It was discovered by Pierre Mechain in 1781. Seventeen eighty one was a pretty good year for Mechain - he discovered at least 17 deep sky objects and 2 comets that year. The dwarf galaxy in the lower right of the image is NGC 5477.