The galaxy is 21 million light-years (six megaparsecs) away from us. It was first discovered by Pierre Méchain on March 27 1781. Charles Messier verified its position and included it in the Messier Catalogue as one of the last entries.
In 2006, NASA and the ESA released a very detailed image of the Pinwheel Galaxy, which was the largest and most detailed image of a galaxy by Hubble Space Telescope at the time. The image was composed from 51 individual exposures, plus some extra ground-based photos.
On August 24 2011, a Type Ia supernova, SN 2011fe, was discovered in M101.
A remarkable property of this galaxy is its huge and extremely bright H II regions, of which about 3,000 can be seen on photographs. H II regions have numbers of short-lived blue stars formed from clouds of high density molecular hydrogen. These regions give off huge amounts of ultraviolet light that ionizes the surrounding gas.
- Shappee, Benjamin & Stanek, Kris 2011. A new Cepheid distance to the giant spiral M101 based on image subtraction of Hubble Space Telescope/Advanced Camera for Surveys observations. Astrophysical Journal 733 (2): 124. 
- HubbleSite – NewsCenter – Hubble's largest galaxy portrait offers a new high-definition view (02/28/2006) – Introduction