The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy 21 million light-years away from Earth in the constellation Ursa Major. It was discovered by French astronomer Pierre Méchain in 1781. M101 is a large galaxy, with a diameter of 170,000 light-years and has a high population of H II regions, many of which are very large and bright. M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress interstellar hydrogen gas, which then triggers strong star formation activity in M101’s spiral arms that can be detected in ultraviolet images.