The M101 (Messier 101), also known as the Pinwheel Galaxy, is a face-on spiral galaxy located in the constellation Ursa Major. It lies approximately 21 million light-years away and has a diameter of about 170,000 light-years, making it significantly larger than the Milky Way. M101 is noted for its well-defined spiral arms and large population of H II regions, indicating active star formation. Its asymmetrical shape is thought to be the result of gravitational interactions with nearby galaxies. The Pinwheel Galaxy is a favorite among astrophotographers due to its expansive, detailed structure and vivid star-forming regions.