M101

The Pinwheel Galaxy is a large face-on spiral galaxy in Ursa Major, located about 21 million light-years from Earth. It is one of the largest spiral galaxies in the nearby universe, spanning about 170,000 light-years — significantly larger than the Milky Way. M101 is notably asymmetric, with its core displaced from the geometric center of its disk, likely due to gravitational interactions with companion galaxies. It contains many large and luminous HII regions, some so bright they have their own NGC designations.

Located about 5.5 degrees northeast of Mizar (the bend star in the Big Dipper's handle). Despite its size (nearly as large as the full Moon), M101 has very low surface brightness, making it one of the trickiest Messier objects. It is nearly invisible from light-polluted sites. From a dark location, binoculars show a very faint, large smudge. A telescope at low power with a wide-field eyepiece and excellent dark skies reveals a large, diffuse glow. Patience and dark adaptation are essential. Best observed from March through August.

Discovered by Pierre Mechain on March 27, 1781, and added to Messier's catalog shortly after. Due to its low surface brightness, it was one of the more difficult objects cataloged by Messier. The galaxy's face-on orientation and clear spiral arms have made it a favorite photographic target. A Type Ia supernova (SN 2023ixf) was observed in M101 in May 2023, reaching naked-eye visibility in large amateur telescopes.