M40

M40 (Winnecke 4) is one of the most unusual Messier objects — it is simply a double star in Ursa Major, about 510 light-years from Earth. The two stars (magnitudes 9.0 and 9.3) are separated by about 49 arcseconds. Most astronomers believe the pair is an optical double (not gravitationally bound) rather than a true binary system.

Located about 1.5 degrees northeast of Megrez (Delta Ursae Majoris). At the eyepiece, M40 appears as two faint stars close together — nothing more. A telescope at 50-80x easily splits the pair. Don't expect a deep-sky spectacle — M40 is essentially just two stars. It is primarily of historical interest as one of Messier's mistakes.

Cataloged by Charles Messier on October 24, 1764, while searching for a 'nebula' reported by Johannes Hevelius in this location in the late 1600s. Messier found no nebula but recorded the double star instead. The pair was independently cataloged by Friedrich August Theodor Winnecke in 1863 as Winnecke 4.