Messier 40

Winnecke 4 (also known as Messier 40 or WNC 4) is an optical double star consisting of two unrelated stars in a northerly zone of the sky, Ursa Major.

The pair were discovered by Charles Messier in 1764 while he was searching for a nebula that had been reported in the area by Johannes Hevelius. Not seeing any nebulae, Messier catalogued this apparent pair instead. In reality, Hevelius (who had made naked eye observations of the sky) had spotted 74 and 75 Ursae Majoris, an unrelated pair of 5th-6th magnitude stars separated by 22 arcminutes, a degree to the East of Winnecke 4, and whose faintness and close separation makes them potentially appear as a single nebulous star to the unaided eye. It is unclear why Messier determined this would correspond to a pair of 9-10th magnitude stars, as Winnecke 4 is far fainter than the naked eye can see even in ideal conditions, although he may have been attempting to catalog it for the sake of pointing out Hevelius's error. The pair were rediscovered by Friedrich August Theodor Winnecke in 1863, and included in the Winnecke Catalogue of Double Stars as number 4. Burnham calls M40 "one of the few real mistakes in the Messier catalog," faulting Messier for including it when all he saw was a double star, not a nebula of any sort.