M40 - WINNECKE 4

Messier 40 (Winnecke 4) is the most unusual entry in the Messier catalog: it is not a deep sky object at all, but rather an optical double star in the constellation Ursa Major. Located approximately 510 light-years from Earth for the closer component, the pair consists of two unrelated stars that appear close together by chance line of sight rather than being a gravitationally bound binary system. The two stars, of magnitudes 9.6 and 10.1, are separated by about 49 arcseconds. Messier observed and cataloged this pair on October 24, 1764, apparently while searching for a nebula reported by the 17th-century astronomer Johannes Hevelius in this area. Finding no nebula but noting the close star pair, Messier included it in his catalog. The entry has been a source of puzzlement and mild embarrassment for Messier enthusiasts ever since, as it is clearly not the type of fuzzy, comet-like object the catalog was designed to document. Modern measurements have confirmed that the two stars are at vastly different distances and are merely aligned by coincidence. Despite its underwhelming nature as a deep sky object, M40 has become a curiosity that Messier marathon observers must check off their list. It requires a telescope to observe and appears simply as two faint stars close together, indistinguishable from countless other double stars in the sky except for its Messier designation. Some observers enjoy the historical detective story of tracking down exactly what Hevelius and Messier observed, while others consider M40 to be the least rewarding Messier object. It remains a reminder that even great astronomers occasionally cataloged objects that did not merit inclusion.

An optical double star with components of magnitude 9.6 and 10.1 separated by 49 arcseconds, at different distances and not physically related.