Winnecke 4

Messier 40 (Winnecke 4) is the most unusual object in Charles Messier's catalog — not a nebula, cluster, or galaxy, but a pair of faint stars. Messier entered it on October 24, 1764, while searching for a nebula that Johannes Hevelius had reported in the constellation Ursa Major. Finding no nebula at the indicated position, Messier recorded the two faint stars he did find, noting that Hevelius must have been mistaken. The pair was later independently catalogued in 1863 by A. Winnecke of the Pulkovo Observatory, giving it the designation Winnecke 4. The inclusion of M40 in the catalog is sometimes cited as evidence that Messier's method, while enormously valuable, was not perfectly systematic — he was primarily interested in avoiding false-positive comet identifications rather than in classifying all types of celestial objects.

The two stars that make up Winnecke 4 are a wide visual double — separated by about 50 arc-seconds on the sky. Modern measurements suggest they lie at different distances, approximately 500 and 900 light-years respectively, and are not physically bound to each other: the apparent pairing is a line-of-sight coincidence rather than a true gravitational binary system. Each star is a modestly luminous dwarf, both close to solar luminosity, without any of the striking visual qualities of Messier's showpiece nebulae. Neither star has spectroscopic companions or other notable properties.

In a small telescope Winnecke 4 appears simply as two faint stars close together, easily missed without knowing what to look for. This image was made at the Kitt Peak National Observatory 0.9-meter telescope in February 1996.