M40 Winnecke 4

Object Type Pair of Stars
Constellation Ursa Major
Magnitude 9.65 + 10.10
Separation 52"

M 40, also known as Winnecke 4, is one of the few mistakes in the Messier catalog. Messier observed this pair of stars in 1764 while he was searching for a nebula that had been reported in the area by Johann Hevelius. Not seeing any nebulae, Messier catalogued this double star instead. It is not a true binary, but rather a chance line-up of two stars.

Combination of 16, 1 minute images.
20" f/6.8 Dall-Kirkham cassegrain telescope at prime focus.

It is possible that the "nebula" reported by Johann Hevelius was the 12th magnitude galaxy, NGC 4290, visible in the above image on the left. This galaxy may possibly have been visible in the largest telescopes of the time, but would have been too faint for those owned by Messier.