An unusual entry within the Messier catalog is Messier 40 (M40), also known as Winnecke 4 (WNC 4), is an optical double star located in the constellation Ursa Major, the Great Bear (the lower pair).
M40 lies at a distance of 510 light years from Earth. It is one of the few Messier objects that are not deep sky objects, but were catalogued by mistake. It was not listed in the New General Catalogue (NGC).
Messier 40 is composed of a G0-type star and an F8-type star with apparent magnitudes of 9.65 and 10.10. The stars are separated by 52.8 arc seconds and form an optical pair, lying along the same line of sight, but not physically connected to each other as they are known to be at different distances from Earth. The brighter component (G0) is designated as HD 238107 and the fainter (F8), HD 238108. The more distant component lies more than 1,900 light years away.
Winnecke 4 was discovered by Charles Messier on October 24, 1764. Messier was looking for a nebula reported in the area by the Polish astronomer Johannes Hevelius in the 17th century, but did not find one, so he catalogued the double star instead. The “nebula” reported by Hevelius may have been the 12th magnitude barred spiral/early type ring galaxy NGC 4290 (visiable at top of image directly above M40), which may have been visible in large telescopes at the time, but not in those Messier used.
A small 17.4 magnitude spiral galaxy PGC 39934 is located just below NGC 4290. It measures only 0.7'x0.2'.
(messier-objects.com)
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