M40 - Winnecke 4

Messier 40 or M40 (also designated Winnecke 4 or W) is an optical double star in the constellation Ursa Major. Its combined magnitude is 8.4 and the angular separation of the two components is 0.8 arc-minutes. M40 lies at an estimated distance of 510 light years. The Equinox 2000 coordinates are RA= 12h 22.4m, Dec= +58° 05´ which makes M40 best seen during the spring. The Messier Spring Star Chart shows the position of all Messier objects visible during that season. As one of the more famous objects in the Messier Catalog, it is commonly known as the xx.

The image above shows the uncropped view of M40 through the Takahashi E-180 Astrograph (North is up). A 3x enlargement of this image appears to the right.

This apparent double star was discovered by Messier in 1764. According to Stoyan et al. (2010), the distances of the two stars in M40 are 490 and 1860 light years. They are not physically asociated with each other but simply a optical pair that happen to be along the same line of sight.

For more information, see the Messier Catalog as well as specific entries for M40 in Wikipedia and SEDS.

Messier's Description of M40

October 24, 1764) [1771] `The same night of October 24 to 25, I searched for the nebula above the tail of the Great Bear, which is indicated in the book Figure of the Stars, second edition: its should have, in 1660, the right ascension 183d 32' 41", and the northern declination 60d 20' 33". I have found, by means of this position, two stars very near to each other and of equal brightness, about the 9th magnitude, placed at the beginning of the tail of Ursa Major: one has difficulty to distinguish them with an ordinary [nonachromatic] refractor of 6 feet. Here are their position: right ascension, 182 deg 45' 30", and 59 deg 23' 50" northern declination. There is reason to presume that Hevelius mistook these two stars for a nebula.'

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