M40 is one of three curiosities in the Messier catalogue (along with M73 and M102). It's a faint double star in the constellation of Ursa Major that was discovered by Charles Messier on October 24, 1764. Messier was searching for a nebula reported in the area by Johann Hevelius. Although he didn't see any nebula, Messier catalogued this double star instead. However, despite no nebulosity existing, the double star remained on the list.
American astronomer Robert Burnham called M40, "one of the few real mistakes in the Messier catalogue". He faulted Messier for including it, when he found no trace of a nebula and all he saw was a double star. In 1863, it was rediscovered by Friedrich August Theodor Winnecke and hence is sometimes referred to as Winnecke 4 or WNC 4. M40 is best seen from the Northern Hemisphere during the months of February, March and April.
Locating M40 is easy. It lies about 1.5 degrees to the northeast of Megrez (δ UMa). With an apparent mag. of +3.3, Megrez is the dimmest of the seven stars that make up the famous "Big Dipper" or "Plough" asterism of Ursa Major. Positioned 17 arc minutes southwest of M40 is star 70 UMa (mag. +5.5).
The two component stars of M40 are of magnitudes +9.65 and +10.1 and are currently separated by 51.7 arc seconds. The double star is faintly visible in 10x50 binoculars, although much easier through larger 20x80 binoculars, where it's splittable. A small or medium size telescope, reveals a pair of widely spaced unimpressive stars. The brighter star is orange-yellow in colour, the fainter one white.
The separation between the components of M40 has increased since Messer's days, strongly suggesting that this is merely an optical double rather than a physically connected system.
The galaxy NGC 4290, a barred spiral of 12th magnitude lies nearby M40. However, this galaxy is faint and couldn't have been the nebula recorded by Hevelius.
M40 Data Table
|Object Type||Double Star|
|RA (J2000)||12h 22m 13s|
|DEC (J2000)||+58d 04m 59s|
|Apparent Separation (arc secs)||51.7|
|Number of Stars||2|
|Star A - Apparent Mag.||+9.65|
|Star A - Spectral Type||G0|
|Star B - Apparent Mag.||+10.1|
|Star B - Spectral Type||F8|
|Other Name||Winnecke 4|