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.

M40 Double Star - Winnecke 4 (credit:- NOAO/AURA/NSF)

Finder Chart for M40 (credit:- freestarcharts)

Finder Chart for M40 - pdf format (credit:- freestarcharts)

Finder Chart for M81 (also shown M40, M97, M82, M108 and M109) (credit:- freestarcharts)

Finder Chart for M81 (also shown M40, M97, M82, M108 and M109) - pdf format (credit:- freestarcharts)

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

Messier40
Object TypeDouble Star
ConstellationUrsa Major
Distance (light-years)510
Apparent Mag.+9.0
RA (J2000)12h 22m 13s
DEC (J2000)+58d 04m 59s
Apparent Separation (arc secs)51.7
Number of Stars2
Star A - Apparent Mag.+9.65
Star A - Spectral TypeG0
Star B - Apparent Mag.+10.1
Star B - Spectral TypeF8
Other NameWinnecke 4

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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