M46 is a rich open cluster, easily visible with binoculars, that's located in the Milky Way star fields of the constellation of Puppis. The cluster is a relatively large object with an apparent diameter almost equal to that of the full Moon. It was discovered by Charles Messier on February 19, 1771 and is best seen from southern and equatorial latitudes during the months of December, January and February.

Puppis borders many constellations including Canis Major, the home of the brightest star in the sky, Sirius (mag. -1.46). To locate M46, imagine a line connecting Mirzam (mag. +1.98) with Sirius and extend it for 14 degrees in a north-easterly direction to arrive at the cluster. In addition to M46, there are two further Messier objects in Puppis, M47 and M93. All three are superb open clusters, with M47 positioned just over a degree northwest of M46. The third cluster M93 is located about 9 degrees south of the M46, M47 pair.

M46 Open Cluster (credit:- NOAO/AURA/NSF)

Finder Chart for M46 (credit:- freestarcharts)

Finder Chart for M46 - pdf format (credit:- freestarcharts)

Finder Chart for M50 (also shown M46 and M47) (credit:- freestarcharts)

Finder Chart for M50 (also shown M46 and M47) - pdf format (credit:- freestarcharts)

With a magnitude of +6.1, M46 is on the fringe of naked eye visibility. When viewed through 10x50 binoculars, it appears mottled and more like a nebula than an open cluster. Also visible in the same field of view is the brighter, similar sized, but less richer cluster M47. About 0.5 degrees to the north of M47 is another open cluster NGC 2423 (7th magnitude). In larger 20x80 binoculars, the view of M46 is enhanced. The cluster appears large and dense with at least 50 fainter members visible. Through small telescopes, M46 looks like a sprinkling of stardust. An 80mm (3.1-inch) scope at high powers reveals many fainter stars of remarkably uniform brightness, especially when using averted vision. M46 is a superb sight through a 200mm (8-inch) instrument, with dozens of white and blue-white stars visible. At a magnification of about 100x, the field of view is completely filled with stars. In total, there are up to 150 stars brighter than mag. +13 with the brightest stars being of mag. +9. It's believed that M46 contains at least 250 stars.

One notable feature of M46 is a planetary nebula (NGC 2438), which is located at the northern edge of the cluster. It's visible in small telescopes of the order of 100mm (4-inch). The nebula appears as a diffuse circular patch embedded within the cluster, but is actually unrelated and therefore purely a line of site phenomena. It's estimated that M46 is 5,400 light-years distant, with the planetary closer at 3,000 light-years.

M46 has an estimated age of about 300 million years and a linear diameter of 30 light-years. It's a compact cluster with an apparent diameter of 27 arc minutes, almost the same as that of the full Moon. The cluster is a nice sight when viewed through all types of optical instruments. In the same wide field of view as M46 is M47, a brighter but less compact cluster. Also visible, is an unrelated planetary nebula (NGC 2438).

M46 Data Table

Object TypeOpen Cluster
Distance (light-years)5,400
Apparent Mag.+6.1
RA (J2000)07h 41m 47s
DEC (J2000)-14d 48m 36s
Apparent Size (arc mins)27 x 27
Radius (light-years)15
Age (years)300 Million
Number of Stars250
Other NameCollinder 159

Sky Highlights - March 2017

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

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
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)
Southeast:- Jupiter (mag. -2.3 to -2.5)
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
West:- Venus (first half of month), Mars, Uranus
North:- Jupiter
East:- Saturn
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)

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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