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M103 is a small loose but sparkling open cluster of at least 40 stars located among the Milky Way star fields of Cassiopeia. At magnitude +7.4, it's beyond naked eye visibility but an easy binocular target.

M103 was discovered by Pierre Méchain in either March or April of 1781. He reported his discovery to Charles Messier, who would normally take the opportunity to observe the object himself, but this time he didn't probably due to lack of time or unfavourable weather conditions. This cluster along with M101 and M102 were the last objects added by Messier. Items M104 to M110 were included much later during the 20th century.

Messier 103 Open Cluster (credit:- NOAO/AURA/NSF)

Finder Chart for M103 (credit:- freestarcharts)

Finder Chart for M103 - pdf format (credit:- freestarcharts)

M103 is easy to find as it's located east of the well-known Cassiopeia "W" asterism. The cluster is positioned a degree east of Ruchbah (δ Cas - mag. +2.7) and almost exactly along a line connecting Ruchbah with epsilon Cassiopeiae (ε Cas - mag. +3.4). Situated nearby are a number of other fine open clusters, including NGC 654, NGC 659 and NGC 663. Of these, the latter is occasionally confused with M103.

Located some 10,000 light-years away, M103 has an actual diameter of 17.5 light-years. The cluster is easy to identify with binoculars, where it appears like a diffuse fan or wedge-shaped patch of light. Through 100mm (4-inch) telescopes, the brightest four stars are resolvable and look somewhat like the Greek letter lambda (λ). When viewed with averted vision a triangle shaped patch of light is revealed, which extends beyond the main group of stars. One star in particular, Struve 131, dominates the scene. It's a 7th magnitude multiple that's easily split in small scopes. However, this is not a true cluster member. It's a foreground star that just happens to be in the same line of sight. The brightest individual cluster members shine at tenth magnitude and at the centre of the cluster is a prominent red giant star. With larger scopes, M103 can be confused with other star groups or clusters in the vicinity. Of course, larger telescopes will show many more fainter members. For example, a 200mm (8-inch) reflector at about 100x magnification reveals a dozen or so stars in a wedge shape. Even larger instruments resolve the cluster completely.

It's estimated that M103 is about 25 million years old. The cluster is one of the more distant in the Messier catalogue and is best seen from Northern Hemisphere latitudes during the winter months. From latitudes greater than 30N, it's circumpolar and therefore never sets.

M103 Data Table

Object TypeOpen Cluster
Distance (light-years)10,000
Apparent Mag.+7.4
RA (J2000)01h 33m 22s
DEC (J2000)+60d 39m 29s
Apparent Size (arc mins)6.0 x 6.0
Radius (light-years)8.75
Age (years)25 Million
Number of Stars>40
Other NameCollinder 14