M103 is a small loose but sparkling open cluster of at least 40 stars located amongst the Milky Way star fields of Cassiopeia. At magnitude +7.4, it's beyond naked eye visibility but an easy binocular target, appearing somewhat like a fan shaped hazy patch of light spanning about 6 arc minutes across.

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

M103 is easy to find as it's located on the eastern side of the well-known Cassiopeia "W" shape. Its positioned 1 degree east of Ruchbah (δ Cas - mag. 2.7) and almost along the line connecting Ruchbah with epsilon Cas (ε Cas - mag. 3.4). Situated nearby are a number of other open clusters, including NGC 654, NGC 659 and NGC 663. The latter is occasionally confused with M103.

Messier 103 Open Cluster (NOAO/AURA/NSF)

Finder Chart for M103 (also shown M52 and M76)

Finder Chart for M103 (also shown M52 and M76) - pdf format

Located some 10,000 light-years away and spanning 17.5 light-years, this wide spaced cluster is easy to find and identify with binoculars where it appears as a fan or wedge-shaped diffuse patch of light. Through a 100mm (4-inch) telescope the brightest four stars of the cluster are resolvable looking somewhat like the Greek letter lambda (λ), with averted vision revealing a nebulous triangle shaped patch of light that extends beyond the brightest stars.

One star in particular, Struve 131 dominates the scene. It's a 7th magnitude multiple star that's easily split in telescopes of 100mm (4-inch) aperture. However, it's not actually a true member of the cluster; it's much closer to us and just happens to be just in the line of sight. The brightest members of M103 are of magnitude +10.5 and at the centre of the cluster lies a prominent red giant star.

With larger scopes, the cluster is not so easy to identify due to its looseness and is easy to confuse with other star groups or clusters in the vicinity. Of course, larger telescopes will show many fainter member stars. A 200mm (8-inch) scope at about 100x magnification reveals a dozen or so stars with the brighter stars anchoring the points of the wedge. Still larger sized amateur scopes reveal at least 20 stars.

Its estimated that M103 is about 25 million years old. It's one of the more distant open clusters in the Messier catalogue and is best seen during the Northern Hemisphere winter months when it appears high in the sky. From latitudes greater than 30N it's circumpolar and therefore never sets.

M103 Data Table

Messier103
NGC581
Object TypeOpen Cluster
ConstellationCassiopeia
Distance (kly)10.0
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)25M
Number of Stars>40
Other NameCollinder 14

Sky Highlights - April 2017

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