Right Ascension	01 : 33.2 (h:m)
Declination	+60 : 42 (deg:m)
Distance	8.5 (kly)
Visual Brightness	7.4 (mag)
Apparent Dimension	6.0 (arc min)

Discovered by Pierre Méchain in 1781.

M103 is one of the "latest additions" (together with M101 and 102) which Messier included from Méchain's report, but had not the time to observe before publication.

This cluster of at least 40 proven members is one of the more remote open clusters in Messier's catalog, at about 8,000 light years (Wallenquist, Mallas/Kreimer) or slightly more; the Sky Catalogue 2000.0 gives 8,500 (quoted also e.g. by Kenneth Glyn Jones and Robert Garfinkle), Pennington 9,000 and Kepple's and Sanner's Night Sky Observer's Handbook 9,200 light years - the uncertainty mainly due to the less wellknown amount of obstruction for this cluster which lies well within the band of the Milky Way. Its appearance is dominated by the non-member binary Struve 131 (ADS 1209; components A, 7.3 mag, and B, 10.5 mag, separated 13.8" in position angle 142 deg, at 1956); nevertheless, John Herschel has always referred to this double star in context of this cluster. The two brightest cluster members, of about mag 10.5, are a B5 Ib supergiant and a B2 III giant, while the lot of main sequence stars indicate an age of about 9 million years, according to Cecilia Payne-Gaposhkin's "Stars and Clusters", while the Star Catalog 2000 gives 22 million years. New calculations of the Geneva team of G. Meynet have provided a significantly higher age of 25 million years for M103.

M103 contains one red giant star, obvious in color photos of the cluster; this star is of about 10.8 magnitude and spectral type gM6, according to Burnham.

Adopting a distance of 8,500 light years, M103's angular diameter of 6 arc minutes corresponds to 15 light-years linear extension. This stellar swarm is approaching us at 37 km/sec. Its Trumpler type has been given as II,3,m (Trumpler, according to Glyn Jones), III,2,p (Sky Catalog 2000) and II,2,m (Götz).

M103 is well visible in binoculars as a nebulous fan-shaped patch. It is not so easy to identify in telescopes because it is quite loose and poor, and may be confused with star groups or clusters in the vicinity.

This cluster is quite easy to find from Delta or 37 Cassiopeiae (named Ruchbah), a blue-white 2.7-mag star of spectral type A5 III-IV, 1/2 deg N and 1 deg E, close to the line to Epsilon (Segin; mag 3.38, spectrum B3 III). Situated nearby are a number of other open clusters, including Trumpler 1, NGC 654, NGC 659 and NGC 663. The latter is sometimes mentioned as a candidate to be confuswed with M103.

Observing Reports for M103 (IAAC Netastrocatalog)

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Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

Last Modification: 9 Dec 1999, 22:59 MET