Discovered by Pierre Méchain in 1781.

Open cluster M103 is one of the "latest additions" (together with M101 and 102) to his catalog, which Charles Messier included from Pierre Méchain's report, but had no occasion and no time to observe before publication.

Harlow Shapley, who classified M103 as a loose and poor cluster, and thus classified it as of type 'd', once doubted its existance and considered it may be just an accidental grouping of physically unrelated stars at different distances. However, we now know that this is a physical cluster, from the common proper motion of its member stars. The number of proven cluster members has been given as 40 by Wallenquist, while Becvar has 60, the Sky Catalogue 2000.0 only 25, and Archinal and Hynes (2003) give a modern number of 172 member stars.

M103 is one of the more remote open clusters in Messier's catalog, at about 8,000 light years (Wallenquist, Mallas/Kreimer, Burnham) 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.

M103's appearance is dominated by the non-member binary Struve 131 (ADS 1209; components A, 7.3 mag, Sp B3, and B, 10.5 mag (or 9.9 mag, according to Mallas/Kreimer), 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. M103 contains one red giant star, of spectral type M6 III, or gM6, and mag 10.8. The red giant is obvious in color photos of the cluster such as this Kitt Peak National Observatory image. 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 Sky Catalogue 2000.0 gives the significantly higher value of 22 million years. New calculations of the Geneva team of G. Meynet have provided an age of 25 million years for M103.

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 Catalogue 2000.0) and II,2,m (Götz).

In binoculars, M103 is easy to find and identify, and well visible as a nebulous fan-shaped patch. Mallas states that a 10x40 finder resolves the cluster into stars; however, this is so only under very good viewing conditions. The object 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. But telescopes show many fainter member stars.

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 confused with M103.