Messier 103 (also known asM103, orNGC 581) is a smallopen cluster of many faint stars inCassiopeia. It was discovered on 27 March 1781 byPierre Méchain, but later added asCharles Messier's last deep-sky object in his catalogue.[3]
It is located 9,400light-years from theSun[1][3] and is about 15 light years across. It holds two prominent stars, of which the brightest is magnitude 10.5, and in the center of the cluster, another magnitude 10.8 red giant. Another bright foreground object is the double star Struve 131,[4] but is not a member of the cluster. Cluster membership is about 172 stars based on >50% probability of gravitational attachment that binds the cluster together.[4] M103 is between 12.6[1] to 25 million years[5] in age.
After the discovery ofMessier 101 through 103 byPierre Méchain, Messier later added this open cluster to his own catalogue.[4][6] In 1783,William Herschel described the region of M103 as 14 to 16pL (pretty large stars) and with great manyeS or extremely faint ones.[7]Åke Wallenquist first identified 40 stars in M103 whileAntonín Bečvář raised this to 60. Archinal and Hynes suggest that the cluster has 172 stars.[4]Admiral William Henry Smyth pointed out the cluster's 10.8-magnitude red giant, citing it was a double star on Cassiopeia's knee, about 1° northeast ofDelta Cassiopeiae, sometimes called as Ruchbah or Rukhbah.
Messier 103 is an easy object to find and the cluster is visible in binoculars or a small telescope.[8][4][9] M103 can be seen as a nebulous fan-shaped patch, and is about a fifth the apparent diameter of the Moon or 6 arcminute (6′) or 0.1° across. To find M103, it is suggested that the observer center on Ruchbah or the lowest star of the signature “W” asterism of Cassiopeia. The cluster will appear as a hazy patch in a field about1⁄3 the length of an imaginary line towardsEpsilon Cassiopeiae, a northern endpoint of the 'W', and placed on the outer side of the 'W'.
