Messier 103 (also known as M103, or NGC 581) is a small open cluster of many faint stars in Cassiopeia. It was discovered on 20 September 1781 by Pierre Méchain, but later added as Charles Messier's last deep-sky object in his catalogue.[3]
It is located between 9400 light-years from the Sun[1][3] and is about 15 light years across. It holds two prominent stars, of which the brightest is magnitudes 10.5, and in the center of the cluster, a 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 about 22 million years in age.[5]
After the discovery of Messier 101 through 103 by Pierre 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 16 pL (pretty large stars) and with great many eS or extremely faint ones.[7]Åke Wallenquist then identified 40 stars in M103 while Antonín 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 of Delta Cassiopeiae, sometimes called as Ruchbah or Rukhbah.
Messier 103 is an easy object to find[8] and the cluster is visible in binoculars or a small telescope.[9] object to find and the cluster is visible even with the use of binoculars.[4][10] 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 about 1⁄3 the length of an imaginary line towards Epsilon Cassiopeiae, a northern endpoint of the 'W', and placed on the outer side of the 'W'.
M103 image from Aladin Sky Atlas
Finder chart for M103
