Messier 103 (also known NGC 581) is an open cluster in the constellation Cassiopeia. It was discovered by Pierre Mechain in 1781, and was the last object that Charles Messier added to his original catalog.
Observing Messier 103
This cluster is quite easy to find, 1.5° NE of δ Cassiopeiae (Ruchbah). In binoculars, M 103 is visible 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 other clusters in the vicinity. But telescopes show many fainter member stars. The cluster is bright, large, fairly rich, moderately compressed, and has forty 8th- to 12th- magnitude stars. The field is liberally sprinkled with 8th and 9th magnitude stars, the brighter stars lying to the north.
Messier 103 is an exquisite arrowhead-shaped cluster, with a total magnitude of 7.4. It is dominated by the pretty double star Struve 131 (magnitudes 7.3 and 10.5, separated by 13.8") to the northwest, which is not a member. The two brightest cluster members, of about mag 10.5, are a B5 Ib supergiant and a B2 III giant. M 103 also contains one obvious red giant star, of mag 10.8 and spectral type M6 III.
The area nearby is rich in open clusters, including Trumpler 1, NGC 654, NGC 659 and NGC 663. The latter is sometimes mentioned as a candidate to be confused with M 103.
Physical Properties
M 103 is one of the more remote open clusters in Messier's catalog. It lies at about 8,000 - 9200 light years' distance, the uncertainty mainly due to the amount of interstellar absorption by interstellar dust; for this cluster lies well within the band of the Milky Way. M 103 has a true diameter of about 15 light years, and is about 25 million years old.
M 103 as a loose and poor cluster, once considered an accidental grouping of physically unrelated stars. However, we now know that it is a physical cluster, from the common proper motion of its member stars. As of 2003, the proven number of cluster members is 172.