- M 103 (NGC 581)
M 103 (NGC 581) is a compact open cluster lying within the Winter Milky Way in the constellation Cassiopeia. It was discovered by Pierre Mechain (Messier's collaborator) in March of 1781 and is the last entry in Messier's original list of deep-sky objects.
Visually, M 103 is relatively small (only 6 arc-min across), a feature that is related to its distance from us. This cluster is around 7150 LY away, which makes it the most distant open cluster in Messier's list. This is around the same distance as the Double Cluster in Perseus. M 103 contains around 80 member stars. Of them, the brightest is the blue giant Struve 131, which shines with a visual magnitude of 7.2 along the upper right edge of the cluster. It has two companions, and all three of the stars are spectral class B. The bright red class M giant star in the heart of the cluster is also a physical member of the cluster. The age of M 103 is estimated to be 16-25 million years.
M 103 is part of a stellar association (Cas OB8) that includes several other nearby open clusters. These include NGC 654, NGC 659 and NGC 663.
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