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NGC 581

A relatively loose open star cluster of the northern sky. It falls close to the star Ruchbah (also called Ksora or Delta Cassiopeiae), one of the five stars that make up the distinctive shape of Cassiopeia. Though the star and the cluster appear close together in the sky, M103 is actually immensely more distant from the Sun than Ruchbah.

The exact distance from the Solar System to M103 has proved difficult to establish, but most estimates place the cluster about 9,000 light years across the Galactic disc. The diameter of the cluster cannot be calculated without knowing its distance, so this value is also approximate, but M103 appears to enclose an area of space some fifteen light years across.

There are a total of nearly two hundred stars within the M103 cluster. Viewed from Earth, the brightest appears to be HD 9311, but this is a line-of-sight effect: at 1,300 light years' distance, HD 9311 is much closer than M103 itself, and cannot therefore be a true member of the cluster. The actual member stars are generally blue in colour, with a single exception. The star designated SAO 11826 has evolved to the red giant phase of its existence, and stands out against the backdrop of its blue companions with a prominent red-orange glow.


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