The exact distance from the Solar System to M103 has proved difficult to establish, but most estimates place the cluster about 7,500 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.
The bright northern star seen here is HD 9311, actually a foreground star unconnected to the cluster itself. Also unconnected is the prominent central red-orangestar, the supergiant BD +59 274, which - at an estimated distance of some 9,500 light years - is even more distant than M103 and shines through the stars of the cluster from beyond. Imagery provided by Aladin sky atlas
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 redgiant phase of its existence, and stands out against the backdrop of its blue companions with a prominent red-orange glow.
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