Messier 103 (NGC 581) is Cassiopeia’s second Messier-designated open cluster, after Messier 52. As such it’s an impressive-looking object, though it’s usually overlooked in favour of the aforementioned M52 and the marvellous ET Cluster (NGC 457) and Caroline’s Rose (NGC 7789). Nevertheless, observers slewing its way will be delighted by what they see.
Messier 103 has an integrated magnitude of +7.4 and is very easy to locate through 10 x 50 binoculars lying just a degree north-east of Ruchbah (beta [b] Cassiopeiae; magnitude +2.7), one of the stars in Cassiopeia’s familiar ‘W’ asterism. As darkness falls in early October, Cassiopeia, which is circumpolar from UK shores, is over 50 degrees high in the east. Messier 103 sits high overhead by midnight BST (01:00 UT) and can be observed for the remainder of the long night.
Through a small telescope M103 reveals a distinctive fan shape, orientated south-east to north-west across an apparent diameter of around 6’. A 150mm (six-inch) telescope can pick out around 15 stars of its full complement of nearly 200. Images reveal a splash of colour from the magnitude +8.5 red star near the centre of M103.
Messier 103 lies 8,130 light years away and is a very young, metal-rich cluster formed about 25 million years ago. It was discovered by Pierre Méchain, in 1781.