C10

The band of the Milky Way as it passes behind the 'W' shape of Cassiopeia presents a view of the Perseus Arm of the Galaxy, looking outward from Earth towards the Galaxy's edge. Along this arm are a string of open clusters, especially towards the eastern parts of Cassiopeia's Chair in the area between the stars Segin and Ruchbah. Among these is the bright cluster M103, but slightly brighter even than that Messier cluster is Caldwell 10 or NGC 663. Together with numerous other related clusters in the same area, these form a stellar association designated Cassiopeia OB8.

C10 consists of about four hundred stars in total in an area fourteeen arcminutes across, as seen from Earth (that is, about half the apparent diameter of the Moon's disc). Two pairs of brighter eighth magnitude stars stand out from the others, and the entire cluster lies about 6,400 light years from the Sun (though some estimates make it rather more distant than this).

This is a relatively young open cluster (estimates of its age vary from nine to twenty-five million years) and is notable for its population of 'Be' stars. These are blue stars that emit hydrogen, which gathers in a swirling orbit around the star to create a ring or disc of material, and they are found in an unusually high concentrations within the C10 cluster.