NGC 663

NGC 663是在仙后座内，拥有大约400颗恒星的一个年轻的疏散星团，这400颗星星在天空中的跨距估计大约是1/4度。据说这个星团仅凭裸眼就能看见，但使用望远镜观看可以得到更佳的效果，而使用双筒望远镜就能看见星团中最亮的恒星。虽然表中列出最亮恒星的视星等只有7.1等，但有几位观测者报告估计有更亮的恒星

在修正了星际尘埃造成的红化之后，距离模数估计的星等是11.6等。估计这个星团的距离约为2,100秒差距，年龄大约在2000-2500万年。这意味着恒星的光谱类型为B2或者更高（感觉上有更大的质量），已经到达主序带的顶端。这个星团出现在一个分子云的前方，但两这之间并无物理上的关联。这团云气距离星团约300秒差距，已经影响到该星团在目视观测上的背景恒星亮度景观

这个星团令人感兴趣的是有大量的Be恒星，发现的数量已经高达24颗。这是一种光谱类型为B，但在光谱中有着突出的氢发射光谱。多数在该星团中的Be星光谱类型在B0和B3之间 这个星团的一颗候选者， LS I +61° 235，是一颗有X-射线伴星的Be恒星，互绕的周期大约是3年。在这个星团内至少有5颗蓝掉队星，它们都是由两颗恒星合并所形成的恒星。星团中也有两颗周期分别是0.6天和1.03天的食双星。

NGC 663 (also known as Caldwell 10) is a young open cluster located 8,800 light years from Earth in the constellation of Cassiopeia. It has an estimated 400 stars and spans about a quarter of a degree across the sky. It can reportedly be detected with the unaided eye, although a telescope is recommended for best viewing. The brightest members of the cluster can be viewed with binoculars. Although the listed visual magnitude is 7.1, several observers have reported higher estimates.

Based on its parallax as measured by Gaia, it is located about 2,700 parsecs distant with an estimated age of 20–25 million years. This means that stars of spectral class B2 or higher (in the sense of higher mass), are reaching the end of their main sequence lifespan. This cluster appears to be located in front of a molecular cloud, although the two are not physically associated. This cloud has the effect of blocking background stars from the visual image of the cluster as it lies at a distance of 300 parsecs.

This cluster is of interest because of the high number of Be stars, with a total of about 24 discovered. These are spectral class B stars that show prominent emission lines of hydrogen in their spectrum. Most of the Be stars in the cluster lie between spectral class B0 and B3. A candidate member of the cluster, LS I +61° 235, is a Be star with an X-ray binary component that has a period of about three years. There are at least five blue stragglers in the cluster. These are stars that formed by the merger of two other stars. Two of the cluster's star systems are likely eclipsing binaries with p…

The star cluster is assumed to form part of the stellar association Cassiopeia OB8, that is located in the Perseus Arm of the Milky Way, along with the open clusters M103, NGC 654, NGC 659, and some supergiant stars scattered between them, all of them having similar ages and distances. It is likely the most massive star cluster in the Cassiopeia OB8 association, and possibly the Perseus Arm.