| NGC 663 | |
|---|---|
 NGC 663 in Cassiopeia | |
| Observation data (J2000epoch) | |
| Right ascension | 01h 46.0m |
| Declination | +61° 15′ |
| Distance | 6,850ly (2,100pc) |
| Apparent magnitude (V) | 7.1 |
| Apparent dimensions (V) | 16.0′ |
| Physical characteristics | |
| Other designations | Caldwell 10,Cr 20 |
| Associations | |
| Constellation | Cassiopeia |
| See also:Open cluster,List of open clusters | |
NGC 663 (also known asCaldwell 10) is a youngopen cluster in theconstellation 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.[1]
After adjusting for reddening due to interstellar dust, thedistance modulus is estimated as 11.6 magnitudes. It is located about 2,100 parsecs distant with an estimated age of 20–25 million years.[2] This means that stars of spectral class B2 or higher (in the sense of higher mass), are reaching the end of theirmain sequence lifespan.[3] This cluster appears to be located in front of amolecular 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.[4]
This cluster is of interest because of the high number ofBe stars, with a total of about 24 discovered. These arespectral class B stars that show prominentemission lines ofhydrogen in their spectrum. Most of the Be stars in the cluster lie between spectral class B0 and B3.[2] A candidate member of the cluster, LS I +61° 235, is a Be star with anX-ray binary component that has a period of about three years.[5] There are at least fiveblue stragglers in the cluster. These are stars that formed by the merger of two other stars.[3] Two of the cluster's star systems are likelyeclipsing binaries with periods of 0.6 and 1.03 days.[6] NGC 663 also has twored supergiant stars, both located on its periphery[7][8]
The star cluster is assumed to form part of thestellar association Cassiopeia OB8, that is located in thePerseus arm of theMilky Way, along with the open clustersM103,NGC 654,NGC 659, and somesupergiant stars scattered between them, all of them having similar ages and distances.[9][10]
