NGC 663

Lore & Background

NGC 663 is a young open cluster estimated to contain about 400 stars and spanning roughly a quarter of a degree across the sky. It can reportedly be detected with the unaided eye, though a telescope is recommended for best viewing; the brightest members are visible with binoculars. Based on Gaia parallax measurements, the cluster lies about 2,700 parsecs distant and has an age of 20–25 million years, meaning stars of spectral class B2 or higher are reaching the end of their main sequence lifespan. The cluster appears in front of a molecular cloud at a distance of 300 parsecs, which blocks background stars from the visual image, though the two are not physically associated.

The cluster is of particular interest for its high number of Be stars—about 24 have been discovered—most lying between spectral class B0 and B3. A candidate member, LS I +61° 235, is a Be star with an X-ray binary component having a period of about three years. At least five blue stragglers, stars formed by the merger of two other stars, are present. Two star systems are likely eclipsing binaries with periods of 0.6 and 1.03 days, and two red supergiant stars lie on the cluster's periphery. NGC 663 is assumed to form part of the stellar association Cassiopeia OB8 in the Perseus Arm of the Milky Way, along with M103, NGC 654, NGC 659, and scattered supergiant stars of similar ages and distances. It is likely the most massive star cluster in that association and possibly in the Perseus Arm.

Reader's Guide

NGC 663 holds significance as a rich laboratory for studying Be stars, with about 24 such objects identified, most in the B0–B3 range. The presence of a candidate Be star with an X-ray binary component adds to its astrophysical interest. The cluster also contains at least five blue stragglers, which provide insight into stellar merger processes, and two likely eclipsing binaries with short periods. Its two red supergiants on the periphery offer further opportunities for studying evolved massive stars. As a member of the Cassiopeia OB8 association, NGC 663 is considered the most massive cluster in that group and possibly in the entire Perseus Arm, making it a key object for understanding star formation and cluster dynamics in that spiral arm. The cluster's location in front of an unrelated molecular cloud creates a visual effect that blocks background stars, a feature that observers may note. Despite a listed visual magnitude of 7.1, several observers have reported higher estimates, indicating some variability in perceived brightness. Its young age of 20–25 million years places it in a phase where higher-mass stars are ending their main sequence lives, offering a snapshot of stellar evolution in progress.

Did You Know?

• NGC 663 is also known as Caldwell 10.
• It contains about 24 Be stars, most between spectral class B0 and B3.
• The cluster has at least five blue stragglers formed by stellar mergers.
• Two red supergiant stars are located on the cluster's periphery.