NGC 654

NGC 654 is an open star cluster in the northern constellation Cassiopeia, discovered by William Herschel in 1787. It has an apparent magnitude of 6.5, making it visible through binoculars. The cluster lies about 2.5° northeast of the star Delta Cassiopeiae, and in the same low-power field of view, the open clusters NGC 663 and NGC 659 can also be seen. At its center is a yellowish 7th-magnitude star—an F5Ia supergiant—that may belong to the cluster.

The cluster is roughly 2,400 parsecs away and is quite young, with an estimated age of 15 million years, though it could be as old as 40 million years, and star formation appears to have spanned at least 20 million years. Its central region shows less reddening than the outer parts, possibly because two dust layers lie between the Solar System and the cluster—one at 200 parsecs and another at 1 kiloparsec—with an additional dust layer behind the cluster. NGC 654 contains about 80 members, including three Be stars and a few luminous stars such as HD 10494 and the F5Ia supergiant. The earliest spectral type among its stars is around B0.

This cluster is thought to be part of the Cassiopeia OB8 stellar association, located in the Perseus Arm of the Milky Way. Other open clusters in this association include M103, NGC 663, and NGC 659, along with scattered supergiant stars, all with similar ages and distances.