NGC 637

NGC 637 is an open cluster of stars in the northern constellation of Cassiopeia, positioned about 1.5° to the WNW of the star Epsilon Cassiopeiae. The cluster was discovered on 9 November 1787 by German-born English astronomer William Herschel. It is located in the Perseus Arm of the Milky Way, at a distance of approximately 2.160 kpc from the Sun. The cluster is small but compact, and is readily visible in a small telescope.

This is a young cluster with an estimated age of 5–15 million years. It has a Trumpler class of I2m, indicating it is strongly concentrated (I) with an intermediate range of brightness variation (2) and a moderate richness of stars (m). The cluster has 55 members and an angular radius of 4′.2, corresponding to a physical radius of 3.0 pc. It has a core radius of $0.36 arcminute$.

The seven brightest members are all over 10th magnitude, with five known to be variable. A total of four β Cephei-type variables have been identified, one of the highest such totals for an open cluster. A classical Be star candidate has been detected. The distribution of the cluster's stars on the Hertzsprung–Russell diagram shows a noticeable gap on the main sequence, which is not explained by missing data.