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.^[2][5] The cluster was discovered on 9 November 1787 by German-born English astronomer William Herschel.^[6] It is located in the Perseus Arm of the Milky Way,^[7] at a distance of approximately 7,045 light years from the Sun.^[1] The cluster is small but compact, and is readily visible in a small telescope.^[2]

NGC 637
Observation data (J2000.0 epoch)
Right ascension	01h 43m 04.0s[1]
Declination	+64° 02′ 24″[1]
Distance	7.045 ± 1.409 kly (2.160 ± 0.432 kpc)[1]
Apparent magnitude (V)	8.2[2]
Apparent dimensions (V)	4.2′[3]
Physical characteristics
Estimated age	10 ± 5 Myr[3]
Other designations	Cr 17, NGC 637[4]
Associations
Constellation	Cassiopeia
See also: Open cluster, List of open clusters

This is a young cluster with an estimated age of 5–15 million years.^[3] 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).^[7] The cluster has 55^[2] members and an angular radius of 4′.2, corresponding to a physical radius of 9.8 ly (3.0 pc).^[7] It has a core radius of 0.36′±0.13′.^[8]

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.^[9] A classical Be star candidate has been detected.^[7] 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.^[8]