November 2026 - Picture of the Month

NGC 869 and NGC 884 in Perseus

This month I have a favourite and familiar sight of the Double Cluster in Perseus. To the usual view David has added some narrowband data to show the nebulosity that surrounds this region of the Perseus arm of our galaxy. The Double Cluster comprises the two massive open clusters NGC 869 (on the left) and NGC 884 (on the right), also known as η Persei and χ Persei respectively. They share a common origin in a massive molecular cloud and they now form the core of the Perseus OB association.

All open clusters form inside molecular gas clouds, and I frequently feature images of this in action, but the Double Cluster is a great example of a close binary open cluster (CBOC). These clusters are physically associated in the sense that they're identified not only by their proximity in space but also their similar ages and movement through space. CBOCs are thought to form by either the fragmentation of their shared molecular cloud or through tidal capture. The Double Cluster appears belong to the first of these categories.

The pair are amongst the most massive young open clusters in the Milky Way and in a 2001 study photometric estimates of the their ages found that NGC 869 appeared to be older at nearly 20 Myr than NGC 884 which was only 12.6–14.1 Myr old.¹ Using data collected by the Mosaic Imager at the 4m Mayall telescope at the Kitt Peak National Observatory in 2006, Currie et al. estimated NGC 869 and NGC 884 to have cores containing 4,700 and 3,700 solar masses respectively.

In addition they conclude that the pair's extensive halo must add at least another 11,000 solar masses, giving a combined mass of 20,000 solar masses and about 15,000 stars, 5,000 of which lie within 10 arcminutes of the cluster centres.² It's suspected that the halo may extend well beyond 30 arcminutes and Zhong et al. have extended NGC 869 to 5,500 and NGC 884 4,300 solar masses with an analysis of Gaia DR2 data.³

Very recently, based on a probablistic analysis of the Double Cluster using Gaia DR3 data, Tașdemir et al. found distances of 2,378pc (7,750 light-years) for NGC 869 and 2,279pc (about 7,430 light-years) for NGC 884 and both clusters to be about 20 Myr old. The same analysis also identified 808 and 707 high probablity members of NGC 869 and NGC 884 respectively ⁴, somewhat less than the studies above, but certainly more than the 317 and 303 given by Archinal and Hynes.

Further, their analysis appears to confirm the their common origin and also suggests that the two clusters might be in for some interaction in about 11 Myr when they're expected to be 26 pc (85 light-years) apart.⁴

It's entirely possible to see the Double Cluster as a hazy patch with the naked eye, and it's a much more impressive sight in a pair of 10x50 binoculars. Both clusters are at least 18 arcminutes in diameter with condensed cores, as reflected in their I3r Trumpler classification. The clusters contain a large number of Washington Double Star Catalog pairs within these boundaries: 10 for NGC 884 and 22 for NGC 869.

This part of the night sky is full of deep-sky objects and David's also captured the nearby open cluster NGC 957, on the extreme left of his image, which is home to double star HJ 2143.

James Whinfrey - Website Administrator.

References

1. Marco, A. and Bernabeu, G., “Photometric study of the double cluster h & chi Persei”, Astronomy and Astrophysics, vol. 372, EDP, pp. 477–494, 2001. doi:10.1051/0004-6361:20010441.
2. Currie, T., “The Stellar Population of h and χ Persei: Cluster Properties, Membership, and the Intrinsic Colors and Temperatures of Stars”, The Astrophysical Journal Supplement Series, vol. 186, no. 2, IOP, pp. 191–221, 2010. doi:10.1088/0067-0049/186/2/191.
3. Zhong, J., Chen, L., Kouwenhoven, M. B. N., Li, L., Shao, Z., and Hou, J., “Substructure and halo population of Double Cluster h and χ Persei”, Astronomy and Astrophysics, vol. 624, Art. no. A34, EDP, 2019. doi:10.1051/0004-6361/201834334.
4. Tașdemir, S., Elsanhoury, W. H., Çınar, D. C., Haroon, A., and Bilir, S., “Comprehensive Gaia DR3-based Astrometric, Photometric, and Kinematic Studies of the Binary Open Cluster h and χ Persei”, Research in Astronomy and Astrophysics, vol. 26, no. 4, Art. no. 045016, IOP, 2026. doi:10.1088/1674-4527/ae3d11.
5. Brent Archinal and Steven Hynes, Star Clusters, Willman-Bell, 2003.