- NGC 869 and NGC 884 The Double Cluster
The Double Cluster, NGC 869 and NGC 884, is one of the most beautiful objects that can be seen in the night sky. It lies within the stream of the Winter Milky Way between the constellations Cassiopeia and Perseus. Visible to the naked eye (visual magnitude 3.7) as a small glowing patch, it is spectacular even in binoculars. The object has been known since antiquity, and was cataloged by Hipparchus in 130 BC. However, it was William Herschel in the early 19th century who recognized that it consisted of two separate star clusters. The great mystery is why Charles Messier did not include it in his list of "nebulous objects", since it was certainly known to astronomers of his time (he included several other well-known objects, including the Pleiades and Praesepe clusters).
In my image, NGC 869 is on the left while NGC 884 is on the right. These two clusters are a true physical pair, and lie at a distance of around 7500 LY. The clusters are relatively young, with an estimated age of 3.2 to 5.6 million years. This young age is evident in the blue-white color of many of the cluster stars.
It is common to see the Double Cluster called "χ - h", where χ refers to NGC 884 and h refers to NGC 869. However, this appears to be a misunderstanding by more recent astronmers stemming from early measurements of the positions of the clusters. The bright yellow star lying below NGC 869 in my image is actually χ Persei.
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