The "double cluster" (NGC 869 and NGC 884) also known as h and X (chi) Persei are a highly renowned pair of open clusters residing some 7500 light years from our sun in the Perseus arm of the Milky Way. The clusters are among the brightest, densest, and closest of the open clusters containing moderately massive stars. The two clusters are separated by only a few hundred light years and probably form the core of the immense stellar association Perseus OB1. Intervening dust from our galaxy's disk slightly obscures the view, dimming the pairs overall brightness by 1.6 magnitude (4.4 times). Despite this fact h and Chi Persei are two of the brightest clusters in the sky. Many of its members are hot and luminous O and B type giants, some shining 60,000 times brighter than our sun.
The two clusters are strikingly similar in many ways and are believed to have their origin from a single ancestral cloud some 12.8 million years ago. Similar velocities (21 km/sec toward our sun) and similar overall mass ( 3700 solar masses for h and 2800 solar masses for chi) are strong evidence pointing to a common origin of the two clusters. A few red supergiants are seen at considerable projected distances from the double cluster and may either belong to the background Perseus OB1 association or alternatively could represent massive stars previously ejected from the dense clusters. Although the clusters show impressive density they lack the overall mass to qualify as superclusters. For a comparison each cluster carries about 1/10th the mass of a super cluster like R136 in the LMC (see 30 Doradus).
The existence of dynamically related multiple
clusters is not unique and can be found in several other known
cluster systems. However the remarkable similarity of the two
clusters in distance, mass, reddening, ages, and physical size
is remarkable and considered especially unique.