The open star clusters NGC 869 and NGC 884 (also known as h and χ Per) form the famous Double Cluster in Perseus. The clusters mark the Scimitar with which Perseus decapitated the Medusa.
The famous double cluster in Perseus has perhaps been known since pre-historic times, and was first cataloged by the Greek astronomer Hipparcos in 130 B.C. For unknown reasons, Messier did not include the Double Cluster in his famous catalogue.
There is some historical confusion about the designations h and χ Persei. Since the 1840s, the name χ Per is attributed to NGC 884, and h Per to NGC 869. However, Tycho Brahe probably measured one position for the "nebulous star" that is actually the double cluster, and Johann Bayer designated this "star" as Chi. Probably, Bayer used the name h Per for a fainter star nearby.
These two clusters are a stunning sight in low power, wide field eyepiece. Each cluster is half a degree in diameter. NGC 869 is the more compressed of the two, and has over 200 white and bluish-white members. NGC 884 to the east has 175 mostly white and bluish-white stars. NGC 884 contains about 150 stars, and judging from its stellar population, is significantly older than NGC 869.
Both clusters are situated in the Perseus OB 1 association. At a distance of about 7000 light years, the clusters are only a few hundred light-years apart. They are both quite young, with NGC 869 being 5.6 million years old and NGC 884 at 3.2 million years old. Their hottest main sequence stars are of spectral type B0. By comparison, the Pleiades have an estimated age ranging from 75 million years to 150 million years.
Both clusters are also blue-shifted, with NGC 869 approaching Earth at a speed of 22 km/s and NGC 884 approaching at a similar speed of 21 km/s.