Double cluster — 페르세우스의 산개성단

NGC 884 is the eastern half of the famous Double Cluster in Perseus — historically designated chi Persei, while its western twin NGC 869 is h Persei. The pair, together cataloged as Caldwell 14, lies about 7,500 light-years away and consists of two young open clusters separated by only a few hundred light-years. Each cluster contains hundreds of hot blue and yellow supergiant stars roughly 12-14 million years old. Together they are one of the most spectacular binocular sights in the entire northern sky. NGC 884 on its own is slightly more compact and star-rich than its partner, with a distinct splash of contrasting colored supergiants.

Visible to the naked eye from dark skies as a conspicuous fuzzy knot between Cassiopeia and Perseus. Binoculars reveal the Double Cluster's overall shape beautifully; a small refractor at 30-40x frames both halves in the same low-power field. For NGC 884 specifically, a 4- to 6-inch telescope at 50-80x shows several orange and yellow-red supergiants scattered among the bluer cluster stars — the color contrast is often cited as the most striking feature of the cluster. Best observed from August through February when Perseus climbs high.

Both halves of the Double Cluster have been known since antiquity as a naked-eye pair. Hipparchus cataloged them around 130 BC as a single object. They appear in star charts across cultures for millennia. The 'h and chi Persei' letters were assigned by Johann Bayer in 1603, and William Herschel later gave them separate NGC-precursor designations. Photographic studies in the early 20th century confirmed that the two clusters are at nearly the same distance and similar age — they are almost certainly a physical double formed from the same molecular cloud.