NGC 188 is an open cluster located in the far northern constellation of Cepheus. It was discovered by John Herschel - the son of William Hershel - on November 3, 1831. He originally recorded it as h 34 in his 1833 catalogue and then included it as GC 92 in his subsequent General Catalogue of 1864. The cluster finally became NGC 188 in John L.E. Dreyer's New General Catalogue of 1888.
NGC 188 is the northernmost open cluster in the sky, it's positioned only 4.75 degrees from the North Celestial Pole. Located at such a northerly declination means the cluster is circumpolar from almost the entire northern hemisphere. It can be seen from the Southern Hemisphere but only from latitudes north of 5 degrees south. Even then it never climbs more than a few degrees above the northern horizon.
With an apparent magnitude of +8.1, NGC 188 is visible through binoculars but not easily. On dark nights of excellent seeing it can be spotted with 7x50 or 10x50 binoculars, appearing as a faint dim patch of light. The cluster has a low surface brightness and is better seen using at least a medium size telescope. A 150mm (6-inch) instrument reveals a soft glow that initially appears unresolved but on further inspection a sprinkling of faint stars can be seen. The brightest cluster members shine at 12th magnitude and there are at least 130 stars brighter than 17th magnitude. Through a 250mm (10-inch) scope, NGC 188 is a wonderful sight with dozens of stars scattered like stardust across the entire cluster face, which spans 15 arc minutes in diameter.
Studies indicate that NGC 188 is the most ancient of all known open clusters. It's estimated to be at least five billion years old. For comparison, M45 The Pleiades cluster in Taurus is a youthful 115 million years ago. Once factor that certainly contributed to the longevity of NGC 188 is that it lies well away from the plane of the galaxy and therefore rarely passes through high-density galactic regions.
Since most open star clusters are young they typically contain many luminous hot blue giant stars. These type of stars are short lived and as a result old clusters like NGC 188 tend to contain more yellow and red stars and are almost devoid of blue stars.
NGC 188 has an actual diameter of 33 light-years and is estimated to be about 5,000 light-years distant. It's also the first object in the Caldwell catalogue.
NGC 188 Data Table
|Object Type||Open Cluster|
|RA (J2000)||00h 47m 30s|
|DEC (J2000)||85h 14m 29s|
|Apparent Size (arc mins)||15 x 15|
|Age (years)||5 Billion|
|Number of Stars||>130|
|Other Name (s)||Collinder 6|
|Notable Feature||Most northerly open cluster in the sky|