NGC 6231 is a bright open cluster of approx. 120 stars spread over 15 arc minutes of sky in Scorpius. With a combined magnitude of +2.6 the group is visible to the naked eye and a stunning object through binoculars and telescopes. At its heart are numerous hot luminous fifth and sixth magnitude blue-white type B supergiants. They appear similar to a smaller version of M45, the famous Pleiades cluster in Taurus.
NGC 6231 is located half a degree north of bright star Zeta Scorpii whose Bayer designation is shared by zeta1 Scorpii (ζ1 Sco - mag. +4.7) and zeta2 Scorpii (ζ2 Sco - mag. +3.6). The two stars are separated by 7 arc minutes but this is no physical double star, just a line of sight effect. Zeta2 is much closer at 155 light-years while zeta1 - an outlying member of NGC 6231 - lies 5,700 light-years distant. NGC 6231 is also known as the Northern Jewel Box.
NGC 6231 was discovered by Giovanni Batista Hodierna sometime before 1654. Edmond Halley independently rediscovered in 1678. It was observed by Philippe Loys De Chéseaux in 1745-46 and Nicholas Louis de Lacaille in 1751-52. The cluster was just too far south to be seen by Charles Messier.
Through popular 7x50 or 10x50 binoculars it appears bright and condensed with many stars resolvable. With a small 80mm (3.1-inch) scope, NGC 6231 is superb with many more stars visible including a nice double towards the centre. Its resemblance to the Pleiades is apparent and if it were as close to us as the Pleiades, it would be the same size in the sky but 50x brighter. NGC 6231 is connected to a larger scattered cluster of fainter stars called H12, which lies 1 degree to the north. The chain of stars linking the two clusters outlines one of the spiral arms of our galaxy.
NGC 6231 is a spectacular open cluster 5,900 light-years distant. It has a spatial diameter of 26 light-years and is number 76 in the Caldwell catalogue. The cluster is best seen from southern and equatorial regions during the months of June, July and August.
NGC 6231 Data Table
|Name||Northern Jewel Box|
|Object Type||Open Cluster|
|RA (J2000)||16h 54m 00s|
|DEC (J2000)||-41d 48m 00s|
|Apparent Size (arc mins)||15 x 15|
|Age (years)||3.2 Billion|
|Number of Stars||120|
|Other Name||Collinder 315|