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, this group of stars is visible to the naked eye and a stunning sight through binoculars and telescopes. At its heart are numerous hot luminous fifth and sixth magnitude blue-white type B supergiants. It appears 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). These two stars are separated by 7 arc minutes. However, this is a line of sight effect as Zeta2 is much closer at 155 light-years while Zeta1 lies 5,700 light-years distant. Zeta1 is an outlying member of the cluster. 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 it in 1678. It was observed by Philippe Loys De Chéseaux in 1745-46 and Nicholas Louis de Lacaille in 1751-52. Through popular 7x50 or 10x50 binoculars, NGC 6231 appears bright and condensed with many stars resolvable. Through a small 80mm (3.1-inch) scope it's a superb sight with many more stars visible including a nice double towards the centre. The cluster's resemblance to the Pleiades is apparent, and if it were as close as the Pleiades, it would be about 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 a degree to the north. A chain of stars linking these two clusters outlines one of the spiral arms of our galaxy.
NGC 6231 is a spectacular open cluster that's 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|