- NGC 6231 Northern Jewel Box
NGC 6231 is a compact, bright open cluster lying in the southern part of the constellation Scorpius near the bright pair of stars ζ1 and ζ2 Scorpii. In my image, ζ1 and ζ2 lie near the bottom edge, with the brighter ζ2 on the left. The other star forming a triangle with ζ1 and ζ2 is HD 152293. The stars ζ1, ζ2 and HD 152293 have visual magnitudes of 4.70, 3.62 and 5.84 and lie at distances of 2510, 133 and 1418 LY, respectively.
NGC 6231 is one of the most beautiful of the open clusters, but it's low elevation (declination of -41.8 degrees) in North American skies makes it a difficult object for many to observe. The common name "Northern Jewel Box" alludes to another beautiful cluster, the better-known Jewel Box Cluster (NGC 4755) in the southern constellation Crux. NGC 6231 lies at a distance of around 5900 LY at the center of the Scorpius OB1 Association of stars, which includes ζ1 Scorpii. With a visual magnitude of 2.6, NGC 6231 is easily visible to the naked eye as a "star" in the curving tail of the scorpion. It contains around 90 brighter member stars and has a visual diameter of 15 arc-min. Along with ζ1, ζ2 and HD 152293, NGC 6231 forms the head of the "False Comet", an asterism that contains several other groups of stars, including the large open clusters Trumpler 24 and Collinder 316.
NGC 6231 was first observed by Giovanni Batista Hodierna some time before 1654. He listed as a luminosa (a "nebula" that could be resolved into stars) in his catalog of deep-sky objects published in Palermo, Sicily, in 1654.
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