Constellation Scorpius Declination −41° 48′ Apparent magnitude (V) 2.6 | Right ascension 16 54 Distance 5.9 kly Apparent dimensions (V) 15.0′ | |
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NGC 6231 is an open cluster located near Zeta Scorpii. Zeta1 (HR 6262) is a member of this star cluster. (Its brighter apparent partner, Zeta2 (HR 6271), is only 150 ly from Earth and so is not a cluster member.)
This cluster is estimated about 3.2 million years old, and is approaching the Solar System at 22 km/s. The cluster belongs to the young Scorpius OB1 association. Zeta1 Scorpii (spectral type O8 and magnitude 4.71.) is the brightest star in the association, and one of the most radiant stars known in the galaxy.
NGC 6231 also includes three Wolf-Rayet stars: HD 151932, HD 152270, and HD 152408.
The cluster was discovered by Giovanni Batista Hodierna before 1654. Hodierna listed it as Luminosae in his catalogue of deep sky observations. This catalogue was included in his book De Admirandis Coeli Characteribuse published in 1654 at Palermo. It was independently observed by other astronomers after Hodierna.
The cluster forms the head of the "false comet", the tail being the dimmer cluster Trumpler 24 extending to the north, also part of the Scorpius OB1 association.