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NGC 6231 | |
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![]() NGC 6231 (top) with Zeta2 and Zeta1 Scorpii (bottom) | |
Observation data (J2000.0 epoch) | |
Right ascension | 16h 54m |
Declination | −41° 48′ |
Distance | 5,600±400 ly (1,700±130 parsec[1]) |
Apparent magnitude (V) | 2.6 |
Apparent dimensions (V) | 15.0′ |
Physical characteristics | |
Estimated age | 2–7 million years[2][3] |
Other designations | NGC 6171, Caldwell 76, Collinder 315, Melotte 153, De Cheseaux 9, Dunlop 499, Ha. I.7, Lacaille II.13 |
Associations | |
Constellation | Scorpius |
NGC 6231 (also known as Caldwell 76 or the Baby Scorpion Cluster[4][5]) is an open cluster in the southern sky located half a degrees north of Zeta Scorpii. NGC 6231 is part of a swath of young, bluish stars in the constellation Scorpius known as the Scorpius OB1 association.[6] The star Zeta1 (HR 6262) is a member of this association, while its brighter apparent partner, Zeta2 (HR 6271), is only 150 ly from Earth and so is not a member.[citation needed]
This cluster is estimated to be about 2–7 million years old,[2][3] and is approaching the Solar System at 22 km/s. The cluster and association lie in the neighboring Sagittarius Arm of the Milky Way. Zeta1 Scorpii (spectral type O8 and magnitude 4.71.[7]) is the brightest star in the association, and one of the most radiant stars known in the galaxy.[8] NGC 6231 was used to measure the binary fraction of B-type stars: 52 ± 8%, indicating that B-type stars are commonly found in binary systems, but not as commonly as in O-type stars.[3]
NGC 6231 also includes three Wolf-Rayet stars: HD 151932, HD 152270,[9] and HD 152408.[10]
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