NGC 6231

NGC 6231 is a bright open star cluster associated with nebulosity in Scorpius. It is also catalogued as Melotte 153 and Collinder 315.

NGC 6231 was one of the discoveries of Giovanni Batista Hodierna, published in his 1654 catalog in Palermo. Hodierna listed it as "Luminosae" in his catalogue of deep sky observations. This catalogue was included in his book, "De Admirandis Coeli Characteribus", published in 1654 at Palermo, but forgotten until its rediscovery in the early 1980s. NGC 6231 was independently rediscovered by Edmond Halley in 1678, by Philippe Loys De Cheseaux 1745-46, and by Abbe Lacaille in 1751-52.

NGC 6231 is located near Zeta Scorpii. ζ1 and ζ2 Sco are members of this cluster, and account for its very bright total magnitude of 2.6. Dreyer describes NGC 6231 as bright, considerably large (its apparent diameter is 15'), pretty rich, and containing stars of magnitude 10 to 13.

NGC 6231 is 5,900 light years away, and is approaching us at 22 km/sec. The cluster is extremely young, estimated to be only about 3.2 million years old. Its hottest star is ζ1 Sco, of spectral type O8 and visual magnitude 4.71. The cluster belongs to the Scorpius OB association of very young stars.

Over a degree to the NW of NGC 6231 lies a curious collection of un-named bright and dark nebulae. Their swept-back shape is moulded by radiation from very luminous stars of NGC 6231. The flow pattern in the dark cloud indicate the direction of the radiation source; its extensive bright red rims are emission nebulae, caused where the gassy dust is exposed to radiation from the hot stars of NGC 6231. The open cluster and the dark cloud it is eroding are at least 100 light years apart.