NGC 6231, in the Sco OB association
AAO image reference AAT 104.    « Previous || Next »

NGC 6231, in the Sco OB association, ngc6231.jpg
Top left is NE. Image width is about 13 arc min
Image and text © 1992-2010, Australian Astronomical Observatory, photograph by David Malin.

The constellation of Scorpius is scattered through with spectacular individual stars forming loose groups. If such groups contain many hot, extremely luminous OB-type stars they are known as OB associations. This is more than merely a cataloguing device to bring some order to the apparent randomness of nature, rather it is an identification of the site of recent star formation, usually a broadly spread group of stars that has a strong influence on its surroundings. The stars in such groups are mostly not gravitationally bound but are expanding away from some common center, which presumably marks their birthplace.

Close to the center of the Scorpius OB association is the cluster NGC 6231. This group is gravitationally bound and it is very young, with an age of about 3.5 million years. The cluster is also very luminous and its radiation, and that of the ultra-luminous star zeta-1 Scorpii (about one degree to the south of this image) affects a vast volume of space, turning dark and dusty clouds into vivid nebulae such as that seen in AAT 72a. It is not surprising therefore to find the names of OB associations linked to the most spectacular regions in the sky, in Orion, (the nearest and richest), in Carina, in Cygnus and Perseus.

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Updated by David Malin, 2010, August 1