AAO image reference AAT 104. « Previous || Next »
Top left is NE. Image width is about 13 arc min
Image and text © 1992-2002, Anglo-Australian 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. 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 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.
Entry from NGC 2000.0 (R.W. Sinnott, Ed.) © Sky Publishing Corporation, 1988:
NGC 6231 C+N 16 54.0 -41 48 s Sco 15. 2.6 Cl, B, cL, pRi, st 10...13