Updated: 19 Sep 00

NGC 6231


Synopsis:  CR 315, Mel 153; open cluster; located in Scorpius and including z Scorpii; R.A. 16:54.0, dec. -41:47; culmination late July; magnitude 2.6; distance 1800 pc.


Description:  A very young open cluster, perhaps as little as 3 million years old.

It is rich in extremely hot, high-luminosity, young giant and supergiant stars.

One cluster member, z1 Scorpii, has an absolute magnitude of -8.7 in visual wavelengths (a luminosity of 250,000 suns) and a bolometric absolute magnitude of -10.8.  It is one of the most brilliant stars known from our galaxy.

This cluster is also notable for containing two of the rare Wolf-Rayet stars.



Fig 1:  Finder chart for NGC6231 showing stars of magnitude 5 and greater.  North is down (southern hemisphere view).  Easily located on the scorpion's tail, by observing the prominant optical double, z Sco.

SAO Number m(V) Spectral Type
227375 (z1) 4.2 B1
227402 (z2) 3.8 K5
227356 7.4 B0
227430 6.4 B0
227374 6.3 B0
227328 6.5 Oc (WN7)
227336 7.1 B0
227313 5.4 Oe
227425 5.8 Oe
Table 1:  Table of the brightest cluster members.   Key refers to fig. 2, m is the apparent visual magnitude, and spect. refers to the spectral type.  After Burnham 1992, p. 1722.


Fig 2:  Map of NGC6231 showing stars of magnitude 8 and greater.  North is down (southern hemisphere view).


Observation:  Reportedly visible in binoculars but I cannot see the cluster in 7 50 mm.



Burnham, Robert (1992): Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.

Malin, David (1993):  A View of the Universe. Cambridge.


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