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.
References
Burnham, Robert (1992): Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.
Malin, David (1993): A View of the Universe. Cambridge.