…continued
Clusters of Clusters: Globular PairingsGlobular Groupings
Although we have reached the end of our tour, there are plenty of other pairs and chains of globulars in this region. In fact, M22 and NGC 6642 represent just one side of a compact trapezium of globulars that includes M28 and NGC 6638. If you find yourself under a moonless sky, perhaps at a star party, locate a spot with a good southern horizon and enjoy an evening with some of summer's clusters of globulars!
| Globular Groupings | ||||||
| Name | Const. | R.A. (2000.0) | Dec. (2000.0) | Diam. | Mag. | Concen. |
| M4 | Sco | 16h 23.6m | -26°32' | 35' | 5.4 | IX |
| NGC 6144 | Sco | 16h 27.3m | -26° 02' | 17' | 9.0 | II |
| M9 | Oph | 17h 19.2m | -18° 31' | 11' | 7.8 | VIII |
| NGC 6342 | Oph | 17h 21.2m | -19° 35' | 5' | 9.5 | IV |
| NGC 6356 | Oph | 17h 23.6m | -17° 49' | 8' | 8.2 | II |
| NGC 6522 | Sgr | 18h 03.6m | -30° 02' | 7' | 9.9 | VIII |
| NGC 6528 | Sgr | 18h 04.8m | -30° 03' | 5' | 9.6 | VIII |
| MGC 6642 | Sgr | 18h 31.9m | -23° 29' | 9' | 8.9 | V? |
| M22 | Sgr | 18h 36.4m | -23° 54' | 33' | 5.2 | VIII |
Note: the diameter (Diam.) and magnitude (Mag.) must be considered together when judging the difficulty of detecting any type of deep-sky object, but with globular clusters there is also a third important parameter — the Shapley-Sawyer concentration class (Concen.). Class I globulars are the most concentrated and have a high surface brightness, while Class XII are low-surface-brightness loose clusters.
