| NGC 6633 | |
| Constellation: | Ophiuchus |
| Epoch: | J2000.0 |
| Ra: | 18h 27.7m |
| Dec: | 6° 34 |
| Dist Ly: | 1.04 kly |
| Appmag V: | 4.6 |
| Size V: | 27 |
| Age: | 660 million years |
| Names: | NGC 6633, De Cheseaux No. 3 H VIII.72, Cr 380, Mel 201 |
NGC 6633 is a large bright open cluster in the constellation Ophiuchus. Discovered in 1745-46 by Philippe Loys de Chéseaux, it was independently rediscovered by Caroline Herschel in 1783 and included in her brother William's catalog as H VIII.72.[1] Bright enough to be seen with the naked eye, the cluster is considered a fine object for binoculars or small telescopes.[2]
NGC 6633 is also known as the Tweedledum Cluster (paired with IC 4756 as Tweedledee), also as the Captain Hook Cluster and the Wasp Cluster.[3] It is also designated Collinder 380 or Melotte 201. Nearly as large as the full moon, the cluster contains 38 known stars and shines with a total magnitude of 4.6; the brightest star is of mag 7.6. Its age has been estimated at 660 million years.[1]
The cluster contains at least one chemically peculiar star - NGC 6633 48 (BD+06 3755).
The 8th-magnitude binary star HD 169959 (NGC 6633 58) is within the line-of-sight of the open cluster but is not physically associated with it.