|Observation data (J2000.0 epoch)|
|Right ascension||18h 27.7m|
|Apparent magnitude (V)||4.6|
|Apparent dimensions (V)||27′|
|Estimated age||660 million years|
|Other designations||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. Bright enough to be seen with the naked eye, the cluster is considered a fine object for binoculars or small telescopes.
NGC 6633 is also known as the Tweedledum Cluster (paired with IC 4756 as Tweedledee), also as the Captain Hook Cluster. 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.
The 8th-magnitude binary star HD 169959 (NGC 6633 58) is within the line-of-site of open cluster but is not physically associated with it.
- "NGC 6633". messier.seds.org. Retrieved 2020-10-05.
- Michael Bakich. "Open cluster NGC 6633, a complex of dark nebulae, and globular cluster M54". astronomy.com. Retrieved 2020-10-05.
- O'Meara, Stephen James (2007). Deep-Sky Companions: Hidden Treasures. Cambridge University Press. p. 457-458. ISBN 978-0521-83704-0.
- Photoelectric search for peculiar stars in open clusters. XV. Feinstein 1, NGC 2168, NGC 2323, NGC 2437, NGC 2547, NGC 4103, NGC 6025, NGC 6633, Stock 2, and Trumpler 2, 2014, arXiv:1403.3538
- Media related to NGC 6633 at Wikimedia Commons
- NGC 6633 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images