Coordinates: Sky map 18h 27m 42s, +06° 34′ 00″

NGC 6633

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NGC 6633
NGC 6633.png
NGC 6633 (Stellarium image) (Roberto Mura)
Observation data (J2000.0 epoch)
Right ascension18h 27.7m
Declination6° 34′
Distance1.04 kly
Apparent magnitude (V)4.6
Apparent dimensions (V)27′
Physical characteristics
Estimated age660 million years
Other designationsNGC 6633, De Cheseaux No. 3
H VIII.72, Cr 380, Mel 201
See also: Open cluster, List of open clusters

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.[3] It is also designated Collinder 380 or Melotte 201. Nearly as large as the full moon, the cluster contains 38 known stars[4] 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).[4]

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.[4]

Map showing location of NGC 6633 (Roberto Mura)


  1. ^ a b "NGC 6633". Retrieved 2020-10-05.
  2. ^ Michael Bakich. "Open cluster NGC 6633, a complex of dark nebulae, and globular cluster M54". Retrieved 2020-10-05.
  3. ^ O'Meara, Stephen James (2007). Deep-Sky Companions: Hidden Treasures. Cambridge University Press. pp. 457–458. ISBN 978-0521-83704-0.
  4. ^ a b c 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

External links[edit]

NGC 6633 - Tweedledum Cluster (Kurt Zeppetello)