NGC 6633 is a relatively nearby open cluster visible to the naked eye under good conditions and an easy target for binoculars and small to medium telescopes. It spans nearly the apparent diameter of the full Moon, making it ideal for wide-field and short-focal-length astrophotography. The cluster contains several dozen member stars (commonly quoted ≈30–40), with the brightest cluster members around magnitude ~7–8, producing a pleasing, resolved starfield. It is sometimes paired visually and historically with the nearby cluster IC 4756 (the “Tweedledee / Tweedledum” pairing) and lies close to some dark nebula patches in the same rich Milky Way field.
Names and Catalog numbers
- NGC 6633
- Cr 380 (Collinder catalog)
- Mel 201 (Melotte catalog)
Position and the cosmic neighborhood
NGC 6633 sits low in northern Ophiuchus near the border with Serpens, at J2000 coordinates approximately RA 18h 27.7m, Dec +06°34′. The cluster lies against a rich Milky Way background so the field contains many foreground and background stars as well as faint dark nebulae in the same patch of sky; IC 4756 is a nearby open cluster often mentioned together with NGC 6633. Because of its modest declination (+6°), the cluster is well placed for observers across most of the northern hemisphere and into mid-southern latitudes during its season.
Nice to Know
- Age: NGC 6633 is moderately old for an open cluster — roughly a few hundred million years (commonly cited ~600–700 Myr), which explains the lack of very massive blue supergiants and the presence of somewhat evolved members.
- Magnitude & apparent size: It has a combined (integrated) magnitude of about +4.6 and an apparent diameter near 27 arcminutes — almost the apparent size of the full Moon.
- Chemical peculiarity: At least one chemically peculiar star (catalogued as a peculiar member) has been identified among its members, which makes it of minor interest for stellar studies beyond visual observing and imaging.
- Historical discovery and nicknames: Discovered by Philippe Loys de Chéseaux in the 18th century and later catalogued by Herschel, the cluster carries several colloquial nicknames (Tweedledum, Captain Hook, Wasp) used in observing lore and by some amateur writers.
Brightnes, distance and size
NGC 6633 has an integrated visual magnitude of approximately 4.6, making it a naked-eye object under dark skies and easily visible in binoculars; the brightest individual cluster members are near magnitude 7–8. Modern distance estimates place the cluster at roughly 1.04 kiloyears (≈1,040 light-years) from Earth. With an apparent diameter of about 27 arcminutes, the cluster’s projected physical diameter corresponds to roughly 8 light-years across at that distance (angular diameter in radians ≈0.00785 × distance ≈ 8 ly), though literature sometimes quotes a wider range (≈8–15 ly) depending on membership criteria. The compact, resolved starfield and large apparent size make it an excellent wide-field astrophotography target that fills many common imaging sensors without mosaics.
