NGC 6633 is a bright, loosely scattered open cluster that makes an inviting target for wide-field astrophotography. Spanning about 0.7° (43′) across the sky, it comfortably fits within the frame of a short telescope or a fast telephoto lens, leaving room to showcase the rich surrounding star fields of the Milky Way. Its generous angular size and modest star density reward exposures that bring out the varied colors of its member stars without crowding them together. For imagers, NGC 6633 pairs beautifully with the nearby open cluster IC 4756, and the two can be captured together in a single sweeping mosaic.
Names, key data and data sources
All values come from the astronomical database SIMBAD (CDS Strasbourg). Tap a row to see where the value comes from.
Catalogue numbersNGC 6633 · Mel 201 (Melotte 201)Primary designation: NGC 6633SIMBAD
TypeOpen ClusterSIMBAD
ConstellationOphiuchus (Oph)computed
Position (J2000)RA 18h 27m 23s · Dec +06° 36′ 54″SIMBAD
Galactic coordinatesl = 36.1° · b = 8.3°Position relative to the Milky Way plane: 8° north of itcomputed
Apparent size0.7° (43′)SIMBAD
Orientation0°Position angle of the major axis, measured from north through eastSIMBAD
Distance1,300 light-years390 pcSIMBAD
Diameter16 light-yearscomputed
Blueshiftz = −0.000096−29 km/s: approaching usSIMBAD
Sources and methods (10)
- Catalogue numbers: All identifiers under which SIMBAD lists the object; order according to the Astrocamp catalogue rule.
- Type: Entry in SIMBAD, object type “OpC”. Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Constellation: Computed from the position (IAU constellation boundaries). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Position (J2000): Entry in SIMBAD, quality E (SIMBAD scale A–E). Publication: 2021A&A...647A..19T (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Galactic coordinates: Converted from the position (IAU 1958 galactic system). Publication: 2021A&A...647A..19T (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Apparent size: One catalogue value, measured in visible light, quality D (SIMBAD scale A–E). Publication: 2020A&A...633A..99C (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Orientation: One catalogue value, measured in visible light, quality D (SIMBAD scale A–E). Publication: 2020A&A...633A..99C (NASA ADS). Value since Oct 20, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Distance: Median of all 9 measurements listed in SIMBAD (methods: paral 3, ST-L 1), middle half 1,300 light-years – 1,400 light-years, full range 1,200 light-years – 1,400 light-years, rounded according to the scatter. Value since Oct 20, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Diameter: Computed from the major axis and the distance; takes the weaker rating of the two values.
- Blueshift: One catalogue value, error ± 0.1 km/s, quality A (SIMBAD scale A–E). Publication: 2022A&A...658A..14C (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
Checked against SIMBAD on Oct 20, 2026 · Open NGC 6633 in SIMBAD
Position and the cosmic neighborhood
NGC 6633 sits in the northern reaches of Ophiuchus at a right ascension of 18h 27m and a declination of +06° 37′, placing it close to the border with neighboring Serpens Cauda. This region of sky lies along a dense lane of the summer Milky Way, so wide-field frames of the cluster are naturally flecked with countless faint background stars. Its celebrated companion, the open cluster IC 4756, lies just to the east, and together the pair is sometimes informally known as the “Tweedledum and Tweedledee” clusters by visual observers and imagers alike. The star-rich setting makes careful framing and background gradient control key to a clean, balanced image.
Click marks an object (its path tonight) · click again for its constellation, once more to zoom to its field · click a frame to see it, again to zoom · its name shows details · click outside zooms out · drag to pan
Nice to Know
- NGC 6633 carries a second catalog designation, Mel 201, from the Melotte catalogue of star clusters.
- Its distance of around 1,300 light-years represents the median of nine published measurements, giving it a reliable, well-constrained value.
- The cluster measures about 16 light-years across, a true physical scale that translates into its broad 0.7° (43′) footprint on the sky.
- Its large apparent size and low surface density make it an easy, forgiving target well suited to short focal lengths and wide-field astrophotography.
