Open Clusters · Constellation Sagittarius

Messier 23 (NGC 6494)

Coordinates, Gaia DR3 distance (728 pc), magnitude 5.5, foreground Sagittarius dust lane geometry, and observing guide for M23.

Right Ascension (J2000.0)17h 57m 04.00s
Declination (J2000.0)-18° 59′ 06.0″
Primary ClassificationOpen star cluster
Best Observing SeasonSummer (June–August)

Provenance-Backed Scientific Measurements

Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.

Astrometric, Photometric, and Physical Parameters for Messier 23 (NGC 6494)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Integrated visual magnitude (V)5.5 mag± 0.1 magWide-aperture integrated photometrydefinitiveRevised New General Catalogue and Index Catalogue (RNGC/IC)
Heliocentric distance2375 ly± 35 lyGaia DR3 astrometric parallax of cluster members (728 ± 11 pc) (J2016.0)definitiveESA Gaia Archive — Data Release 3
Apparent diameter27 arcmin± 2 arcminMember star-count boundaryhigh-confidenceRevised New General Catalogue and Index Catalogue (RNGC/IC)
Main-sequence turnoff age0.33 Gyr± 0.04 GyrGaia DR3 color-magnitude diagram isochrone fittinghigh-confidenceESA Gaia Archive — Data Release 3
Measurement Uncertainties & Competing Solutions: Gaia DR3 parallaxes resolve earlier photometric distance discrepancies (600–750 pc) caused by patchy foreground dust extinction (E(B−V) = 0.36 ± 0.04 mag) along the Great Rift border, fixing M23 at 728 ± 11 pc.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Messier 23 (NGC 6494 Sagittarius Cluster) at RA 17h 57m 04.00s, Dec -18° 59′ 06.0″ within Sagittarius, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.

J2000 Equatorial Sky Position (Sagittarius) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 31° 40°N Lat 79° 30°S Lat RA: 17h 57m 04.00s Dec: -18° 59′ 06.0″ Epoch: J2000.0 · V = 5.5 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Messier 23 (NGC 6494) is a wide, uniform open cluster located 2,375 light-years (728 ± 11 parsecs) away in northwestern Sagittarius, perched directly in front of the dark dust clouds bordering the Small Sagittarius Star Cloud (M24). Discovered by Charles Messier on June 20, 1764, M23 spans 27 arcminutes across the sky—nearly the full diameter of the Moon—corresponding to a physical width of about 19 light-years.

Classified as Trumpler Class III 1 m, M23 is striking because its ~400 confirmed member stars have a remarkably narrow brightness range: lacks a dense central cusp, and its brightest 100 stars all shine closely between visual magnitudes 9.4 and 11.5. Isochrone fitting to Gaia DR3 astrometry yields an age of ~330 million years and near-solar metallicity ([Fe/H] ≈ −0.04 dex), with the cluster's main sequence turning off around spectral type B9 V alongside half a dozen evolved red-clump giants.

Because M23 sits on the near side of the Carina–Sagittarius arm while dark molecular clouds lie immediately behind it, the dark background acts like a natural velvet curtain that blocks out distant Milky Way bulge stars and makes M23's looping chains of stars stand out in sharp relief.

Practical Visual Observing Guide

Minimum Instrument7×50 binoculars
Recommended Magnification25×–60× wide-field
Best Evening SeasonSummer (June–August)
  • Located 4.5° northwest of Mu Sagittarii (Polis) and 2.5° northwest of the Trifid Nebula (M20).
  • In 7×50 or 10×50 binoculars M23 is a large, mottled 5.5-magnitude patch bordered on the northwest by a bright 6.5-magnitude foreground star (HD 163245).
  • Best viewed in an 80–150 mm telescope at low power (25×–50×), where over 100 stars of similar 10th-magnitude brightness form graceful bat-wing arcs radiating from the center.

Catalog Cross-Identifications & Aliases

Common Aliases: M23 · NGC 6494

Cross-Catalog Identifiers for Messier 23 (NGC 6494)
Catalog AuthorityIdentifier
MESSIERM 23
NGCNGC 6494
SIMBADM 23
WIKIDATAQ11369
Related Object · Sagittarius

Lagoon Nebula (Messier 8 / NGC 6523)

Coordinates, Gaia DR3 distance (1.25 kpc), NGC 6530 cluster, Hourglass core, and observing guide for the Lagoon Nebula (M8) in Sagittarius.

Related Object · Sagittarius

Trifid Nebula (Messier 20 / NGC 6514)

Coordinates, Gaia DR3 distance (1.26 kpc), Barnard 85 trisecting dust lanes, HN 40 multiple star, and observing guide for M20 in Sagittarius.

Reference Guide

Apparent vs Absolute Magnitude Explained

Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.

Reference Guide

Trigonometric Parallax, Parsecs & Gaia DR3

How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.

Scientific Sources & Dataset Provenance

  • ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
  • Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC
  • SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg