Messier 71 (NGC 6838)
Coordinates, Gaia DR3 distance (4.00 kpc), magnitude 8.19, metal-rich thick-disk chemistry, and observing guide for Messier 71 in Sagitta.
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.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Integrated visual magnitude (V) | 8.19 mag | ± 0.06 mag | Integrated photoelectric & CCD photometry | definitive | Catalog of Parameters for Milky Way Globular Clusters |
| Heliocentric distance | 13050 ly | ± 170 ly | Gaia EDR3 parallax + HST proper motions (4.00 ± 0.05 kpc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 7.2 arcmin | ± 0.6 arcmin | Outer surface-density profile fitting | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Main-sequence turnoff age | 11.2 Gyr | ± 0.5 Gyr | HST ACS metal-rich isochrone fitting | model-dependent | Catalog of Parameters for Milky Way Globular Clusters |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Messier 71 (NGC 6838 Disk Globular Cluster) at RA 19h 53m 46.49s, Dec +18° 46′ 45.1″ within Sagitta, 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.
Astrophysical Context & Structure
Messier 71 (NGC 6838) is a compact, low-mass globular cluster embedded in the rich Milky Way star clouds of the tiny arrow constellation Sagitta, just 13,050 light-years (4.00 ± 0.05 kiloparsecs) from Earth. Discovered by Jean-Philippe Loys de Chéseaux in 1745 and added to Charles Messier's catalog in October 1780 after Pierre Méchain observed it, M71 spans only about 27 light-years in physical diameter and has a modest total mass of ~30,000 solar masses.
Throughout much of the 20th century, M71 was the subject of a famous classification debate: because it lacks a sharp central condensation (Shapley-Sawyer Class XI), possesses no RR Lyrae variable stars, moves on a low-inclination Galactic disk orbit, and has a relatively high iron abundance of [Fe/H] = −0.78 dex (~17% solar, similar to 47 Tucanae), many catalogs listed it as a rich, compressed open cluster.
Modern Hubble Space Telescope photometry and high-resolution spectroscopy settled the question decisively: M71 has a classic 11.2-billion-year globular turnoff age, a stubby red horizontal branch characteristic of metal-rich thick-disk globular clusters, five millisecond pulsars (including eclipsing black-widow systems), and second-generation light-element abundance anomalies.
Practical Visual Observing Guide
- Exceptionally easy to locate along the shaft of the Arrow in Sagitta, sitting almost halfway between Gamma Sagittae (γ Sge) and Delta Sagittae (δ Sge), just 4° south of the Dumbbell Nebula (M27).
- In 10×50 binoculars M71 appears as a faint 8.2-magnitude smudge set against a dazzling backdrop of Summer Triangle Milky Way field stars.
- Through a 100–200 mm telescope at 80×–150×, M71 resolves into a arrowhead- or V-shaped wedge of 12th-to-14th magnitude stars that looks remarkably like a rich open cluster.
Catalog Cross-Identifications & Aliases
Common Aliases: M71 · NGC 6838 · Angelfish Cluster
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 71 |
| NGC | NGC 6838 |
| SIMBAD | M 71 |
| WIKIDATA | Q11491 |
Related Objects, Comparisons & Guides
Dumbbell Nebula (Messier 27 in Vulpecula)
Coordinates, Gaia DR3 distance (389 pc), magnitude 7.4, central white dwarf, and observing guide for the Dumbbell Nebula (M27).
47 Tucanae (NGC 104 Globular Cluster)
Coordinates, Gaia distance (4.52 kpc), magnitude 4.09, millisecond pulsar core, and observing guide for 47 Tucanae (NGC 104).
King Cobra Cluster (Messier 67 / NGC 2682)
Coordinates, Gaia DR3 distance (855 pc), 4.0 Gyr solar-twin age, exoplanet hosts, and observing guide for ancient open cluster M67 in Cancer.
Astronomical Catalogs Explained (M, NGC, Gaia)
How astronomical designations work across the Messier (M), NGC/IC, Bayer/Flamsteed, Henry Draper (HD), Hipparcos (HIP), and Gaia DR3 catalogs.
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.
Scientific Sources & Dataset Provenance
- Catalog of Parameters for Milky Way Globular Clusters (2010 / 2021 Gaia EDR3 revision): Harris, W. E. 1996 (2010 ed.), AJ, 112, 1487; Vasiliev, E. & Baumgardt, H. 2021, MNRAS, 505, 5978 — McMaster University / Gaia EDR3 Cluster Survey
- 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
- 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