Messier 4 (NGC 6121)
Coordinates, Gaia DR3 distance (1.85 kpc), magnitude 5.63, central stellar bar, and Antares finder guide for Messier 4 in Scorpius.
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) | 5.63 mag | ± 0.05 mag | Wide-aperture integrated photometry | definitive | Catalog of Parameters for Milky Way Globular Clusters |
| Heliocentric distance | 6030 ly | ± 70 ly | Gaia DR3 trigonometric parallax of cluster members (1.85 ± 0.02 kpc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 36 arcmin | ± 2.5 arcmin | Outer tidal radius surface-density fit | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| White-dwarf cooling & turnoff age | 12.2 Gyr | ± 0.5 Gyr | HST white-dwarf cooling sequence + main-sequence turnoff | high-confidence | Catalog of Parameters for Milky Way Globular Clusters |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Messier 4 (NGC 6121 Nearest Globular) at RA 16h 23m 35.22s, Dec -26° 31′ 32.7″ within Scorpius, 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 4 (NGC 6121) is the closest major globular cluster to the Solar System, located just 6,030 light-years (1.85 ± 0.02 kiloparsecs) away in Scorpius. Discovered by Jean-Philippe Loys de Chéseaux in 1745 and cataloged by Charles Messier on May 8, 1764, M4 was the very first globular cluster ever resolved into individual stars—a feat achieved by Messier himself using a modest 3.5-inch reflector because M4's proximity makes its brightest red giants shine at magnitude 10.8.
If our sightline to M4 were completely clear of interstellar dust, it would blaze as one of the most spectacular naked-eye clusters in the sky. Instead, its starlight passes through the southern outskirts of the Rho Ophiuchi molecular cloud complex, suffering more than a full magnitude of visual extinction and taking on a warm golden-amber hue. Despite its 12.2-billion-year age and modest total mass (~90,000 solar masses), M4 follows an eccentric, low-inclination orbit that repeatedly plunges through the dense Milky Way disk, stripping away its outer halo stars.
In 1987, M4 yielded the first millisecond pulsar ever discovered inside a globular cluster (PSR B1620−26, spinning at 90.3 Hz). Subsequent Hubble and radio timing measurements revealed that this pulsar and its white-dwarf companion are orbited by a 2.5-Jupiter-mass circumbinary exoplanet (PSR B1620−26 b)—one of the oldest known planets in the universe.
Practical Visual Observing Guide
- Exceptionally easy to locate just 1.3° due west of 1st-magnitude red supergiant Antares (α Scorpii), sharing the same low-power binocular or finder field.
- Because its Shapley-Sawyer Class IX profile is loose and spread across 36 arcminutes, low surface brightness can wash out under light pollution; swing Antares just outside the eyepiece field to reduce glare.
- Even an 80–100 mm telescope at 60×–100× resolves dozens of 11th-magnitude stars, including the famous 2.5′-long north-south 'spine' or bar of 11th-magnitude giants bisecting the core.
Catalog Cross-Identifications & Aliases
Common Aliases: M4 · NGC 6121 · Cat's Eye Globular
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 4 |
| NGC | NGC 6121 |
| SIMBAD | M 4 |
| WIKIDATA | Q11196 |
Related Objects, Comparisons & Guides
Messier 80 (NGC 6093 Dense Scorpius Globular)
Coordinates, Gaia DR3 distance (10.34 kpc), magnitude 7.33, Nova T Scorpii 1860, blue-straggler core, and observing guide for M80 in Scorpius.
Messier 22 (NGC 6656 Sagittarius Cluster)
Coordinates, Gaia DR3 distance (3.30 kpc), magnitude 5.10, stellar-mass black holes, and observing guide for Messier 22 in Sagittarius.
Omega Centauri (NGC 5139 Cluster)
Coordinates, Gaia DR3 distance (5.43 kpc), magnitude 3.68, and multi-population core data for Omega Centauri (NGC 5139) in Centaurus.
Surface Brightness & Extended Objects Guide
Why surface brightness (mag/arcsec²) governs nebula and galaxy visibility more than integrated magnitude, and how exit pupil controls eyepiece contrast.
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
- 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