Ptolemy's Cluster (Messier 7 / NGC 6475)
Coordinates, Gaia DR3 distance (279 pc), magnitude 3.3, Baade's Window background globulars, and observing guide for M7 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) | 3.3 mag | ± 0.1 mag | Wide-aperture integrated photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric distance | 910 ly | ± 10 ly | Gaia DR3 astrometric parallax of ~750 member stars (279 ± 3 pc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 80 arcmin | ± 5 arcmin | Wide-field member star counts (~1.3°) | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Cluster turnoff & lithium age | 0.22 Gyr | ± 0.03 Gyr | Gaia DR3 isochrone + gyrochronology & lithium depletion | high-confidence | ESA Gaia Archive — Data Release 3 |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Ptolemy's Cluster (Messier 7 / NGC 6475) at RA 17h 53m 51.00s, Dec -34° 47′ 34.0″ 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
Ptolemy's Cluster (Messier 7, NGC 6475) is the southernmost object in the entire Messier catalog (declination −34°48′) and one of the brightest, most prominent naked-eye open clusters in the sky. Described in 138 CE by Greco-Roman astronomer Claudius Ptolemy in the Almagest as 'the nebulous following the sting of Scorpius,' M7 lies just 910 light-years (279 ± 3 parsecs) from Earth and spans 80 arcminutes—more than 2.5 times the width of the full Moon.
Gaia DR3 astrometry and radial-velocity surveys have identified ~750 gravitationally bound member stars inside a physical diameter of 25 light-years, totaling roughly 735 solar masses. At an age of ~220 million years, M7's main sequence turns off around spectral type B6 V, while its most massive surviving star has already evolved into the 5.8-magnitude yellow giant HD 162391 near the cluster center.
Because M7 sits directly in front of the brilliant Sagittarius-Scorpius Galactic bulge star clouds through a low-extinction window, long exposures and big-aperture views through the cluster also capture the distant 11th-magnitude background globular cluster NGC 6453 (lying 37,000 light-years away) on M7's northwestern rim, plus several planetary nebulae belonging to the Milky Way bulge.
Practical Visual Observing Guide
- Located 4.5° northeast of Shaula (λ Scorpii) at the tip of the Scorpion's stinger and 3.5° southeast of the Butterfly Cluster (M6).
- At magnitude 3.3, M7 stands out effortlessly to the naked eye as a bright, detached patch against the Milky Way.
- Because it spans 1.3°, 7×50 or 10×50 binoculars or a low-power rich-field telescope (15×–25×) give the best view, showing a bold cross-shaped central pattern of 6th- and 7th-magnitude stars; use 120× in a 150 mm+ scope on the northwestern edge to spot background globular NGC 6453.
Catalog Cross-Identifications & Aliases
Common Aliases: M7 · NGC 6475 · Ptolemy's Cluster
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 7 |
| NGC | NGC 6475 |
| SIMBAD | M 7 |
| WIKIDATA | Q11234 |
Related Objects, Comparisons & Guides
Butterfly Cluster (Messier 6 / NGC 6405)
Coordinates, Gaia DR3 distance (459 pc), magnitude 4.2, BM Scorpii orange supergiant contrast, and observing guide for M6 in Scorpius.
Pleiades (Messier 45 Open Cluster)
Coordinates, Gaia DR3 distance (136.2 pc), magnitude 1.6, reflection nebulosity, and observing guide for the Pleiades (M45) in Taurus.
Beehive Cluster (Messier 44 Praesepe)
Coordinates, Gaia DR3 distance (186.2 pc), magnitude 3.1, exoplanet hosts, and observing guide for the Beehive Cluster (M44 / NGC 2632).
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.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
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