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.
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) | 4.2 mag | ± 0.1 mag | Wide-aperture integrated photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric distance | 1497 ly | ± 16 ly | Gaia DR3 astrometric parallax of co-moving members (459 ± 5 pc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 25 arcmin | ± 2 arcmin | Visual and Gaia DR3 member core extent | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Isochrone turnoff age | 0.095 Gyr | ± 0.015 Gyr | Gaia DR3 B-star main-sequence turnoff fitting | high-confidence | ESA Gaia Archive — Data Release 3 |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Butterfly Cluster (Messier 6 / NGC 6405) at RA 17h 40m 10.00s, Dec -32° 13′ 12.0″ 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
The Butterfly Cluster (Messier 6, NGC 6405) is a brilliant naked-eye open cluster situated 1,497 light-years (459 ± 5 parsecs) away near the stinger of Scorpius. First recorded by Italian astronomer Giovanni Battista Hodierna before 1654 and cataloged by Charles Messier on May 23, 1764, the cluster owes its popular nickname—coined by 20th-century observer Robert Burnham Jr.—to two symmetrical wings of 7th-to-10th magnitude stars and twin curving stellar antennae spanning 25 arcminutes across the sky.
With an age of roughly 95 million years and a physical diameter of about 11 light-years, M6 is dominated by dozens of hot, blue-white B4 to A0 main-sequence stars. Contrasting dramatically with this icy blue ensemble is the cluster's brightest member on its eastern wingtip: BM Scorpii (HD 160371), a luminous K2 Ib–II orange supergiant and semiregular variable whose visual brightness fluctuates sluggishly between magnitude 5.25 and 6.46 over a cycle of roughly 815 days.
Spectroscopic abundance surveys indicate that M6 formed with slightly supersolar metallicity ([Fe/H] ≈ +0.07 dex) inside the Carina–Sagittarius spiral arm, and several of its B-type stars exhibit chemically peculiar mercury-manganese or silicon atmospheric enhancements.
Practical Visual Observing Guide
- Located 5° north-northwest of Shaula (λ Scorpii) in the Scorpion's stinger and 3.5° northwest of Ptolemy's Cluster (M7).
- Easily visible to the naked eye under moderately dark skies at magnitude 4.2, and fits together with M7 inside a single 7×50 or 10×50 binocular field of view.
- A 70–120 mm rich-field telescope at 25×–50× frames the entire butterfly silhouette and highlights the deep amber-orange tint of BM Scorpii against ~80 blue-white stars.
Catalog Cross-Identifications & Aliases
Common Aliases: M6 · NGC 6405 · Butterfly Cluster
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 6 |
| NGC | NGC 6405 |
| SIMBAD | M 6 |
| WIKIDATA | Q11225 |
Related Objects, Comparisons & Guides
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.
Messier 23 (NGC 6494 Sagittarius Cluster)
Coordinates, Gaia DR3 distance (728 pc), magnitude 5.5, foreground Sagittarius dust lane geometry, and observing guide for M23.
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.
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.
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