Open Clusters · Constellation Crux

Jewel Box Cluster (NGC 4755 / Kappa Crucis)

Coordinates, Gaia DR3 distance (1.97 kpc), magnitude 4.2, supergiant color contrast in the Herschel 'A' asterism, and observing guide for NGC 4755.

Right Ascension (J2000.0)12h 53m 40.00s
Declination (J2000.0)-60° 21′ 42.0″
Primary ClassificationYoung supergiant open star cluster (Centaurus OB1 core)
Best Observing SeasonSouthern Autumn (March–June)

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 Jewel Box Cluster (NGC 4755 / Kappa Crucis)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Integrated visual magnitude (V)4.2 mag± 0.1 magIntegrated photoelectric & CCD photometrydefinitiveRevised New General Catalogue and Index Catalogue (RNGC/IC)
Heliocentric distance6425 ly± 130 lyGaia DR3 astrometric parallax + detached eclipsing binary (1.97 ± 0.04 kpc) (J2016.0)definitiveESA Gaia Archive — Data Release 3
Apparent diameter10 arcmin± 1 arcminVisual and photographic supergiant core extenthigh-confidenceRevised New General Catalogue and Index Catalogue (RNGC/IC)
OB supergiant turnoff age0.014 Gyr± 0.002 GyrB0–B2 supergiant isochrone & VLT spectroscopyhigh-confidenceESA Gaia Archive — Data Release 3
Measurement Uncertainties & Competing Solutions: Gaia DR3 parallaxes and eclipsing binary orbits constrain NGC 4755 to 1.97 ± 0.04 kpc (6,425 ly) and an age of 14 ± 2 Myr—virtually identical in age and stellar population to the northern Perseus Double Cluster.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Jewel Box Cluster (NGC 4755 / Kappa Crucis) at RA 12h 53m 40.00s, Dec -60° 21′ 42.0″ within Crux, 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 (Crux) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 0° 40°N Lat 60° 30°S Lat RA: 12h 53m 40.00s Dec: -60° 21′ 42.0″ Epoch: J2000.0 · V = 4.2 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

The Jewel Box Cluster (NGC 4755, Kappa Crucis, Caldwell 94) is the most celebrated colorful open cluster of the southern sky, situated 6,425 light-years (1.97 ± 0.04 kiloparsecs) away in the Southern Cross (Crux). Discovered by Nicolas-Louis de Lacaille from South Africa in 1751–1752, the cluster received its famous name from Sir John Herschel in the 1830s, who described its telescopic appearance as 'a casket of variously coloured precious stones.'

Only ~14 million years old and forming the dense core of the Centaurus OB1 association, NGC 4755 is dominated by a dazzling central triangle ('the Traffic Light' or 'Letter A' asterism) of massive supergiants shining between magnitudes 5.7 and 7.5. Four of the brightest stars—including HD 111904 (B9 Ia), HD 111973 (κ Crucis, B3 Ia), and HD 111990 (B1/B2 Ib)—are icy blue-white supergiants, each shining with 30,000 to 100,000 solar luminosities.

Right in the center of the crossbar of that blue supergiant 'A' sits DU Crucis (HD 111934), a cool M2 Iab red supergiant variable (magnitude ~7.0 to 7.6) whose deep ruby-orange color creates one of the most vivid chromatic contrasts in visual astronomy.

Practical Visual Observing Guide

Minimum InstrumentNaked eye / 7×50 binoculars (Southern Hemisphere)
Recommended Magnification60×–130×
Best Evening SeasonSouthern Autumn (March–June)
  • Located just 1.0° southeast of 1.25-magnitude Mimosa (β Crucis), perched right on the northern border of the dark Coalsack Nebula in Crux (visible south of ~25°N).
  • Visible to the naked eye as a 4.2-magnitude fuzzy 'star' (κ Crucis), and resolves into its triangular 'A' frame in 10×50 binoculars.
  • In any 75–150 mm telescope at 60×–120×, the sapphire-blue B supergiants and the central ruby-red M2 supergiant DU Crucis glow against a dark backdrop of ~100 fainter stars.

Catalog Cross-Identifications & Aliases

Common Aliases: NGC 4755 · Jewel Box Cluster · Kappa Crucis Cluster · Caldwell 94

Cross-Catalog Identifiers for Jewel Box Cluster (NGC 4755 / Kappa Crucis)
Catalog AuthorityIdentifier
NGCNGC 4755
SIMBADNGC 4755
WIKIDATAQ14266
Related Object · Perseus

Double Cluster (NGC 869 & NGC 884)

Coordinates, Gaia DR3 distance (2.29 kpc), age 14 Myr, supergiant population, and observing guide for NGC 869 and NGC 884 in Perseus.

Related Object · Cassiopeia

Owl Cluster (NGC 457 / ET Cluster)

Coordinates, Gaia DR3 distance (2.88 kpc), magnitude 6.4, Phi Cassiopeiae supergiant eyes, and observing guide for NGC 457 in Cassiopeia.

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