Acrux (Alpha Crucis / α¹ & α² Crucis)
Coordinates, Hipparcos distance (321 ly), magnitude 0.76, 4.0″ B-star visual binary + α Cru C triple, and observing guide for Acrux.
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 |
|---|---|---|---|---|---|
| Combined apparent visual magnitude (V) | 0.76 mag | ± 0.01 mag | Johnson UBV photometry (α¹ Cru = 1.33 mag, α² Cru = 1.73 mag) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 10.13 mas | ± 0.5 mas | Hipparcos New Reduction astrometric solution (98.7 ± 4.9 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 321.9 ly | ± 15.8 ly | Derived from Hipparcos parallax 10.13 ± 0.50 mas | high-confidence | Hipparcos New Reduction Catalog |
| Visual α¹–α² angular separation | 0.067 arcmin | ± 0.002 arcmin | Washington Double Star Catalog astrometry (4.0 arcseconds at PA 112°) | definitive | SIMBAD Astronomical Database (CDS) |
| Combined bolometric luminosity | 45000 L☉ | ± 5000 L☉ | UV/optical B0.5 IV + B1 V spectrophotometry (~25,000 + 20,000 L☉) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Acrux (Alpha Crucis Southern Cross Multiple) at RA 12h 26m 35.92s, Dec -63° 05′ 56.7″ 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.
Astrophysical Context & Structure
Acrux (Alpha Crucis) anchors the southern foot of the Southern Cross (Crux) and is the thirteenth-brightest star in Earth's night sky at combined apparent magnitude 0.76—making it the southernmost first-magnitude star in the heavens (declination −63° 06′). Featured on the national flags of Australia, New Zealand, Brazil, Papua New Guinea, and Samoa, Acrux was first resolved into a telescopic double star by Jesuit missionary astronomers Fontenay and Richaud in 1685–1689.
Located 98.7 ± 4.9 parsecs (322 light-years) away in the Lower Centaurus–Crux subgroup of the Scorpius–Centaurus OB association, Acrux resolves in any small telescope into two scorching blue-white diamonds separated by 4.0 arcseconds along position angle 112°: α¹ Crucis (magnitude 1.33, spectral class B0.5 IV, ~28,000 K) and α² Crucis (magnitude 1.73, spectral class B1 V, ~26,000 K, 13 solar masses). Separated by at least 430 AU in space, the pair takes upwards of 1,500 years to complete one orbit.
Furthermore, α¹ Crucis is itself a double-lined spectroscopic binary composed of two massive stars (~14 and ~10 solar masses) whipping around each other every 75.77 days at a separation of barely 1 AU. Ninety arcseconds south-southwest sits a third visual component, magnitude 4.86 HD 108250 (sometimes called α Crucis C), which is itself a 1.22-day spectroscopic binary—bringing the total count in the eyepiece field to five hot stars!
Practical Visual Observing Guide
- In any 60–80 mm telescope at 100×–150×, Acrux is one of the most dazzling double stars in the entire sky: two almost equally brilliant icy-white headlights (magnitudes 1.3 and 1.7) separated by 4.0 arcseconds, joined 90 arcseconds away in the same field by 4.9-magnitude HD 108250.
- Extend the long axis of the Southern Cross from Gacrux (γ Cru) through Acrux 4.5 times southward to locate the unmarked South Celestial Pole.
Catalog Cross-Identifications & Aliases
Common Aliases: α Crucis · α¹ & α² Crucis · HD 108248 / HD 108249 · HIP 60718
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 60718 |
| HD | HD 108248 |
| SIMBAD | * alf Cru |
| WIKIDATA | Q13020 |
Related Objects, Comparisons & Guides
Hadar (Beta Centauri Triple B-Giant System)
Coordinates, interferometric distance (390 ly), magnitude 0.61, twin 10.7-M☉ Beta Cephei binary orbit, and observing guide for Hadar.
Proxima Centauri (Nearest Star to Sun)
Coordinates, Gaia DR3 parallax (768.07 mas, 4.2465 ly), magnitude 11.13, flare activity, and planetary system guide for Proxima Centauri.
Miaplacidus (Beta Carinae Southern Subgiant)
Coordinates, Hipparcos distance (113.2 ly), magnitude 1.67, Diamond Cross asterism anchor, and observing guide for Miaplacidus in Carina.
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.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
Scientific Sources & Dataset Provenance
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- 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