Acrux
 The position of Acrux | |
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Crux |
| Pronunciation | /ˈeɪkrʌks/ |
| α Crucis AB | |
| Right ascension | 12h 26m 35.89522s |
| Declination | −63° 05′ 56.7343″ |
| Apparent magnitude (V) | 0.76 (combined) 1.33 (A) + 1.75 (B) |
| α Crucis C | |
| Right ascension | 12h 26m 30.8829s |
| Declination | −63° 07′ 20.047″ |
| Apparent magnitude (V) | 4.86 (Ca + Cb) 13.36 (D) |
| Characteristics | |
| Spectral type | B0.5 IV (Aa) B1 V (B) B5 V (Ca) M0 V (D) |
| U−B color index | −0.59 (C) |
| B−V color index | −0.26 (AB) −0.12 (C) |
| Variable type | β Cep + β Cep |
| Astrometry | |
| α Crucis AB | |
| Radial velocity (Rv) | +19.2±0.8 km/s |
| Proper motion (μ) | RA: −35.83 mas/yr Dec.: −14.86 mas/yr |
| Parallax (π) | 10.13±0.50 mas |
| Distance | 348±5 ly (106.7±1.5 pc) |
| Absolute magnitude (MV) | −3.77 |
| α Crucis C | |
| Radial velocity (Rv) | +27 km/s |
| Proper motion (μ) | RA: −39.548 mas/yr Dec.: −13.760 mas/yr |
| Parallax (π) | 9.3964±0.1289 mas |
| Distance | 348±5 ly (106.7±1.5 pc) |
| Absolute magnitude (MV) | −0.98 |
| Orbit | |
| Primary | α Crucis Aa |
| Name | α Crucis Ab |
| Period (P) | 75.7469±0.0017 d |
| Semi-major axis (a) | (9.48±0.30)×10−3" (1.01±0.04 au) |
| Eccentricity (e) | 0.369±0.015 |
| Inclination (i) | 66.2±1.4° |
| Longitude of the node (Ω) | 225.0±2.0° |
| Periastron epoch (T) | 2,458,183.1±0.4 JD |
| Argument of periastron (ω) (secondary) | 229.5±2.2° |
| Semi-amplitude (K1) (primary) | 40.6±0.9 km/s |
| Orbit | |
| Primary | α Crucis Ba |
| Name | α Crucis Bb |
| Period (P) | 405.4+0.4 −0.3 d |
| Semi-major axis (a) | (28.2±0.8)×10−3" (3.0 au) |
| Eccentricity (e) | 0.36+0.45 −0.07 |
| Inclination (i) | 94.4+2.6 −0.3° |
| Longitude of the node (Ω) | 86.0+3.4 −6.1° |
| Periastron epoch (T) | 2,458,099+258 −145 JD |
| Argument of periastron (ω) (secondary) | 105.2+32.1 −55.3° |
| Orbit | |
| Primary | α Crucis Ca |
| Name | α Crucis Cb |
| Period (P) | 1.225155±0.000005 d |
| Semi-major axis (a) | ≥1.044 R☉ |
| Eccentricity (e) | 0.024±0.014 |
| Periastron epoch (T) | 2438903.314±0.003 JD |
| Argument of periastron (ω) (secondary) | 314±34° |
| Semi-amplitude (K1) (primary) | 43.1±0.6 km/s |
| Details | |
| α Crucis Aa | |
| Mass | 17.2±1.2 M☉ |
| Radius | 6.8 R☉ |
| Luminosity | 31,110+3,190 −2,910 L☉ |
| Temperature | 28,950 K |
| Rotational velocity (v sin i) | 84 km/s |
| Age | 7.1 Myr |
| α Crucis Ab | |
| Mass | 6.8±0.3 M☉ |
| Radius | 3.2 R☉ |
| Luminosity | 1,400 L☉ |
| Temperature | 19,750 K |
| Age | 7.1 Myr |
| α Crucis Ba | |
| Mass | 12.4 M☉ |
| Radius | 5.4 R☉ |
| Luminosity | 14,000 L☉ |
| Surface gravity (log g) | 4.00 cgs |
| Temperature | 26,950 K |
| Rotational velocity (v sin i) | 200 km/s |
| Age | 7.1 Myr |
| α Crucis Bb | |
| Mass | 9.8 M☉ |
| Radius | 4.4 R☉ |
| Luminosity | 6,100 L☉ |
| Temperature | 24,350 K |
| α Crucis Ca | |
| Mass | 4.5 M☉ |
| Radius | 2.5 R☉ |
| Luminosity | 380 L☉ |
| Surface gravity (log g) | 4.05 cgs |
| Temperature | 15,900 K |
| Rotational velocity (v sin i) | 131 km/s |
| α Crucis Cb | |
| Mass | ≥0.64 M☉ |
| α Crucis D | |
| Mass | 0.5 M☉ |
| Age | 12 Myr |
| Other designations | |
| α Crucis, CPD−62°2745, HIP 60718, CCDM J12266-6306, WDS J12266-6306 | |
| α Cru A: Acrux, α1 Crucis, 26 G. Crucis, FK5 462, GC 16952, HD 108248, HR 4730 | |
| α Cru B: α2 Crucis, 27 G. Crucis, GC 16953, HD 108249, HR 4731, 2MASS J12263615-6305571 | |
| α Cru C: 25 G. Crucis, CPD−62°2742, GC 16951, HD 108250, HR 4729 | |
| Database references | |
| SIMBAD | α Cru |
| α Crucis A | |
| α Crucis B | |
| α Crucis C | |
Acrux is a multiple star system and the brightest star in the southern constellation of Crux. It has the Bayer designation α Crucis, which is Latinised to Alpha Crucis and abbreviated Alpha Cru or α Cru. With a combined visual magnitude of +0.76, it is the 13th-brightest star in the night sky. It is the most southerly star of the asterism known as the Southern Cross and is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri. This system is located at a distance of 348 light-years (106.7 parsecs) from the Sun.
To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing seven components. Through optical telescopes, Acrux appears as a triple star, whose two brightest components are visually separated by about 4 arcseconds and are known as α Cru A and α Cru B, α1 Crucis and α2 Crucis, or α Crucis A and α Crucis B. Both components are B-type stars, and are many times more massive and luminous than the Sun. This system was the second ever to be recognized as a binary, in 1685 by a Jesuit priest. α1 and α2 Crucis are close binaries themselves, with components designated α Crucis Aa (officially named Acrux, historically the name of the entire system), α Crucis Ab, α Crucis Ba, and α Crucis Bb.
α Cru C, also known as HR 4729, is a more distant companion, forming a triple star through small telescopes. C is a spectroscopic binary and has a faint companion, which brings the total number of stars in the system to seven.