Acrux
Acrux is 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 321 light-years from the Sun.
The position of Acrux | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Crux |
| Pronunciation | /ˈeɪkrʌks/ |
| Right ascension | 12h 26m 35.89522s |
| Declination | −63° 05′ 56.7343″ |
| Apparent magnitude (V) | 0.76 (1.33 + 1.75) |
| Characteristics | |
| Spectral type | B0.5IV + B1V |
| B−V color index | −0.26 |
| Variable type | β Cep |
| Astrometry | |
| Radial velocity (Rv) | −11.2 / −0.6 km/s |
| Proper motion (μ) | RA: −35.83 mas/yr Dec.: −14.86 mas/yr |
| Parallax (π) | 10.13 ± 0.50 mas |
| Distance | 320 ± 20 ly (99 ± 5 pc) |
| Absolute magnitude (MV) | −3.77 (−2.2 + −2.7) |
| Orbit | |
| Primary | α Crucis Aa |
| Companion | α Crucis Ab |
| Period (P) | 75.7794±0.0037 d |
| Eccentricity (e) | 0.46±0.03 |
| Periastron epoch (T) | 2,417,642.3±1.6 JD |
| Argument of periastron (ω) (secondary) | 21±6° |
| Semi-amplitude (K1) (primary) | 41.7±1.2 km/s |
| Details | |
| α1 | |
| Mass | 17.80 + 6.05 M☉ |
| Radius | 7.8 R☉ |
| Luminosity | 25,000 L☉ |
| Temperature | 24,000 K |
| Rotational velocity (v sin i) | 120 km/s |
| α2 | |
| Mass | 15.52 M☉ |
| Radius | 5.4 R☉ |
| Luminosity | 16,000 L☉ |
| Temperature | 28,000 K |
| Rotational velocity (v sin i) | 200 km/s |
| Age | 10.8 Myr |
| Other designations | |
| α1 Cru: Acrux, 26 G. Crucis, FK5 462, GC 16952, HD 108248, HR 4730 | |
| α2 Cru: 27 G. Crucis, GC 16953, HD 108249, HR 4731, 2MASS J12263615-6305571 | |
| Database references | |
| SIMBAD | α Cru |
| α1 Cru | |
| α2 Cru | |
To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing six 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 Acrux A and Acrux 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. α1 Crucis is itself a spectroscopic binary with components designated α Crucis Aa (officially named Acrux, historically the name of the entire system) and α Crucis Ab. Its two component stars orbit every 76 days at a separation of about 1 astronomical unit (AU). HR 4729, also known as Acrux C, is a more distant companion, forming a triple star through small telescopes. C is also a spectroscopic binary, which brings the total number of stars in the system to at least five.