Alnitak
Alnitak is a triple star system in the constellation of Orion. It has the designations ζ Orionis, which is Latinised to Zeta Orionis and abbreviated Zeta Ori or ζ Ori, and 50 Orionis, abbreviated 50 Ori. The system is located at a distance of several hundred parsecs from the Sun and is one of the three main stars of Orion's Belt along with Alnilam and Mintaka.
Alnitak (in lower right) and the Flame Nebula | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Orion |
| Pronunciation | /ælˈnaɪtæk/, |
| Right ascension | 05h 40m 45.52666s |
| Declination | −01° 56′ 34.2649″ |
| Apparent magnitude (V) | 1.77 (2.08 + 4.28 + 4.01) |
| Characteristics | |
| Spectral type | O9.5Iab + B1IV + B0III |
| U−B color index | −1.06 |
| B−V color index | −0.11 (A) −0.20 (B) |
| Astrometry | |
| Radial velocity (Rv) | 18.50 km/s |
| Proper motion (μ) | RA: 3.19 mas/yr Dec.: 2.03 mas/yr |
| Parallax (π) | 4.43 ± 0.64 mas |
| Distance | 1,260 ± 180 ly (387 ± 54 pc) |
| Absolute magnitude (MV) | −6.0 / −3.9 / −4.1 |
| Orbit | |
| Primary | Aa |
| Companion | Ab |
| Period (P) | 2,687.3±7.0 d |
| Semi-major axis (a) | 35.9±0.2 mas |
| Eccentricity (e) | 0.338±0.004 |
| Inclination (i) | 139.3±0.6° |
| Longitude of the node (Ω) | 83.8±0.8° |
| Periastron epoch (T) | JD 2452734.2±9.0 |
| Argument of periastron (ω) (secondary) | 204.2±1.2° |
| Semi-amplitude (K1) (primary) | 10.1 km/s |
| Semi-amplitude (K2) (secondary) | 19.6 km/s |
| Orbit | |
| Primary | A |
| Companion | B |
| Period (P) | 1,508.6 yr |
| Semi-major axis (a) | 2.728″ |
| Eccentricity (e) | 0.07 |
| Inclination (i) | 72.0° |
| Longitude of the node (Ω) | 155.5° |
| Periastron epoch (T) | 2,070.6 |
| Argument of periastron (ω) (secondary) | 47.3° |
| Details | |
| Alnitak Aa | |
| Mass | 33±10 M☉ |
| Radius | 20.0±3.2 R☉ |
| Luminosity | 250,000 L☉ |
| Surface gravity (log g) | 3.2±0.1 cgs |
| Temperature | 29,500±1,000 K |
| Rotation | 6.67 days |
| Rotational velocity (v sin i) | 110±10 km/s |
| Age | 6.4 Myr |
| Alnitak Ab | |
| Mass | 14±3 M☉ |
| Radius | 7.3±1.0 R☉ |
| Luminosity | 32,000 L☉ |
| Temperature | 29,000 K |
| Age | 7.2 Myr |
| Alnitak B | |
| Rotational velocity (v sin i) | 350 km/s |
| Age | ~7 Myr |
| Other designations | |
| A: HD 37742, HR 1948 | |
| B: HD 37743, HR 1949 | |
| Database references | |
| SIMBAD | AB |
| A | |
| B | |
The primary star, Alnitak Aa, is a hot blue supergiant with an absolute magnitude of −6.0 and is the brightest class O star in the night sky with a visual magnitude of +2.0. It has two companions—Ab and B, the latter known for the longest time and the former discovered recently, producing a combined magnitude for the trio of +1.77. The stars are members of the Orion OB1 association and the Collinder 70 association.