Alnitak
| 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 | |
| Evolutionary stage | Blue supergiant (Aa) |
| 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 | |
| 126 G. Orionis, ζ Orionis, 50 Orionis, BD−02°1338, HIP 26727, SAO 132444, WDS J05407-0157 | |
| A: HD 37742, HR 1948 | |
| B: HD 37743, HR 1949 | |
| Database references | |
| SIMBAD | AB |
| A | |
| B | |
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.
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.