Alnitak
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Orion |
| 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 ± 0.64mas |
| Distance | 387 ± 54pc |
| Absolute magnitude (MV) | −6.0/−3.9/−4.1 |
| Orbit | |
| Primary | Aa |
| Companion | Ab |
| Period (P) | 2,687 days |
| Semi-major axis (a) | 0.0359" |
| Eccentricity (e) | 0.338 |
| Inclination (i) | 139.3° |
| Longitude of the node (Ω) | 83.8° |
| Periastron epoch (T) | JD 2452734.2 |
|
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) | 2.7280" |
| Eccentricity (e) | 0.0700 |
| Inclination (i) | 72.0° |
| Longitude of the node (Ω) | 155.5° |
| Periastron epoch (T) | 2070.6 |
| Details | |
| Mass | 33/14/16 M☉ |
| Radius | 20/7.3/7.2 R☉ |
| Luminosity | 250,000/31,600/35,000 L☉ |
| Temperature | 28,000/28,000/29,000 K |
| Rotation | 140/?/350 |
| Age | ~7 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Alnitak, designated Zeta Orionis (ζ Orionis, abbreviated Zeta Ori, ζ Ori) and 50 Orionis (50 Ori), is a multiple star several hundred parsecs from the Sun in the constellation of Orion. It is part of Orion's Belt along with Alnilam and Mintaka.
The primary star 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 bluish 4th magnitude companions, one finely resolved and one only detected interferometrically and spectroscopically, producing a combined magnitude for the trio of +1.77. The stars are members of the Orion OB1 association and the association.
Alnitak has been known since antiquity and, as a component of Orion's belt, has been of widespread cultural significance. It was reported to be a double star by amateur German astronomer George K. Kunowsky in 1819. Much more recently, in 1998, the bright primary was found by a team from the Lowell Observatory to have a close companion; this had been suspected from observations made with the Narrabri Stellar Intensity Interferometer in the 1970s. The stellar parallax derived from observations by the Hipparcos satellite imply a distance around 225pc, but this does not take into account distortions caused by the multiple nature of the system and larger distances have been derived by many authors.
Alnitak is a binary star system at the eastern end of Orion's belt, the second magnitude primary having a 4th magnitude companion nearly 3 arc-seconds distant, in an orbit taking over 1,500 years. The primary (Alnitak A) is itself a close binary, comprising Alnitak Aa (a blue supergiant of spectral type O9.5Iab with an absolute magnitude of -6.0 and an apparent magnitude of 2.0) and Alnitak Ab (a blue sub-giant of spectral type B1IV with an absolute magnitude of -3.9 and an apparent magnitude of 4.3, discovered in 1998 ). Aa is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 21,000 times brighter than the sun, with a surface brightness (luminance) some 50 times greater. It is the brightest star of class O in the night sky. Alnitak B is a 4th magnitude B-type star which orbits Alnitak A every 1500 years. A fourth star, 9th magnitude Alnitak C, has not been confirmed to be part of the Aa-Ab-B group, and may simply lie along the line of sight.
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