Zeta Orionis

Alnitak

Alnitak (in lower right corner) and Flame Nebula
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
Alnitak Aa
Mass	33 ± 10 M☉
Radius	20.0 ± 3.2 R☉
Luminosity	160,000 L☉
Surface gravity (log g)	3.2 ± 0.1 cgs
Temperature	29,500 ± 1000 K
Rotation	6.67 days
Rotational velocity (v sin i)	110 ± 10 km/s
Age	~7 Myr
Alnitak Ab
Mass	14 ± 3 M☉
Radius	7.3 ± 1.0 R☉
Luminosity	31,600 L☉
Temperature	29,000 K
Age	~7 Myr
Alnitak B
Mass	16 M☉
Radius	7.2 R☉
Luminosity	35,000 L☉
Temperature	29,000 K
Rotational velocity (v sin i)	350 km/s
Age	~7 Myr
Other designations
ζ Orionis, 50 Orionis, 126 G. Orionis, HR 1948/9, BD−02°1338, HD 37742, SAO 132444, HIP 26727, TD1 5127, 參宿一
Database references
SIMBAD	AB
	A
	B

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.