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Alnitak, Zeta Orionis, 50 Orionis, HD37742, HIP26727, HR1948

Alnitak (Zeta Orionis) is a blue very luminous supergiant star that can be located in the constellation of Orion. Alnitak is the 34th brightest star in the night sky and the 5th brightest star in Orion based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Zeta Orionis is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR1948. HIP26727 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD37742. In Arabic, it is known as An-Nitaq.

Location of Alnitak

The location of the star in the galaxy is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For Alnitak, the location is 05h 40m 45.52 and -01d 56` 33.3 .

Proper Motion of Alnitak

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving 2.03 ± 0.25 miliarcseconds/year towards the north and 3.19 ± 0.64 miliarcseconds/year east if we saw them in the horizon.

Physical Properties (Colour, Temperature, Radius) of Alnitak

Alnitak has a spectral type of O9.5Ib SB. This means the star is a blue supergiant star. The star is 7688.00000000 Parsecs from the Galactic Centre or terms of Light Years is 25075.4378867200000000s. The star has a B-V Colour Index of -0.19 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 20,357 Kelvin.

Alnitak Radius has been calculated as being 8.46 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 5,888,598.14.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 7.61. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Alnitak Apparent and Absolute Magnitudes

Alnitak has an apparent magnitude of 1.74 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of -5.26 If you used the 2007 Parallax value, you would get an absolute magnitude of -5.03. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to Alnitak

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 3.99 which gave the calculated distance to Alnitak as 817.45 light years away from Earth or 250.63 parsecs. It would take a spaceship travelling at the speed of light, 817.45 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 4.43 which put Alnitak at a distance of 736.26 light years or 225.73 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated.

The star's Galacto-Centric Distance is 7,688.00 Parsecs or 25,075.44 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.

Alnitak Facts

Alternative Names

Traditional/Proper NameAlnitak
Flamsteed Name50 Orionis
Flamsteed Short Name50 Ori
Arabic NameAn-Nitaq
English MeaningThe belt
Bayer DesignationZeta Orionis
Hipparcos Library I.D.26727
Yale Bright Star Catalogue (HR) Id1948
Bonner DurchmusterungBD-02 1338
Henry Draper Designation37742

Visual Facts

Star Typesupergiant star
Absolute Magnitude-5.26 / -5.03
Visual / Apparent Magnitude1.74
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)05h 40m 45.52
Declination (Dec.)-01d 56` 33.3
Galactic Latitude-16.59 degrees
Galactic Longitude206.45 degrees
1997 Distance from Earth3.99 Parallax (milliarcseconds)
 817.45 Light Years
 250.63 Parsecs
2007 Revised Distance from Earth4.43 Parallax (milliarcseconds)
 736.26 Light Years
 225.73 Parsecs
Galacto-Centric Distance25,075.44 Light Years / 7,688.00 Parsecs
Proper Motion Dec.2.03 ± 0.25 milliarcseconds/year
Proper Motion RA.3.19 ± 0.64 milliarcseconds/year
B-V Index-0.19
Radial Velocity18.50 ± 1.30 km/s
Spectral TypeO9.5Ib SB
Brightest in Night Sky34th
Colour(O) blue

Estimated Facts

Calculated Effective Temperature20,357 Kelvin

Sources and Links

SIMBAD SourceLink

Related Stars

Multi-Star System

The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.

Proper Motion mas/yr
H.D. IdB.D. IdStar CodeMagnitudeR.A.Dec.SpectrumColourYear
37742-02 1338.0A2.00000-2.00000-2.00000B0Blue/White

Location of Alnitak in Orion

Alnitak (Zeta Orionis) Location in Orion

The map was generated using Night Vision, an awesome free application by Brian Simpson.

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