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

Alnitak Location in Orion

Primary Facts on Alnitak

  • Alnitak's star type is supergiant star that can be located in the constellation of Orion. The description is based on the spectral class.
  • Alnitak is a main star of the constellation outline.
  • Based on the spectral type (O9.5Ib SB) of the star, the star's colour is blue .
  • Zeta Orionis is the Bayer name for the star. It was assigned this name by Johann Bayer in 1603. The closer to the start of the Greek Alphabet the name, the brighter the star is. Alpha stars tend to be the brightest in the constellation. A notable exception is Pollux (Beta Geminorum) which is the brighest star in the Gemini constellation.
  • Alnitak is the 33rd 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.
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 736.26 light years away from us.

Alnitak's Alternative Names

Zeta Orionis (Zet Ori) 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.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 50 Orionis with it shortened to 50 Ori.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD-02 1338.

More details on star alternative names can be found at Star Names .

Location of Alnitak

The location of the star in the night sky 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 -01° 56` 33.3 .

Radial Velocity and 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.

The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is 18.50 km/s with an error of about 1.30 km/s . When the value is negative then the star and the Sun are getting closer to one another, likewise, a positive number means that two stars are moving away. Its nothing to fear as the stars are so far apart, they won't collide in our life-time, if ever.

Alnitak Luminosity

Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 78,179.80 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Physical Properties (Colour, Temperature) of Alnitak

Alnitak has a spectral type of O9.5Ib SB. This means the star is a blue supergiant star. The star is 7,688.00 Parsecs from the Galactic Centre or in terms of Light Years is 25,075.44 s. 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. If you need the diameter of the star, you just need to multiple the radius by 2. 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.

Using the 2007 distance, the star is roughly 46,559,830.28 Astronomical Units from the Earth/Sun give or take a few. An Astronomical Unit is the distance between Earth and the Sun. The number of A.U. is the number of times that the star is from the Earth compared to the Sun.

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.

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Additional Alnitak Facts and Figures

Visual Facts

Primary / Proper / Traditional NameAlnitak
Alternative NamesZeta Orionis, Zet Ori, An-Nitaq, HD 37742, HIP 26727, HR 1948, 50 Orionis, 50 Ori, BD-02 1338
Spectral TypeO9.5Ib SB
Constellation's Main StarYes
Multiple Star SystemYes
Star Type very luminous Supergiant Star
Colour blue
GalaxyMilky Way
Absolute Magnitude -5.26 / -5.03
Visual / Apparent Magnitude1.74
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)05h 40m 45.52
Declination (Dec.)-01° 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 Distance from Earth4.43 Parallax (milliarcseconds)
 736.26 Light Years
 225.73 Parsecs
 46,559,830.28 Astronomical Units
Galacto-Centric Distance 25,075.44 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
Semi-Major Axis8538.00
Stellar Luminosity (Lsun)78,179.80
Brightest in Night Sky33rd
Associated / Clustered StarsZeta Orionis A
Zeta Orionis B

Companions (Multi-Star and Exoplanets) Facts

Exoplanet CountNone/Unaware

Estimated Facts

Calculated Effective Temperature20,357 Kelvin

Sources and Links

SIMBAD SourceLink

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

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