Universe Guide


Alnilam, Epsilon Orionis, 46 Orionis, HD37128, HIP26311, HR1903

Alnilam (Epsilon Orionis) is a blue pulsating very luminous supergiant star that can be located in the constellation of Orion. Alnilam is the 30th brightest star in the night sky and the 4th 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.

Epsilon Orionis is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR1903. HIP26311 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD37128.

Alnilam has alternative name(s), eps Ori. In Arabic, it is known as An-Nidham.

Location of Alnilam

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 Alnilam, the location is 05h 36m 12.81 and -01d12`06.9 .

Proper Motion of Alnilam

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 -0.78 ± 0.19 miliarcseconds/year towards the north and 1.44 ± 0.45 miliarcseconds/year east if we saw them in the horizon.

Alnilam 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 320000.0000000 that I have given is based on the Spectral Types page that I have found on the Internet. You might find a different figure, one that may have been calculated rather than generalised that I have done. The figure is always the amount times the luminosity of the Sun. It is an imprecise figure because of a number of factors including but not limited to whether the star is a variable star and distance.

Physical Properties (Colour, Temperature, Radius) of Alnilam

Alnilam has a spectral type of B0Ia. This means the star is a blue supergiant star. The star is 7758.00000000 Parsecs from the Galactic Centre or terms of Light Years is 25303.7522275200000000s. The star has a B-V Colour Index of -0.18 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 19,382 Kelvin.

Alnilam Radius has been calculated as being 15.64 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 10,880,611.09.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 23.02. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Alnilam Apparent and Absolute Magnitudes

Alnilam has an apparent magnitude of 1.69 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 -6.38 If you used the 2007 Parallax value, you would get an absolute magnitude of -7.22. 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 Alnilam

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 2.43 which gave the calculated distance to Alnilam as 1342.24 light years away from Earth or 411.52 parsecs. It would take a spaceship travelling at the speed of light, 1342.24 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 1.65 which put Alnilam at a distance of 1976.75 light years or 606.06 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 7,758.00 Parsecs or 25,303.75 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Variable Type of Alnilam

The star is a pulsating Alpha Cygnus variable type which means that its size changes over time. The Variable Type is usually named after the first star of that type to be spotted. Alnilam brightness ranges from a magnitude of 1.648 to a magnitude of 1.603 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.0 days (variability).

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.

Alnilam Facts

Alternative Names

Traditional/Proper NameAlnilam
Flamsteed Name46 Orionis
Flamsteed Short Name46 Ori
Short Nameeps Ori
Arabic NameAn-Nidham
English MeaningThe string of pearls
Bayer DesignationEpsilon Orionis
Hipparcos Library I.D.26311
Yale Bright Star Catalogue (HR) Id1903
Bonner DurchmusterungBD-01 969
Henry Draper Designation37128

Visual Facts

Star Typesupergiant star
Absolute Magnitude-6.38 / -7.22
Visual / Apparent Magnitude1.69
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)05h 36m 12.81
Declination (Dec.)-01d12`06.9
Galactic Latitude-17.24 degrees
Galactic Longitude205.21 degrees
1997 Distance from Earth2.43 Parallax (milliarcseconds)
 1342.24 Light Years
 411.52 Parsecs
2007 Revised Distance from Earth1.65 Parallax (milliarcseconds)
 1976.75 Light Years
 606.06 Parsecs
Galacto-Centric Distance25,303.75 Light Years / 7,758.00 Parsecs
Proper Motion Dec.-0.78 ± 0.19 milliarcseconds/year
Proper Motion RA.1.44 ± 0.45 milliarcseconds/year
B-V Index-0.18
Radial Velocity27.30 ± 0.80 km/s
Spectral TypeB0Ia
Brightest in Night Sky30th
Colour(B) blue

Variable Star Details

Variable Star ClassPulsating
Variable Star TypeAlpha Cygnus
Mean Variability Period in Days0.035
Variable Magnitude Range (Brighter - Dimmer)1.603 - 1.648

Estimated Facts

Luminosity (x the Sun)320,000.0000000
Calculated Effective Temperature19,382 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
37128-01 969.0A1.80000-3.00000-2.00000B0Blue/White

Location of Alnilam in Orion

Alnilam (Epsilon Orionis) Location in Orion

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

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