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Alhena (Gamma Geminorum) - HD47105 - HIP31681 - HR2421

Alhena (Gamma Geminorum) is a blue subgiant star that can be located in the constellation of Gemini. It is the 44th brightest star in the night sky. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Gamma Geminorum is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR2421. HIP31681 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD47105.

Alhena has alternative name(s), 24 Geminorum , 24 Gem. In Arabic, it is known as Al-Han'ah.

Location of Alhena

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 Alhena, the location is 06h 37m 42.70 and +16d23`57.9 .

Proper Motion of Alhena

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 -54.96 ± 1.53 miliarcseconds/year towards the north and 13.81 ± 2.23 miliarcseconds/year east if we saw them in the horizon.

Physical Properties (Colour, Temperature, Radius) of Alhena

Alhena has a spectral type of A0IV. This means the star is a blue subgiant star. The star is 7432.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24240.4597260800000000s. The star has a B-V Colour Index of 0 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 9,531 Kelvin.

Alhena Radius has been calculated as being 4.51 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 3,141,353.29.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 4.73. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's Iron Abundance is -0.28 with an error value of 0.04 Fe/H with the Sun has a value of 1 to put it into context.

Alhena Apparent and Absolute Magnitudes

Alhena has an apparent magnitude of 1.93 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 -0.60 If you used the 2007 Parallax value, you would get an absolute magnitude of -0.70. 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 Alhena

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 31.12 which gave the calculated distance to Alhena as 104.81 light years away from Earth or 32.13 parsecs. It would take a spaceship travelling at the speed of light, 104.81 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 29.84 which put Alhena at a distance of 109.30 light years or 33.51 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 7,432.00 Parsecs or 24,240.46 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.

Alhena Facts

Alternative Names

Traditional/Proper NameAlhena
Short Name24 Gem
Arabic NameAl-Han'ah
English MeaningThe brand-mark
Bayer DesignationGamma Geminorum
Alternative Name(s)24 Geminorum
Hipparcos Library I.D.31681
Yale Bright Star Catalogue (HR) Id2421
Bonner DurchmusterungBDD+16 1223
Henry Draper Designation47105

Visual Facts

Star Typesubgiant star
Absolute Magnitude-0.60 / -0.70
Visual / Apparent Magnitude1.93
Naked Eye VisibleYes - Ref: Wiki
Right Ascension (R.A.)06h 37m 42.70
Declination (Dec.)+16d23`57.9
Galactic Latitude4.45 degrees
Galactic Longitude196.77 degrees
1997 Distance from Earth31.12 Parallax (milliarcseconds)
 104.81 Light Years
 32.13 Parsecs
2007 Revised Distance from Earth29.84 Parallax (milliarcseconds)
 109.30 Light Years
 33.51 Parsecs
Galacto-Centric Distance24,240.46 Light Years / 7,432.00 Parsecs
Proper Motion Dec.-54.96 ± 1.53 milliarcseconds/year
Proper Motion RA.13.81 ± 2.23 milliarcseconds/year
B-V Index0.00
Radial Velocity-12.63 ± 0.02 km/s
Iron Abundance-0.28 ± 0.04 Fe/H
Spectral TypeA0IV
Brightest in Night Sky44th
Colour(A) blue

Estimated Facts

Calculated Effective Temperature9,531 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
47105+16 1223.0A1.9000043.00000-44.00000A0White
B11.200001907
C10.900001907

Location of Alhena in Gemini


Alhena (Gamma Geminorum) Location in Gemini

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


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