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Tejat Prior, Eta Geminorum, 7 Geminorum, HD42995, HIP29655, HR2216

Tejat Prior (Eta Geminorum) is a red pulsating giant star that can be located in the constellation of Gemini. Tejat Prior is the brightest star in Gemini 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.

Eta Geminorum is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR2216. HIP29655 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD42995.

Tejat Prior has alternative name(s), Propus , eta Gem.

Location of Tejat Prior

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 Tejat Prior, the location is 06h 14m 52.70 and +22d30`24.6 .

Proper Motion of Tejat Prior

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 -12.12 ± 0.61 miliarcseconds/year towards the north and -62.46 ± 1.23 miliarcseconds/year east if we saw them in the horizon.

Tejat Prior 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 10.0000000 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 Tejat Prior

Tejat Prior has a spectral type of M3III. This means the star is a red giant star. The star is 7514.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24507.9136681600000000s. The star has a B-V Colour Index of 1.6 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 3,779 Kelvin.

Tejat Prior Radius has been calculated as being 50.84 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 35,371,388.69.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 56.00. 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 0.00 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

Tejat Prior Apparent and Absolute Magnitudes

Tejat Prior has an apparent magnitude of 3.31 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 -1.84 If you used the 2007 Parallax value, you would get an absolute magnitude of -2.05. 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 Tejat Prior

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 9.34 which gave the calculated distance to Tejat Prior as 349.21 light years away from Earth or 107.07 parsecs. It would take a spaceship travelling at the speed of light, 349.21 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 8.48 which put Tejat Prior at a distance of 384.63 light years or 117.92 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 7,514.00 Parsecs or 24,507.91 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 Tejat Prior

The star is a pulsating Semi-Regular Star which are giants or supergiants of intermediate and late spectral and Eclipsing 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. Tejat Prior brightness ranges from a magnitude of 3.392 to a magnitude of 3.183 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 233.0 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.

Tejat Prior Facts

Alternative Names

Traditional/Proper NameTejat Prior
Flamsteed Name7 Geminorum
Flamsteed Short Name7 Gem
Short Nameeta Gem
Bayer DesignationEta Geminorum
Alternative Name(s)Propus
Hipparcos Library I.D.29655
Yale Bright Star Catalogue (HR) Id2216
Bonner DurchmusterungBD+22 1241
Henry Draper Designation42995

Visual Facts

Star Typegiant star
Absolute Magnitude-1.84 / -2.05
Visual / Apparent Magnitude3.31
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)06h 14m 52.70
Declination (Dec.)+22d30`24.6
Galactic Latitude2.52 degrees
Galactic Longitude188.85 degrees
1997 Distance from Earth9.34 Parallax (milliarcseconds)
 349.21 Light Years
 107.07 Parsecs
2007 Revised Distance from Earth8.48 Parallax (milliarcseconds)
 384.63 Light Years
 117.92 Parsecs
Galacto-Centric Distance24,507.91 Light Years / 7,514.00 Parsecs
Proper Motion Dec.-12.12 ± 0.61 milliarcseconds/year
Proper Motion RA.-62.46 ± 1.23 milliarcseconds/year
B-V Index1.60
Radial Velocity22.39 ± 0.36 km/s
Iron Abundance0.00 ± 9.99 Fe/H
Spectral TypeM3III
Colour(M) Red

Variable Star Details

Variable Star ClassPulsating
Variable Star TypeSemi-Regular Star which are giants or supergiants of intermediate and late spectral and Eclipsing
Mean Variability Period in Days233.000
Variable Magnitude Range (Brighter - Dimmer)3.183 - 3.392

Estimated Facts

Luminosity (x the Sun)10.0000000
Calculated Effective Temperature3,779 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
42995+22 1241.0A-66.00000-12.00000M1Red
B8.800001958

Location of Tejat Prior in Gemini


Tejat Prior (Eta Geminorum) Location in Gemini

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


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