Universe Guide


Prima Giedi, HIP81157

Prima Giedi is a white to yellow subgiant star that can be located in the constellation of Capricornus. The star can not be seen by the naked eye, you need a telescope to see it.

HIP81157 is the reference name for the star in the Hipparcos Star Catalogue.

Prima Giedi has alternative name(s), RT TrA.

Location of Prima Giedi

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 Prima Giedi, the location is 16h 34m 30.90 and -63d08`00.7 .

Proper Motion of Prima Giedi

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 -14.25 ± 0.97 miliarcseconds/year towards the north and -4.00 ± 1.50 miliarcseconds/year east if we saw them in the horizon.

Physical Properties (Colour, Temperature, Radius) of Prima Giedi

Prima Giedi has a spectral type of G2IV:. This means the star is a white to yellow subgiant star. The star has a B-V Colour Index of 0.76 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 5,391 Kelvin.

Prima Giedi Radius has been calculated as being 5.12 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 3,564,785.96.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 9.76. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Prima Giedi Apparent and Absolute Magnitudes

Prima Giedi has an apparent magnitude of 9.70 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 1.60 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.20. 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 Prima Giedi

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 2.40 which gave the calculated distance to Prima Giedi as 1359.01 light years away from Earth or 416.67 parsecs. It would take a spaceship travelling at the speed of light, 1359.01 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 1.26 which put Prima Giedi at a distance of 2588.60 light years or 793.65 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. Prima Giedi brightness ranges from a magnitude of 10.325 to a magnitude of 9.576 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 1.9 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.

Prima Giedi Facts

Alternative Names

Traditional/Proper NamePrima Giedi
Short NameRT TrA
Hipparcos Library I.D.81157

Visual Facts

Star Typesubgiant star
Absolute Magnitude1.60 / 0.20
Visual / Apparent Magnitude9.70
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)16h 34m 30.90
Declination (Dec.)-63d08`00.7
Galactic Latitude-10.40 degrees
Galactic Longitude325.26 degrees
1997 Distance from Earth2.40 Parallax (milliarcseconds)
 1359.01 Light Years
 416.67 Parsecs
2007 Revised Distance from Earth1.26 Parallax (milliarcseconds)
 2588.60 Light Years
 793.65 Parsecs
Proper Motion Dec.-14.25 ± 0.97 milliarcseconds/year
Proper Motion RA.-4.00 ± 1.50 milliarcseconds/year
B-V Index0.76
Spectral TypeG2IV:
Colour(G) White to Yellow

Variable Star Details

Mean Variability Period in Days1.946
Variable Magnitude Range (Brighter - Dimmer)9.576 - 10.325

Estimated Facts

Calculated Effective Temperature5,391 Kelvin

Sources and Links

SIMBAD SourceLink

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