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.
More details on star alternative names can be found at Star Names.
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 -63° 08` 00.7 .
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.
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 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.
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).
|Alternative Names||HIP 81157, RT TrA|
|Star Type||Subgiant Star|
|Absolute Magnitude||1.60 / 0.20|
|Visual / Apparent Magnitude||9.70|
|Naked Eye Visible||Requires a 7x50 Binoculars - Magnitudes|
|Right Ascension (R.A.)||16h 34m 30.90|
|Declination (Dec.)||-63° 08` 00.7|
|Galactic Latitude||-10.40 degrees|
|Galactic Longitude||325.26 degrees|
|1997 Distance from Earth||2.40 Parallax (milliarcseconds)|
|1359.01 Light Years|
|2007 Revised Distance from Earth||1.26 Parallax (milliarcseconds)|
|2588.60 Light Years|
|Proper Motion Dec.||-14.25 ± 0.97 milliarcseconds/year|
|Proper Motion RA.||-4.00 ± 1.50 milliarcseconds/year|
|Colour||(G) White to Yellow|
|Mean Variability Period in Days||1.946|
|Variable Magnitude Range (Brighter - Dimmer)||9.576 - 10.325|
|Calculated Effective Temperature||5,391 Kelvin|