Alpha1 Capricorni

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For other star systems with this Bayer designation, see Alpha Capricorni.
α1 Capricorni
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Capricornus
Right ascension  20h 17m 38.86987s[1]
Declination −12° 30′ 25.5594″[1]
Apparent magnitude (V) 4.27[2]
Spectral type G3 Ib[3]
U−B color index +0.70[2]
B−V color index +1.07[2]
Proper motion (μ) RA: 22.98 ± 0.32[1] mas/yr
Dec.: 1.28 ± 0.28[1] mas/yr
Parallax (π)5.73 ± 0.33[1] mas
Distance570 ± 30 ly
(170 ± 10 pc)
Absolute magnitude (MV)−1.90[4]
Mass5.3 M
Luminosity1,047 L
Temperature5,300 K
Metallicity [Fe/H]+0.22 dex
Rotational velocity (v sin i)7.3 km/s
Other designations
Prima Giedi, Algiedi Prima,[5], α1 Ari, 5 Capricorni, BD−12° 5683, HD 192876, HIP 100027, HR 7747
Database references

Alpha1 Capricorni, Latinized from α1 Capricorni, is a double star in the southern constellation of Capricornus. It also has the traditional star names Prima Giedi /ˌprmə ˈdi/ and Algiedi Prima /æˈdi ˈprmə/. It is approximately 690 light years from Earth. It is separated from the brighter Alpha2 Capricorni by 0.11° on the sky, and resolvable with the naked eye, similar to Mizar and Alcor.

α1 Capricorni has three faint companions within one arc-minute. The brightest of these is 10th magnitude and on this basis it has often been considered as a double star. The separation of the stars is increasing rapidly due to the high proper motion of the primary star. The Hipparcos satellite also detected a new very close companion, 0.6" away and four magnitudes fainter.[6]

The primary star is a yellow G3 supergiant with an apparent magnitude of +4.3. This star has 5.3 times the mass of the Sun and is radiating around 1,047 times the Sun's luminosity.[3]

In popular culture[edit]

The name Giedi Prime is used for a fictional planet in Frank Herbert's 1965 science fiction novel Dune. This Giedi Prime is a planet of the star 36 Ophiuchi B, and is the home world of the villainous House Harkonnen.[7]


  1. ^ a b c d e van Leeuwen, F. (2007). "Validation of the New Hipparcos Reduction". Astronomy and Astrophysics. 474 (2): 653–64. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. ^ a b c Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  3. ^ a b c Smiljanic, R.; et al. (April 2006), "CNO in evolved intermediate mass stars", Astronomy and Astrophysics, 449 (2): 655–671, arXiv:astro-ph/0511329, Bibcode:2006A&A...449..655S, doi:10.1051/0004-6361:20054377
  4. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  5. ^ Rumrill, H. B. (1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. 48: 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681.
  6. ^ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  7. ^ Herbert, Frank (1965). Dune. ISBN 978-0441172719.