Alpha2 Capricorni

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Alpha2 Capricorni
Capricornus constellation map.svg
Red circle.svg
α¹ and α² are combined in this chart, to be held upwards, so west is right of page. Nu Capricorni to the east is not to be confused, nor 3 Capricorni even closer in angular distance to the west.
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Capricornus
Right ascension  20h 18m 03.25595s[1]
Declination −12° 32′ 41.4684″[1]
Apparent magnitude (V) +3.57[2]
Spectral type G8.5III-IV[3]
U−B color index +0.69[2]
B−V color index +0.94[2]
Radial velocity (Rv)−0.47±0.47[4] km/s
Proper motion (μ) RA: +62.63[1] mas/yr
Dec.: +2.66[1] mas/yr
Parallax (π)31.9795 ± 0.3485[5] mas
Distance102 ± 1 ly
(31.3 ± 0.3 pc)
Absolute magnitude (MV)+0.98+0.07
α2 Cap A
Mass2.05±0.29 M
Radius8.38±0.58 R
Luminosity40.4±2.2 L
Surface gravity (log g)3.0[4] cgs
Temperature5,030±160 K
Metallicity [Fe/H]−0.15±0.10 dex
Rotational velocity (v sin i)2.7[4] km/s
Age1.30±1.04 Gyr
Other designations
Gredi, Algedi, Secunda Giedi, Algiedi Secunda, α2 Cap, 6 Cap, ADS 13645, BD−12° 5685, FK5 761, HD 192947, HIP 100064, HR 7754, SAO 163427, WDS J20181-1233A,BC[8]
Database references

Alpha2 Capricorni (α2 Capricorni), or Algedi /ælˈdi/,[9][10] is a triple star system[11] in the southern constellation of Capricornus. It is visible to the naked eye with an apparent visual magnitude of +3.57.[2] It is separated from the fainter α¹ Capricorni by 0.11° of the sky, a gap just resolvable with the naked eye, similar to Mizar and Alcor. Based on parallax shift as refined from orbits around the sun of the Gaia spacecraft at earth's Lagrange point 2, the star is 101 to 103 light years from the solar system.


The primary, component A, is an evolved G-type star with a stellar classification of G8.5III-IV,[3] indicating that the spectrum displays mixed traits of a giant and subgiant star. At the age of 1.3 billion years, is currently on the red giant branch[7] and is generating energy through hydrogen fusion along a shell surrounding an inert helium core. The star has around double the mass of the Sun and has expanded to more than eight times the Sun's radius. The star is radiating 40 times the solar luminosity from its photosphere at an effective temperature of 5,030 K.[7]

The secondary components B and C form a binary system that orbit each other with a period of about 244 years. Both stars have masses about half that of the Sun. They orbit the primary with an estimated period of around 1,500 years.[12] As of 2010, the pair lies at an angular separation of 6.6 arc seconds from the primary along a position angle of 196°.[13]


α² Capricorni (Latinised to Alpha² Capricorni) is the star's Bayer designation. It bore the traditional names Secunda Giedi or Algiedi Secunda and shared the name Algedi (from the Arabic الجدي al-jadii 'the goat') with α¹ Capricorni. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN approved the name Algedi for α² Capricorni on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[10]

In Chinese, 牛宿 (Niú Xiù), meaning Ox (asterism), refers to an asterism consisting of α² Capricorni, Beta Capricorni, ξ² Capricorni, Pi Capricorni, Omicron Capricorni and Rho Capricorni.[15] Consequently, α² Capricorni itself is known as 牛宿二 (Niú Xiù èr, English: the Second Star of Ox.)[16]


  1. ^ a b c d van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. ^ a b c d Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
  3. ^ a b Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637.
  4. ^ a b c Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209.
  5. ^ Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  6. ^ Carney, Bruce W.; et al. (March 2008), "Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars", The Astronomical Journal, 135 (3): 892–906, arXiv:0711.4984, Bibcode:2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892.
  7. ^ a b c Reffert, Sabine; et al. (2015), "Precise radial velocities of giant stars. VII. Occurrence rate of giant extrasolar planets as a function of mass and metallicity", Astronomy and Astrophysics, 574A (2): 116–129, arXiv:1412.4634, Bibcode:2015A&A...574A.116R, doi:10.1051/0004-6361/201322360, hdl:10722/215277.
  8. ^ "alf02 Cap -- Double or multiple star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-05-15.
  9. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  10. ^ a b Mamajek, Eric (February 1, 2017), IAU Catalog of Star Names, IAU Division C Working Group on Star Names (WGSN), retrieved 2017-05-17.
  11. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
  12. ^ Tokovinin, A. (September 2008), "Comparative statistics and origin of triple and quadruple stars", Monthly Notices of the Royal Astronomical Society, 389 (2): 925–938, arXiv:0806.3263, Bibcode:2008MNRAS.389..925T, doi:10.1111/j.1365-2966.2008.13613.x.
  13. ^ Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122: 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920, retrieved 2015-07-22
  14. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  15. ^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  16. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.

External links[edit]

  • Kaler, James B., "Algedi", Stars, University of Illinois, retrieved 2017-05-17.</ref>