Eta Capricorni

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Eta Capricorni
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Capricornus
Right ascension 21h 04m 24.30132s[1]
Declination −19° 51′ 17.9711″[1]
Apparent magnitude (V) +4.84[2] (5.02 + 7.39)[3]
Spectral type A4 V + F2 V[4]
U−B color index +0.09[2]
B−V color index +0.17[2]
Proper motion (μ) RA: −37.62±0.90[1] mas/yr
Dec.: −24.95±0.88[1] mas/yr
Parallax (π) 20.20 ± 0.82[1] mas
Distance 161 ± 7 ly
(50 ± 2 pc)
Period (P) 27.85±0.15 yr
Semi-major axis (a) 0.265±0.003
Eccentricity (e) 0.410±0.005
Inclination (i) 162.6±0.5°
Longitude of the node (Ω) 171.1±15.0°
Periastron epoch (T) 2002.46±0.09
Argument of periastron (ω)
η Cap A
Mass 2.03±0.12[5] M
η Cap B
Mass 1.21±0.07[5] M
Other designations
η Cap, 22 Cap, BD−20° 6115, HD 200499, HIP 104019, HR 8060, SAO 189986, WDS J21044-1951AB[6]
Database references

Eta Capricorni (η Cap, η Capricorni) is a binary star[5] system in the southern constellation of Capricornus. Sometimes, this star is called by the name Armus.[7] It can be seen with the naked eye, having a combined apparent visual magnitude of +4.84.[2] Based upon an annual parallax shift of 20.20 mas as seen from the Earth,[1] the star is located about 161 light years from the Sun.

The pair orbit each other with a period of 27.85 years, a semimajor axis of 0.265 arc seconds, an eccentricity of 0.410.[5] The primary member, component A, is a white-hued A-type main sequence star with an apparent magnitude of +5.02.[3] Its companion, component B, has an apparent magnitude of +7.39.[3]

Chinese Name[edit]

In Chinese, 十二國 (Shíer Guó), meaning Twelve States, refers to an asterism which is represent twelve ancient states in the Spring and Autumn period and the Warring States period, consisting of η Capricorni, φ Capricorni, ι Capricorni, 38 Capricorni, 35 Capricorni, 36 Capricorni, χ Capricorni, θ Capricorni, 30 Capricorni, 33 Capricorni, ζ Capricorni, 19 Capricorni, 26 Capricorni, 27 Capricorni, 20 Capricorni and 21 Capricorni.[8] Consequently, η Capricorni itself is known as 周一 (Zhou yī, English: the First Star of Zhou), meaning that this star (together with 21 Capricorni and β Serpentis in Right Wall of Heavenly Market Enclosure (asterism)[9]) represent Zhou () (possibly Chow, the dynasty in China).[10]


  1. ^ a b c d e f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, Bibcode:2007A&A...474..653V, arXiv:0708.1752Freely accessible, doi:10.1051/0004-6361:20078357. 
  2. ^ a b c d Nicolet, B. (1978), "Catalogue of homogeneous data in the UBV photoelectric photometric system", Astronomy and Astrophysics, 34: 1, Bibcode:1978A&AS...34....1N. 
  3. ^ a b c 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, Bibcode:2008MNRAS.389..869E, arXiv:0806.2878Freely accessible, doi:10.1111/j.1365-2966.2008.13596.x. 
  4. ^ ten Brummelaar, Theo; et al. (May 2000), "Binary Star Differential Photometry Using the Adaptive Optics System at Mount Wilson Observatory", The Astronomical Journal, 119 (5): 2403−2414, Bibcode:2000AJ....119.2403T, doi:10.1086/301338. 
  5. ^ a b c d e Docobo, J. A.; Andrade, M. (2013), "Dynamical and physical properties of 22 binaries discovered by W. S. Finsen", Monthly Notices of the Royal Astronomical Society, 428 (1): 321–339, Bibcode:2013MNRAS.428..321D, doi:10.1093/mnras/sts045. 
  6. ^ "eta Cap -- Double or multiple star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-05-15. 
  7. ^ Armus,, retrieved 2017-05-16. 
  8. ^ (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.
  9. ^ "LacusCurtius • Allen's Star Names — Serpens". 
  10. ^ Allen, Richard Hinckley (1963), "Capricornus", Star Names, Their Lore and Meaning, Dover, retrieved 2017-05-09.