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
Distance161 ± 7 ly
(50 ± 2 pc)
Absolute magnitude (MV)+1.35[5]
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
Mass2.03±0.12[6] M
Luminosity24[5] L
Metallicity [Fe/H]−0.01[5] dex
η Cap B
Mass1.21±0.07[6] M
Other designations
η Cap, 22 Cap, BD−20° 6115, HD 200499, HIP 104019, HR 8060, SAO 189986, WDS J21044-1951AB[7]
Database references

Eta Capricorni, Latinized from η Capricorni, is a binary star[6] system in the southern constellation of Capricornus. 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.[6] 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]


η Capricorni (Latinised to Eta Capricorni) is the system's Bayer designation. It also has the Flamsteed designation 22 Capricorni.

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, the Chinese name for η Capricorni itself is 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]

Sometimes, this star is called by the name Armus in an astrological context.[11]


  1. ^ a b c d e f 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), "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, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, 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 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.
  6. ^ 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.
  7. ^ "eta Cap". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-05-15.
  8. ^ ‹See Tfd›(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.
  11. ^ Armus,, retrieved 2017-05-16.