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Eta Capricorni
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Capricornus
Right ascension	21h 04m 24.30132s
Declination	−19° 51′ 17.9711″
Apparent magnitude (V)	+4.84 (5.02 + 7.39)
Characteristics
Spectral type	A4 V + F2 V
U−B color index	+0.09
B−V color index	+0.17
Astrometry
Proper motion (μ)	RA: −37.62±0.90 mas/yr ; Dec.: −24.95±0.88 mas/yr
Parallax (π)	20.20 ± 0.82 mas
Distance	161 ± 7 ly ; (50 ± 2 pc)
Absolute magnitude (MV)	+1.35
Orbit
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); (secondary)	238.2±15.0°
Details
η Cap A
Mass	2.03±0.12 M☉
Luminosity	24 L☉
Metallicity [Fe/H]	−0.01 dex
η Cap B
Mass	1.21±0.07 M☉
Other designations
	η Cap, 22 Cap, BD−20° 6115, HD 200499, HIP 104019, HR 8060, SAO 189986, WDS J21044-1951AB
Database references
SIMBAD	data

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]

Nomenclature

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 (周) (possibly Chow, the dynasty in China).^[10]

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

References

^ ^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, S2CID 18759600.
^ ^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.
^ ^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, S2CID 14878976.
^ 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.
^ ^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, S2CID 119257644.
^ ^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.
^ "eta Cap". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-05-15.{{cite web}}: CS1 maint: postscript (link)
^ (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.
^ "LacusCurtius • Allen's Star Names — Serpens".
^ Allen, Richard Hinckley (1963), "Capricornus", Star Names, Their Lore and Meaning, Dover, retrieved 2017-05-09.
^ Armus, constellationsofwords.com, retrieved 2017-05-16.