Eta Cassiopeiae

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Eta Cassiopeiae
Cassiopeia constellation map.svg
Eta Cassiopeiae is just left of Alpha Cassiopeiae
Observation data
Epoch J2000      Equinox J2000
Constellation Cassiopeia
Right ascension  00h 49m 06.29070s[1]
Declination +57° 48′ 54.6758″[1]
Apparent magnitude (V) 3.44[2]/7.51[3]
Spectral type G0 V[4] + K7 V[3]
U−B color index +0.02[2]/1.03
B−V color index +0.58[2]/1.39
Variable type RS CVn?[5]
Radial velocity (Rv)+10.0 ± 0.1[6] km/s
Proper motion (μ) RA: 1086.59[1] mas/yr
Dec.: –559.43[1] mas/yr
Parallax (π)167.98 ± 0.48[1] mas
Distance19.42 ± 0.06 ly
(5.95 ± 0.02 pc)
Absolute magnitude (MV)4.57[7]
CompanionEta Cassiopeiae B
Period (P)480 yr
Semi-major axis (a)11.9939″
Eccentricity (e)0.497
Inclination (i)34.76°
Longitude of the node (Ω)98.42°
Periastron epoch (T)1889.6
Argument of periastron (ω)
η Cas A
Mass0.972 ± 0.012[9] M
Radius1.0386 ± 0.0038[10] R
Luminosity1.2321 ± 0.0074[10] L
Temperature5973 ± 8[10] K
Metallicity [Fe/H]–0.31[3] dex
Rotational velocity (v sin i)3.15[4] km/s
Age5.4 ± 0.9[9] Gyr
η Cas B
Mass0.57 ± 0.07[3] M
Radius0.66[11] R
Luminosity0.06[3] L
Temperature4036 ± 150[3] K
Other designations
Achird, η Cas 24 Cassiopeiae, ADS 671, BD+57°150, GCTP 155, GJ 34, HD 4614, HIP 3821, HR 219, LHS 123/122, LFT 74, LTT 10287, SAO 21732, Wolf 24, Struve 60, GC 962, CCDM J00491+5749.[6]
Database references
SIMBADThe system

Eta Cassiopeiae (η Cassiopeiae, abbreviated Eta Cas, η Cas) is a binary star system in the northern circumpolar constellation of Cassiopeia. Its binary nature was first discovered by William Herschel in August 1779. Based upon parallax measurements, the distance to this system is 19.42 light-years (5.95 parsecs) from the Sun.[1] The two components are designated Eta Cassiopeiae A (officially named Achird /ˈərd/, the traditional name for the system)[12] and B.


η Cassiopeiae (Latinised to Eta Cassiopeiae) is the system's Bayer designation. The designations of the two constituents as Eta Cassiopeiae A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[13]

The proper name Achird was apparently first applied to Eta Cassiopeiae in the Skalnate Pleso Atlas of the Heavens published in 1950, but is not known prior to that.[14] Richard Hinckley Allen gives no historical names for the star in his book Star Names: Their Lore and Meaning.[15] In 2016, the IAU organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[17] It approved the name Achird for the component Eta Cassiopeiae A on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[12]

In Chinese astronomy, Eta Cassiopeiae is within the Legs mansion, and is part of the 王良 (Wáng Liáng) asterism named for a famous charioteer during the Spring and Autumn period. The other components are Beta Cassiopeiae (Caph), Kappa Cassiopeiae, Alpha Cassiopeiae (Schedar) and Lambda Cassiopeiae.[18] Consequently, the Chinese name for Eta Cassiopeiae itself is 王良三 (Wáng Liáng sān, English: the Third Star of Wang Liang.)[19]


Eta Cassiopeiae's two components are orbiting around each other over a period of 480 years.[8] Based on an estimated semi-major axis of 12″ and a parallax of 0.168″, the two stars are separated by an average distance of 71 AU, where an AU is the average distance between the Sun and the Earth.[20] However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU, with an apoapsis of about 106 AUs. For comparison, the semi-major axis of Neptune is 30 AU.

There are six dimmer optical components listed in the Washington Double Star Catalog. However, none of them are related to the Eta Cassiopeiae system and are in reality more distant stars. The primary has been reported to be a spectroscopic binary, but this has never been confirmed.[14]

Eta Cassiopeiae A has a stellar classification of G0 V,[4] which makes it a G-type main-sequence star like the Sun. It therefore resembles what the Sun might look like were humans to observe it from Eta Cassiopeiae. The star has 97%[9] of the mass of the Sun and 101%[11] of the Sun's radius. It is of apparent magnitude 3.44,[2] radiating 129%[3] of the luminosity of the Sun from its outer envelope at an effective temperature of 6,087 K.[3] It appears to be rotating at a leisurely rate, with a projected rotational velocity of 3.15 km s−1.[4]

The cooler and dimmer (magnitude 7.51[3]) Eta Cassiopeiae B is of stellar classification K7 V;[3] a K-type main-sequence star. It has only 57%[3] of the mass of the Sun and 66%[11] of the Sun's radius. Smaller stars generate energy more slowly, so this component radiates only 6%[3] of the luminosity of the Sun. Its outer atmosphere has an effective temperature of 4,036 K.[3]

Compared to the Sun, both components show only half the abundance of elements other than hydrogen and helium — what astronomers term their metallicity.[3]

A necessary condition for the existence of a planet in this system are stable zones where the object can remain in orbit for long intervals. For hypothetical planets in a circular orbit around the individual members of this star system, this maximum orbital radius is computed to be 9.5 AU for the primary and 7.1 AU for the secondary. (Note that the orbit of Mars is 1.5 AU from the Sun.) A planet orbiting outside of both stars would need to be at least 235 AU distant.[21]

See also[edit]


  1. ^ a b c d e f van Leeuwen, F. (November 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 Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J
  3. ^ a b c d e f g h i j k l m n Fernandes, J.; et al. (1998), "Fundamental stellar parameters for nearby visual binary stars: eta Cas, XI Boo, 70 OPH and 85 Peg", Astronomy and Astrophysics, 338: 455–464, Bibcode:1998A&A...338..455F
  4. ^ a b c d Martínez-Arnáiz, R.; et al. (September 2010), "Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter" (PDF), Astronomy and Astrophysics, 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725
  5. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  6. ^ a b "eta Cas". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2008-04-10.
  7. ^ Holmberg, J.; et al. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics, 501 (3): 941–947, arXiv:0811.3982, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191.
  8. ^ a b Strand, K. A. (1969), "The orbit of Eta Cassiopeiae", Astronomical Journal, 74: 760–763, Bibcode:1969AJ.....74..760S, doi:10.1086/110853
  9. ^ a b c Boyajian, Tabetha S.; et al. (February 2012), "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars", The Astrophysical Journal, 746 (1): 101, arXiv:1112.3316, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101.
  10. ^ a b c Boyajian, Tabetha S.; et al. (July 2013), "Stellar Diameters and Temperatures. III. Main-sequence A, F, G, and K Stars: Additional High-precision Measurements and Empirical Relations", The Astrophysical Journal, 771 (1): 40, arXiv:1306.2974, Bibcode:2013ApJ...771...40B, doi:10.1088/0004-637X/771/1/40.
  11. ^ a b c Johnson, H. M.; Wright, C. D. (1983), "Predicted infrared brightness of stars within 25 parsecs of the sun", Astrophysical Journal Supplement Series, 53: 643–711, Bibcode:1983ApJS...53..643J, doi:10.1086/190905—see p. 647.
  12. ^ a b "Naming Stars". Retrieved 16 December 2017.
  13. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  14. ^ a b Hoffleit, D.; Warren, W. H. (1995). "VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR On-line Data Catalog: V/50. Originally published in: 1964BS....C......0H. 5050. Bibcode:1995yCat.5050....0H.
  15. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 473. ISBN 0-486-21079-0.
  16. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  17. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  18. ^ (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.
  19. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2010-09-03 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  20. ^ Unsöld, Albrecht; Baschek, Bodo (2001), The New Cosmos: An Introduction to Astronomy and Astrophysics (5th ed.), Springer, p. 186, ISBN 3-540-42177-7
  21. ^ Jaime, Luisa G.; et al. (December 2012), "Regions of dynamical stability for discs and planets in binary stars of the solar neighbourhood", Monthly Notices of the Royal Astronomical Society, 427 (4): 2723–2733, arXiv:1208.2051, Bibcode:2012MNRAS.427.2723J, doi:10.1111/j.1365-2966.2012.21839.x.

External links[edit]