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
Radial velocity (Rv) +10.0 ± 0.1[5] 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
Distance 19.42 ± 0.06 ly
(5.95 ± 0.02 pc)
Companion Eta 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
Mass 0.972 ± 0.012[7] M
Radius 1.0386 ± 0.0038[8] R
Luminosity 1.2321 ± 0.0074[8] L
Temperature 5973 ± 8[8] K
Metallicity [Fe/H] –0.31[3] dex
Rotational velocity (v sin i) 3.15[4] km/s
Age 5.4 ± 0.9[7] Gyr
η Cas B
Mass 0.57 ± 0.07[3] M
Radius 0.66[9] R
Luminosity 0.06[3] L
Temperature 4036 ± 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.[5]
Database references
SIMBAD The system

Eta Cassiopeiae (η Cas, η Cassiopeiae) is a star system in the northern circumpolar constellation of Cassiopeia. It has the traditional name Achird. Based upon parallax measurements, the distance to this system is 19.42 light-years (5.95 parsecs) from Earth.[1]

In Chinese, 王良 (Wáng Liáng), meaning Wang Liang,[10] refers to an asterism consisting of η Cassiopeiae, β Cassiopeiae, κ Cassiopeiae, α Cassiopeiae and λ Cassiopeiae.[11] Consequently, η Cassiopeiae itself is known as 王良三 (Wáng Liáng sān, English: the Third Star of Wang Liang.)[12]


The primary star in the Eta Cassiopeiae system has a stellar classification of G0 V,[4] which makes it a G-type main-sequence star like the Sun. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae. The star has 97%[7] of the mass of the Sun and 101%[9] 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] companion 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%[9] 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, this star, and the primary component, show only half the abundance of elements other than hydrogen and helium—what astronomers term their metallicity.[3]

This is a binary star first discovered by William Herschel in August 1779. The pair are orbiting around each other over a period of 480 years.[6] Based on an estimated semimajor 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.[13] 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.

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.[14]

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", Astronomy and Astrophysics 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725 
  5. ^ a b "eta Cas -- Spectroscopic binary", Simbad Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2008-04-10 
  6. ^ a b Strand, K. A. (1969), "The orbit of Eta Cassiopeiae", Astronomical Journal 74: 760–763, Bibcode:1969AJ.....74..760S, doi:10.1086/110853 
  7. ^ 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 
  8. ^ 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. 
  9. ^ 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.
  10. ^ Wang Liang was a famous charioteer during the Spring and Autumn Period
  11. ^ (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.
  12. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
  13. ^ Unsöld, Albrecht; Baschek, Bodo (2001), The New Cosmos: An Introduction to Astronomy and Astrophysics (5th ed.), Springer, p. 186, ISBN 3-540-42177-7 
  14. ^ 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]

  • "Eta Cassiopeiae 2". SolStation. Retrieved 2005-11-03. 
  • Kaler, Jim. "Achird". University of Illinois at Urbana-Champaign. Retrieved 2008-04-10.