Eta Ceti

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η Ceti
Cetus constellation map.svg
Red circle.svg
Location of η Ceti (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cetus
Right ascension  01h 08m 35.39148s[1]
Declination –10° 10′ 56.1570″[1]
Apparent magnitude (V) +3.446[2]
Spectral type K1 III[3]
U−B color index +1.194[2]
B−V color index +1.161[2]
Radial velocity (Rv)+11.74 ± 0.30[4] km/s
Proper motion (μ) RA: +215.61[1] mas/yr
Dec.: –139.02[1] mas/yr
Parallax (π)26.32 ± 0.14[1] mas
Distance123.9 ± 0.7 ly
(38.0 ± 0.2 pc)
Absolute magnitude (MV)+0.68[5]
Mass1.84[6] M
Radius15.10 ± 0.10[7] R
Luminosity74.0 ± 3.7[7] L
Surface gravity (log g)2.5[4] cgs
Temperature4,543±24[6] K
Metallicity [Fe/H]–0.03[4] dex
Rotational velocity (v sin i)4.8[4] km/s
Age1.80[6] Gyr
Other designations
Deneb Algenubi, Algenudi, 31 Ceti, BD–10 240, FK5 40, HD 6805, HIP 5364, HR 334, SAO 147632.[8]
Database references

Eta Ceti (η Cet, η Ceti) is a star in the equatorial constellation of Cetus. It has the traditional name Deneb Algenubi or Algenudi. The apparent visual magnitude of this star is +3.4,[2] making it the fourth-brightest star in this otherwise relatively faint constellation. The distance to this star can be measured directly using the parallax technique, yielding a value of 123.9 light-years (38.0 parsecs).[1]

This is a giant star with a stellar classification of K1 III,[3] having exhausted the hydrogen at its core and evolved away from the main sequence of stars like the Sun. (The classification is sometimes listed as K1.5 IIICN1Fe0.5, indicating a higher than normal abundance of cyanogen and iron relative to other stars of its class.)[9] It is a red clump star that is generating energy through the nuclear fusion of helium at its core.[10]

Eta Ceti may have slightly more mass than the Sun and its outer envelope has expanded to 15 times the Sun's radius,[7] it is radiating 74[7] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 4,356 K.[7] This heat gives the star the orange-hued glow of a K-type star.[11]

In culture[edit]

The name Deneb Algenubi was from Arabic ذنب القيطس الجنوبي - al-dhanab al-qayṭas al-janūbī, meaning the southern tail of the sea monster. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Aoul al Naamat (أول ألنعمة - awwil al naʽāmāt), which was translated into Latin as Prima Struthionum, meaning the first ostrich;[12] this star, along with θ Cet (Thanih al Naamat), τ Cet (Thalath Al Naamat), ζ Cet (Baten Kaitos) and υ Cet, were Al Naʽāmāt (ألنعمة), the Hen Ostriches.[13]

In Chinese, 天倉 (Tiān Cāng), meaning Square Celestial Granary, refers to an asterism consisting of η Ceti, ι Ceti, θ Ceti, ζ Ceti, τ Ceti and 57 Ceti.[14] Consequently, the Chinese name for η Ceti itself is 天倉二 (Tiān Cāng èr, English: the Second Star of Square Celestial Granary.)[15]

Planetary system[edit]

In 2014, two exoplanets around the star were discovered using the radial velocity method. Planets discovered by radial velocity have poorly known masses because if the orbit of the planets were inclined away from the line of sight, a much larger mass would have to compensate for the angle.[16]

Eta Ceti b has a minimum mass of 2.55 MJ and an orbital period of 403.5 days (about 1.1 years), while Eta Ceti c has a minimum mass of 3.32 MJ and an orbital period of 751.9 days (2.06 years). Assuming the orbits of the two are coplanar, then the two planets must be locked in a 2:1 orbital resonance, otherwise the system would become dynamically unstable. Although the inclinations from the line of sight are unknown, the value is constrained to be 70° or less: if any higher, the higher masses would render the system dynamically unstable, with no stable solutions.[17]

The Eta Ceti planetary system[17]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b ≥2.55 ± 0.13 MJ 1.27 403.5 ± 1.5 0.13 ± 0.05
c ≥3.32 ± 0.18 MJ 1.93 751.9 ± 3.8 0.1 ± 0.06


  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 Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants.", Monthly Notices of the Royal Astronomical Society, 172: 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
  3. ^ a b Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
  4. ^ a b c d Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
  5. ^ Cardini, D. (January 2005), "Mg II chromospheric radiative loss rates in cool active and quiet stars", Astronomy and Astrophysics, 430: 303–311, arXiv:astro-ph/0409683, Bibcode:2005A&A...430..303C, doi:10.1051/0004-6361:20041440.
  6. ^ a b c Luck, R. Earle (2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 88, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88.
  7. ^ a b c d e Berio, P.; et al. (November 2011), "Chromosphere of K giant stars. Geometrical extent and spatial structure detection", Astronomy & Astrophysics, 535: A59, arXiv:1109.5476, Bibcode:2011A&A...535A..59B, doi:10.1051/0004-6361/201117479
  8. ^ "eta Cet". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-03-17.
  9. ^ 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
  10. ^ Puzeras, E.; et al. (October 2010), "High-resolution spectroscopic study of red clump stars in the Galaxy: iron-group elements", Monthly Notices of the Royal Astronomical Society, 408 (2): 1225–1232, arXiv:1006.3857, Bibcode:2010MNRAS.408.1225P, doi:10.1111/j.1365-2966.2010.17195.x
  11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  12. ^ Knobel, E. B. (June 1895), "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket", Monthly Notices of the Royal Astronomical Society, 55: 429–438, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429
  13. ^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc., p. 162, ISBN 0-486-21079-0, retrieved 2010-12-12
  14. ^ ‹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.
  15. ^ ‹See Tfd›(in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived September 29, 2009, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  16. ^ "Radial Velocity". The Planetary Society. Retrieved 3 March 2017.
  17. ^ a b Trifonov, Trifon; Reffert, Sabine; Tan, Xianyu; Lee, Man Hoi; Quirrenbach, Andreas (2014). "Precise radial velocities of giant stars. VI. A possible 2:1 resonant planet pair around the K giant star η Ceti". Astronomy & Astrophysics. 568: A64. arXiv:1407.0712. Bibcode:2014A&A...568A..64T. doi:10.1051/0004-6361/201322885.