Astronomy:Gamma Cephei

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Short description: Binary star system in the constellation Cephei
Gamma Cephei
Cepheus constellation map.png
Location of γ Cephei (top center)
Observation data
Equinox J2000.0]] (ICRS)
Constellation Cepheus
Right ascension  23h 39m 20.852s[1]
Declination +77° 37′ 56.19″[1]
Apparent magnitude (V) 3.21[2]
Gamma Cephei A
Spectral type K1III-IV CN1[3]
U−B color index +0.94[2]
B−V color index +1.03[2]
Variable type Suspected[4]
Gamma Cephei B
Spectral type M4V[5]
Radial velocity (Rv)−42.82±0.30[6] km/s
Proper motion (μ) RA: –47.96 ± 0.45[1] mas/yr
Dec.: 126.59 ± 0.40[1] mas/yr
Parallax (π)72.5167 ± 0.1470[7] mas
Distance44.98 ± 0.09 ly
(13.79 ± 0.03 pc)
Absolute magnitude (MV)2.62[8]
Period (P)67.5 ± 1.4 yr
Semi-major axis (a)1.467 ± 0.046"
(19.56±0.18 AU[9])
Eccentricity (e)0.4112 ± 0.0063
Inclination (i)119.3 ± 1.0°
Longitude of the node (Ω)18.04 ± 0.98°
Periastron epoch (T)1,991.605 ± 0.031
Argument of periastron (ω)
161.01 ± 0.40°
Gamma Cephei A
Mass1.294±0.081[9] M
Radius4.93±0.04[10] R
Luminosity11.6±0.6[10] L
Surface gravity (log g)+3.18[6] cgs
Temperature4,792±62[10] K
Metallicity [Fe/H]−0.05[6] dex
Rotational velocity (v sin i)1.63[6] km/s
Age3.25±0.63[10] Gyr
Gamma Cephei B
Mass0.384±0.013[9] M
Other designations
Errai, 35 Cep, Gl 903, HR 8974, BD +76°928, HD 222404, GCTP 5725.00, SAO 10818, FK5 893, HIP 116727
Database references
Exoplanet Archivedata
Extrasolar Planets

Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the constellation of Cepheus. The primary (designated Gamma Cephei A, officially named Errai /ɛˈr./, the traditional name of the system)[11][12] is a stellar class K1 orange giant or subgiant star; it has a red dwarf companion (Gamma Cephei B). An exoplanet (designated Gamma Cephei Ab, later named Tadmor) has been confirmed to be orbiting the primary.

Gamma Cephei is the naked-eye star that will succeed Polaris as the Earth's northern pole star, due to the precession of the equinoxes. It will be closer to the northern celestial pole than Polaris around 3000 CE and will make its closest approach around 4000 CE. The 'title' will pass to Iota Cephei some time around 5200 CE.


Gamma Cephei has an apparent magnitude of 3.21, nearly all of which is accounted for by Gamma Cephei A. The primary is about 3.25 billion years old and has evolved off the main sequence, having burned all of the hydrogen in its core.[10]

The spectrum of this star has served as one of the stable anchor points by which other stars are classified. It was listed as a standard star for the spectral class K1 IV in 1943, 1953 and 1973.[13] However, in 1989, it was given as a spectral standard for K1 III-IV. Its spectrum is notable for the strength of the cyano radical (CN) bands.[3] Analysis of the spectrum in 2018 gave a best match for a spectral type of K1 III.[10]

Gamma Cephei B has a mass approximately 0.409 times that of the Sun.[5] It is probably a red dwarf of class M4, 6.2 magnitudes fainter than the primary.[5] It is assumed to be of similar age to its primary.

γ Cephei is catalogued as a suspected variable star with a brightness range between magnitudes 3.18 and 3.24,[4] based on its inclusion in an 1884 list of suspected variable stars.[14]


γ Cephei (Latinised to Gamma Cephei) is the system's Bayer designation. Under the rules for naming objects in multiple star systems the two components are designated A and B.[15] Following its discovery the planet was designated Gamma Cephei Ab.

The system bore a traditional name variously spelled as Errai, Er Rai or Alrai, deriving from the Arabic الراعي (ar-rā‘ī), meaning 'the shepherd'. (The star Beta Ophiuchi is sometimes also called Alrai, but it is more commonly known as Cebalrai or Kelb Alrai, meaning 'shepherd's dog'.) In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[17] included a table of the first two batches of names approved by the WGSN; which included Errai for Gamma Cephei A.

In July 2014 the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets.[18] The process involved public nomination and voting for the new names.[19] In December 2015, the IAU announced the winning name was Tadmor for this planet.[20] It was submitted by the Syrian Astronomical Association and is the ancient Semitic name and modern Arabic name for the city of Palmyra, a (UNESCO) World Heritage Site.[21]

In Chinese, the star is named 少衛增八 (Shàowèi Zēng Bā, literally, the 8th added star of the Xingguan Shaowei, Shaowei: the Minor Guard[22]) belonging to the Left Wall of the Purple Forbidden enclosure (紫微左垣, Zǐwēi Zuǒyuán), which refers to an asterism consisting of Gamma Cephei, Iota Draconis, Theta Draconis, Eta Draconis, Zeta Draconis, Upsilon Draconis, 73 Draconis and 23 Cassiopeiae.[23]

Planetary system

A planet orbiting Gamma Cephei A with a 2.7-year period was discovered in 1988.[24] Its existence was also announced in 1989.[25] This was the first-confirmed extrasolar planet and its ostensible discovery was based on the same radial velocity technique later used successfully by others. However, the claim was challenged in 1992 by a paper[26] which favoured K-giant variability with a period equal to the stellar rotation, but in 2002, the existence of a planet with an orbital period of about 2.5 years was confirmed.[27]

The secondary star B orbits A at only 9.8 times the semimajor axis of A's planet. Despite how compact the system is, the planet's orbit is stable if it is coplanar with that of the binary companion.[28]

The Gamma Cephei A planetary system[29]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (Tadmor) ≥1.85±0.16 MJ 2.05±0.06 903.3±1.5 0.049±0.034


  1. 1.0 1.1 1.2 1.3 van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode2007A&A...474..653V. Vizier catalog entry
  2. 2.0 2.1 2.2 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: V/50. Bibcode1995yCat.5050....0H. 
  3. 3.0 3.1 Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins catalog of revised MK types for the cooler stars". The Astrophysical Journal Supplement Series 71: 245. doi:10.1086/191373. Bibcode1989ApJS...71..245K. 
  4. 4.0 4.1 Samus, N. N. 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: B/gcvs. Bibcode2009yCat....102025S. 
  5. 5.0 5.1 5.2 5.3 Neuhäuser, R. et al. (2007). "Direct detection of exoplanet host star companion γ Cep B and revised masses for both stars and the sub-stellar object". Astronomy and Astrophysics 462 (2): 777–780. doi:10.1051/0004-6361:20066581. Bibcode2007A&A...462..777N. 
  6. 6.0 6.1 6.2 6.3 Jofré, E; Petrucci, R; Saffe, C; Saker, L; Artur de la Villarmois, E; Chavero, C; Gómez, M; Mauas, P. J. D (2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets". Astronomy & Astrophysics 574: A50. doi:10.1051/0004-6361/201424474. Bibcode2015A&A...574A..50J. 
  7. Brown, A. G. A. (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics 649: A1. doi:10.1051/0004-6361/202039657. Bibcode2021A&A...649A...1G.  Gaia EDR3 record for this source at VizieR.
  8. Park, Sunkyung et al. (2013), "Wilson-Bappu Effect: Extended to Surface Gravity", The Astronomical Journal 146 (4): 73, doi:10.1088/0004-6256/146/4/73, Bibcode2013AJ....146...73P. 
  9. 9.0 9.1 9.2 Follow-up observations of the binary system γ Cep, 2022 
  10. 10.0 10.1 10.2 10.3 10.4 10.5 Baines, Ellyn K. et al. (2018). "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer". The Astronomical Journal 155 (1): 30. doi:10.3847/1538-3881/aa9d8b. Bibcode2018AJ....155...30B. 
  11. Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific (San Francisco, California) 48 (283): 139. doi:10.1086/124681. Bibcode1936PASP...48..139R. 
  12. "IAU Catalog of Star Names". 
  13. Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode1993AAS...183.1710G,, retrieved 2012-02-04 
  14. Gore, J. E (1884). "A Catalogue of Suspected Variable Stars. With Notes and Observations". Proceedings of the Royal Irish Academy. Science 4: 267–410. Bibcode1885PRIA....4..411G. 
  15. Hartkopf, William I.; Mason, Brian D.. "Addressing confusion in double star nomenclature: The Washington Multiplicity Catalog". U.S. Naval Observatory. 
  16. "IAU Working Group on Star Names (WGSN)". 
  17. "Bulletin of the IAU Working Group on Star Names, No. 1". 
  18. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. 9 July 2014
  19. "NameExoWorlds The Process". 
  20. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  21. NameExoWorlds The Approved Names
  22. Richard Hinckley Allen: Star Names — Their Lore and Meaning: Cepheus
  23. (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
  24. Campbell, Bruce et al. (1988). "A search for substellar companions to solar-type stars". The Astrophysical Journal 331: 902–921. doi:10.1086/166608. Bibcode1988ApJ...331..902C. 
  25. Lawton, A. T.; Wright, P. (1989). "A planetary system for Gamma Cephei?". British Interplanetary Society 42: 335–336. Bibcode1989JBIS...42..335L. 
  26. Walker, Gordon A.H.; Bohlender, David A.; Walker, Andrew R. (1992). "Gamma Cephei: Rotation or Planetary Companion?". Astrophysical Journal Letters 396: L91–L94. doi:10.1086/186524. Bibcode1992ApJ...396L..91W. 
  27. Hatzes, Artie P. et al. (2003). "A Planetary Companion to Gamma Cephei A". The Astrophysical Journal 599 (2): 1383–1394. doi:10.1086/379281. Bibcode2003ApJ...599.1383H. 
  28. Torres, Guillermo (2007). "The Planet Host Star γ Cephei: Physical Properties, the Binary Orbit, and the Mass of the Substellar Companion". The Astrophysical Journal 654 (2): 1095–1109. doi:10.1086/509715. Bibcode2007ApJ...654.1095T. 
  29. Endl, Michael; Cochran, William D.; Hatzes, Artie P.; Wittenmyer, Robert A. (2011). "News from the γ Cephei Planetary System". PLANETARY SYSTEMS BEYOND THE MAIN SEQUENCE: Proceedings of the International Conference.. 1331. pp. 88–94. doi:10.1063/1.3556187. Bibcode2011AIPC.1331...88E. 

External links

Preceded by
Pole Star
Succeeded by
Iota Cephei

Coordinates: Sky map 23h 39m 20.8s, +77° 37′ 56″