Gamma Cephei

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Alrai, γ Cep
Cepheus constellation map.png
Location of γ Cephei (top center).
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension 23h 39m 20.852s[1]
Declination +77° 37′ 56.19″[1]
Apparent magnitude (V) 3.22
Gamma Cephei A
Spectral type K1IVe
U−B color index 0.94
B−V color index 1.03
Variable type Suspected
Gamma Cephei B
Spectral type M4V[2]
Radial velocity (Rv) 8.8 km/s
Proper motion (μ) RA: –47.96 ± 0.45[1] mas/yr
Dec.: 126.59 ± 0.40[1] mas/yr
Parallax (π) 72.69 ± 0.41[2] mas
Distance 44.9 ± 0.3 ly
(13.76 ± 0.08 pc)
Absolute magnitude (MV) 2.51
Primary A
Companion B
Period (P) 67.5 ± 1.4 yr
Semi-major axis (a) 1.467 ± 0.046"
(20.18 ± 0.66 AU)
Eccentricity (e) 0.4112 ± 0.0063
Inclination (i) 119.3 ± 1.0°
Longitude of the node (Ω) 18.04 ± 0.98°
Periastron epoch (T) 1991.605 ± 0.031
Argument of periastron (ω)
161.01 ± 0.40°
Gamma Cephei A
Mass 1.40 ± 0.12[2] M
Radius 4.79 ± 0.06[3] R
Age 6.6[4] Gyr
Gamma Cephei B
Mass 0.409 ± 0.018[2] M
Other designations
35 Cep, Gl 903, HR 8974, BD +76°928, HD 222404, GCTP 5725.00, SAO 10818, FK5 893, HIP 116727.
Database references
Exoplanet Archive data
Extrasolar Planets

Gamma Cephei (γ Cep, γ Cephei), traditionally named Errai, Er Rai, and or Alrai, is a binary star system approximately 45 light-years away in the constellation of Cepheus. Gamma Cephei contains an apparent magnitude of 3.22. The visible part of the system is a stellar class K1III-IV orange subgiant star on its first ascent off the main sequence. It is about 6.6 billion years old (based on Fe/H metallicity).[4] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[5]

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 ι Cephei some time around 5200 CE.

The star has a companion star with a mass approximately 0.409 times that of our Sun.[2] Gamma Cephei B is of stellar mass and is assumed to be of similar age to its primary. It is probably a red dwarf of class M4, 6.2 degrees of magnitude fainter than the K-type primary star.[2]


Gamma Cephei's traditional name derives from the Arabic الراعي ar-rā‘ī, meaning "the shepherd". Confusingly, the star β Ophiuchi is sometimes also called Alrai, but it is more commonly known as Cebalrai or Kelb Alrai, meaning "shepherd's dog".

In Chinese, 紫微左垣 (Zǐ Wēi Zuǒ Yuán), meaning Left Wall of Purple Forbidden Enclosure, refers to an asterism consisting of γ Cephei, ι Draconis, θ Draconis, η Draconis, ζ Draconis, υ Draconis, 73 Draconis and 23 Cassiopeiae.[6] Consequently, γ Cephei itself is known as 紫微左垣七 (Zǐ Wēi Zuǒ Yuán qī, English: the Seventh Star of Left Wall of Purple Forbidden Enclosure.),[7] representing 右樞 (Shǎowèi), meaning The Second Imperial Guard[8] or Minor Guard[9]

Planetary system[edit]

A planet orbiting Gamma Cephei A was tentatively identified by a Canadian team consisting of Bruce Campbell, Gordon Walker and Stephenson Yang in 1988.[10] Its existence was also announced by Anthony Lawton and P Wright in 1989.[11] This would have been the first confirmed extrasolar planet and its ostensible discovery was based on the same radial velocity technique later used successfully by others. The claim was retracted in 1992 because the quality of the data was not good enough to establish discovery. In 2002, evidence of the planet was considerably strengthened by new measurements from Artie Hatzes and his collaborators at the McDonald Observatory.[12]

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.[4]

The Gamma Cephei A planetary system
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b ≥1.60 ± 0.13 MJ 2.044 ± 0.057 902.9 ± 3.5 0.115 ± 0.058

The planet is one of those selected by the International Astronomical Union as part of their public process for giving proper names to exoplanets.[13][14] The process involves public nomination and voting for the new name, and the IAU plans to announce the new name in mid-November 2015.[15]

See also[edit]


  1. ^ a b c d 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. Vizier catalog entry
  2. ^ a b c d e f g 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. arXiv:astro-ph/0611427. Bibcode:2007A&A...462..777N. doi:10.1051/0004-6361:20066581. 
  3. ^ Nordgren, Tyler E.; et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal 118 (6): 3032–3038, Bibcode:1999AJ....118.3032N, doi:10.1086/301114 
  4. ^ a b c 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. arXiv:astro-ph/0609638. Bibcode:2007ApJ...654.1095T. doi:10.1086/509715. 
  5. ^ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04 
  6. ^ (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.
  7. ^ (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.
  8. ^ (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.
  9. ^ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Cepheus
  10. ^ Campbell; Walker, G. A. H.; Yang, S. (1988). "A search for substellar companions to solar-type stars". The Astrophysical Journal 331: 902–921. Bibcode:1988ApJ...331..902C. doi:10.1086/166608. 
  11. ^ Lawton, A. T.; Wright, P. (1989). "A planetary system for Gamma Cephei?". British Interplanetary Society 42: 335–336. Bibcode:1989JBIS...42..335L. 
  12. ^ Hatzes, Artie P.; et al. (2003). "A Planetary Companion to Gamma Cephei A". The Astrophysical Journal 599 (2): 1383–1394. arXiv:astro-ph/0305110. Bibcode:2003ApJ...599.1383H. doi:10.1086/379281. 
  13. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. 9 July 2014
  14. ^ NameExoWorlds.
  15. ^ NameExoWorlds.

External links[edit]

Preceded by
Pole Star
Succeeded by
Iota Cephei

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