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 (abbreviated γ Cephei, γ Cep) is a binary star system approximately 45 light-years away in the constellation of Cepheus. It has a traditional Arabic name variously spelled as Errai, Er Rai or Alrai. The primary (designated Gamma Cephei A) is a stellar class K1III-IV orange subgiant star; it has a red dwarf companion (Gamma Cephei B). An extrasolar planet (designated Gamma Cephei Ab, later named Tadmor) is believed 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 ι Cephei some time around 5200 CE.

Gamma Cephei has an apparent magnitude of 3.22, nearly all of which is accounted for by Gamma Cephei A. The primary is about 6.6 billion years old (based on Fe/H metallicity).[4] and is on its first ascent off the main sequence. 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 B has a mass approximately 0.409 times that of the Sun.[2] It is probably a red dwarf of class M4, 6.2 degrees of magnitude fainter than the primary.[2] It is assumed to be of similar age to its primary.


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

Following its discovery the planet was designated Gamma Cephei Ab. In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets.[6] The process involved public nomination and voting for the new names.[7] In December 2015, the IAU announced the winning name was Tadmor for this planet.[8] 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.[9]

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.[10] 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.),[11] representing 右樞 (Shǎowèi), meaning The Second Imperial Guard[12] or Minor Guard[13]

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.[14] Its existence was also announced by Anthony Lawton and P Wright in 1989.[15] 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.[16]

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 (Tadmor) ≥1.60 ± 0.13 MJ 2.044 ± 0.057 902.9 ± 3.5 0.115 ± 0.058

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:0708.1752free to read. 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/0611427free to read. 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/0609638free to read. 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. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. 9 July 2014
  7. ^ NameExoWorlds The Process
  8. ^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  9. ^ NameExoWorlds The Approved Names
  10. ^ (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.
  11. ^ (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.
  12. ^ (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.
  13. ^ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Cepheus
  14. ^ 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. 
  15. ^ Lawton, A. T.; Wright, P. (1989). "A planetary system for Gamma Cephei?". British Interplanetary Society. 42: 335–336. Bibcode:1989JBIS...42..335L. 
  16. ^ Hatzes, Artie P.; et al. (2003). "A Planetary Companion to Gamma Cephei A". The Astrophysical Journal. 599 (2): 1383–1394. arXiv:astro-ph/0305110free to read. Bibcode:2003ApJ...599.1383H. doi:10.1086/379281. 

External links[edit]

Preceded by
Pole Star
Succeeded by
Iota Cephei

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