Epoch J2000.0 Equinox J2000.0
|Right ascension||23h 39m 20.852s|
|Declination||+77° 37′ 56.19″|
|Apparent magnitude (V)||3.21|
|Gamma Cephei A|
|Spectral type||K1III-IV CN1|
|U−B color index||+0.94|
|B−V color index||+1.03|
|Gamma Cephei B|
|Radial velocity (Rv)||−42.82±0.30 km/s|
|Proper motion (μ)|| RA: –47.96 ± 0.45 mas/yr |
Dec.: 126.59 ± 0.40 mas/yr
|Parallax (π)||72.69 ± 0.41 mas|
|Distance||44.9 ± 0.3 ly |
(13.76 ± 0.08 pc)
|Absolute magnitude (MV)||2.62|
|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|
|Surface gravity (log g)||+3.18 cgs|
|Metallicity [Fe/H]||−0.05 dex|
|Rotational velocity (v sin i)||1.63 km/s|
|Gamma Cephei B|
|Mass||0.409 ± 0.018 M☉|
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 //, the traditional name of the system) 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.
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. 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. Analysis of the spectrum in 2018 gave a best match for a spectral type of K1 III.
Gamma Cephei B has a mass approximately 0.409 times that of the Sun. It is probably a red dwarf of class M4, 6.2 magnitudes fainter than the primary. It is assumed to be of similar age to its primary.
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. 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) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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 a process for giving proper names to certain exoplanets. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name was Tadmor for this planet. 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.
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) 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.
A planet orbiting Gamma Cephei A was tentatively identified in 1988. Its existence was also announced in 1989. 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, but in 2002, the existence of a planet with an orbital period of about 2.5 years was confirmed.
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.
(in order from star)
|b (Tadmor)||≥1.85±0.16 MJ||2.05±0.06||903.3±1.5||0.049±0.034||—||—|
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- McDonald Observatory: Planet Search finds first planet orbiting close-in binary star
- A Planetary Companion to the Binary Star Gamma Cephei
- SolStation: Errai 2
| Pole Star