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| Observation data Epoch J2000.0 Equinox J2000.0 |
|
|---|---|
| Constellation (pronunciation) |
Cepheus |
| Right ascension | 23h 39m 20.8s |
| Declination | +77° 37′ 56″ |
| Apparent magnitude (V) | 3.22 / ? |
| Characteristics | |
| Spectral type | K1 IVe / M1V |
| U-B color index | 0.94 / ? |
| B-V color index | 1.03 / ? |
| Variable type | Suspected |
| Astrometry | |
| Radial velocity (Rv) | 8.8 km/s |
| Proper motion (μ) | RA: -48.85 mas/yr Dec.: 127.18 mas/yr |
| Parallax (π) | 72.42 ± 0.52 mas |
| Distance | 45 ± 0.3 ly (13.81 ± 0.1 pc) |
| Absolute magnitude (MV) | 2.51 / ? |
| Visual binary orbit | |
| Companion | Gamma Cephei B |
| Period (P) | 74 yr |
| Semimajor axis (a) | 18.5 AU |
| Eccentricity (e) | 0.36 |
| Details | |
| Mass | 1.6 / 0.4 M☉ |
| Radius | 4.7 / 0.5 R☉ |
| Luminosity | 10.6 / ? L☉ |
| Temperature | 4,900 / 3,500 K |
| Metallicity | 89-186% |
| Rotation | 781 days / ? |
| Age | 3 × 109 years |
| Other designations | |
| Database references | |
| SIMBAD | data |
Gamma Cephei (γ Cep / γ Cephei) is a star in the constellation Cepheus. It also has the traditional name Alrai, also known as Errai (or Er Rai).
Gamma Cephei is a binary star approximately 50 light years from Earth, with an apparent magnitude of 3.22. The larger and brighter star is a stellar class K1 IV orange subgiant star. The smaller is a red dwarf in a poorly studied orbit: the current best guess is that it orbits eccentrically from 10 to 29 AU over the course of 70 yrs ± 16 yrs, with an eccentricity of 0.439 ± 0.06 (Cochran et al 2002).
Gamma Cephei is the naked-eye star that will follow Polaris as the Earth's northern pole star, due to the precession of the equinoxes. Alrai will become closer to the northern celestial pole than Polaris around 3000 AD, and be at its closest approach around 4000 AD. The "title" will pass to ι Cephei some time around 5200 AD.
The 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".
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Planetary system
A planet orbiting Gamma Cephei A was tentatively identified by a Canadian team of Bruce Campbell, Gordon Walker, and Stephenson Yang in 1988. Its existence was also announced by Anthony Lawton and P. Wright in 1989. This would have been the first confirmed extrasolar planet, and it was hypothesized based on the same radial velocity technique later used successfully by others. However, the claim was retracted in 1992 due to the quality of the data not being good enough to make a solid discovery. But in 2002, evidence of the planet was considerably strengthened by the new measurements by Artie Hatzes and his collaborators at the McDonald Observatory.
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity |
|---|---|---|---|---|
| Ab | >1.59 MJ | 2.03 | 902.96 ± 6 | 0.2 ± 0.069 |
See also
References
- Campbell et al. (1988). "A search for substellar companions to solar-type stars". The Astrophysical Journal 331: 902–921. doi:.
- Hatzes et al. (2003). "A Planetary Companion to Gamma Cephei A". The Astrophysical Journal 599 (2): 1383 – 1394. doi:.
External links
- http://jumk.de/astronomie/exoplanets/errai.shtml
- McDonald Observatory: Planet Search finds first planet orbiting close-in binary star
- A Planetary Companion to the Binary Star Gamma Cephei
- SolStation: Errai 2
- Extrasolar Planets Encyclopaedia: Gamma Cephei + Gamma Cephei b
- Extrasolar Visions: Gamma Cephei A + Gamma Cephei Ab + Gamma cephei B
| Preceded by Polaris |
Pole Star 3000–5200 |
Succeeded by Iota Cephei |
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