Gamma Cephei - Errai, Alrai

At magnitude 3.22, Gamma Cephei is the among the visually brightest stars known to have a planet. Gamma Cephei is one of the two planet-hosting stars that have proper names. "Errai", also spelled "Alrai", comes from Arabic and means "the Shepherd." Gamma Cephei also has a low mass companion with a relatively small orbit.

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.

Properties and Companion

At a distance of 45 light years, γ Cep is an evolving class K1 IV subgiant with a surface temperature of 4920 K. It has a luminosity of 10.6 suns, and a radius 4.5 times solar. Direct measure of its angular diameter gives a radius 4.8 times the Sun's, confirming the validity of these parameters. With a mass estimated to be 1.6 times the Sun's, the star is now beginning to become a brighter red giant, with a dead helium core. Spectral variations suggest a rotation period of 781 days.

Gamma Cephei is accompanied by a red dwarf companion with a mass around 0.3 or 0.4 solar masses in a poorly studied orbit. The current best guess is that it orbits with a period between 57 and 66 years, with a substantial eccentricity (e=0.439) that takes it star from 12 to 26 AU from the primary.

Planet

A planet orbiting γ Cep A was tentatively identified by a Canadian team in 1988. 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 However, the claim was retracted in 1992 due to the quality of the data. But in 2002, evidence of the planet was considerably strengthened by new measurements at the McDonald Observatory.

The planet is one of the few known to lie within a double-star system, and orbits the main star γ Cep A with a period of 2.47 years at an average distance of 2.03 AU. A modest eccentricity brings the planet as close as 1.62 AU to its parent star and takes it as far as 2.43 AU. An estimate of the inclination of the system from the stellar rotation gives a true planetary mass between 3 and 16 Jupiters.

The planet is in a double star system whose separation is comparable to the size of our own planetary system, and shows that planets can be formed and survive in relatively close binary systems. From the planet, the secondary star would shine with the light of a couple dozen full Moons.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]