The star Gamma Cephei, Errai or Alrai, is a star located 45 light years from the solar system, in the constellation Cepheus -the 27th largest constellation in the night sky-, named after the mythical king Cepheus of Ethiopia, husband of Cassiopeia and father of Andromeda, both represented by neighboring constellations. If you’re wondering, there are no meteor showers associated with Cepheus. The neighboring constellations are Camelopardalis, Cassiopeia, Cygnus, Draco, Lacerta, and Ursa Minor. Like most other constellations in the Perseus family, Cepheus was listed by the Greek astronomer Ptolemy in the 2nd century.
It is a binary star of apparent magnitude +3.22, and is the second brightest star in this constellation after Alderamin, whose name in Arabic means “the right arm.” The star can be seen with the naked eye, that is, we do not need a telescope / binoculars to see it.
This orange-yellow star is formed, as we have said, by two stars: by an orange giant and a red dwarf. Around the giant star a planet was discovered that was baptized as Tadmor, with a mass of 60%. larger than Jupiter, which takes 903 days to complete one orbit of its star.
Gamma Cephei or Errai is a moderately bright third magnitude star in the northern constellation (it has a surface temperature of 860 ºC cooler than our Sun). The constellation Cepheus resembles a child’s drawing of a house, with Gamma Cephei marking the top of the roof. This fascinating star has played an important role in our understanding of exoplanets, especially multiple star systems.
With an estimated mass of 1.6 times that of the Sun, the star’s core silently fuses helium into carbon and oxygen. Gamma Cephei is accompanied by a low-mass companion (a red dwarf with a mass of about 0.3 or 0.4 solar masses) that orbits with a period of 57 to 66 years at a distance of about 19 astronomical units. From the planet, the secondary star would shine in the light of a couple dozen full moons. Surprising, right?
Errai is one of the few stars that host planets that have proper names; others are Edasich (Iota Draconis), Pollux, Algieba (Gamma Leonis) and Rigil Kentaurus (Alpha Centauri).
When will it become the next pole star?
In the year 4,000 AD, Gamma Cephei will move into position, due to the axial precession of the Earth; It will approach our celestial north pole and will become our polar star, leaving aside Polaris , which is the brightest star in the constellation Ursa Minor. It will be the star with the naked eye closest to the north celestial pole of the Earth. It will remain in a fixed position during the course of the night, which makes it very useful for navigation and as a ground orientation.
The wobble or axial precession of our planet changes the location of the north and south celestial poles with respect to the stars, tracing a circle in the sky approximately every 26,000 years. There are still a few centuries left for Errai to become the North Star. After her, around 7,500 AD, it will be Alderamin who becomes the new North Star. And, in about 26,000 years, Polaris will recover its port after the closing of the circle traced by axial precession.
