The brightest star in Cepheus is Alderamin, or Alpha Cephei, at magnitude 2.44. Though Cepheus, the King, is famed in mythology as father to Andromeda, it is far from being one of the sky's more prominent ancient constellations.
The name Alderamin has a checkered history. Commonly translated from Arabic "ao-oira' al-yaminas", meaning "the right forearm," it seems to have been misapplied from an Arabic name for Castor in Gemini, and then misspelled. Alpha Cephei is known as the Fifth Star of the Celestial Hook in Chinese.
Alderamin is located near the precessional path traced across the celestial sphere by the Earth's north pole, which wobbles in a circle 23.5 degrees across over a period of 25,800 years. That means that periodically this star comes within 3° of being a pole star, as Polaris is at present. This will next occur about the year 7500 A.D.; it happened last in 18,000 B.C.
Properties
This white class A7 V star has a surface temperature of 7600 K. Commonly classified as a hydrogen-fusing, main-sequence star like the Sun, it may be beginning to evolve into a subgiant, implying the impending cessation of its core hydrogen fusion. From its nearby distance of only 49 light years, its luminosity is 18 times solar, and its radius is 2.5 times the Sun's. This 1.9 solar mass star is single, and possibly slightly variable.
Alpha Cephei rotates at a minimum equatorial speed of 246 km/sec, giving it a rotational period of less than half a day (compared with the Sun's rotation period of nearly a month). The rapid spin apparently suppresses the separation of chemical elements common to stars of this class.
The spin may also be related to the star's activity. The Sun is magnetically active partly because its outer third is churning up and down in huge convective currents. Such outer zones are not supposed to exist in class A stars like Alderamin. Yet Alderamin emits about the same amount of X-ray radiation as does the Sun, and has other features that suggest considerable magnetic activity. No one really knows why.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]