ALDERAMIN (Alpha Cephei). 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, its brightest star, Alderamin, just barely second magnitude (2.44). Nevertheless, Alderamin is enough brighter than its constellation- mates that there was little question about Bayer giving it the Alpha designation. The name has a checkered history. Commonly translated from Arabic as meaning the "right forearm," it seems to have been misapplied from an Arabic name for Castor in Gemini and then misspelled to give a wrong translation. Such is the lore of star names. Alderamin's celestial prominence arises not so much from its being the luminary of Cepheus, but from its closeness to the precessional path of the North Celestial Pole, which wobbles in a circle 23.5 degrees across over a period of 25,800 years. As a result our pole stars change. Now near Polaris in Ursa Minor, the north celestial pole pointed to Thuban in Draco in 2700 BC and more or less to Kochab (also in Ursa Minor) about the time of Homer. A wait of 5500 years will bring Alderamin within three degrees of the pole, not as good as Thuban or Polaris, but not too bad either (it was last there in 18,000 BC). This white class A (A7) star has some other curiosities. It is commonly classified as a "dwarf," or hydrogen-fusing star like the Sun, but may be just beginning to evolve into an expanded "subgiant," implying the impending cessation of core hydrogen fusion. From its nearby distance of only 49 light years, we find a modest luminosity 18 times solar, and from that and a temperature of 7600 Kelvin an even more modest radius just 2.5 times that of the Sun. This 1.9 solar mass star is single and just possibly slightly variable, as are so many stars of this kind. It is rather set apart, however, by its mad spin. It rotates at a minimum equatorial speed of 246 kilometers per second, at least 125 times that of the Sun, giving it a rotational period of less than half a day (compared with the solar 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 in broad part because its outer third is churning up and down in huge convective currents, the movement helping to generate a magnetic field. Such outer zones are supposed to disappear 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 together suggest considerable magnetic activity. No one really knows why. Such anomalies, of course, drive the science. Understanding Alderamin will someday help us understand our own star, on which we depend for life.
Written by Jim Kaler. Return to STARS.