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.