EPS CEP (Epsilon Cephei). Tucked into a tight triangle at the
southeastern corner of the irregular pentagon that makes Cepheus (the celestial King) lie Delta, Zeta,
and Epsilon Cephei. The first, Delta Cephei, the prototype of the
Cepheid variables, is among
the most famed stars of the sky, while the second, Zeta Cep, is a
magnificent supergiant that serves nicely as a comparison star
through which we can easily see Delta's variations. The third,
Epsilon, rarely gets much respect, rather too bad as it is an
interesting critter in its own right. Epsilon Cephei is classed as
a warm class F (F0) subgiant, the subgiant status implying that it
is beginning to give up its core hydrogen fusion, if it has not
done so already. Detailed observations, however, indicate
otherwise. At a distance of 84 light years, the star shines at us
with a luminosity 11 times that of the Sun
from a surface heated to 7330 Kelvin, which together give us a
radius almost exactly twice that of the Sun. Coupled with the
theory of stellar structure and evolution, luminosity and
temperature also yield a mass of 1.7 solar and show that the star,
at about a billion years of age, is only about halfway through its
hydrogen-fusing lifetime (very much like the Sun, which, however,
has a much longer total hydrogen-fusing lifetime of 10 billion
years), the spectral class oddly misleading. Like many warmer
stars, Eps Cep is a rapid rotator. Spinning at least 90 kilometers
per second at the equator, it takes under 1.1 days to make a full
rotation. The metal content is roughly similar to that of the Sun,
but with some chemical elements raised up by 60 percent or so,
another depleted by that amount, possibily because of atmospheric
diffusion (some sinking under the effect of gravity, others rising
through radiation pressure). Nearby are a couple "companion stars"
that seem to be just line of sight coincidences, though there is
some suggestion of a close spectroscopic companion. Epsilon
Cephei, however, is best known as a variable star. Overwhelmed in
renown by its neighbor Delta Cephei, Epsilon is a much subtler "Delta Scuti star" that quickly chatters
away by only 0.02 magnitudes (about two percent) in brightness over
a 1.0 hour period on which is superimposed another, longer,
variation period of 1.6 hours (multiple periods rather defining the
class). Such stars are actually dwarf variations on the Delta
Cephei scheme (baby Cepheids, so to speak) in which the variations
are greatly suppressed by inherent dwarf stability, which ties
Cepheus's little triangle even closer together.