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.
Written by Jim Kaler. Return to STARS.