ZETA CEP (Zeta Cephei). Delta, VV, and Mu Cephei (the last "Herschel's Garnet Star") are so well known that we tend to pay little attention to the other stars of Cepheus, the heavenly King, Cassiopeia's husband, Andromeda's father. Delta, as leader of the Cepheid variables, was crucial in establishing the distances of other galaxies. (The absolute brightnesses of Cepheids are tightly tied to their periods of variation; once the true luminosities are known, comparisons with apparent magnitudes give distances.) Third magnitude (3.35) Zeta, notably brighter than Delta, lies with Epsilon Cephei in a little triangle at Cepheus's southeastern corner. Though it has been cited as having a companion that causes an occasional very small eclipse, its light is otherwise steady (and even the "eclipse" is questioned). Delta aside, Zeta is a magnificent star in its own right, a class K (K1.5) lesser supergiant with a temperature of 4310 Kelvin. Parallax (second Hipparcos reduction) gives a distance of 836 light years (give or take 20). Along with Delta Cep, Zeta appears to be part of the Cepheus OB6 association (centered at 880 light years), an expanding group of massive stars that share a common birthplace. After a 0.22 magnitude correction for dimming by interstellar dust (derived from that of Delta Cephei, with which it is apparently related), we find that it shines with a luminosity 5900 times that of the Sun from a surface swollen through stellar evolution to 140 times the solar size, making it two-thirds the size of Earth's orbit. Direct measures of angular diameter give a similar value, 145 times solar, showing that the various parameters are in good shape. Rather metal-rich (the iron-to-hydrogen 1.6 times solar), the star is also losing mass at a (very rough) modest millionth of a solar mass per year. If now evolving toward internal helium fusion, Zeta would have to be some 9 the mass of the Sun to have its current temperature and luminosity, while if already fusing helium to carbon the mass could be lower, a "mere" 7 times solar. Now about 50 million years old, Zeta began life at the hotter end of the range of class B dwarfs. At the edge of the 8- to-10 solar mass limit at which stars develop iron cores and then explode as supernovae, its most likely fate is to produce a massive white dwarf (perhaps one made of neon instead of carbon) near the 1.4 solar limit at which such dense remnants can survive. If there IS a companion, and it is close enough to feed sufficient matter to the white-dwarf-to-be, it is marginally possible that the limit could be overflowed, resulting in the white dwarf's collapse and again a supernova.

Written by Jim Kaler 10/28/05; revised 5/21/13. Return to STARS.