ZETA CAS (Zeta Cassiopeiae). Among the sky's best known stars are the five that make Cassiopeia's famed "W," from west to east Beta, Alpha, Gamma, Delta, and Epsilon. Extending a line from Delta through Epsilon gets you to Iota and the memory-aiding "BAGDEI" sequence. Then add Kappa to make the equally famed "Chair." Falling off the Chair a bit to the south, Zeta Cas (just over the border into fourth magnitude, 3.66) then becomes largely ignored.
Zeta Cas Zeta Cassiopeiae, the bright star just below top center, is set into a dense part of the Milky Way. The brighter of the two stars down and to the left of it is Lambda Cas. Zeta lies to the north (and a bit west) of Shedar, Alpha Cas, seen toward the bottom. To the right of Shedar is a slightly fuzzy patch, the nebula NGC 281.
A magnificent hot (21,000 Kelvin) blue class B (B2) subgiant (but see below), it well deserves a place beside the others. Set within the Milky Way at a distance of 600 light years, the star is far enough to be dimmed some by interstellar dust. If space were clear, Zeta would shine 0.13 magnitude brighter and almost make third magnitude. Taking that and the severe ultraviolet light that comes with high temperature into account leads to a luminosity 6400 times that of the Sun, a radius 6.1 times solar, and a high mass of 9 times solar, which puts Zeta Cas at or near the limit at which stars develop iron cores and explode as supernovae. If below the limit, Zeta Cas will instead become a massive white dwarf, perhaps one made of neon and oxygen rather than carbon and oxygen. Rather than being a subgiant (which suggests the star is near the end of its life), Zeta Cas really seems to be a hydrogen-fusing dwarf, albeit one that is some three-quarters of the way through its 25 million year dwarf lifetime. Tiny variations of under 0.005 magnitudes with a period of 1.56 days reveal Zeta to be member of the "Slowly Pulsating B" (SPB) stars whose prototype is 53 Persei and that are just slightly dimmer and cooler than the better known Beta Cephei pulsators. Zeta Cas stands out as the first of its tribe to be found with a magnetic field (odd for hot stars). Variations in the outflowing wind that is modulated by the field leads to a rotation period of 5.37 days and a rotation speed of 56 kilometers per second, rather low for the class. The observed projected value of 14 km/s shows the star to be spinning with its rotation axis rather pointing toward Earth. The effect of the slow pulsations on the spectrum once fooled us into thinking that the star had a companion. None, however, is seen or even suggested, Zeta Cas heading toward its final state -- whether supernova or white dwarf -- quite alone.
Written by Jim Kaler. Return to STARS.