DELTA CET (Delta Ceti). Shining at mid-fourth magnitude in the neck of Cetus (the Whale, or Sea Monster), Delta Ceti carries no proper name. Though not at all obvious at first glance, the star is rather unusually placed: it is a very hot (23,400 Kelvin) blue class B (B2) subgiant way off the plane of the Milky Way, which holds the vast majority of such stars. Delta's apparent dimness is not intrinsic, but due to its rather large distance of 650 light years. Factoring in the large amount of ultraviolet light that radiates from its hot surface (the "surface" of a star not solid, but an opaque gas) gives a very respectable luminosity 5800 times that of the Sun, which in turn leads to a radius of 4.7 times solar. A minimum projected rotation velocity of just 11 kilometers per second gives a rotation period of under 21 days, not much of a restriction. Since most such stars are rapid rotators, there are good odds that Delta is spinning with its axis more or less directed at us. The star is just a bit deficient in metals (by about 25 percent). The theory of stellar structure and evolution shows Delta to carry a mass of 9.5 times that of the Sun and really to be a core hydrogen-fusing dwarf in middle life (with an age of roughly 10 million years) rather than a subgiant in which the core has given up such fusion. Delta Ceti's mass places it right at the border between stars that become massive neon-oxygen white dwarfs (most final states being carbon-oxygen) and those that blow up as supernovae. In the meantime, Delta Ceti has three features going for it. First, like many of its class, it is a subtly-pulsating "Beta Cephei" star that varies by about 0.05 magnitudes (about five percent) over a short period of 3.867 hours. Long thought to be an unusual "monoperiodic" version (with but one period of oscillation), other periods have now been found (making it fit the breed), the star also pulsating with periods of 1.934 hours (exactly half the primary period), 6.422 hours, 6.534 hours, and 3.14 days. Second, it serves as something of a guide to famed Mira, which lies just to the southwest of it. Finally, Delta Ceti is an excellent "equator star" (in league with such others as Alpha Sextantis, Zeta Virginis, Zeta Aquarii, and Delta Orionis) that lies only a third of a degree north of the celestial equator. Such positioning, however, is variable as a result of the 26,000-year precession (wobble) of the Earth's axis, which causes this part of the equator to shift to the south, and the star therefore to appear to shift to the north, at a current rate of a quarter of a minute of arc per year. Delta Ceti thus made the transition from being a southern hemisphere star to being one of the northern late in the year 1923.
Written by Jim Kaler. Return to STARS.