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DELTA CET (Delta Ceti). Shining at mid-fourth magnitude in theneck ofCetus (the Whale, or SeaMonster), Delta Ceti carries no proper name. Though not at allobvious at first glance, the star is rather unusually placed: it isa very hot (23,400 Kelvin) blue class B (B2) subgiant way off theplane of theMilky Way, which holdsthe vast majority of such stars. Delta's apparent dimness is notintrinsic, but due to its rather large distance of 650 light years. Factoring in the large amount of ultraviolet light that radiatesfrom its hot surface (the "surface" of a star not solid, but anopaque gas) gives a very respectable luminosity 5800 times that oftheSun, which in turn leads to a radius of4.7 times solar. A minimum projected rotation velocity of just 11kilometers per second gives a rotation period of under 21 days, notmuch of a restriction. Since most such stars are rapid rotators,there are good odds that Delta is spinning with its axis more orless directed at us. The star is just a bit deficient in metals(by about 25 percent). The theory of stellar structure andevolution shows Delta to carry a mass of 9.5 times that of the Sunand really to be a core hydrogen-fusing dwarf in middle life (withan age of roughly 10 million years) rather than a subgiant in whichthe core has given up such fusion. Delta Ceti's mass places itright at the border between stars that become massive neon-oxygenwhite dwarfs (most finalstates being carbon-oxygen) and those that blow up assupernovae. In the meantime,Delta Ceti has three features going for it. First, like many ofits class, it is a subtly-pulsating "BetaCephei" star that varies by about 0.05 magnitudes (about fivepercent) over a short period of 3.867 hours. Long thought to be anunusual "monoperiodic" version (with but one period ofoscillation), other periods have now been found (making it fit thebreed), the star also pulsating with periods of 1.934 hours(exactly half the primary period), 6.422 hours, 6.534 hours, and3.14 days. Second, it serves as something of a guide to famedMira, which lies just to the southwest of it. Finally, Delta Ceti is an excellent "equator star" (in league withsuch others asAlpha Sextantis,Zeta Virginis,ZetaAquarii, andDelta Orionis) thatlies only a third of a degree north of thecelestial equator. Such positioning,however, is variable as a result of the 26,000-yearprecession (wobble) of the Earth'saxis, which causes this part of the equator to shift to the south,and the star therefore to appear to shift to the north, at acurrent rate of a quarter of a minute of arc per year. Delta Cetithus made the transition from being a southern hemisphere star tobeing one of the northern late in the year 1923.
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