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TAU CET (Tau Ceti). WhileCetus, theWhale, is not among the brightest constellations, two of its stars(bright-third magnitudeMenkar and secondmagnitudeDeneb Kaitos) nicely markits head and tail. The lower body consists of a lopsided squarepartly outlined by Theta Ceti at the north,Baten Kaitos at the east, and anchored by TauCeti, which lies right at the border of third and fourth magnitude(3.50). While carrying no proper name, and not overwhelminglyobvious, Tau Ceti marks itself by its extreme closeness to theSun. A mere 11.9 light years away, the starranks either as the 29th closest to us (counting all the stars ina double or multiple system) or 19th (counting double or multiplesystems as single units). Much more impressive, of the starswithin its sphere of 12 light years, it is fifth brightest in thenighttime sky, exceeded only bySirius,Procyon, and the two stars that make thedouble ofAlpha Centauri! That it ismuch fainter than Procyon, which at 11.4 light years is almost thesame distance away, tells of a modest body. Tau Ceti is one of thefew stars visible to the naked eye that has a mass less than theSun, only about 70 percent solar, which renders it a cool class G(G8) dwarf. With a surface temperature of 5380 Kelvin (as opposedto the solar value of 5780 K), the star radiates at a rate onlyhalf that of the Sun, its radius about 80 percent solar. As starslike the Sun and Tau Ceti age, their outflowing winds, coupled withtheir magnetic fields, slow them down. With a rotation period of31 days, rather more than that of the Sun, the star is much fartheralong its relative hydrogen-fusing lifetime than the Sun, and isconsidered an "old dwarf." More telling, it is "inactive," showingmuch less evidence for sunspot and related activity (though a weak11-year activity cycle has been noted). Tau Ceti also stands outas a modestly high-velocity (37 kilometers per second) localvisitor from the "thick disk" of the Galaxy that surrounds the thindisk that makes the Milky Way. Older, the thick disk has a lowermetal content, Tau Ceti's about half that of the Sun. Tau Cetiachieved its true fame in 1960, when Frank Drake initiated "ProjectOzma," an attempt to detect intelligent signals from space and theopening salvo in modern SETI, the Search for ExtraterrestrialIntelligence. He picked two nearby sunlike stars, Tau Ceti andEpsilon Eridani. He found nothing, nor hasanyone else since. Epsilon Eridani at least has a giant planet inorbit about it. Alas, Tau Ceti seems to be all alone, no planet asyet discovered, though a 13th magnitude stellar "companion" doesreside 90 seconds of arc away. If a real companion, which is notat all known, it is a low mass class M dwarf cooler thanProxima Centauri that lies at least 325Astronomical Units from Tau proper and takes at least 6000 years toorbit. Most likely the pairing is a line-of-sight coincidence, theold star moving past us all alone. Thanks to John Lindblad, whosuggested this star.
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