TAU CET (Tau Ceti). While Cetus, the Whale, is not among the brightest constellations, two of its stars (bright-third magnitude Menkar and second magnitude Deneb Kaitos) nicely mark its head and tail. The lower body consists of a lopsided square partly outlined by Theta Ceti at the north, Baten Kaitos at the east, and anchored by Tau Ceti, which lies right at the border of third and fourth magnitude (3.50). While carrying no proper name, and not overwhelmingly obvious, Tau Ceti marks itself by its extreme closeness to the Sun. A mere 11.9 light years away, the star ranks either as the 29th closest to us (counting all the stars in a double or multiple system) or 19th (counting double or multiple systems as single units). Much more impressive, of the stars within its sphere of 12 light years, it is fifth brightest in the nighttime sky, exceeded only by Sirius, Procyon, and the two stars that make the double of Alpha Centauri! That it is much fainter than Procyon, which at 11.4 light years is almost the same distance away, tells of a modest body. Tau Ceti is one of the few stars visible to the naked eye that has a mass less than the Sun, only about 70 percent solar, which renders it a cool class G (G8) dwarf. With a surface temperature of 5380 Kelvin (as opposed to the solar value of 5780 K), the star radiates at a rate only half that of the Sun, its radius about 80 percent solar. As stars like the Sun and Tau Ceti age, their outflowing winds, coupled with their magnetic fields, slow them down. With a rotation period of 31 days, rather more than that of the Sun, the star is much farther along its relative hydrogen-fusing lifetime than the Sun, and is considered an "old dwarf." More telling, it is "inactive," showing much less evidence for sunspot and related activity (though a weak 11-year activity cycle has been noted). Tau Ceti also stands out as a modestly high-velocity (37 kilometers per second) local visitor from the "thick disk" of the Galaxy that surrounds the thin disk that makes the Milky Way. Older, the thick disk has a lower metal content, Tau Ceti's about half that of the Sun. Tau Ceti achieved its true fame in 1960, when Frank Drake initiated "Project Ozma," an attempt to detect intelligent signals from space and the opening salvo in modern SETI, the Search for Extraterrestrial Intelligence. He picked two nearby sunlike stars, Tau Ceti and Epsilon Eridani. He found nothing, nor has anyone else since. Epsilon Eridani at least has a giant planet in orbit about it. Alas, Tau Ceti seems to be all alone, no planet as yet discovered, though a 13th magnitude stellar "companion" does reside 90 seconds of arc away. If a real companion, which is not at all known, it is a low mass class M dwarf cooler than Proxima Centauri that lies at least 325 Astronomical Units from Tau proper and takes at least 6000 years to orbit. Most likely the pairing is a line-of-sight coincidence, the old star moving past us all alone. Thanks to John Lindblad, who suggested this star.
Written by Jim Kaler. Return to STARS.