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.