Tau Ceti

At magnitude 3.50, Tau Ceti is not overwhelmingly obvious. But at only 11.9 light-years away, it is the 19th closest star system, and the seventh nearest naked eye star (after α Centauri, Sirius, ε Eridani, ε Indi, 61 Cygni, and Procyon.) Of the stars within 12 light years, it is fifth brightest in the night sky, exceeded only by Sirius, Procyon, and Alpha Centauri A and B.

Properties

Tau Ceti is a yellow class G8 dwarf similar to the Sun, but only 45% as bright, with an absolute magnitude of +5.7. It is one of the few stars visible to the naked eye that has a mass less than the Sun's (only about 70% solar). Its surface temperature of 5380 K, and its half-solar luminosity, give it a radius about 80% of the sun's.

A 13th magnitude stellar companion appears 90" away. If physically related, it is a low mass, class M dwarf at least 325 AU from τ Cet, and takes at least 6000 years to orbit. More likely, the pairing is a line-of-sight coincidence.

Tau Ceti seems to be alone, with no planets yet detected. However, in 2004 a team of UK astronomers discovered that τ Cet is surrounded by a disk of cold dust, centered symmetrically on the star, out to a radius of about 55 AU. The debris disk contains more than 10 times the amount of dust found in our solar system; this implies that 10 times more cometary and asteroidal material orbits τ Cet as the Sun, since the dust is produced by collisions between such small bodies. Sunlike stars with large debris disks may not be uncommon; τ Cet's disk is only one-twentieth as dense as the disk around ε Eridani.

Evolution and SETI

As stars like the Sun and Tau Ceti age, their outflowing winds, coupled to their magnetic fields, slow them down. With a rotation period of 31 days, as opposed to the Sun's 25, τ Cet is much farther along its main-sequence lifetime than the Sun. It is inactive, showing much less evidence of sunspot-related activity (though a weak 11-year activity cycle has been noted).

Tau Ceti's modestly high velocity (37 km/sec) suggests that it is a local visitor from the "thick disk" of old stars that surrounds the plane of the Milky Way. The older thick disk stars have a lower metal content; τ Cet's is about a third of the Sun's; this lower abundance also indicates that τ Cet is older than our Sun, with an estimated age of about 10 billion years.

Tau Ceti achieved fame in 1960, when Frank Drake initiated "Project Ozma," the inaugural search for extraterrestrial intelligence (SETI). He examined two nearby sunlike stars, τ Cet and ε Eri, for indications of artificial radio signals. He found nothing, nor has anyone since.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]