Tau Ceti

Tau Ceti

Also known as Durre Menthor, 52 Ceti, HD 10700, HR 509, BD-16º295, GCTP 365.00, GJ 71,

LHS 146, LTT 935, LFT 159, SAO 147986, LPM 84, FK5 59, HIP 8102

     With Tau Ceti, we start on the last lap of our journey about the stellar neighborhood, entering the realm of naked eye stars. The last four stops on our year-long tour are all readily observable without any optical aid whatsoever. Although not the result of any intention upon my part, there should be no cause for surprise here. For we began our observational odyssey in springtime, as the nocturnal hemisphere of the Earth began to swing round towards the galactic center. Recall from the introductory postings, when our position at the inside margins of the Orion Spur was described. As we gaze towards the great star clouds of the summer Milky Way, we are looking across the relatively unpopulated gap between our spur and the next spiral arm inward. So the light from even the nearest bright stars in the Sagittarius Arm must first traverse thousands of light years to get to us. Conversely, as the Earth moves to the opposite side of its orbit and the nighttime hemisphere in winter now faces outward, away from the core of the galaxy, we find ourselves face to face with the massed splendor of the Orion Spur, right on top of us so to speak.

     So where we began our exploration of nearby stars with the ridiculously faint magnitude 14.78 red dwarf DX Cancri, we will end it with the brightest star in the entire sky – the magnitude -1.46 Sirius (only the Moon, Jupiter, Venus, and the Sun itself are brighter). Rather appropriate bookends.

     Meanwhile, as for Tau Ceti, here we have a star that, although it shares many similarities with our own Sun, the differences ultimately overshadow the likenesses. First of all, it is significantly smaller than the Sun. It weighs in at about two thirds solar mass, shines at only slightly more than one half solar luminosity, with a diameter 20% smaller and a surface temperature 400º cooler than the Sun. Together these characteristics define Tau Ceti as a Class G8V dwarf star (the Sun’s class is G2V). It rotates once every 34 days, and appears to be almost completely quiescent. A sunspot would be a rare event indeed on the surface of Tau Ceti. There is little or no magnetic activity in its chromosphere, and the star has no confirmed sunspot cycle. Its metallicity is about one third solar, which means Tau Ceti is significantly older than the Sun. It was once thought to be more than 10 billion years old, but the general consensus today is for an age of approximately 5.8 billion years (as opposed to 4.57 for the Sun).

     Tau Ceti is a permanent resident of the Milky Way’s Thin Disk, its orbit never taking it far from the galactic equator. It circles the galaxy at a mean distance of 32,000 ly with an orbital eccentricity of 0.22.

     So much for the star itself. Of perhaps even more interest is what has been learned about its immediate surroundings. Tau Ceti is encircled by a gigantic belt of cold material, almost certainly composed of dust, meteoroids, asteroids, comets, and plutoids extending from about 10 Astronomical Units (AU), roughly equivalent to the orbit of Saturn, to 55 AU (or to the distance we believe outer edge of the Kuiper Belt to be) from the star. The total mass of this belt has been estimated to exceed 10 times the mass of all non-planetary material in our own solar system combined.

     Due to its proximity as well as to its superficial similarity to the Sun, Tau Ceti has been the object of intense scrutiny looking for any signs of exoplanets. So far, none have been discovered. It is impossible from our vantage point to use the eclipse method for detection, since we happen to be looking practically straight down onto one of the star’s poles. Any planets would thus from our perspective be circling around the stellar disk, and would never pass in front of it to dim the star’s light. Radial velocity searches have been conducted of an accuracy that would have surely detected any Jupiter-sized masses, but none were found. The Hubble Space Telescope’s Wide Field and Planetary Camera was also employed in an attempt to spot any faint companions, but with negative results. Tau Ceti’s low metallicity normally would not bode well for the presence of small rocky Earthlike bodies, but this notion is at least partially contradicted by the confirmed existence of the cold belt. Also, the inner cutoff of the circumstellar disk at 10 AU strongly argues for there being an undetected “shepherd” body which confines the material outward from that distance.

     In any case, as pointed out by British Astronomer Jane Greaves, co-discoverer of the cold belt surrounding Tau Ceti, even if there were an Earthlike planet about the star, it would likely be subject to “constant bombardment from asteroids of the kind believed to have wiped out the dinosaurs.” Yet again another disappointing result for those eager to find places conducive to the existence of extraterrestrial life (not to mention intelligence).