ZETA TUC (Zeta Tucanae). Most naked-eye stars are considerably brighter than the Sun, else they would not be very visible at all at their mostly great distances. But here and there is a smattering of nearby sunlike stars that are, if nothing else, at least faintly visible without optical aid. Putting a lie to their general faintness of course is Alpha Centauri, which reaches its status of third brightest star in the sky only because it is the closest of all to Earth. Among the top 150 brightest, there are no others. Included among their ranks are fifth and sixth magnitude 9 Ceti, Rho Coronae Borealis, 18 Scorpii (the solar clone), and the binaries 53 Aquarii, 16 Cygni, and Zeta Reticuli. A main criterion is that the spectral class be at least within half a class of solar G2, which all these are. While not as "good" as these, as a class F9 dwarf with a temperature of 6015 Kelvin (just over 200 Kelvin warmer than the Sun), Zeta Tucanae, in the far southern hemisphere Tucana (the Toucan) comes fairly close. Even from a nearby distance of 28.0 light years (known to a precision of less than a tenth of a light year), Zeta Tuc shines at but 4th magnitude (4.23), which still makes it apparently brighter than any in the above list, in part the result of its having a luminosity 24 percent greater than solar, which leads to a radius almost exactly like that of the Sun, a mass maybe 10 percent greater, and a vaguely similar age. Zeta Tuc's rotation speed seems to be zero, which implies that its axis is presented toward us. In spite of searches, no planet has ever been deduced, which is consistent with the somewhat depressed metal abundance of 70 percent solar (planet-holding stars tending to be metal-rich by solar standards). On the other hand, if the rotation (and thus the probable planetary orbital axis) is turned toward us, any planet would be undetectable through its orbiting gravitational influence on the star. Giving some "hope" (if that is indeed called for), infrared observations clearly reveal Zeta Tucanae to have a warm, at least minimal, debris/dust disk around it, which suggests some kind of collisions among orbiting bodies, much as we have with our own asteroid belt and Kuiper Belt of distant cometary bodies that hover outside the orbit of Neptune.
Written by Jim Kaler 8/27/10. Return to STARS.