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.