ZETA LUP (Zeta Lupi). Scorpius is such a dramatic figure that the constellation next door to the southwest, tucked under the Scorpion's curve, Lupus (the Wolf), is often bypassed, though it has a lot to recommend it. Led by second magnitude Kakkab (Alpha Lupi), the ancient pattern contains a number of modestly bright stars, the most southerly of which is Zeta Lupi. Anchoring Lupus's classic outline, Zeta Lup is visible only below a latitude of 38 or so degrees north latitude, On the faint end of third magnitude (3.41), Zeta is a class G (G8, also given as K0) giant that lies 117 light years away, the distance known through Hipparcos satellite observations to an accuracy better than one light year. It's also actually double with a seventh magnitude (6.74) class F (probably F8) dwarf companion 71 seconds of arc away. While little orbital motion is seen, the two have been keeping pace with each other for nearly two centuries, so the linkage is almost certainly real. Moreover, given Zeta Lup's distance, the absolute brightness of the companion is right for the class. With a temperature of 5025 Kelvin, the lead star, third magnitude Zeta Lup A, shines with the light of 59 Suns, which leads to a radius of 10.1 times solar, right on the mark from an earlier estimate of angular diameter. Theory then gives a mass of 2.5 (or a little under) Suns, with considerable sway as in this state of evolution, stars in a range of masses look a lot alike. Having given up core hydrogen fusion at an age of 585 or so million years, Zeta A is fusing helium into carbon and oxygen in its deep core. The seventh magnitude companion, Zeta B, is perhaps more intriguing, as it is not all that far from being something of a "Sun." With about double the solar luminosity, Zeta Lup B has a radius of just 1.3 times solar and a mass of 1.2 Suns, which shows the sensitivity of luminosity to mass. Given the angular separation and distance, Zeta A and B have to be at least 2600 Astronomical Units apart, which with the sum of masses suggests an orbital period of more than 68,000 years. From the companion, Zeta A would appear pointlike to the naked eye, shining with the light of three full Moons. At one time, a pair of stars a bit over three degrees to the north, Kappa-1 and Kappa-2 Lupi, also shared Zeta's motion, but the Kappas are now known to be considerably closer.
Written by Jim Kaler 8/10/12. Return to STARS.