ASELLUS TERTIUS (Kappa Bootis). Just to the northeast of Alkaid (the last star of Big Dipper's handle) in far northwestern Bootes (the Herdsman) lies a small flat triangle of stars that represents Bootes' outstretched fingers, but that for unknown reasons Bayer called the Aselli, Latin for "donkeys." From west to east (going away from Alkaid) they are fourth magnitude Kappa, fifth magnitude Iota (which has a small unrelated star just to the southeast of it), and fourth magnitude Theta that have rarely-used proper names of Asellus Tertius, Secundus, and Primus, the Third, Second, and First Donkey. Together, they form a long thin triangle with another star to the south, one of stranger and least understood of stars, metal-deficient Lambda Boo, which gave its name to a whole class. These four are also together the ancient Arabs' "Al Aulad al Dhibah," the "Young of the Hyena," that animal represented by a trapezoid of stars in Bootes made of Beta Bootis (Nekkar), Gamma (Seginus), Delta, and Mu (Alkalurops). Other than their rather remarkable multiple cultural associations, the four stars are quite unrelated.

Writing from the nineteenth century, Smythe and Chambers call Kappa a "neat double star...pale white and bluish...a fine object." The components, currently 13.5 seconds of arc apart, are each bright and prominent enough to have separate names, the western fainter seventh magnitude (6.69) member called Kappa-1, the brighter eastern fifth magnitude (4.54) component Kappa-2. A distance of 155 light years places them at least 640 Astronomical Units apart, 16 times Pluto's distance from the Sun. The brighter, Kappa-2, is a class A (A8) subgiant, while the fainter is an F1 dwarf, the colors really both quite white, the old colors coming from visual effects caused by the stars' different brightnesses and close juxtaposition. Kappa-2 and Kappa-1 have respective temperatures of 7830 and 6835 Kelvin, luminosities 27 and 3.7 times that of the Sun, and radii of 2.8 and 1.4 times solar. Metal contents are just under solar. Projected equatorial rotation speeds of 123 and 40 kilometers per second lead to rotation periods under 1.2 and 1.7 days. The theory of stellar structure gives Kappa-2 a mass just double solar, and shows that it is indeed a subgiant ending its core-hydrogen-fusing life, one that is preparing to expand to become a red giant. Kappa-1, on the other hand, with 1.4 solar masses, still has a long life left to it. The masses, the lower limit to the separation, and Kepler's laws give an orbital period of at least 8700 years, which is consistent with a period of 6000 years estimated from a VERY partial orbit. But we need not stop there. This most interesting binary just keeps going. Kappa-2 is also variable, a "Delta Scuti Star" that vibrates by about 5 percent with at least one period of just 1.56 hours. Kappa-2, on the other hand, is (from its spectrum) apparently also binary, with a period of 4.9 years. Assuming the companion has a low mass, the average separation must be around 2.5 AU with a close approach of 1.2 AU. So at the end we have a triple star within a doubly-cultural triangle that sits within a cultural foursome that is related to another quartet, all these stars making quite the complex story.
Written by Jim Kaler 5/22/09. Return to STARS.