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
Written by Jim Kaler 5/22/09. Return to STARS.