KAPPA CEN (Kappa Centauri). Centaurus,
the Centaur, blends smoothly into the stars of Lupus, the Wolf, the two constellations filled with bright, prominent
stars. The two are commented through mythology, as in one story
Centaurus sacrifices Lupus upon Ara, the
Altar. The main stellar connection between them is made by notable
third magnitude (2.97) Kappa Centauri and, about a degree to the
south, somewhat brighter Beta Lupi.
Though perhaps looking paired, they are not, Beta Lup more than 500
light years away, Kappa Cen much closer at 383 light years (give or
take 28). Kappa Cen is, however, a member of the Upper Centaurus-
Lupus association of hot stars
that is centered at a distance of 460 light years, showing how
spread out these associations are. Beta Lupi could be a member as well,
thus reconnecting them. A close look, then a very close
look, reveals more relations, that Kappa Cen is actually a triple
star. Visually, the bright star, Kappa Cen A, a hot blue class B
(B2) subgiant, is paired with
11th magnitude Kappa B. Lying four seconds of arc away, from its
brightness Kappa B is a class K2 dwarf. Kappa Cen A is then split
with sophisticated instrumentation into two just a tenth of a
second of arc apart, and is made of the hot B star (now Kappa Cen
Aa, of magnitude 3.4), and Kappa Ab, a 4.7 magnitude star that is
probably a class A0 dwarf. Aa and Ab are difficult to separate, to
say the least. With respective temperatures of 19,845 Kelvin, and
(from the class) 9500 K, after about a 15 percent correction for
dimming by interstellar dust, Aa and Ab have luminosities somewhere
around 3500 and 190 times that of the Sun,
which lead to radii of 5.0 and 5.1 solar. With a slow projected
equatorial rotation speed of 20 kilometers per second, the rotation
period of Aa is under 13 days (the rotation probably more or less
pole-on). Theory then allows masses of 7.5 and 3.0 Suns. It also
shows Aa to be not a subgiant (a star that has given up core
hydrogen fusion, or is about to), but a dwarf about two-thirds of
the way through its lifetime of 33 million years, such divergences
in classification not uncommon. However, Ab then seems to be a
subgiant, which cannot be, as lower mass dwarfs take longer to burn
out than do those of higher mass. Moreover, Aa should be larger
than Ab, not smaller, both of these anomalies showing that one or
more of the input parameters is wrong. We can probably rely on
those of Aa more than those of Ab. From distance and separation,
Ab is separated from Aa by about a dozen Astronomical Units, and assuming
the masses to be correct, take a dozen years to orbit each other.
Out in the distance, Kappa Cen B lies at least 470 Astronomical
Units away from the inner pair. Adopting 0.75 solar for Kappa Cen
B, the little class K star must take at least 3000 years to orbit
the inner pair. From its perspective, Aa and Ab would get as far
as a bit over a degree apart. Not big enough to explode as a supernova, Kappa Cen Aa will
die as a fairly massive white
dwarf rather like Sirius B.
Written by Jim Kaler 6/29/12. Return to STARS.