KAPPA DEL (Kappa Delphini). Kappa Delphini, in Delphinus (the Dolphin), a couple degrees
southwest of Epsilon (the fingertip
of the hand that appears to point south), at first appears to be a
bit of a mess. The Bright Star Catalogue lists Kappa Del as a fifth
magnitude (5.05) star with a combined spectral class, a G5 subgiant (more recently given
as G1) plus a K2 subgiant, giving us some semblance of solar types
to look at. Or perhaps not. The dimmer K2 star, Kappa B, was last
measured 45 seconds of arc from Kappa Del A. But it's moving so fast
relative to Kappa A that it's clearly an "optical" line of sight
coincidence and does not belong to Kappa A at all. Much farther (212
seconds, 3.5 minutes of arc) is ninth magnitude (8.62) Kappa Del C,
which for 160 years has been keeping a fine pace with Kappa A and
clearly DOES belong to it. Its spectral class, however, is not
known. Given an accurate distance of Kappa A (and thus C) of 98.2
light years (the uncertainty but 0.8), it should be a K2 or so dwarf, both B and C then
confusingly sharing part of a spectral class. Given a recent
magnitude of 5.15 for Kappa A, respective temperatures for Kappa A
and C of 5675 (measured) and 5000 (estimated) Kelvin, we get
respective luminosities of 6.85 and 0.34 times that of the Sun and radii of 2.7 and 0.8 times solar. The
theory of stellar structure and evolution then indeed shows Kappa
A to be a subgiant 50 percent more massive than the Sun, one that
after a hydrogen-fusing lifetime of 2.7 billion years is ready to
brighten as a true giant,
following which it will slough off its outer layers to become a white dwarf. The companion, Kappa C, comes in at
0.8 solar masses, theory showing it not to be a subgiant as is Kappa
"B," but a common low mass hydrogen-fusing dwarf. If that is not
enough, subtle wobbles in Kappa A's motion detected by the Hipparcos
parallax satellite suggest a much closer faint companion (more than
4 magnitudes dimmer than Kappa A) at a separation of half a second
of arc (corresponding to 16 AU), probably a white or red dwarf with
a mass of under 0.4 Suns that takes 45 years to orbit. We thus have
ourselves a triple star. Given
the huge separation between A and C, at least 6400 Astronomical Units,
some 200 times greater than Neptune is from the Sun, it's amazing
that they have stayed together and that Kappa C has not broken loose
of its gravitational bonds, leaving the inner two (if indeed there
ARE two) to themselves. If left undisturbed, Kappa C would have an
orbital period around Kappa A of more than 300,000 years. Such
"fragile" binaries are not all that uncommon. We have a more extreme
example in Proxima Centauri, the
nearest star, which orbits Alpha Cen (third brightest in the sky)
with a period of perhaps three-quarters of a million years. From
Kappa A, Kappa C would shine a magnitude or more brighter than our
Venus, while from Kappa C, Kappa A would
be brighter than a quarter Moon, Kappa
B entirely out of the picture as it sails away. (Thanks to Bill
Hartkopf for extensive discussion of this star.)
Written byJim Kaler 10/10/14. Return to STARS.