MARSIC (Kappa Herculis), along with
8 HERCULIS. Marsic (we save 8 Her for later) is a confused, and
confusing, star. Start with the very name, which means "the elbow"
(of Hercules), one of five proper star
names in the Hero, which is a bit strange since Marsic is just
fifth magnitude (4.70). The name derives from the Arabic "al-
marfiq," which also gave us two Marfaks (Marfak-East and West)
in Cassiopeia and Marfik in neighboring Ophiuchus, the names almost interchangeable (our Marsic
commonly also called Marfak as well). We'll let different
spellings do the job. If nothing else, Marsic shows the dangers of
using proper names. It fools physically as well. At first
telescopic glance, Marsic appears as a pretty double star, the first measures
of it going back to 1703. The stars are remarkably similar. The
brighter, fifth magnitude (5.00) Kappa Her A, is a class G (G8)
giant that lies 27 seconds of arc away from the sixth magnitude
(6.25) class K (K1) giant Kappa Her B. The temperatures are thus
nearly the same, estimated at 4990 for A and 4650 for B. In spite
of that, the old observers saw the stars as different colors
(light-yellow and pale garnet; pale yellow and reddish yellow),
showing how proximity can mightily enhance even subtle temperature
and magnitude differences. The luminosities then come out to be
150 and 70 Suns (both helium-fusing giants),
radii 16 and 13 solar, masses 3 and 2.5 solar, rotation periods of
under 80 days for Kappa-A and unknown for Kappa-B, and ages of 400
and 700 million years. Something is wrong. Real double stars have
the same age. Not only that, the measured distances are different,
388 light years for Kappa A, 470 for Kappa B (on which the above
parameters are based). The distance for fainter Kappa B, however,
has a large associated error, typical of doubles, so it COULD be at
the same distance. But we already knew. Over the 300 years that
the star has been observed, the separation has shrunk from 57
seconds of arc to 27 seconds, far greater than would be expected.
This is no binary star, but a most remarkable line-of-sight
coincidence, something that has been suspected for at least 150
years. And the distance of Kappa B seems good, since it gives the
right absolute brightness for its class. They also have different
metal contents, Kappa A 30 percent low compared with the Sun, Kappa
B 30 percent high. Then we get fooled again, as Kappa-A really
DOES seem to be double with a 13th magnitude companion (Kappa C) a
minute of arc away that is probably a class K dwarf, the two
separated by at least 7500 Astronomical Units with a period of at
least 340,000 years. From Marsik A, Marsik C would glow as bright
as Saturn, whereas from B, C would be as bright as a full Moon. So
how then about 8 Herculis? Kappa (also 7 Herculis) is separated
from 8 Herculis (a class A0 dwarf at magnitude 6.1) by a mere 0.2
degrees, making it look like a real naked eye double. And at 367
light years, 8 Herculis has about the same distance as Kappa A, the
errors associated with the two overlapping. Could they be a real
pair? If so, they would be separated by at minimum 1.3 light
years, making such a pairing seem unlikely. Moreover, the angular
motions across the sky are different. Fooled again. Eight Her is,
however, interesting in its own right because of its very rapid
rotation. With an estimated temperature of 10,000 Kelvin, the
luminosity comes in at 44 solar, the radius at 2.2 solar, and the
mass at 2.5. An impressive projected equatorial spin speed of 259
kilometers per second leads to a rotation period of under 10.3
hours! If at the stated distances, Kappa A would shine in 8
Herculis's sky at magnitude -1.3, almost as bright as our Sirius.
Coincidences or no, this trio is well worth an appreciative look
with a telescope, if for no other reason than to see if you too can
see a color difference between quite-similar Marsic A and Marsic B.
Written by Jim Kaler 8/17/07. Return to STARS.