MARFIK (Lambda Ophiuchi). Double
stars are among the prettiest sights the sky has to offer. The
best ones, like Albireo and many others,
display striking color contrasts that are enhanced by visual
proximity effects. But even the white "colorless" ones are
attractive, especially if the two members are close together.
Those of Marfik (the "elbow" of Ophiuchus), or Lambda Ophiuchi, are so close that they
are, however, difficult to separate at the telescope, averaging
just a second of arc -- a 3600th of a degree -- apart. Do not
confuse with the star "Marfak," which is
the "elbow" in Cassiopeia, the two often
spelled the same. Together the two class A stars that belong to
Ophiuchus's Marfik shine at fourth magnitude (3.82) from a distance
of 166 light years, the brighter (magnitude 4.16) a class A1
hydrogen-fusing dwarf, the dimmer (5.22) a class A4 dwarf. Their
respective parameters are: temperatures of 9300 and 8500 Kelvin,
luminosities of 56 and 17 times that of the Sun, radii of 2.5 and 1.9 solar, masses -- from
the theory of stellar structure and evolution -- 2.6 and 2.0 solar.
So far, all is straightforward. But then comes the orbit. The
pair go around each other with a period of 129 years at an average
distance of 46 Astronomical Units (15 percent farther than Pluto is
from the Sun). A high orbital eccentricity takes them from as far
away as 68 AU to as close as 18 AU. They were closest together in
1939 and were farthest apart (best for viewing) in 2004. Orbital
theory then gives a total mass to the system of 6.00 times that of
the Sun, 30 percent greater than is predicted from structure
theory. The mass-sum from binary theory is very sensitive to
separation, an orbital size of 42 AU reconciling the mass
discrepancy. More intriguing, the system might be "hiding mass,"
that is, one of the pair may itself have a closer orbital companion
that is undetectable (as may both of them for that matter). What
IS detectable is a more distant 11th magnitude companion two
minutes of arc away that shares the motion of Marfik proper across
the sky. So far away that no orbital motion can be detected, the
little star (roughly class K6) must be at least 6100 AU away from
the inner pair and must take at least 185,000 years to go around.
From the K star, the bright pair would average the order of half a
degree apart, and would together shine with the light a couple full
Moons, whereas from the duo, the distant companion would appear as
an orange-colored Venus.