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.
Written by Jim Kaler. Return to STARS.