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.