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MARFAK-WEST

(Mu Cassiopeiae). Much of the naming game seems to be location. "Marfak" more oddly applies not to just one, but to TWO stars that represent "the elbow," the western one fifth magnitude (5.17) Mu Cassiopeiae, the eastern one fourth magnitude (4.33) Theta Cas. They are not related, as Mu is only 25 light years away, whereas Theta is 137 light years distant. While Theta is a more or less ordinary class A7 dwarf, Mu is a star with a serious difference. That it is visually rather dim even though close to us reveals its inherent faintness. Indeed, this class G (G5) dwarf is significantly intrinsically less luminous than the Sun, one of the few naked eye stars in the sky to be able to make that claim. With a temperature of 5290 Kelvin (490 Kelvin cooler than the Sun), Marfak-West shines at us with a luminosity only 46 percent solar. More intriguing, the star is moving in angle across the sky at an exceedingly high pace of 3.78 seconds of arc per year (its "proper motion"), over a third that of the fastest known (much dimmer Barnard's Star of Ophiuchus). At Mu Cas's distance, that translates into an "across the line of sight" speed (the "transverse velocity") of a surprising 135 kilometers per second. Combine that with the line of sight speed (the "radial velocity") of 97 kilometers per second (approaching us), and you get a full velocity relative to the Sun of 167 kilometers per second, nearly 10 times the typical value. To that add an unusually low metal content, an iron abundance but 15 percent solar, and we see that Mu Cas is a classic "subdwarf" (which renders the spectrum "peculiar"), a visitor to our neighborhood from the Galaxy's ancient halo, which surrounds the disk that makes our Milky Way. Low metallicities drop the opacities of the stellar gases and make such stars look too dim for their classes, hence the "subdwarf" appellation (whereas in reality subdwarfs have classes too warm for their visual luminosities). In absolute terms, Mu is moving inward toward the center of the Galaxy at 42 km/s, out of the Galactic plane at 35 km/s, and is falling behind the Sun in its Galactic orbit by 61 km/s. Even better, Mu is not just a subdwarf, but a BINARY subdwarf. Orbiting it is a much dimmer class M (probably M5) 11th magnitude (11.5) dwarf that takes 21.75 years to orbit its brighter companion, the two averaging 7.6 Astronomical Units apart, a high eccentricity making them 3.3 AU apart at closest, 11.9 AU at farthest.

Mu Cas The two stars of Mu Cassiopeiae orbit each other on elliptical paths every 22 years. The small inner orbit is that of the bright primary star about the center of mass of the system, while the outer orbit is that of the faint secondary star focussed on the primary star. The numbers are years of observation; the stars were closest in 1998. (From an article in the Astrophysical Journal by J. Drummond, J. Christou, and R. Fugate.)

Orbital analysis shows Mu Cas A to have a mass of 0.74 times that of the Sun, with Mu Cas B weighing in at a miserable 0.17 solar. With a visual luminosity only 0.001 times that of the Sun, from Mu Cas A, Mu Cas B would shine with the light of but 40 or so full Moons. Several other dim stars flock within a few minutes of arc around Mu Cas, one of which (12th magnitude Mu Cas E) may be a real, though very distant, companion. (Thanks to Jack Drummond for suggesting this star.)