ARRAKIS (Mu Draconis). Though at fifth magnitude (4.92) often left out of star charts, from old-time lore Arrakis still serves a role in the constellation Draco as the tip of the Dragon's tongue. And it's not hard to find either as, if you curve north a bit, fairly bright Eltanin and Rastaban (Gamma and Beta Dra) point westerly at it. The name, however, from old Arabia (expressed also as Errakis as derived from the Arabic Al Rakis) refers instead to a "trotting camel" (though Allen suggests a "dancer"). The star is far better known by Bayer's Greek letter name of Mu Draconis. As dim as it is, Mu Dra reaches fifth magnitude only because it is a double that consists of two remarkably similar sixth magnitude stars, Mu-A and Mu-B both class F7 hydrogen-fusing dwarfs at respective magnitudes of 5.66 and 5.69 and separated by only a couple seconds of arc. In the 1880's Smythe and Chambers refer to them as "a very neat binary star," "both white," and reminiscent of Castor but "too nearly equal to bear out the resemblance accurately." Perhaps Porrima (Gamma Virginis) would be a better match. With similar temperatures of 6300 Kelvin, they each shine at us from a distance of 90 light years with luminosities 3.3 times that of the Sun, which leads to similar radii of 1.5 solar and masses of 1.3 times that of the Sun. Both are also confirmed as dwarfs that are roughly halfway through their 4.2-billion-year hydrogen-fusing lifetime. About the only significant difference between the two is that "A" spins with a projected equatorial velocity of 13 kilometers per second (for a rotation period under 6 days), while the numbers for B are 23 km/s and 3.3 days, the pair not in tidal synchrony, which is not surprising given the orbital size of 109 Astronomical Units and a period of 672 years (remarkably close to the 19th century estimate of 600 years). The rotation does, however, help generate magnetic activity and X-rays from and outer thin coronae with temperatures of some 3 million Kelvin, a bit above the solar value. A high orbital eccentricity brings the pair as close as 62 AU and as far apart as 156, the last close approach taking place in 1949.
Mu Dra Slightly fainter Mu Draconis B appears here to orbit Mu-A (at the cross), though in reality the two orbit a common center of mass halfway between them every 672 years at an average separation of 109 Astronomical Units, the high eccentricity causing the distance to change by a factor of 2.5. A tilt to the plane of the sky of 35 degrees distorts the orbital shape. The nearly identical white stars make a fine sight in a small telescope. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
Kepler's Laws then give a combined mass of 2.9 times that of the Sun, satisfyingly close to the evolutionary estimate from luminosity and temperature of 2.6 solar. Shrinking the orbital size by just four percent brings perfect agreement. In the 1940's, possible jiggles in Mu-B's motion suggested a small orbiting companion, Mu Dra Bb, but the orbital size and period of 0.7 AU and 3.2 years are hopelessly inconsistent with stellar-sized masses. There is some hope, however, for 14th magnitude Mu Draconis C, which resides some 13 seconds of arc away from the inner pair. If a real third member of the system, it would be a class M4 dwarf with an orbital size of at least 360 AU and a period greater than 4000 years. If that close, however, the system would probably be gravitationally unstable, leading to the little one's chaotic ejection. It may well just be a line of sight coincidence. (Thanks to Bill Hartkopf for commentary and to Steven Raine for suggesting the star.)
Written by Jim Kaler 8/06/10. Return to STARS.