MU ORI (Mu Orionis). Look just northeast of Betelgeuse in Orion to find fourth magnitude (4.12) Mu Orionis, the star part of the Hunter's upraised right arm (he's drawn facing you). A history spanning three-fourths of a century revealed the star to be double, then triple, then finally quadruple, a "double-double" in the mold of Mizar and Epsilon Lyrae, but all closer together and more like Castor. Mu Ori A, the more massive pair, orbits Mu Ori B with a period of 18.644 years, while the "A" pair (Aa and Ab) orbit in 4.447 days and the "B" pair (Ba and Bb) in a nearly identical 4.784 days. Don't try to separate A and B (apparent magnitudes 4.31 and 6.16) through the telescope, though, as at best they are only a few tenths of a second of arc apart, the star analyzed through motions of absorptions in its spectrum and through sophisticated interferometry.
Mu Ori The elliptical path shows the orbit of Mu Orionis B relative to Mu Ori A (indicated by the "plus sign" toward the top), that is, as if "A" were actually at the orbital focus. In reality both stars orbit a common center of mass. The scale is in seconds of arc. The orbit is seen as projected against the sky. The dot-dash line is the major axis of the true, unprojected ellipse. Not only is the true path quite elliptical, but the orbit is also presented very close to edge-on, making the apparent path even more eccentric. The observations connected to the orbit by dotted lines were not used in the calculations. The two stars, each of which is again double, take 18.644 years to make a complete circuit of each the, and were last closest together in mid-2003. (From F. C. Fekel, C. C. Scarfe, D. J. Barlow, W. I. Hartkopf, B. D. Mason, and H. A. McAlister in the Astronomical Journal, vol. 123, p. 1723, 2002.)
Unlike Mizar, Eps Lyr, and Castor, the light from Mu Ori is dominated by Mu Aa. While given as a class A (A2) dwarf, the class is deceptive, as Mu Ori Aa is a "metallic line" star, one whose spectrum is enhanced in various metals and depleted in others as a result of atmospheric diffusion, some chemical elements rising under the action of radiation pressure, others sinking under the force of gravity. As a result, the class is given as too "hot," A5 being more descriptive of the approximate 8350 Kelvin temperature. Full analysis, which includes that of the orbits of A and B around each other and of the Ba and Bb pair (that of Aa and Ab so far defying accurate study) gives a distance of 155 light years (very close to the 152 light years derived from parallax) and also the characteristics of all but "Ab." All four are hydrogen-fusing dwarfs. Mu Ori A and B (the two doubles) are on the average separated by 12.7 Astronomical Units (33 percent farther than Saturn's distance from the Sun). The average does not mean much, though, as a very high eccentricity runs them between 3.3 to 22 AU apart. Mu Ori Aa, the dominant partner (still magnitude 4.31 as Ab is so faint), carries a mass of 2.1 times that of the Sun, shines with the light of 32 Suns, has as radius of 2.9 solar, and like all metallic line stars is a relatively slow rotator with a period of 13 days (fast rotation stirring up the atoms and preventing diffusion). Mu Ori Ab is probably a class G (or lesser) star with a mass perhaps equal to that of the Sun, making the two a mere 0.077 AU apart (20 percent Mercury's distance from the Sun), the orbit near circular. The Mu B pair are class F5 seventh magnitude (6.91) twins with masses of 1.4 solar, luminosities of three solar, and radii 1.3 solar, separated by 0.078 AU (again in circular orbit), almost exactly the same as Aa and Ab. While Mu Ori seems to be on its way to becoming a quadruple white dwarf system, the eventual evolution to giant states (particularly of the dominant Aa member) will probably mess things up, good evidence being that in spite of the number of double-doubles in the sky, no such quadruple has ever been found. (Most of this discussion is based on the work of F. C. Fekel et al., the reference given in the above illustration.)
Written by Jim Kaler. Return to STARS.