MU CYG (Mu Cygni). "A lovely double star" say Smythe and Chambers, writing to us from the nineteenth century. Discovered by William Herschel, currently separated by 1.7 seconds of arc, over a long period of time one can watch the two stars partially orbit each other. Made of fifth and sixth magnitude (4.75 and 6.18) F6 and G2 dwarfs (with a combined magnitude for the naked eye of 4.49), Mu Cyg is tucked away in far southeastern Cygnus far away from the Swan's classical figure, just over the border with Pegasus, 3.1 degrees north of Kappa Pegasi. The naming is a bit unusual, as the two carry separate Bright Star and Henry Draper catalogue numbers, Mu A and B (brighter and fainter) also respectively called Mu-1 and Mu-2 Cyg. Old-time astronomers appropriately called the brighter "white," while visual effects made the fainter seem "blue," which it isn't; if anything it's more yellow-white.

Mu Cyg The components of Mu Cygni, the "lovely double star," mutually orbit every 789 years, averaging a distance between them of 118 Astronomical Units, which does not mean all that much since the high eccentricity (0.66) takes them between 40 and 196 AU apart. While the two actually go around a common center of mass that lies between them, it's mathematically simpler to graph the fainter member, Mu-2 Cygni, as going around the brighter, Mu-1, which lies at the cross. The major axis of the true orbital ellipse, given by the dot-dash line, is offset from that of the observed ellipse because of the 76-degree orbital tilt and the orientation to the plane of the sky. The direction of motion is shown at the lower right corner. Note that the most recent modern observations (in blue) are much more accurate than the earlier ones obtained by traditional means. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

At a distance from us of 72.5 light years (give or take just 0.7), the two orbit with a period of 789 years at an average separation of 118 Astronomical Units. A rather high eccentricity carries them between 40 and 196 AU apart. While they were last closest in 1958, the orbital tilt (76 degrees) and orientation made them seem closest, just over half a second of arc apart, in 1937. Kepler's Laws give them a total mass (A plus B) of 2.64 times that of the Sun. The classes are not all that well known, opinions about Mu-1 (A) ranging from F4 to F7, Mu-2 (B) from F3 to G2 (F6 and G2 generally adopted). The temperatures are respectively 6325 and (from the sunlike G2 class) 5780 Kelvin. With little infrared or ultraviolet correction, the distance then yields luminosities of 4.7 and 1.4 times that of the Sun and radii of 1.8 and 1.2 times solar. A projected equatorial rotation speed for Mu-1 of 18 kilometers per second yields a rotation period under 5 days. Theory then suggests respective masses of 1.4 and 1.1 Suns, which add to 2.5 solar, in fine agreement with that found from the orbit. If you could travel there, from each the other would look like unresolved points hundreds of times brighter than the full Moon. So far as we know, however, you would find no planet to stand on. Hovering around the pair is a veritable cluster of stars that range from twelfth magnitude Mu Cyg C 77 or so seconds of arc away through fourteenth magnitude Mu Cyg G at 90 seconds. Alas, their rapid movements relative to the AB pair show them all to be line of sight coincidences. Oddly, the DE pair, 13 seconds of arc from each other, might possibly be a real couple. The seventh magnitude A5 primary ("D") should (from its expected absolute brightness) be about 260 light years away, which makes the thirteenth magnitude secondary ("E") a red dwarf. With a separation of at least 1000 AU and given that an A5 dwarf carries a mass of say 1.7 Suns, they would take at least 24,000 years to orbit each other. They are probably not related, but they do make a nice conclusion to the story of the AB pair, one of the nicer double stars that sky has to offer.
Written byJim Kaler 7/03/15. Return to STARS.