36 OPH (36 Ophiuchi). Because of their inherent faintness, class K dwarfs are not common in the naked-eye sky. The best-known is 61 Cygni, the first parallax star and the 13th closest star system, which is made of a pair of them. One-upping it is 36 Ophiuchi. In far southern Ophiuchus (the Serpent Bearer) just 3.5 degrees north of the border with Scorpius and 2 degrees southwest of Theta Oph, it's triple-K, all dwarfs. At a distance of 19.40 light years (give or take 0.05), fourth magnitude (4.32) 36 Oph consists of a double K star (from RECONS K1.5 and just K, with identical magnitudes of 5.12) now 5 seconds of arc apart, plus a distant sixth magnitude (6.32) K5 dwarf 732 seconds of arc (a fifth of a degree) away. Together make the 76th closest system, just after Eta Cassiopeiae.

36 Oph The apparent orbit of 36 Ophiuchi B about brighter 36 Oph A (at the cross) is tilted almost into the line of sight. The actual major axis of the true orbit (dash-dot line) is way down at the bottom, with the cross at the focus of the true orbital ellipse. Given a bit more tilt and the stars could eclipse each other once apiece during the 470-or-so year orbit. Adding to the odd effect is a high eccentricity that takes the two from as far as nearly 160 AU apart to as close as 7. They will make closest approach (at "periastron") around the year 2150. In reality of course the stars orbit a common center of mass, which here remains undefined for lack of sufficient data. The orbital direction of "B" about "A" is clockwise with north presented down as is traditional in visual observations (since telescopes invert the image). The first measurement, made a couple hundred years ago, is obviously terrible. The data then quite noticeably improve to the right as time proceeds. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

A partial orbit from observations made over the past two centuries show that the inner pair (36 Oph A and B) go around each other with a period of 471 years at a mean distance of 82.3 Astronomical Units, about twice Pluto's distance from the Sun, a very high eccentricity of 0.92 running them between 7 and 157 AU apart. They were last most distant from each other in 1913 and will be at their closest in 2148. Be sure to look. Tilted by only 10 degrees, the orbital plane is not far from the line of sight. Kepler's Laws give them a combined mass of 2.5 times that of the Sun, each then 1.25 solar masses, which is much too high for class K dwarfs, so the orbit clearly needs a longer baseline of observations. What's a couple more centuries anyway? With respective temperatures of 5135 and 5100 Kelvin, to account for a bit of infrared radiation, the A and B have luminosities a third that of the Sun and radii of three-quarters solar. Masses are estimated at 0.85 Suns, giving a sum of 1.7 Suns, much lower than the orbital value, as expected. An adjustment to a mean distance of 72 AU would take care of the discrepancy, though the period could well need adjusting too. Both stars seem to have coronae like the one that surrounds the Sun, with temperatures that run between three and four million Kelvin. Metal contents seem to be about 60 percent solar. The distant third member, 36 Oph C, with a temperature of maybe 4500 Kelvin, has a luminosity just 14 percent solar and a radius of 0.63 times that of the Sun. With an estimated mass of 0.71 Suns, it's at least 4400 AU distant from the inner pair and must take at least 180,000 years to orbit. Another, eighth magnitude 36 Oph D, is moving too fast and is clearly just a line of sight coincidence. But twelfth magnitude (12.3) 36 Ophiuchi E may well be real. If so it would be a cool M7 or so red dwarf with a mass of maybe 12 percent solar orbiting at least 230 AU away over a period of at least 2500 years. There is really not all that much unusual here. Of the 100 closest star systems, 26 are double, while 6 (including this one) are triple, and one each quadruple and quintuple, leaving 66 as single. Of the 145 closest stars, about half are in double or multiple systems. Three of the single stars have planets, and who knows how many more there may be. (Thanks to RECONS, the Research Consortium on Nearby Stars for stellar data and statistics.)
Written byJim Kaler 7/01/16. Return to STARS.