XI OPH (Xi Ophiuchi). Amidst the sparkling luminous blue stars of the Milky Way, it comes almost as a surprise to find a modest star of the solar neighborhood, one relatively nearby. Moreover, in between the classical figures of Sagittarius and Scorpius, fourth magnitude (4.39) Xi Ophiuchi hardly looks like it belongs to more northerly Ophiuchus, the Serpent Bearer, but such is the state of constellation boundaries, which bring Ophiuchus south of the ecliptic and make it an unofficial "thirteenth constellation of the Zodiac." Indeed, Xi Oph itself is just a couple degrees north of the solar path. Only 56.6 light years away (give or take a mere 0.3), Xi Oph is a fast mover, zipping along against the distant background at a rate of a third of a second of arc per year (the result of proximity, not speed). Originally called a class F (F1) giant/subgiant, its later listing as an F2 hydrogen-fusing dwarf (which we adopt here) seems more appropriate, but problems remain as a well-determined temperature of 6723 Kelvin is high and better fits the F1 class. Distance and apparent brightness tell of a luminosity 4.1 times that of the Sun, which with temperature gives a radius 1.5 times solar and from theory a mass of 1.35 to 1.4 Suns. Moreover, we see that the star is truly a fairly young dwarf not far from having just started its six-billion-year hydrogen-fusing lifetime. Rotation-velocity estimates are in gross disagreement, and go from nothing to 20 kilometers per second, the latter giving a rotation period of under four days, the star probably seen more or less pole-on. The iron content is about two-thirds solar, the rest of the heavier elements following suit. But out of this seeming ordinariness rises a nice surprise, a ninth magnitude (8.9) companion. If real, from its absolute brightness, it has to be a K (K7) dwarf with a mass of around 0.6 Suns. And it probably really does belong to the brighter star. Over a 57 year interval, the separation between Xi Oph A and B went from 2.5 to 4.4 seconds of arc, whereas over that same interval "A" itself moved by some 19 seconds, strongly suggesting that the stars are moving through space together. From distance, the two have to be at least 17 Astronomical Units apart, which from Kepler's Laws gives an orbital period of at least 48 years, which is consistent with the observed change in separation. No orbit has yet been calculated, but it may not be long (at least on an "astronomical time scale") in coming, leading to a check on the stellar masses.
Written by Jim Kaler 9/9/11. Return to STARS.