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.