70 OPH (70 Ophiuchi). Look to the northeasternpart of Ophiuchus between its main pentagonal figure and Serpens Cauda (the tail of the Serpent that wraps Ophiuchus) to find a "vee" shaped set of stars that reminds one of the "vee" of the Hyades, which makes the head of Taurus the Bull. In parallel, in the latter eighteenth century Ophiuchus's "vee" was called " Poniatowski's Bull" in honor of a King of Poland. On the east side of this asterism -- which never made it as a formal constellation -- is fourth magnitude (4.03) Flamsteed 70, a wonderful double star that begs for close examination. Only 16.6 light years away (the 51st closest star system), 70 Oph is so nearby that its stars are easily separable with a small telescope, which allows one to see nearly a full orbit over a human lifetime, the pair swinging from two to seven seconds of arc apart and back over a period of 88.4 years. The binary is also unusual in that it consists of two low mass cool yellow-orange class K dwarfs, a rarity, since most naked eye stars are hotter and more luminous than the Sun. Contrast effects make the two quite the colorful pair, nineteenth-century Admiral Smythe calling them "pale topaz and violet." The primary (70 Oph A) is a class K0 star with a magnitude of 4.22, A temperature of 5290 Kelvin, a luminosity 0.51 that of the Sun, and a radius of 0.85 solar. The respective parameters for the fainter secondary star (70 Oph B) are magnitude 6.01, 4250 Kelvin, 0.16 solar luminosities, and 0.70 solar radii. (A more typical temperature of 4650 K drops the luminosity a bit as it admits less infrared radiation). The beauty of binaries lies not so much in their aesthetic aspects, but in their use in determining stellar masses through Kepler's Laws. The average separation between the
The binary orbit: 70 Ophiuchi B is seen in elliptical orbit about 70 Ophiuchi A (though in reality, each orbits a common center of mass). Since the orbit is tilted by 59 degrees to the plane of the sky, the true major axis of the ellipse (the dashed red line) does not seem to line up with axix of the projected ellipse. (Yerkes Observatory, used by permission, as rendered in Astronomy, J. B. Kaler, HarperCollins, 1994.)
stars (the semimajor axis of the elliptical orbit of B about A) is just 23.3 Astronomical Units. A fair degree of orbital eccentricity makes them as close as 11.6 AU and as distant as 34.8 AU. The last close approach was in 1984, the next greatest separation will be in 2028. From separation and period, we find a combined mass of 1.60 solar masses. From the way that each star affects the other (the location of the center of mass), we find a mass for 70 Oph A of 0.89 solar and for 70 Oph B a mass of 0.71 solar. Data such as these allow the construction of a "mass- luminosity relation" for hydrogen-fusing dwarfs that is a prime test of the theory of stellar structure. 70 Oph A is, like the Sun, magnetically active, and has a surrounding hot corona. Active regions rotating in and out of view allow the determination of a rotation period of 19.7 days. This remarkable double star is near its closest approach to the Sun, the distance shortening to 15 light years 75,000 years from now.
Written by Jim Kaler. Return to STARS.