XI SCO (Xi Scorpii). In the 1930s modern boundaries were drawn around the 88 accepted constellations such that they included as many of the Bayer Greek-lettered and Flamsteed-numbered stars as possible. As a result, some of the formal outlines look mighty peculiar. Scorpius, for example, has a long thin panhandle only five degrees wide sandwiched between Libra and Ophiuchus that rises from the south near the ecliptic to nearly 12 degrees south of the celestial equator. Within it we find three of Bayer's stars, Phi, Chi, and the brightest, fourth magnitude (4.19) Xi Sco. The star is a bit of a bouncing ball. Ptolemy put in Libra, while Bayer moved it to Scorpius as Xi Sco. Flamsteed then put it back into Libra as Xi Librae. Because it was number 51 on his list in that constellation, it also became known as 51 Librae. But as the star was in more modern times flipped back into Scorpius, "51 Lib" is never used. It's at least triple, and may be sextuple, or something in between. Writing in the 19th century, Smyth and Chambers tell us it's "a fine triple star...A 4 1/2, bright white; B 5, pale yellow, C 7 1/2. grey," the first measures of separation made by William Herschel in 1780-1782. The multiplicity disallowed a Hipparcos parallax, so the distance of 91 light years is only roughly known. However, 4.6 minutes of arc away is ANOTHER triple that seems to be co-moving with Xi, which does have a Hipparcos parallax that gives a supportive 82 light years. It seems fairly safe to stick with 90 l-y. Xi Sco A and B, magnitudes 5.16 and 4.87 ("B" oddly the brighter), are mid- class F stars (a somewhat uncertain F4 and F5, both hydrogen-fusing dwarfs) a mere second or so of arc apart. With estimated temperatures of 6600 and 6530 Kelvin, we find respective luminosities of 5.2 and 6.9 times that of the Sun, which gives similar masses of 1.5 Suns. Both again seem to be evolutionary dwarfs. Seventh magnitude (7.3) Xi C may be on the warm side of class G (G1 to G4), and with a luminosity near 80 percent solar carries roughly a solar mass.
Lambda Lup Xi Scorpii A is seen to go about Xi Sco B (or B around A: it's not clear) with a period just short of 46 years in a rather highly elliptical orbit in which they average 18 Astronomical Units apart, about the distance Uranus is from the Sun. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
We can check the numbers by analyzing the stellar orbits. Xi A and B go around each other with a period of 45.9 years at an average separation of 18.2 Astronomical Units, a rather large eccentricity carrying them between 32 and 5 AU apart. They were last closest in 1997. Kepler's third law then gives a combined mass of 2.9 times that of the Sun, very close to the evolutionary values. Xi Sco C, now 7.0 seconds of arc from the AB pair, has gone far enough around to have had an orbit fitted to the data. It takes the outer star 1500 years to go around the inner double in a more circular path at an average separation of 215 AU. Now Kepler's laws give a combined ABC mass of 4.3 solar. Subtracting the AB mass from the ABC mass then allows a mass for "C" of 1.4 Suns, too high but not bad given the uncertainties of the big orbit.

Then next door, too faint to have a Greek-lettered name, is the other apparent triple. It's rather similar, with seventh and eighth magnitude A and B (G8 and K3) 12 seconds of arc apart, eleventh magnitude AC separated by 83 seconds. From its motion "C" is probably a line of sight coincidence, which is hardly surprising given the density of the stars in Scorpius. Stranger still, Xi Sco is also listed as the "D" companion to the fainter triple. On the whole it looks like we may have a quintuple, with the fainter set at least 7400 AU away from Xi, taking at least a quarter of a million years to make the journey all the way around. It's rather hard to see how they can stick together. (The discussion of the star's name is from Morton Wagman's "Lost Stars," McDonald and Woodward, 2003.)
Written byJim Kaler 8/12/16. Return to STARS.