XI BOO (Xi Bootis). Rare is the naked-eye star that has a luminosity and mass less than that of the Sun. Fifth magnitude (at 4.6 just barely) Xi Bootis offers not just one, but two subsolar stars, presenting itself to us as a fine, easily-viewed double star, the two components lying anywhere from 2.5 to 7 seconds of arc apart, depending on where they are on their 152-year mutual orbit.
Xi Bootis Xi Bootis B goes around Xi Boo A (the brighter and more massive of the two, placed at the cross) in the clockwise direction with an orbital period of 151.6 years at an average separation of 33.5 Astronomical Units. In reality, both go around each other. North is down, and the scale around the edges is in seconds of arc. The dot-dash line is the orbit's major axis. The orbital plane is tilted to the line of sight by 41 degrees, so that Xi Boo A does not appear at the focus of the orbit (where it actually is). Binaries like this one are critical in the determination of stellar masses. (From W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, U. S, Naval Observatory.)
Though not within Bootes' classical outline, the star is easily found about nine degrees due east of Arcturus. Both members of the binary are hydrogen-fusing dwarfs. The brighter (Xi Boo A) is a fifth magnitude (4.76) class G8 star with a temperature of 5550 Kelvin, while the fainter (Xi B) is a seventh magnitude (6.78) cooler (4600 Kelvin) class K4 star. Contrast effects make them more colorful than they really are, Xi Boo A given as yellow, B as "reddish- violet." Smythe called them orange and purple. Were it not for their proximity to us, the pair would never have made naked-eye status. One of the closest star-systems to Earth, it lies a mere 22.1 light years away, and was only recently bumped from the closest-100 star-system list by discoveries of closer very faint red dwarfs. Xi Boo A and B take just over a century and a half to orbit about each other at a mean separation of 33.5 Astronomical Units, just a bit farther than Neptune is from the Sun. A rather high eccentricity takes them between 50.5 and 16.4 AU apart. They were last closest together in 1909, and will not be again until 2054. Application of Kepler's laws give a combined mass of just 1.6 times that of the Sun, the individual masses coming in at 0.9 and 0.7 solar. Distance and temperature give respective luminosities of 0.5 and 0.1 times that of the Sun, Xi Boo B being underluminous for its mass. Radii then come in at 0.75 and 0.5 solar. Both are, like the Sun, magnetically active, the effect dominated by Xi Boo A, which seems to have an activity period of two to four years with an outer corona estimated to be at a temperature of more than 10 million Kelvin. A longer cycle may take 25-30 years. Other variations give a rotation period of 6.4 days, a quarter that of the Sun. Too bad the activity is not being seen up close. A low metal content, three quarters that of the Sun, coupled with the eccentric orbit, would seem to preclude any planets (planet-holding stars tending to be metal-rich), and indeed, none has ever been found or even indicated. An old study suggested the gravitational effect of a third low-mass companion, but it has never been confirmed. Several other fainter stars hover nearby, but these are all just in the line of sight.
Written by Jim Kaler 8/22/08. Return to STARS.