ZETA BOO (Zeta Bootis). A rather dim star to the southeast of brilliant Arcturus, one that is hardly ever paid attention, harbors a quite-marvelous secret: just one, but that is all it needs. The modest Zeta Star -- no proper name at all -- of Bootes, the celestial Herdsman, is a double. That in itself is hardly unusual. Zeta Boo shines at fourth magnitude (3.78) from a distance of 180 light years. The individuals -- Zeta Boo A about a second of arc to the east of fainter Zeta Boo B -- are usually called class A (A2) giants, and come in at nearly identical magnitudes of 4.52 and 4.55. So far again nothing really unusual. The distance and a slight correction for ultraviolet radiation from 8750 Kelvin surfaces give luminosities of 38 times that of the Sun. The combination of luminosity and temperature show clearly that the stars are still hydrogen-fusing (though advanced and bright) dwarfs, which is where we place them here. But arguments about such details are still nothing unusual. Though of long period, the orbit of the pair about each other has been mapped. The stars average 33 Astronomical Units apart, and go about each other with a period of 123 years. From Kepler's laws (via Newton) we calculate a combined mass of 2.3 solar. The luminosity and temperature, however, in conjunction with theory, suggests a combined mass of double that, which simply reveals minor errors in the orbit. At the end, the surprise is the orbital eccentricity. The stars loop about each other on hugely elongated orbits, which carries them from (from the computed orbit) 64 Astronomical Units apart (half again Pluto's distance from the Sun) to a mere 1.4, close to Earth's distance, which is close to a record for such ellipticity. The double thus is separable by telescope during farthest separation, and quite inseparable at close passage, which occurred in 1897 and which will take place again in 2021. Your best view will come in 2082. With a stellar invasion like this one, no planets would be at all possible. The large eccentricity suggests some kind of violent encounter with another star. Perhaps a third member was lost in the process. We will never know.
Written by Jim Kaler. Return to STARS.