ZETA SGE (Zeta Sagittae). A quick glance overhead (or so) at the mid-northern hemisphere northern sky settles in on the three stars of the Summer Triangle of Vega, Deneb and Altair (the respective bright luminaries of Lyra, Cygnus, and more southerly Aquila). Oft-times ignored is a string of four smaller figures that run from Albireo (Cygnus's head and the foot of the Northern Cross) to the southeast, starting with dim modern Vulpecula (the Fox), then proceeding through the ancient figures of Sagitta (the Arrow), Delphinus (the Dolphin), and Equuleus (the Little Horse), which in turn leads us to western Pegasus of the Andromeda myth. There is no question why Sagitta was named after a celestial arrow, as indeed it looks like one. Our star, Zeta Sagittae, is a fifth magnitude (5.0) white class A (A3, though warmer classification seems popular) star that lies 255 light years (give or take 13) away. Easily found, it's less than a degree to the northeast of brighter Delta Sge at the Arrow's center. Though seemingly yet another of its kind, Zeta Sge stands out as a close double whose similar components lie just a couple tenths of a second of arc apart and that is orbited by a third fainter ninth magnitude (9.0) member, making it triple. The tight AB pair, with magnitudes of 5.6 and 6.0 (making the fainter secondary about one subclass cooler than Zeta A), orbit each other every 23.2 years at a nominal mean separation of 10.6 Astronomical Units with a large eccentricity that takes them between 19 and 2.2 AU apart. Kepler's Laws then give up a total mass of 2.2 times that of the Sun, clearly too low.
Zeta Sge Zeta Sge B goes around somewhat brighter Zeta A (at the cross) on a quite eccentric orbit that takes 23.24 years to complete at a calculated average separation of 10.6 (though probably larger) Astronomical Units. In reality, the two go about a center of mass that lies very nearly between them. The major axis (the dot-dash line) does not quite fit because of the tilt of the orbit to the plane of the sky. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
Approaching the masses through theory, using temperatures of 8800 and 8600 Kelvin, we get luminosities of 28 and 19 times that of the Sun, radii of 2.4 and 2.0 solar, and masses of 2.1 and 2.0 times that of the Sun, which at a sum of 4.4 solar nearly doubles the sum of masses found from the orbit, showing that continued orbital observation is necessary. The distance may be off, too. The two mass-sums can be reconciled by increasing the mean separation to 13 AU. A projected equatorial rotation speed of 225 kilometers per second applied to the brighter member of Zeta Sge gives a rotation period of under half a day. Off in the distance, 8 seconds of arc away, orbits the fainter star, which from its magnitude of 9.0 seems to be roughly of the solar variety. If so, it would be at least 700 AU from the inner AB pair and would take at least 8000 years to make a full turn. From Zeta Sge C, the inner AB pair would each shine on average at 50 or so full Moons separated by up to about a degree. It would be a spectacular sight, were anyone actually there, which seems more than unlikely. A more distant fainter 11th magnitude component (Zeta Sge D) 76 seconds of arc away is, from its motion, almost certainly a line of sight coincidence. That at least is more than can be said about the uncertainties that seem to plague the inner trio.
Written by Jim Kaler 8/03/12. Return to STARS.