ZETA AQR (Zeta Aquarii). Though Aquarius is not among the sky's brighter constellations, it has one of the most prominent and loved of all asterisms (the set of informal constellations, of the which the Big Dipper is the finest example), the quite-prominent Water Jar. At the Water Jar's center is Zeta Aquarii, which rather curiously carries no proper name, even though many others throughout the constellation have them. Zeta almost exactly rides the celestial equator, and is notably closer than the bright "equator star" Mintaka (Delta Orionis). Until 2003, the star was a member of the southern hemisphere. However, precession -- the 26,000-year wobble in the Earth's axis -- is bringing it to the north at a rate of 0.3 minutes of arc per year. Around November 21, 2003, it crossed the equator into the northern hemisphere. Physically, Zeta Aqr is a fine double star 103 light years away, the duo together shining at the bright end of fourth magnitude (3.7). Their current separation of only 1.7 seconds of arc make them a fine sight in the telescope, but also render their individual characteristics difficult to measure. The eastern star, Zeta-2 Aquarii, at magnitude 4.36, is slightly the brighter and thus the primary, allowing it to be called Zeta Aquarii A. Zeta-1 Aquarii (Zeta Aquarii B) comes in just a bit fainter, at magnitude 4.57 (just over the line to fifth magnitude). Both are class F, Zeta-2 an F3 dwarf, Zeta-1 and F2 subgiant (though its luminosity and temperature clearly indicate it to be a hydrogen-fusing dwarf). The result is a pair of white jewels that in the nineteenth century were called "white and very white." Temperatures of about 7000 Kelvin ensure that nearly all the stars' radiation falls in the optical spectrum, Zeta-2 12 times more luminous than the Sun, Zeta-2 15 times more luminous, from which we deduce masses of 1.8 times solar, Zeta-2 just the bit more massive. They orbit each other with an uncertain period of 760 years at an average separation of 140 Astronomical Units (3.5 times Pluto's average distance from the Sun) with a crudely known eccentricity that takes them from 210 AU apart to 95 AU. The orbit gives a total system mass of 5 times that of the Sun against 3.6 as derived from luminosity and temperature, which clearly shows the orbital characteristics to be somewhat in error. From each, the other would on the average shine with the light of some 70 full Moons.
Written by Jim Kaler. Return to STARS.