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.