AL THALIMAIN POSTERIOR (Iota Aquilae). Stars often come in pairs.
A great many, far too many to give here, are real double stars that
orbit each other, while other close pairings (Algedi, Alpha Capricorni, comes to mind) are
merely caused by two stars lying coincidentally along nearly the
same line of sight. Still others, often quite widely spaced, are
mythological duos from a variety of cultures, of which the two Al
Thalimain stars are a good example. Though both are in the ancient
Greek constellation Aquila, the Eagle,
the names come from the ancient Arabic tradition, and refer to "the
Ostriches," which are oddly paired even though eight degrees apart.
Here we distinguish them by calling the westerly one (the one ahead
in the direction of the sky's apparent rotation) Al Thalimain Prior, while the other
one (the focus of current attention) then becomes Al Thalimain
Posterior. Bayer more simply called them Lambda and Iota Aquilae. As if the
ancients somehow knew their natures, both are oddly class B stars.
But the coincidence stops there, as Lambda is on the cool side of
class B, a B9 dwarf, while Iota Aquilae is classed as a much warmer
B5 giant (the evolutionary
class clearly wrong, however: see below). With a reasonably well-
determined temperature of 14,020 Kelvin (the average of five
measures), Iota Aql shines from a distance of 307 light years (as
opposed to Lambda's much closer 125 light years) with the light of
445 Suns, from which we find a radius of 3.6
times solar. An ill-determined projected equatorial rotation speed
of 73 kilometers per second then yields a rotation period of less
than 2.5 days, rather typical of the class. Luminosity and
temperature combined with theory lead to a mass of 4.3 times that
of the Sun and clearly show that the star is not a giant, but
instead is a fairly young hydrogen-fusing dwarf with quite a bit of
time left on its 140 million-year journey to gianthood, when it
runs out of hydrogen fuel in its core. Somewhat less than a minute
of arc away is a 13th magnitude star sometimes listed as a companion, but the two are
separating at such a large rate that they are clearly just an
accidental line-up. From Al Thalimain Posterior (our Iota), the
Sun would appear considerably fainter than from Al Thalimain Prior
(Lambda), fading to an anonymous 10th magnitude a few degrees south
of Procyon, which itself would shrink to
a mere 8th magnitude and no longer merit any kind of proper name.
Written by Jim Kaler 11/07/08. Return to STARS.