DENEB AL OKAB AUSTRALIS (Zeta Aquilae). Altair of Aquila, the Eagle, flanked by two outlying stars, Alshain and Tarazed, looks for all the world like a
bird with two outstretched wings. Yet the classical picture of the
constellation is quite different, as the trio is depicted in the
neck of the bird, whose tail feathers are stretched out to
encompass a pair of stars together known as "Deneb," the Arabic
word for "tail." But since the name is already in use for Deneb of Cygnus, a
longer part of the original phrase is needed, hence "Deneb al
Okab," the "Eagle's Tail." And since a pair of stars is involved,
the southern will informally be called "Deneb al Okab Australis"
(south). This southern one was given the appellation Zeta Aquilae
by Bayer, while the northern (
Deneb al Okab Borealis) was called
Epsilon, rather odd since Zeta, which comes after Epsilon in the Greek alphabet, is at dead-on third magnitude
(2.99) and much the brighter of the two, indeed it is the third
brightest in the whole constellation after Altair and Tarazed. In
one way it is just another white class A (A0, right at the head of
the class) hydrogen-fusing dwarf star, but like so many seemingly
ordinary stars, it has a secret. At a distance of 83 light years,
our Deneb Okab shines with the light of 39 Suns from a hot 9680 Kelvin surface, from which
we derive a radius 2.2 times that of the Sun and a mass of 2.4
solar. The star's big surprise is its rotation. One of the
fastest spinners known, and close to a record for a class A star,
Deneb Okab "Aust" turns at an equatorial speed of 330 kilometers
per second, 165 times that of the Sun, giving it a rotation period
of only 16 hours. The huge speed strongly suggests that the star's
pole is nearly perpendicular to the line of sight, which gives us
the full treatment of its rotation. Stars turn more slowly with
age, and consistently, Zeta Aql is also young, having changed
little from the temperature and luminosity given it at birth. Two
companions hover, one near one far, both faint M dwarfs (at least
so their dimness would suggest). Close in, at a separation of only
6 or so seconds of arc, the inner must be at least 125 Astronomical
units away from the Zeta proper, and take over 800 years for an
orbital revolution. The farther, if in fact it is a companion at
all and not just a line-of-sight coincidence, is at least 6000
Astronomical Units away and takes at least 250,000 years to make a
full turn. From the outer star, the brighter of the inner pair
would shine with the light of our full Moon about a degree from the
dimmer star, which would appear something like our Jupiter.