AL DHIBAIN "POSTERIOR" (Zeta Draconis). The Zeta star of Draco (the celestial Dragon) has an
interesting variety of cultural relations, and for multiple reasons
is also something of an oddity in itself. The Arabic proper name,
Al Dhibain, refers not to one, but to a pair of physically
unrelated stars, third magnitude (3.17) Zeta and brighter (2.74)
Eta, the stars representing hyenas or wolves. Since Eta precedes
Zeta across the sky, we can distinguish them by calling Eta Al Dhibain Prior and Zeta Al Dhibain
Posterior. Zeta Dra also sits at the third of the twists or
"nodes" of the Dragon, and is hence "Nodus III," I, II, and IV
being Grumium (Xi Dra in the Dragon's
head), Altais (Delta Dra), and Theta Dra (or possibly Edasich, Iota). Third, Zeta Dra vies with
fourth magnitude Phi Draconis as the closest Greek-lettered star to
the north ecliptic pole (the northern pole of the orbit of the
Earth, the southern one in Doradus). But
since it is much the brighter of the two, it more deserves the name
"Polaris Eclipticus Borealis" (though at five degrees to the west
of the actual pole, it is not all that good a marker). It's
clearly best to stick with "Zeta Draconis," which is a hot class B
(B6) giant some 340 light
years away that is a bit far (though not all that much) from the
path of the Milky Way where the large
majority of hot, massive stars are found. With a temperature of
12,900 Kelvin, Zeta Dra shines with the power of 1150 Suns; together they yield a radius of 6.8 times
that of the Sun and a mass between 4.5 and 5 times solar.
Comparison to theoretical prediction shows the star to be right at
the end of its hydrogen fusing lifetime and thus technically more
of a subgiant than a giant. Most class B stars are fast rotators.
Zeta Dra, however, spins at a relatively slow equatorial rate of 43
kilometers per second (giving a rotation period of less than 8
days), suggesting that its rotation axis may be more or less
pointing at us and that we are not getting the full rotational
effect. Zeta also has a curiously low iron abundance, measured at
just a tenth that of the Sun, odd for such a young massive star,
one that seems to move through space all alone, no companion being
registered at all. (Thanks to David Bright.)
Written by Jim Kaler 9/05/08. Return to STARS.