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.