ZETA SCL (Zeta Sculptoris). Dim Sculptor's (the Sculptor's Studio) chief claim to fame is that it holds the South Galactic Pole, which marks the vertical direction to the plane of the Galaxy. Its brightest star, Alpha Sculptoris, is but fourth magnitude. The constellation, however, contains a couple stellar surprises. Hot massive stars, those of class O and B, are largely confined to the Galactic plane and the Milky Way. It's then a bit unusual to see class B stars so far from the Milky circle. At class B7, Alpha is odd enough. But fifth magnitude (5.01) Zeta, a hotter class B4 "giant" (but see below) with a temperature of 15,300 Kelvin, outdoes it. At a distance of 503 light years (give or take just 19), the star shines with the light of 710 Suns, which with temperature gives it a radius of 3.8 times solar. Rotation velocity measures range from 11 to 47 kilometers per second, the former giving a rotation period of under 17 days, slow for the class, implying that the rotation axis is more or less pointed at us. Temperature, luminosity, and theory then yield a substantial mass of 4.8 Suns and reveal that the star is quite distinctly not a giant, but a hydrogen-fusing dwarf a good part of the way through its 100 million year dwarf lifetime. A 13th magnitude "companion" 3 seconds of arc away is, from its motion over just a couple years, clearly a line-of- sight coincidence, while a listing as a spectroscopic binary has no other support. More important, the star is in the direction of an obscure open cluster called Blanco-1, enough so that the loose assembly is commonly called "the Zeta Sculptoris Cluster." While Zeta Scl is frequenty taken as a cluster member, as we saw with the"companion," alignment does not a partner make. The cluster is 70 percent farther away than the star and, equally important, the motions across the sky do not agree, so in spite of the cluster's name, the star is not actually a cluster member! In another but more local alignment, Zeta Scl is a close "equinoctial colure" star, the colure a great circle in the sky that connects the equinoxes and the celestial rotation poles. In 1925, precession (the 26,000-year wobble in the Earth's axis) moved the colure across the star, giving Zeta a "right ascension" (the sky's version of terrestrial longitude) of exactly zero.
Written by Jim Kaler 12/30/11. Return to STARS.