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.