BETA SCL (Beta Sculptoris). Just to the east of wedge-shaped Pisces Austrinus, about five degrees
southeast of its luminary, first magnitude Fomalhaut, lies Gamma Sculptoris of
Sculptor (the Sculptor's Studio). The star, appropriately third
brightest in the constellation, really
looks more like it belongs to the Southern Fish and making the
eastern point of the wedge. About the same angle farther to the
southeast is obscure Beta Scl, at fourth magnitude (4.37) the
number two star in Sculptor and just missing out on being the
luminary. That honor still, however, belongs to Alpha Scl, the top stars all in nice
order, as Delta is in fact number 4. A class B (but at B9.5, just
barely) subgiant (by classification only: see below), Beta
Sculptoris certainly deserves more respect than it gets, as it is
a mercury-manganese star with a strong magnetic field. A
precisely-known distance of 174 light years (give or take only 1!)
coupled with a temperature of 12,220 Kelvin (to allow for some
ultraviolet light) gives Beta Scl a luminosity of 82 times that of
the Sun. The temperature, though, is high,
more like that of a B8 dwarf or subgiant, such oddly-composed stars
difficult to classify. A radius of double that of the Sun together
with a projected rotation velocity of 29 kilometers per second give
it a rotation period less than 3.4 days. The theory of stellar
structure and evolution then yields a mass 3.1 times solar, and
reveals that the star, rather than being an ageing subgiant, is
really a very young dwarf just beginning its 355-million-year
hydrogen-fusing life, again showing the difficulty of
classification when the stellar chemistry is so odd. Mercury-
manganese stars (the brightest of which is Alpheratz, Alpha Andromedae) and their
various odd cousins are caused by chemical separation in relatively
quiet stellar atmospheres (unstirred by rapid rotation), wherein
some elements sink under the effects of gravity, while others are
lofted upward by absorption of stellar radiation. Relative to
hydrogen, manganese is up over the solar value by a factor of 115,
xenon by 15,000, and mercury by a whopping 300,000! Rare earths
(elements numbers 57 through 70, from lanthanum to ytterbium) are
probably enhanced too. Though in this particular star their actual
abundances have not been measured, europium clearly has a strong
presence. Beta Scl is also rich in iron, up by 40 percent over
solar. The magnetic field comes in at over a thousand times that
of Earth, and though there is some indication that the star varies
in light by about three percent, it is unclear as to whether it is
a magnetic variable like Cor Caroli.
What IS clear is that Sculptor's Beta could use a lot more
attention than it has been getting.
Written by Jim Kaler 12/10/10. Return to STARS.