FUM AL SAMAKAH (Beta Piscium). Three classic fish swim the eternal
sea of the sky. Two are tied together to make the zodiacal figure
of Pisces (the Fishes), while one, Piscis Austrinus (the Southern Fish), glides
silently to the south of Aquarius. The
southern fish's mouth is marked by first magnitude Fomalhaut, while that of the western of
Pisces' pair is denoted by much fainter fifth magnitude (4.53) Fum
al Samakah (which from Arabic also refers to the "fish's mouth"),
and is much better known as Beta Piscium. Far from the second
brightest star in the constellation, the Greek
letter "Beta" was probably applied because of the star's
prominent position both anatomically and as the western-most in the
figure's classic outline. Curiously, Fum al Samakah is almost
exactly due north of its much brighter counterpart Fomalhaut,
displaced just a degree and a half to the east. At a distance of
495 light years, it is also much farther than Fomalhaut (25 light
years), accounting for Beta's faintness. Beta Psc (much easier to
write), a class B (B6) hydrogen-fusing dwarf, is also a "Be" star
(the "B" and the "e" pronounced separately) that has emission lines in its spectrum
that arise from a rotating circumstellar disk. Such stars are
rapid rotators, Beta spinning at greater than 104 kilometers per
second. If the disks of Be stars are presented edge-on to us,
absorptions appear in the spectra, and the stars are called "shell
stars" (Alpha Arae and Eta Centauri good examples). Beta Psc is
NOT one of these, so the rotation axis must tilted. The star must
therefore be spinning much faster than observed "projected"
rotational velocity. Since it gets in the way, the disk (whose
cause is not understood) makes something of a mess of temperature
measurement. So does the rotation. Rapidly rotating stars (Altair, Achernar), are distorted into ellipsoids,
which causes temperature variations across the stellar surfaces
(hotter at the poles, cooler at the equators). Temperature
estimates for Beta Psc fall between 13,500 and 15,500 Kelvin.
Using an appropriate 14,000 Kelvin (to account for ultraviolet
radiation), Beta Psc shines with the light of 750 Suns, which gives a radius of 4.7 solar, a
rotation period well under two days, a mass of 4.8 or so solar, and
an age of somewhere around 60 million years (just over halfway
through its hydrogen-fusing lifetime). The surface gravity derived
from the spectrum gives a
somewhat higher 5.5 solar masses. Whatever the details, Beta Psc
will someday morph into a massive white dwarf of about 0.85
solar masses, in its giant
phases losing over 80 percent of itself back into interstellar
space through violent winds.