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.
Written by Jim Kaler. Return to STARS.