FOMALHAUT (Alpha Piscis Austrinus). This wonderful first magnitude (1.16, ranking 18th) star of northern-hemisphere autumn, usually pronounced "fo-ma-low," slides slowly in lonely grandeur above the southern horizon (as seen from the north) during the months of October and November. From the southern hemisphere, it's a glory, passing overhead at 30 degrees south latitude. Well to the south of the Great Square of Pegasus, Fomalhaut marks for us the otherwise dim constellation Piscis Austrinus, the Southern Fish, not surprisingly also south of the more well-known zodiacal constellations Pisces, the Fishes, and Aquarius, the Water Bearer. The Alpha star of the constellation, the name comes from an Arabic phrase meaning "the mouth of the southern fish." It at first seems like yet another ordinary white class A (A3) dwarf similar to, though a bit cooler than, Vega in Lyra (which passes nearly overhead in temperate latitudes) with a surface temperature of about 8500 Kelvin. It is quite close, only 25.1 light years away (give or take just 0.1), from which we calculate a luminosity 16 times greater than the Sun. Almost the same distance as Vega, it is over a full magnitude fainter to the eye as a result of a somewhat lower mass of 2.0 solar (recently refined to 1.9 Suns, with an age of 449 million years), which results in a lower surface temperature and smaller size of 1.8 times that of the Sun (a figure supported by measuring the angular diameter). A minimum equatorial spin speed of 102 kilometers per second leads to a rotation period of under a day. In 1983 an orbiting satellite called IRAS discovered far more infrared radiation coming from the star than expected. Infrared -- radiation which has waves longer than red light -- is a signature of a cool source. The radiation is coming from a huge disk of matter five times the size of the orbit of Pluto that surrounds the star much like the disks that encompass Vega and Denebola. The disk is thought to be made of icy dust particles that have been warmed by the star. The planets of our Solar System almost certainly formed from the accumulation of dust in just such a disk. Fomalhaut seems to have a pair of very distant companions that are moving through space with it. Fomalhaut B (also called TW PsA) is a 13th magnitude class K4 dwarf with a mass three-quarters that of the Sun that lies just under two degrees, at least 54,000 AU, away, and must take at least 7.6 million years to orbit. Fomalhaut C (LP876- 10), a 13th magnitude M4 dwarf, lies even farther, 5.7 degrees (at least 150,000 AU), and must take at least 35 million years to orbit the inner pair.
The Planet. Observation of Fomalhaut's disk shows a hole in the middle. Could the hole be the result of planets that have removed the dust? So far none has been detected within the hole. But in a stunning discovery announced in November of 2008, one was found by direct imaging to be orbiting within the disk.
With a mass of under 3 Jupiters, it lies at a remarkable distance of 177 Astronomical Units from its parent star, a high eccentricity taking it between 320 and 32 AU. The orbital period must be about 1700 years. Along with the planets of HR 8799 Pegasi, Fomalhaut's planet has the honor of being the first to be directly seen. The odd orbit suggests perturbations by an inner unseen planet. (Data on Fomalhaut, its companions, and planet, are in part from P. Kalas et al. in the Astrophysical Journal for Sept. 20, 2013.)
- - Written by Jim Kaler