Alpha Piscis Austrini - Fomalhaut

Fomalhaut, at magnitude 1.16, is the brightest star in the constellation Piscis Austrinus, and the 18th brightest star in the sky. Fomalhaut is very prominent, since it lies in an area of the sky where there are very few bright stars. In fact, Fomalhaut is the first magnitude star furthest from any other first magnitude star.

In 2008, Fomalhaut became the first stellar system with an extrasolar planet (Fomalhaut b) to be imaged at visible wavelengths.

Name and Mythology

The name Fomalhaut comes from the Arabic phrase "Fum al Hut", which means "mouth of the fish". An alternate Arabic name, Difda al Auwel, comes from the colloquial "ad-difdi al-'awwal", meaning "the first frog" (the second frog is Diphda or β Cet). Its Latin names are Os Piscis Meridiani, Os Piscis Meridionalis, Os Piscis Notii, "the mouth of the southern fish".

Fomalhaut has had various other names ascribed to it through time. One such name is the Lonely Star of Autumn, because it is the only first-magnitude star visible in the autumn sky at mid-northern latitudes. Archaeological evidence links Fomalhaut to rituals dating back to about 2500 BCE. Around 1000 B.C, the Sun would rise near Fomalhaut on the winter solstice, and it is one of the Persians' four "royal stars".

Properties

Fomalhaut is a class A3 V main-sequence star, located 25 light-years from Earth. Its surface temperature is around 8500 K; its luminosity is about 15 suns, and its diameter is roughly 1.7 times the Sun's. It contains 2.3 solar masses, with a potential main-sequence lifespan of only a billion years. Fomalhaut is believed to be a young star, only 200 to 300 million years old. When it exhausts its core hydrogen, it will turn into a red giant, then puff away its outer layers, leaving the remnant core behind as a white dwarf.

A nearby orange-red class K4-5 V dwarf star, discovered in 1897, is a distant physical companion. It is a flare star, designated TW PsA, located within a light year of Fomalhaut; the two share a common motion through space, and may share a common origin in the same star cluster. Another K5 dwarf (LTT 8273), later observed in proper motion studies, is believed to be an optical companion, although it may also be a remaining member of a low-density star cluster that includes Fomalhaut, Vega, and Castor, and has gradually dispersed over time.

Dust Disk and Planet Fomalhaut b

In 1983, the IRAS satellite discovered an excess of infrared radiation coming from Fomalhaut. The radiation is emitted by a huge disk of matter in a belt that surrounds the star, inclined 24 degrees from edge-on. The disk has a very sharp inner edge at a distance of 133 AU from the star, and is about 25 AU wide. The geometric center of the disk is offset by about 15 AU from Fomalhaut. In 2007, astronomers argued that the disk likely contains planetary embryos as large as Pluto undergoing runaway growth into larger bodies; and that the disk's off-center position around Fomalhaut and sharp inside edge were probably due to the presence of a Neptune-sized planet orbiting just inside.

The dust is densest around the same distance from Fomalhaut as our solar system's Kuiper Belt is from the Sun. The Kuiper Belt consists of icy objects - dormant comets and larger bodies - that surround the solar system beyond the orbit of Neptune. Fomalhaut's dust disk is believed to be debris that is forming, or being fragmented from, such bodies. The dust may have disappeared from Fomalhaut's inner region because it has already coalesced into planets. Fomalhaut may look today very much how our Solar System appeared four billion years ago.

In November 2008, astronomers announced the discovery of a planet, named Fomalhaut b, orbiting within the disk. Imaged by the Hubble Space Telescope, this extrasolar planet was the first to be seen directly, rather than inferred by indirect means. With a mass of under 3 Jupiters, Fomalhaut b lies at a huge distance of 115 AU from its parent star, and has an orbital period estimated at around 872 years. A modest eccentricity (e=0.11) causes its distance to vary between 102 to 127 AU from the star.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]