Fomalhaut (UK: /ˈfɒməloʊt/, US: /ˈfoʊməlhɔːt/[15]) is the brightest star in the southern constellation of Piscis Austrinus, the Southern Fish, and one of the brightest stars in the night sky. It has the Bayer designationAlpha Piscis Austrini, which is an alternative form of α Piscis Austrini, and is abbreviated Alpha PsA or α PsA. This is a class Astar on the main sequence approximately 25 light-years (7.7 pc) from the Sun as measured by the Hipparcos astrometry satellite.[16] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[17]
It is classified as a Vega-like star that emits excess infrared radiation,[18][19] indicating it is surrounded by a circumstellar disk.[20] Fomalhaut, K-type main-sequence star TW Piscis Austrini, and M-type, red dwarf star LP 876-10 constitute a triple system, even though the companions are separated by approximately 8 degrees.[21][22]
Fomalhaut was the first stellar system with an extrasolar planet candidate imaged at visible wavelengths, designated Fomalhaut b. However, analyses in 2019 and 2023 of existing and new observations indicate that Fomalhaut b is not a planet, but rather an expanding region of debris from a massive planetesimal collision.[23][24][19]
α Piscis Austrini, or Alpha Piscis Austrini, is the system's Bayer designation. It also bears the Flamsteed designation of 24 Piscis Austrini. The classical astronomer Ptolemy put it in Aquarius, as well as Piscis Austrinus. In the 17th century, Johann Bayer firmly planted it in the primary position of Piscis Austrinus. Following Ptolemy, John Flamsteed in 1725 additionally denoted it 79 Aquarii. The current designation reflects modern consensus on Bayer's decision, that the star belongs in Piscis Austrinus.[25] Under the rules for naming objects in multiple star systems, the three components – Fomalhaut, TW Piscis Austrini and LP 876-10 – are designated A, B and C, respectively.[26]
The star's traditional name derives from Fom al-Haut from scientific Arabic فم الحوت fam al-ḥūt (al-janūbī) "the mouth of the [Southern] Fish" (literally, "mouth of the whale"), a translation of how Ptolemy labeled it.[27][28] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[29] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[30] included a table of the first two batches of names approved by the WGSN, which included the name Fomalhaut for this star.
In July 2014, the International Astronomical Union (IAU) launched NameExoWorlds, a process for giving proper names to certain exoplanets.[31] The process involved public nomination and voting for the new names.[32] In December 2015, the IAU announced the winning name was Dagon for Fomalhaut b.[33] The winning name was proposed by Todd Vaccaro and forwarded by the St. Cloud State University Planetarium of St. Cloud, Minnesota, United States of America, to the IAU for consideration.[34] Dagon was a Semitic deity, often represented as half-man, half-fish.[35]