Properties
Fomalhaut is a young star, for many years thought to be only 100 to 300 million years old, with a potential lifespan of a billion years. A 2012 reanalysis of the age for Fomalhaut by astronomer Eric Mamajek yielded an age of 440±40 million years. The surface temperature of the star is around 8,590 K (8,320 °C). Fomalhaut's mass is about 1.92 times that of the Sun, its luminosity is about 16.6 times greater, and its diameter is roughly 1.84 times as large.
Fomalhaut is slightly metal-deficient as compared to the Sun, which means it is composed of a smaller percentage of elements other than hydrogen and helium. The metallicity is typically determined by measuring the abundance of iron in the photosphere relative to the abundance of hydrogen. A 1997 spectroscopic study measured a value equal to 93% of the Sun's abundance of iron. A second 1997 study deduced a value of 78% by assuming Fomalhaut has the same metallicity as the neighboring star TW Piscis Austrini, which has since been argued to be a physical companion. In 2004, a stellar evolutionary model of Fomalhaut yielded a metallicity of 79%. Finally, in 2008, a spectroscopic measurement gave a significantly lower value of 46%.
Fomalhaut is a member of the 16 stars belonging to the Castor Moving Group. This is an association of stars that share a common motion through space and have been claimed to be physically associated. Other members of this group include Castor and Vega. This moving group has an estimated age of 200±100 million years and originated from the same location.
Fomalhaut is a binary star with the nearby star TW Piscis Austrini. TW Piscis Austrini lies 0.28 parsecs away from Fomalhaut, and its space velocity agrees with that of Fomalhaut within 0.1±0.5 km/s, consistent with being a bound companion. A recent age estimate for TW PsA (400±70 million years), agrees very well with the isochronal age for Fomalhaut (450±40 million years), further arguing for the two stars forming a physical binary.
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