Fomalhaut
Primary of Tros.
Fomalhaut (Alpha Piscis Austrini, Alpha PsA, α Piscis Austrini, α PsA) is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not reappear until Antares sets. Its name derives from Arabic fum al-ḥawt, meaning "mouth of the [Southern] Fish" (فُمْ اَلْحَوْتْ).
This is a class A star on the main sequence approximately 25 light-years (7.7 pc) from Earth as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk. Fomalhaut and the K-type star TW Piscis Austrini constitute a binary system.
Fomalhaut holds a special significance in extrasolar planet research, as it is the center of the first stellar system with an extrasolar planet (Fomalhaut b) imaged at visible wavelengths. The image was published in Science in November 2008. It is the third brightest star known to have an orbiting planet, after Pollux and the Sun.
Fomalhaut was a later designation of 79 Aquarii.
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%.[3] 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.