Fomalhaut, also known as Alpha Piscis Austrini, a bright white star in the southern constellation Piscis Austrinus, the Southern Fishes. The name Fomalhaut comes from the Arabic phrase Fam al-Hut al-Janubi, which means “The Mouth of the Southern Fish” and refers to the star’s position in the constellation. Fomalhaut is located about 30° south of the celestial equator, an imaginary plane formed by projecting the earth’s equator into space. At its position, it is visible from September through December throughout the southern hemisphere and in the northern hemisphere up to about the latitude of northern Scotland, southern Scandinavia, and Juneau, Alaska.

Fomalhaut is one of the 20 brightest stars in the earth’s night sky. Stars that are visible to the unaided eye, such as Fomalhaut, belong to the earth’s home galaxy, the Milky Way, and tend to be very bright or relatively close. Fomalhaut is very close to the earth’s solar system at a distance of 23 light-years. For comparison, the most distant easily seen stars are up to 5000 light-years from the earth. Fomalhaut’s intrinsic luminosity, or total light output, rates an absolute magnitude of +2.0 (bright stars have low or even negative magnitude values). This magnitude corresponds to the light output of about 13 suns. At its distance from the earth, Fomalhaut shines in the earth’s night sky with an apparent magnitude—a measure of how bright it appears to an observer on the earth—of +1.16.

Fomalhaut’s surface temperature is 8700° C (16,000° F), which is about 1.5 times the surface temperature of the sun and gives the star a blue-white color. Its diameter is about 2.1 million km (about 1.3 million mi), which is about 1.5 times the diameter of the sun. From its composition, temperature, and size, astronomers classify Fomalhaut as a main sequence star, which means that its primary source of energy is the fusion of hydrogen nuclei to form helium in its core. The main sequence stage is the first and longest phase in the life of a star (see Star: Evolution of Stars). Fomalhaut is only about 200 million years old, much younger than our sun.

In 1998 a group of American astronomers using the James Clerk Maxwell Telescope, a large radio telescope at Mauna Kea Observatory in Hawaii, found that Fomalhaut is surrounded by a disk of dust. The astronomers believe that the star is the center of a solar system in formation. The disk of dust around Fomalhaut has an empty space near the star. The dust on the inner edge of the ring may be in the process of condensing into planets.