Star Facts: Fomalhaut

Image Credit: Fred Espenak

Fomalhaut (Alpha Piscis Austrini) is the most luminous star in the constellation Piscis Austrinus (Southern Fish), and the 18th brightest in the entire night sky. Like the star Vega, Fomalhaut emits an excess of infrared radiation, which has been found to be the result of a series of dust-rich discs that surrounds the three stars that make up the Fomalhaut system. These stars include Fomalhaut A and Fomalhaut B in the constellation Piscis Austrinus , which are separated by about one light year, and the more distant Fomalhaut C in the constellation Aquarius, which lies between 2.5 and 3.2 light years from the main AB pairing, respectively.


Having a declination of -29.6°, Fomalhaut is placed below the celestial equator, but it isn’t placed so far south that is cannot be observed from large parts of the northern hemisphere. For instance, from latitude 40 degrees N, the star is visible for at least eight hours, albeit at an elevation of only 20 degrees or so.

From the UK, the star never brightens to more than magnitude 2.2, due to the fact that it remains even closer to the horizon, and from further north, such as from Alaska and Scandinavia, the star is not visible at all, at any time. From the northern hemisphere, look for Fomalhaut by using the two western stars which form one side of the Great Square of Pegasus, which then point in the direction of Fomalhaut, situated about 45 degrees to the southward of the star Alpha Pegasi, with no other bright stars between them.

Fomalhaut A

Quick Facts

• Constellation: Piscis Austrinus
• Coordinates: RA: 22h 57m 39s | Dec: -29° 37′ 19″
• Distance: 25.13 light years
• Star Type: A3 V
• Mass: 1.92 sol
• Radius: 1.842 sol
• Apparent Magnitude: +1.16
• Luminosity: 16.63 sol
• Surface Temperature: 8,590K
• Rotational Velocity: 93 km/sec
• Age: 440 million years
• Other Designations: a Piscis Austrini, a PsA, Alpha PsA, 24 Piscis Austrini, CPD -30° 6685

Fomalhaut is a blue dwarf (A3V) star situated 25.13 light years from Earth that is 1.842 times bigger than the Sun, with 1.92 times its mass, and 16.63 times its luminosity. It is around 440 million years old, or less than halfway through its one billion year lifespan, and has a surface temperature of around 8,500 kelvins compared to 5,778 K for the Sun. A spectroscopic study performed in 2008 showed that Fomalhaut’s metalicity amounted to only 46% of that of the Sun. It is also surrounded by several dust-rich discs, with the innermost disc consisting of small carbon-rich “ash” particles concentrated at a distance of about 0.1 AU (astronomical units) from the star. The next disc out consists of slightly larger particles, with a clearly defined inner edge about 0.4-1 AU from the star, while the outer-most disc has a toroidal shape, and has a width of about 25 AU, but with a sharply defined inner edge about 133 AU distant from the star.

Fomalhaut B (TW Piscis Austrini)

Quick Facts

• Constellation: Piscis Austrinus
• Coordinates: RA 22h 56m 24.05327s |Dec. -31° 33′ 56.0351″
• Distance to Earth: 24.8 light years
• Star Type: K5Vp
• Mass: 0.725 sol
• Radius: 0.629 sol
• Apparent Magnitude: 6.48
• Luminosity: 0.19 sol
• Surface Temperature: 4,711 K
• Rotational Velocity: 2.93 km/sec
• Age: 450 million years
• Other Designations: Fomalhaut B, TW PsA, Gl 879, HR 8721, CD -32°17321, HD 216803

Fomalhaut B (TW Piscis Austrini) is an orange dwarf (K5Vp) that is separated from Fomalhaut A by a distance of 0.91 light years. Recent studies have shown that both TW Piscis Austrini’s age of 450 million years, and its proper motion is consistent with the star being a true companion star. TW Piscis Austrini is a variable star in the BY Draconis class, with a variation in luminosity of between magnitudes 6.44 and 6.49 over a period of 10.3 days. This star is also a flare star, which is typical of M-type dwarf stars, and although it is significantly smaller and less massive than the Sun, it is much bigger than most flare stars.

Fomalhaut C (LP 876-10)

Quick Facts

• Constellation: Aquarius
• Coordinates: RA 22h 48m 04.47s |Dec. -24° 22′ 07.5″
• Star Type: M4V
• Apparent Magnitude: 12.618
• Other Designations: Fomalhaut C, 2MASS J22480446-2422075, NLTT 54872

LP 876-10 , the third star in the Fomalhaut system, is a red dwarf (M4V) located about 2.5 light years from Fomalhaut A itself and about 3.2 light years away from Fomalhaut B. However, while these distances place LP 876-10 in the neighbouring Aquarius constellation, they still fall well within the Fomalhaut system’s zone of tidal influence, which stretches to about 6.2 light years around the system. In December of 2013, the existence of a cold dust disc was demonstrated around this star, which discovery places the Fomalhaut system among the handful of known multiple-star systems that contain multiple dust discs.


Piscis AustrinusFomalhaut has been in the human consciousness for several thousand years, and it has played a major part in the Arab, Persian, and Chinese cultures. The star’s name derives from the Arabic phrase “Fum al Hut”, meaning the “Mouth of the Fish”, coinciding with its location in the constellation of Piscis Austrinus (Southern Fish). From southern regions, this first magnitude star appears in a relatively empty area of the southern autumn sky, earning it the title of “The Solitary One”.

Being associated with the onset of autumn, Fomalhaut, was considered by the ancient Persians to be one of the four “Royal Stars” which guarded the sky, the others including Aldebaran (winter), Regulus (spring) and Antares (summer). As Martha Evans Martin noted in her book entitled “The Friendly Stars”:

“.. the loneliness of this star, added to the somber signs of approaching autumn, sometimes gives one a touch of melancholy. In November and December, when the winter stillness has fallen upon us, a glance toward the southwest will discover Fomalhaut, still placid and alone”

Fomalhaut is further famous for the fact that it is the first star to hosts a planet in which a direct image in visible light was captured. The historic photo was taken by the Hubble Space Telescope in 2005, with Fomalhaut b, later named Dagon, orbiting its parent star situated 11 billion miles away once every 872-years.

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