Fomalhaut is the brightest star in the constellation of Piscis Austrinus. It is the 18th brightest star in the night sky and the first stellar system with an extrasolar planet candidate imaged at visible wavelengths.
Key Facts & Summary
- Fomalhaut is a bluish-white hydrogen-fusing dwarf star classified as an A3 V-class star on the main sequence, and it is around 25 light-years / 7.7 parsecs away from the Sun.
- The spectrum characteristics of Fomalhaut have served as one of the stable anchor points by which other stars are classified.
- Classifications point out that Folamhaut is a Vega-like star that emits excess infrared radiation, thus an indication that it is surrounded by a circumstellar disk, and indeed it is.
- This circumstellar disk has been nicknamed the “eye of Sauron”, referencing Tolkien’s mythos due to the fact that, indeed, it bears a striking resemblance.
- Fomalhaut is actually a triple star system consisting of Fomalhaut A – a bluish-white main-sequence star, TW Piscis Austrini or Fomalhaut B, an orange main-sequence star, and LP 876-10 or Fomalhaut C, a red dwarf.
- Apart from the other two stars, Fomalhaut A also has an exoplanet which was named Fomalhaut b, also known as Dagon.
- This exoplanet, Dagon, was the first planet to be captured in visible light. It was named after an ancient Mesopotamian deity which was associated with fertility and often depicted as a half-man, half-fish.
- Fomalhaut A is almost twice as massive as our Sun, with an estimated 1.91 solar masses and 1.8 solar radii.
- Fomalhaut B is smaller than our sun, with an estimated 0.7 solar masses, and a radius of 63% the sun’s radius.
- Fomalhaut C is the smallest star, with an estimated 0.18 solar masses while its radius is unknown.
- The primary star, Fomalhaut A, is the brightest at around 16.6 times brighter than our sun. Fomalhaut B is much fainter, at only 0.19 solar luminosities. The dimmest star is Fomalhaut C, at only 0.0046 solar luminosities.
During the time of the ancient Persians, the sky was divided into four districts that were guarded by four royal stars. All of these stars were first magnitude stars and close to the ecliptic, however, one of them lacked these characteristics.
The royal stars were Aldebaran, Regulus, and Antares. The fourth district which was representative of autumn lacked a bright star. It was known as the “celestial sea” since it is a dark and wide area with little bright stars. All the star patterns in this region have an association with water.
The constellations of Capricornus (the sea-goat), Aquarius, and Pisces (fish) – also lacked bright stars. Thus, the ancient Persians had no choice but to designate a fourth star that wasn’t in any zodiac constellation and chose Fomalhaut.
Today, the modern constellation in which Fomalhaut resides in is called Piscis Austrinus (the southern fish). It is sometimes referred to as “The Solitary One” – since it stands out in a rather empty region of space.
Probably the story behind its name is the fact that Piscis Austrianus was depicted many times like a fish with an open mouth drinking the water that flows from the water jar of Aquarius. Fomalhaut might well be a distortion of the Arabic words “Fum Al Hut” – which translates to “mouth of the fish.”
Fomalhaut A formed around 440 million years ago from a molecular cloud or a nebula, of gas and dust. Fomalhaut B seems to be around 400 million years old or even older, this suggests that the stars may be physically related, perhaps they formed at the same time and are now gravitationally bound to each other. Both stars are much younger than our sun, while Fomalhaut C is theorized to be even older than the other two.
Distance, Size, and Mass
Fomalhaut is around 25 light-years / 7.7 parsecs away from the Sun. The primary star, Fomalhaut A, is almost twice as massive as our Sun, with an estimated 1.91 solar masses and 1.8 solar radii.
The secondary star, Fomalhaut B, is smaller than our sun, with an estimated 0.7 solar masses, and a radius of 63% the sun’s radius or 0.63 solar radii. The third star, Fomalhaut C, is the smallest star, with an estimated 0.18 solar masses, its radius is yet unknown.
The primary star, Fomalhaut A, is the brightest at around 16.6 times brighter than our sun. Fomalhaut B is much fainter, at only 0.19 solar luminosities. The dimmest star is Fomalhaut C, at only 0.0046 solar luminosities.
The hottest star, is Fomalhaut A, with average surface temperatures of around 8.590 K, the secondary is Fomalhaut B, with temperatures of around 4.711 K. Fomalhaut A appear to have a lower metallicity than our Sun, with 3 studies estimating different percentages of metallicity: 78%, 93%, and the more accurate 2008 spectroscopic measurements – 46% the Sun’s metallicity.
Fomalhaut is surrounded by several disks of debris with the closest one being at 0.1 AU away, the middle one at 0.4 or 1 AU away, and the largest outermost disk lies at a radial distance of 133 AU. It is nicknamed the Fomalhaut’s Kuiper Belt, stretching for more than 25 AU.
Fomalhaut B is classified as a BY Draconis variable, showing sudden dramatic increases in brightness for short periods of time.
Its brightness varies from magnitude 6.44 to 6.49, over a period of 10.3 days. Fomalhaut C has an apparent magnitude of 12.624 and was discovered in 2013 by Dr. Eric Mamajek. A cold debris disk is also around this star with estimated temperatures at around 24 K.
Perhaps one of the biggest stellar systems in terms of how far the stars stretch from one another. Fomalhaut B is at least 54.000 AU from Fomalhaut A and completes one orbit around it every 7.6 million years, or more.
Fomalhaut C, on the other hand, is located in the constellation of Aquarius and completes one orbit around the primary star once every 22 million years. Fomalhaut C lies about halfway between Fomalhaut A and the Helix Nebula – 2.5 light-years from A, and 3.2 light-years from C.
This stellar system’s most interesting part is that it posses a disk of debris similar to the one surrounding our solar system – the Kuiper Belt.
This circumstellar disk has been nicknamed the “eye of Sauron”, referencing Tolkien’s mythos due to the fact that, indeed, it bears a striking resemblance. It’s also nicknamed Fomalhaut’s Kuiper belt due to its size and other similarities.
The disk lies in Fomalhaut’s equatorial plane. The disk is composed of cosmic dust, gas, and comets as they crash into one another. The outermost disk of debris in the Fomalhaut system is around 20 billion km / 12.4 billion mi from Fomalhaut A, and 2 billion km / 0.6 billion mi wide.
This somewhat narrow and eccentric disk can only be shaped by the gravitational influence of planets such as the case in our own solar system. Jupiter for example, with its great gravitational influence, shapes the asteroid belt.
The discovery of a planet in the Fomalhaut system did indeed occur, but only in 2008.
The discovery of the planet Fomalhaut b – also called Dagon – first occurred in 2008, it is the first exoplanet captured in visible light by the Hubble Space Telescope.
It was located in the outermost debris disk. Its existence was dismissed for a couple of years and it was called a “zombie” planet since many believed that it was just gas and dust, until in 2012 when it was confirmed again.
Studies suggested that the planet had a mass between that of Neptune and Saturn, and a semi-major axis of around 119 AU. However, the photos taken in 2008 suggested that the planet was more close to Jupiter in mass. Further studies concluded that Dagon is a real object, but its true nature remains unknown. Whether it is a planet or not is still under investigation, and other studies suggest that if it is, it might also have a disc of its own, like Saturn.
Even more, observations point out that there could be more than one planet in the Fomalhaut system, but are elusive and remain to be concisely established. One of the great impediments to these observations are the comets. They are quite large and frequently collide with each other – possibly giving an illusion of a bigger object.
Fomalhaut is located in the southern sky in the constellation of Piscis Austrinus. It can easily be seen from most locations north of the equator. Its more specific location is on the western border of the constellation.
In the mid-northern latitudes, it appears above the horizon for at least 8 hours, similarly to Antares. Fomalhaut is located in a pretty dark region, but other deep sky objects such as the Helix Nebula are relatively close to it. The constellation of Piscis Austrinus was once known as Piscis Notius – representing the Southern Fish.
Fomalhaut A is around 440 million years old, much younger than our Sun. Though it is younger, it has evolved faster and it will exhaust its hydrogen supplies within a billion years. After this process, it will evolve into a red giant before expelling its outer layers thus forming a planetary nebula, and leaving behind a white dwarf.
Did you know?
- In October, the constellation of Piscis Austrinus, and consequently Fomalhaut, is the most visible than any other month.
- This constellation is associated with the fish that saved the life of the goddess Isis, in Egyptian lore.
- Fomalhaut is one of the 58 stars selected for navigation. The closest neighbors to Fomalhaut that are also in this category of stars are Ankaa, Diphda, and Alnair.
- In the 2nd century, the Greek astronomer Ptolemy placed Fomalhaut in both the Piscis Austrinus and Aquarius constellations.
- Fomalhaut has been used as a spectral standard for its class A3 V – since 1943.
- Fomalhaut is used in astrology and it symbolizes success in science and writing, as well as some forms of art.
- Fomalhaut was known to the Chinese as the North Gate of the Military Camp. An asterism formed by Fomalhaut alone.
- In the Latin language, Fomalhaut was called Os Piscis Meridiani, and other derivations that usually meant “the mouth of the Southern Fish.”
- The aboriginal people of Australia such as the Moporr, knew Fomalhaut as Buunjill – referring to a masculine being, while the Wardaman people called Fomalhaut Menggen – white cockatoo.