Fomalhaut, Alpha Piscis Austrini (α PsA), is a white main sequence star located in the constellation Piscis Austrinus, the Southern Fish. With an apparent magnitude of 1.16, it is the brightest star in Piscis Austrinus and the 18th brightest star in the sky. It is slightly fainter than Antares in the constellation Scorpius, Spica in Virgo and Pollux in Gemini, but outshines Deneb in Cygnus, Mimosa in Crux and Regulus in Leo. The star lies at a distance of about 25 light years from Earth.
Fomalhaut has two of the widest known companions discovered to date and is one of brightest stars with a candidate planet. The planet, Fomalhaut b, is also known by the proper name Dagon.
The star is also known for its vast circumstellar debris ring, which has earned it the nickname “the Eye of Sauron,” a reference to J.R.R. Tolkien’s mythos.
Alpha Piscis Austrini is a triple star system consisting of Fomalhaut, a bluish-white main sequence star, TW Piscis Austrini (commonly referred to as Fomalhaut B), an orange main sequence star, and LP 876-10 (Fomalhaut C), a red dwarf. The three components appear several degrees apart in the sky. Fomalhaut B and C were only confirmed as companions in the last decade (in 2012 and 2013).
Fomalhaut B is at least 54,000 astronomical units away from the primary star and takes at least 7.6 million years to complete an orbit, while Fomalhaut C has an estimated orbital period of 22 million years.
Fomalhaut (Alpha Piscis Austrini A)
Fomalhaut has the stellar classification A3 V, indicating a hydrogen-fusing dwarf, appearing white or blue-white in colour. The star has almost twice the Sun’s mass (1.91 M☉) and radius (1.842 R☉). With an estimated surface temperature of 8,590 K, it shines with 16.63 solar luminosities.
A 2012 study of the star’s age and binarity gave an estimated age of 440 million years. The same study unequivocally confirmed that TW Piscis Austrini was a physical companion to Fomalhaut, with a similar velocity and at a separation of 0.280 parsecs from the primary.
Even though Fomalhaut is much younger than the Sun (4.5 billion years), as a more massive star, it has evolved faster. It will exhaust the supply of hydrogen in its core within a billion years and evolve into a red giant before expelling its outer layers to form a planetary nebula, leaving behind a white dwarf.
Fomalhaut has a significantly lower metallicity than the Sun. Different studies have given different values, in the range from 78% to 93%, of the Sun’s iron abundance. In 2008, spectroscopic measurements yielded a value of 46% of the Sun’s metallicity.
Fomalhaut is a Vega-like star, showing excess emissions in infrared wavelengths, which indicates the presence of a circumstellar disk. The star is in fact surrounded by several disks of debris. The one closest to the star lies at a distance of 0.1 astronomical units, while the middle disk is between 0.4 and 1 astronomical units from the star, and the outermost disk, sometimes called “Fomalhaut’s Kuiper belt,” lies at a radial distance of 133 astronomical units. The dust in the outer disk spreads in a belt some 25 astronomical units wide.
The star’s debris disk emits a significant amount of infrared radiation and is thought to be protoplanetary. It lies in Fomalhaut’s equatorial plane.
The discovery of Fomalhaut b, an extrasolar planet orbiting Fomalhaut, was announced on November 13, 2008. The planet was imaged by the Hubble Space Telescope orbiting the star inside the outermost debris ring. It was the first extrasolar planet captured in visible light.
Fomalhaut b does not appear in images taken in infrared wavelengths and in spite of being extensively studied, its nature is still uncertain. It has been called a “zombie” planet because not long after its discovery, its existence was dismissed as dust and gas, and then confirmed again in 2012.
The planet’s existence was proposed two years prior to the announcement, based on the eccentricity and sharpness of the edge of the debris ring. Based on observations with the Hubble Space Telescope’s Advanced Camera for Surveys, the study suggested that the suspected planet had a mass between those of Neptune and Saturn and a semi-major axis of about 119 astronomical units.
The Hubble images released in 2008 revealed a planet close to Jupiter in mass (between 0.2 and 2 Jovian masses) and were consistent with the earlier predictions of the planet orbiting at a distance of 119 astronomical units, taking 872 years to complete an orbit around Fomalhaut.
Fomalhaut b is believed to have a vast planetary ring system and may show us what Jupiter’s rings looked like before the debris consolidated into the four Galilean moons, and what Jupiter and Saturn looked like when the solar system was only a million years old.
Follow-up modelling of the way Fomalhaut’s debris disk is gravitationally perturbed by a single planet indicated that the planet’s orbit may not be apsidally aligned with the debris disk, which would mean that it is possible that there are other planets shaping the structure of the dust disk.
An M-band survey of Fomalhaut carried out in December 2006 and published in 2009 ruled out the presence of planets with more than 2 Jupiter masses from 13 to 40 astronomical units from the star and objects with more than 13 Jupiter masses (brown dwarfs) from 8 to 40 astronomical units.
In 2012, observations with the Spitzer Space Telescope suggested that Fomalhaut b was not a planet, but a transient or semi-transient dust cloud, ruling out the possibility that flux from a giant planet was responsible for the flux seen in the visible light images.
However, two other independent studies published in 2012 and 2013 confirmed the existence of Fomalhaut b. They, however, could not confirm that it was a whole planet. The first study indicated that it is enveloped in debris which is likely associated with a massive body, ruling out the possibility that it is a dust cloud. The second study considered two possibilities – a large debris disk around an unseen planet and the result of a collision of two Kuiper-belt-like objects with radii of about 50 km – and confirmed that Fomalhaut b is a real object orbiting Fomalhaut, but the object’s nature was uncertain.
In 2012, far-infrared images taken by the Herschel Space Observatory revealed large amounts of fluffy micrometer-sized dust in Fomalhaut’s outer debris belt, indicating constant collisions of planetesimals, with a collision rate of about 2,000 kilometre-sized comets per day. The dust grains are believed to be fluffy aggregates, pointing to a cometary origin.
Observations of the Fomalhaut debris ring with the Atacama Large Millimeter Array (ALMA) lead to the idea that two shepherd planets were responsible for the ring’s morphology, with neither having an orbital radius proposed for the planet discovered by the Hubble Space Telescope.
In 2013, narrowband observations with the Very Large Telescope/NaCo suggested that any additional planets from 4 to 10 astronomical units would have to have under 20 Jupiter masses and ruled out the existence of any companion with more than 30 Jupiter masses from 2.5 astronomical units outward.
Fomalhaut b was given the name Dagon after a public nomination and vote. The International Astronomical Union (IAU) announced the elected name in December 2015. The planet is named after the ancient Mesopotamian and Canaanite deity Dagon, associated with fertility and often depicted as a half-man, half-fish.
The star Fomalhaut B, also designated TW Piscis Austrini, is located 0.91 light-years, or 0.28 parsecs from the primary component. Both its velocity and estimated age (400 ± 70 million years) indicate that the two stars are physically related.
Fomalhaut B has an estimated mass of 0.725 solar masses and a radius 63% that of the Sun. Its estimated surface temperature is 4,711 K and its luminosity only 0.19 L☉.
Fomalhaut B is classified as a BY Draconis variable, a star exhibiting variations in luminosity due to the effects of rotation and starspots. It is a flare star, showing sudden dramatic increases in brightness for very short periods (minutes). While it is considerably smaller and less massive than the Sun, TW PsA is significantly larger than most flare stars. The star’s brightness varies only slightly, from magnitude 6.44 to 6.49, over a period of 10.3 days.
In 2018, observations with the Transiting Exoplanet Survey Satellite (TESS) revealed that Fomalhaut B may also host a planet, one with 92% of the Earth’s radius, but the evidence for the planet’s existence is weak and will not be confirmed until the next survey. The proposed planet lies about 0.082 astronomical units from the star and completes an orbit every 10.05 days.
Fomalhaut C, also designated LP 876-10, is located in the neighbouring constellation Aquarius, about 5.7 degrees (about 11 full Moons) from the primary star, making Alpha PsA the widest star system ever discovered. Fomalhaut C lies about halfway between Fomalhaut and the Helix Nebula, 2.5 light years from the primary and 3.2 light years from TW Piscis Austrini.
The star is a red dwarf of the spectral type M4V. It has 0.18 solar masses and (barely) shines with 0.0046 solar luminosities. It has an apparent magnitude of 12.624. Its estimated orbital period is 22 million years. Taking the system’s age into account, this means that it has only completed an orbit around the primary star 22 times.
Fomalhaut C was confirmed to be a member of the Alpha Piscis Austrini system in October 2013, when Dr Eric Mamajek of the University of Rochester announced that the star’s velocity, distance and colour-magnitude position indicate that it is a member of the Fomalhaut system.
In 2013, infrared observations with the Herschel Space Observatory revealed a cold debris disk around the star. The disk has an estimated temperature of only 24 K and orbits at a distance between 10 and 40 astronomical units from the star. Like the disk around Fomalhaut A, it is believed to host many comets.
Alpha Piscis Austrini is the first star system to have an extrasolar planet candidate imaged at visible wavelengths. The image was released in November 2008. It is also the third brightest star (after the Sun and Pollux) with a confirmed planet in its orbit.
Fomalhaut is one of the 58 stars selected for navigation. Navigational stars have a special role in the field of celestial navigation because they are some of the brightest and most identifiable stars in the sky. Fomalhaut’s neighbours Ankaa (Alpha Phoenicis), Diphda (Beta Ceti), and Alnair (Alpha Gruis) are also included on the list.
Fomalhaut used to belong both to Piscis Austrinus and Aquarius. The Greco-Roman astronomer Ptolemy placed the star in both constellations in the 2nd century CE. German uranographer Johann Bayer gave it the designation Alpha Piscis Austrini, placing it definitively in Piscis Austrinus as the constellation’s brightest star in his Uranometria (1603). The 17th century English astronomer John Flamsteed designated the star 24 Piscis Australis and assigned it the additional designation 79 Aquarii.
Fomalhaut was once considered to be a member of the Castor Moving Group, a stellar association that was believed to also include Castor and Vega. However, more recent studies have demonstrated that the stars have very different ages and velocities, and were not likely associated with one another millions of years ago.
Fomalhaut has served as a spectral standard for its class, A3 Va, since 1943. Its spectrum is one of the stable anchor points used to classify other stars.
Fomalhaut lies at a similar distance to Earth as Vega (another class A main sequence star), but it is a full magnitude fainter because it has a lower mass, smaller size and lower surface temperature.
Fomalhaut is one of the “fixed stars” used in astrology. If tied up with personal planets, it is believed to bring success in science and writing, as well as in creative and artistic pursuits.
Like many other bright and nearby stars, Fomalhaut has been used in countless works of fiction. Notable uses and mentions include the novel Pebble in the Sky (1950) by Isaac Asimov, Gordon R. Dickson’s Dorsai! (1960), Stanislaw Lem’s Return from the Stars (1961), Jack Vance’s Star King (1964), Ursula K. Le Guin’s Rocannon’s World (1966), Philip K. Dick’s The Unteleported Man (1966), The Divine Invasion (1981) and Radio Free Albemuth (1985), The Zero Stone (1968) by Andre Norton, Joe Haldeman’s The Forever War (1974), Richard Avery’s The War Games of Zelos (1975), Frank Herbert’s Children of Dune (1976), Greg Egan’s Diaspora (1997), and Paul J. McAuley’s In the Mouth of the Whale (2012).
The name Fomalhaut (pronunciation: /ˈfoʊməl.hɔːt/) comes from the Arabic phrase fam al-ḥūt (al-janūbī), meaning “the mouth of the whale,” referring to the mouth of the Southern Fish. The name was officially approved for Alpha Piscis Austrini A by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on December 15, 2015.
The star has been known by many other names across different cultures. The Latin names for the star were Os Piscis Meridiani, Os Piscis Meridionalis and Os Piscis Notii, meaning “the mouth of the Southern Fish.”
The Persians called it Hastorang in 2582 BCE and considered it one of the four “royal stars.” The Royal Stars of Persia – Fomalhaut, Aldebaran in Taurus, Regulus in Leo and Antares in Scorpius – were regarded as the guardians of heaven.
Fomalhaut’s other Arabic name was Difda al Auwel, derived from aḍ-ḍifdiˤ al-’awwal, meaning “the first frog.” The second frog was the nearby Diphda, Beta Ceti.
The Chinese knew Fomalhaut as 北落師門/北落师门 (Běiluòshīmén), or North Gate of the Military Camp. The star was a sole member of the North Gate of the Military Camp asterism in the Encampment mansion, which corresponds to the constellations Piscis Austrinus, Andromeda, Aquarius, Capricornus, Cassiopeia, Cepheus, Cygnus, Lacerta, Pegasus, and Pisces.
The Moporr people of Victoria, Australia called the star Buunjill (the name referred to a masculine being), and the Wardaman people of the Northern Territory knew it as Menggen, meaning “white cockatoo.”
Fomalhaut lies in the southern sky, but not as far south as Alpha Centauri, Canopus and Acrux, and can be seen from most locations north of the equator. Its declination is similar to Antares, which means that, from mid-northern latitudes, it appears above the horizon for at least 8 hours, but never climbs very high in the sky. The easiest way to identify it from northern locations is by using the Great Square of Pegasus, formed by the stars Alpheratz, Scheat, Markab and Algenib. The western side of the asterism points towards the star and a line drawn from Scheat through Markab leads to it. Even though Markab and Fomalhaut are separated by about 45 degrees, there are no other bright stars in the area between.
Fomalhaut does not have any exceptionally bright neighbours and appears quite isolated in the sky, which can make it challenging to identify the star despite its brightness. Seen from southern latitudes, Fomalhaut lies along an imaginary line drawn from Canopus, the second brightest star in the sky, through Achernar, the 9th brightest star. It forms a roughly equilateral triangle with the brighter Achernar in the constellation Eridanus and fainter Peacock in Pavo, and a smaller triangle with the fainter Alnair in Grus and Ankaa in Phoenix.
Fomalhaut lies in the same region of the sky as several notable deep sky objects. The best known of these, the Helix Nebula (NGC 7293), is a bright (mag. 7.6) planetary nebula located in the constellation Aquarius that has also been called the “Eye of Sauron,” but is probably better known as the “Eye of God.” The Helix Nebula lies about 10 degrees northwest of the star. The Whale Galaxy (NGC 55), a bright (mag. 7.87) Magellanic-type barred spiral galaxy, lies in the opposite direction. The galaxy is located in the constellation Sculptor and should not be confused with the other Whale Galaxy (NGC 4631), located in Canes Venatici.
The Sculptor Galaxy (NGC 253), a starburst spiral galaxy also known as the Silver Coin Galaxy, is located northeast of the star, near the border with the constellation Cetus.
Fomalhaut is the luminary of Piscis Austrinus, a small constellation in the southern sky that does not contain any other stars brighter than magnitude 4.00. Once also known as Piscis Notius, the constellation represents the Southern Fish, or the Great Fish in Greek mythology. It is usually depicted as swallowing the water poured by the neighbouring Aquarius, the Water Bearer. In Egyptian lore, the constellation is associated with the fish that saved the life of the goddess Isis. Fomalhaut is located on the constellation’s western border.
Piscis Austrinus does not contain many bright deep sky objects. It is home to several relatively bright galaxies – NGC 7172, NGC 7174, NGC 7314 and NGC 7259 – the last of which hosted a supernova seen in 2009, designated SN 2009ip.
The best time of year to observe Fomalhaut and other stars of Piscis Austrinus is during the month of October.
The 10 brightest stars in Piscis Austrinus are Fomalhaut (Alpha PsA, mag. 1.16), Epsilon Piscis Austrini (mag. 4.17), Delta Piscis Austrini (mag. 4.175), Beta Piscis Austrini (mag. 4.29), Iota Piscis Austrini (mag. 4.35), Gamma Piscis Austrini (mag. 4.448), Mu Piscis Austrini (mag. 4.49), Tau Piscis Austrini (mag. 4.945), Upsilon Piscis Austrini (mag. 4.98), and Theta Piscis Austrini (mag. 5.01).
Alpha Piscis Austrini
|Spectral class||A3 V / K5Vp / M4V|
|Constellation||Piscis Austrinus (Fomalhaut A and B), Aquarius (Fomalhaut C)|
|Designations||Fomalhaut, Alpha Piscis Austrini, α PsA, 24 Piscis Austrini|
Fomalhaut – Alpha Piscis Austrini A
|Spectral class||A3 V|
|U-B colour index||0.08|
|B-V colour index||0.09|
|Angular size||0.212 arcminutes|
|Distance||25.13 ± 0.09 light years (7.70 ± 0.03 parsecs)|
|Parallax||129.81 ± 0.47 mas|
|Radial velocity||+6.5 km/s|
|Proper motion||RA: +328.95 mas/yr|
|Dec.: -164.67 mas/yr|
|Mass||1.92 ± 0.02 M☉|
|Luminosity||16.63 ± 0.48 L☉|
|Radius||1.842 ± 0.019 R☉|
|Metallicity||-0.03 to -0.34 dex|
|Age||440 ± 40 million years|
|Rotational velocity||93 km/s|
|Surface gravity||4.21 cgs|
|Right ascension||22h 57m 39.0465s|
|Declination||-29° 37′ 20.050”|
|Designations||Fomalhaut, Alpha Piscis Austrini, α PsA, 24 Piscis Austrini, HD 216956, HR 8728, HIP 113368, GC 32000, SAO 191524, GCRV 14409, LTT 9292, FK5 867, CPD −30° 6685, CD-30 19370, GJ 881, IRAS 22549-2953, 2MASS J22573901-2937193, PPM 274426, TYC 6977-1267-1|
Fomalhaut B – TW Piscis Austrini
|Variable type||BY Draconis|
|U-B colour index||1.02|
|B-V colour index||1.10|
|Distance||24.81 ± 0.02 light years (7.608 ± 0.005 parsecs)|
|Parallax||131.4380 ± 0.0856 mas|
|Radial velocity||+7.217 km/s|
|Proper motion||RA: -329.577 mas/yr|
|Dec.: -158.293 mas/yr|
|Mass||0.725 ± 0.036 M☉|
|Radius||0.629 ± 0.051 R☉|
|Temperature||4,711 ± 134 K|
|Age||440 million years|
|Rotational velocity||2.93 km/s|
|Right ascension||22h 56m 24.05327s|
|Declination||-31° 33′ 56.0351”|
|Designations||Fomalhaut B, Alpha Piscis Austrini B, TW Piscis Austrini, TW PsA, HD 216803, HR 8721, HIP 113283, SAO 214197, LTT 9283, GCTP 5562.00, GJ 879, CD -32°17321, CP(D)-32 6550, GC 31978, GCRV 14398, IRAS 22536-3150, 2MASS J22562403-3133559, PPM 303193, TYC 7505-100-1, Gaia DR2 6604147121141267712|
Fomalhaut C – LP 876-10
|Absolute magnitude||13.21 ± 0.02|
|Angular size||0.0083 arcminutes|
|Distance||24.7 ± 0.2 light years (7.57 ± 0.07 parsecs)|
|Parallax||132.07 ± 1.19 mas|
|Radial velocity||6.5 km/s|
|Proper motion||RA: 333.8 mas/yr|
|Dec.: -177.5 mas/yr|
|Mass||0.18 ± 0.02 M☉|
|Radius||3,132 ± 65 R☉|
|Right ascension||22h 48m 04.47s|
|Declination||-24° 22′ 07.5”|
|Designations||Fomalhaut C, Alpha Piscis Austrini C, LP 876-10, NLTT 54872, GSC 06964-01226, 2MASS J22480446-2422075, WDS J22577-2937C, Gaia DR2 6623351805412369024|