Fomalhaut

Fomalhaut

Debris ring around Fomalhaut showing location of planet Fomalhaut b—imaged by Hubble Space Telescope's coronagraph. NASA photo.
Observation data Epoch J2000      Equinox J2000
Constellation	Piscis Austrinus
Pronunciation	/ˈfoʊməlhɔːt/
Right ascension	22h 57m 39.1s
Declination	-29° 37′ 20″
Apparent magnitude (V)	1.16
Characteristics
Spectral type	A3 V
U-B color index	0.08
B-V color index	0.09
Astrometry
Radial velocity (Rv)	+6.5 km/s
Proper motion (μ)	RA: 329.22 mas/yr Dec.: −164.22 mas/yr
Parallax (π)	130.58 ± 0.65 mas
Distance	25 ± 0.1 ly (7.66 ± 0.04 pc)
Absolute magnitude (MV)	1.73
Details
Mass	2.1 ± 0.2[1] M☉
Radius	1.832[1] R☉
Surface gravity (log g)	4.21[1]
Luminosity	17.66[1] L☉
Temperature	8,751[1] K
Metallicity	[Fe/H] = −0.10[1]
Rotational velocity (v sin i)	93[1] km/s
Age	(2 ± 1) × 108[1] years
Other designations
α Piscis Austrini, Alp PsA, Alf PsA, 24 PsA, Gl 881, HR 8728, CD -30°19370, HD 216956, GCTP 5565.00, LTT 9292, SAO 191524, FK5 867, HIP 113368.
Database references
SIMBAD	data
NStED	data
ARICNS	data
Extrasolar Planets Encyclopaedia	data

Fomalhaut (α PsA / α Piscis Austrini / Alpha Piscis Austrini) 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 means "mouth of the whale", from the Arabic فم الحوت fum al-ḥawt. It is a class A star on the main sequence approximately 25 light-years (7.7 pc) from Earth. It is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk.

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.^[2]

Properties

Fomalhaut is believed to be a young star, only 100 to 300 million years old, with a potential lifespan of a billion years.^{[citation needed]} The surface temperature of the star is around 8,751 K (15,292 °F; 8,478 °C). Compared to the Sun, its mass is about 2.1, its luminosity is about 18, and its diameter is roughly 1.8. Fomalhaut has a slightly lower proportion of elements with higher atomic numbers than helium: about 79% as much as the Sun.^{[1][note 1]}

Fomalhaut is a member of the 16 stars belonging to the Castor Moving Group. This is an association of stars that shares a common motion through space and are therefore likely 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.^[3] The nearby star TW Piscis Austrini, a member of this group, may form a physical pair with Fomalhaut.^[4]

System

Comparison between our Solar system and the system around Fomalhaut.

Fomalhaut is surrounded by a debris disk of dust in a toroidal shape with a very sharp inner edge at a radial distance of 133 AU (1.99×10¹⁰ km; 1.24×10¹⁰ mi), inclined 24 degrees from edge-on.^[5][6] The dust is distributed in a belt about 25 AU wide. The geometric center of the disk is offset by about 15 AU (2.2×10⁹ km; 1.4×10⁹ mi) from Fomalhaut.^[7] The disk is sometimes referred to as "Fomalhaut's Kuiper belt". Fomalhaut's dusty disk is believed to be protoplanetary,^[8] and emits considerable infrared radiation. Measurements of Fomalhaut's rotation indicate that the disk is located in the star's equatorial plane, as expected from theories of star and planet formation.^[9]

On November 13, 2008, astronomers announced the discovery of an extrasolar planet orbiting just inside the debris ring. This was the first extrasolar planet to be seen with visible light, captured by the Hubble Space Telescope.^[10][11] The planet's existence had been previously suspected from the sharp, elliptical inner edge of Fomalhaut's debris disk.^[12] The mass of the planet, Fomalhaut b, is estimated to be no more than three times the mass of Jupiter and at least the mass of Neptune. There are indications that the planet's orbit is not apsidally-aligned with the dust disk, which may indicate that additional planets may be responsible for the dust disk's structure.^[13] However M-band images taken from the MMT Observatory put strong limits on the existence of gas giants within 40 AU of the star.^[14]

The much-fainter flare star TW Piscis Austrini is located within a light year of Fomalhaut and the two share a common proper motion through the sky. They are believed to be companion stars and may have had a common origin in a star cluster.

Visibility

At dec=-29° 37′ 20″, Fomalhaut is some way south of the celestial equator. This means it is best viewed from the Southern Hemisphere; it passes virtually overhead from places just south of the Tropic of Capricorn, for example Brisbane, Johannesburg and Easter Island. However its southerly declination is not as great as that of stars such as Acrux, Alpha Centauri and Canopus, meaning that unlike them it is visible from a large part of the Northern Hemisphere as well. South of latitude 40˚ north it is quite easy to see on autumn evenings; further north it is low on the southern horizon- it reaches a maximum altitude of only 9˚ from London, 4˚ from Edinburgh or Copenhagen, and it just barely rises from around latitude 60˚ north (Oslo, Helsinki, southern Alaska). It can be located in these northern latitudes by the fact that the western (right-hand) side of the Square of Pegasus points to it. Continue the line from Beta to Alpha Pegasi towards the southern horizon; Fomalhaut is about 45˚ south of Alpha Pegasi, with no bright stars in between.

Etymology and cultural significance

Fomalhaut has had various names ascribed to it through time. One such name in common use is the Lonely Star of Autumn, because it is the only first-magnitude star in the autumn sky of mid-northern latitudes. It has been recognized by many cultures of the northern hemisphere, including the Arabs, Persians and Chinese. Archaeological evidence links it to rituals dating back to about 2500 BC. It is one of the Persians' four "royal stars".

• The name Fom al-Haut comes from scientific Arabic فم الحوت fam al-ħūt (al-janūbī) "the mouth of the (southern) fish/whale"
• The Latin names are ōs piscis merīdiāni, ōs piscis merīdionālis, ōs piscis notii "the mouth of the southern fish"
• The name Difda al Auwel comes from the colloquial Arabic الضفدع الأول aḍ-ḍifdiˤ al-’awwal "the first frog" (the second frog is Beta Ceti)

See also

• Fomalhaut in fiction
• HR 8799

Notes

References

External links

• "Fomalhaut". SolStation. http://www.solstation.com/stars/fomalhau.htm. Retrieved November 23, 2005. 
• Preprint of planet discovery paper

Coordinates: 22^h 57^m 39.1^s, −29° 37′ 20″