Altair is the brightest

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Aquila and the twelfth-brightest star in the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

Alpha Aquilae, which is Latinised from α Aquilae and abbreviated Alpha Aql or α Aql. Altair is an A-type main-sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle asterism; the other two vertices are marked by Deneb and Vega. It is located at a distance of from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. Altair is currently in the G-cloud—a nearby interstellar cloud formed from an accumulation of gas and dust. Altair rotates rapidly, with a velocity at the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

of approximately 286 km/s.From values of ''v'' sin ''i'' and ''i'' in the second column of Table 1, Monnier et al. 2007. This is a significant fraction of the star's estimated breakup speed of 400 km/s. A study with the Palomar Testbed Interferometer revealed that Altair is not spherical, but is flattened at the poles due to its high rate of rotation. Other interferometric studies with multiple telescopes, operating in the

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

, have imaged and confirmed this phenomenon. See second column of Table 1 for stellar parameters.

Nomenclature

''α Aquilae'' ( Latinised to ''Alpha Aquilae'') is the star's

. The traditional name ''Altair'' has been used since medieval times. It is an abbreviation of the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

phrase ''Al-Nisr Al-Ṭa'ir'', "". In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included ''Altair'' for this star. It is now so entered in the IAU Catalog of Star Names.

Physical characteristics

Along with

β Aquilae Beta Aquilae is a triple star system in the equatorial constellation of Aquila. Its name is a Bayer designation that is Latinized from β Aquilae, and abbreviated Beta Aql or β Aql. This system is visible to the naked eye as a po ...

and γ Aquilae, Altair forms the well-known line of stars sometimes referred to as the ''Family of Aquila'' or ''Shaft of Aquila''. Altair is a type-A main-sequence star with about 1.8 times the mass of the Sun and 11 times its luminosity. It is thought to be a young star close to the zero age main sequence at about 100 million years old, although previous estimates gave an age closer to one billion years old. Altair rotates rapidly, with a rotational period of under eight hours; for comparison, the equator of the

makes a complete rotation in a little more than 25 days. Altair's rotation is similar to, and slightly faster than, those of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

and

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

. Like those two planets, its rapid rotation causes the star to be oblate; its equatorial diameter is over 20 percent greater than its polar diameter. AlphaAqlLightCurve

Satellite measurements made in 1999 with the Wide Field Infrared Explorer showed that the brightness of Altair fluctuates slightly, varying by just a few thousandths of a magnitude with several different periods less than 2 hours. As a result, it was identified in 2005 as a Delta Scuti variable star. Its

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

can be approximated by adding together a number of

sine wave A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...

s, with periods that range between 0.8 and 1.5 hours. It is a weak source of coronal

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

emission, with the most active sources of emission being located near the star's equator. This activity may be due to

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

cells forming at the cooler equator.

Rotational effects

The angular diameter of Altair was measured interferometrically by R. Hanbury Brown and his co-workers at Narrabri Observatory in the 1960s. They found a diameter of 3 milliarcseconds. Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Later, using

interferometric measurements made by the Palomar Testbed Interferometer in 1999 and 2000, Altair was found to be flattened. This work was published by G. T. van Belle, David R. Ciardi and their co-authors in 2001. Theory predicts that, owing to Altair's rapid rotation, its surface gravity and

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

should be lower at the equator, making the equator less luminous than the poles. This phenomenon, known as gravity darkening or the von Zeipel effect, was confirmed for Altair by measurements made by the Navy Precision Optical Interferometer in 2001, and analyzed by Ohishi et al. (2004) and Peterson et al. (2006). Also, A. Domiciano de Souza et al. (2005) verified gravity darkening using the measurements made by the Palomar and Navy interferometers, together with new measurements made by the VINCI instrument at the VLTI. Altair is one of the few stars for which a resolved image has been obtained. In 2006 and 2007, J. D. Monnier and his coworkers produced an image of Altair's surface from 2006 infrared observations made with the MIRC instrument on the CHARA array interferometer; this was the first time the surface of any main-sequence star, apart from the Sun, had been imaged. The false-color image was published in 2007. The equatorial radius of the star was estimated to be 2.03

solar radii Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicat ...

, and the polar radius 1.63 solar radii—a 25% increase of the stellar radius from pole to equator. The polar axis is inclined by about 60° to the line of sight from the Earth.

Etymology, mythology and culture

The term ''Al Nesr Al Tair'' appeared in Al Achsasi al Mouakket's catalogue, which was translated into

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

as ''Vultur Volans''. This name was applied by the Arabs to the asterism of Altair,

and γ Aquilae and probably goes back to the ancient Babylonians and Sumerians, who called Altair "the eagle star". The spelling ''Atair'' has also been used. Medieval

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

s of England and Western Europe depicted Altair and Vega as birds. The Koori people of Victoria also knew Altair as ''Bunjil'', the

wedge-tailed eagle The wedge-tailed eagle (''Aquila audax'') also known as the eaglehawk, is the largest bird of prey in the continent of Australia. It is also found in southern New Guinea to the north and is distributed as far south as the state of Tasmania. A ...

, and β and γ Aquilae are his two wives the

black swan The black swan (''Cygnus atratus'') is a large Anatidae, waterbird, a species of swan which breeds mainly in the southeast and southwest regions of Australia. Within Australia, the black swan is nomadic, with erratic migration patterns dependent ...

s. The people of the

Murray River The Murray River (in South Australia: River Murray; Ngarrindjeri language, Ngarrindjeri: ''Millewa'', Yorta Yorta language, Yorta Yorta: ''Dhungala'' or ''Tongala'') is a river in Southeastern Australia. It is List of rivers of Australia, Aust ...

knew the star as ''Totyerguil''.''Aboriginal mythology: an A-Z spanning the history of aboriginal mythology from the earliest legends to the present day'', Mudrooroo, London: HarperCollins, 1994, . The Murray River was formed when ''Totyerguil'' the hunter speared ''Otjout'', a giant Murray cod, who, when wounded, churned a channel across southern Australia before entering the sky as the constellation Delphinus. In Chinese astronomy, the asterism consisting of Altair, β Aquilae and γ Aquilae is known as ''Hé Gǔ'' (; lit. "river drum"). The Chinese name for Altair is thus ''Hé Gǔ èr'' (; lit. "river drum two", meaning the "second star of the drum at the river"). However, Altair is better known by its other names: ''Qiān Niú Xīng'' ( / ) or ''Niú Láng Xīng'' (), translated as the ''cowherd star''.p. 72, ''China, Japan, Korea Culture and Customs: Culture and Customs'', Ju Brown and John Brown, 2006, . These names are an allusion to a love story, '' The Cowherd and the Weaver Girl'', in which Niulang (represented by Altair) and his two children (represented by

and γ Aquilae) are separated from respectively their wife and mother Zhinü (represented by Vega) by the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

. They are only permitted to meet once a year, when magpies form a bridge to allow them to cross the Milky Way. In the Japanese version of this legend, celebrated in the

Tanabata , also known as the , is a Japanese festival originating from the Chinese Qixi Festival. It celebrates the meeting of the deities Orihime and Hikoboshi (represented by the stars Vega and Altair respectively). According to legend, the Milk ...

festival, Altair is known as ''Hikoboshi'' (彦星). The people of

Micronesia Micronesia (, ) is a subregion of Oceania, consisting of approximately 2,000 small islands in the Northwestern Pacific Ocean. It has a close shared cultural history with three other island regions: Maritime Southeast Asia to the west, Poly ...

called Altair ''Mai-lapa'', meaning "big/old breadfruit", while the

Māori people Māori () are the Indigenous peoples of Oceania, indigenous Polynesians, Polynesian people of mainland New Zealand. Māori originated with settlers from East Polynesia, who arrived in New Zealand in several waves of Māori migration canoes, c ...

called this star ''Poutu-te-rangi'', meaning "pillar of heaven". In Western

astrology Astrology is a range of Divination, divinatory practices, recognized as pseudoscientific since the 18th century, that propose that information about human affairs and terrestrial events may be discerned by studying the apparent positions ...

, the star was ill-omened, portending danger from

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s. This star is one of the asterisms used by

Bugis The Bugis people, also known as Buginese, are an Austronesian ethnic groupthe most numerous of the three major linguistic and ethnic groups of South Sulawesi (the others being Makassarese and Torajan), in the south-western province of Sula ...

sailors for navigation, called ''bintoéng timoro'', meaning "eastern star". A group of

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

ese scientists sent a radio signal to Altair in 1983 with the hopes of contacting extraterrestrial life. NASA announced ''Altair'' as the name of the Lunar Surface Access Module (LSAM) on December 13, 2007. The Russian-made Beriev Be-200 Altair seaplane is also named after the star.

Visual companions

The bright primary

has the multiple star designation WDS 19508+0852A and has several faint visual companion stars, WDS 19508+0852B, C, D, E, F and G. All are much more distant than Altair and not physically associated.

Notes

References

External links

Star with Midriff Bulge Eyed by Astronomers
JPL press release, July 25, 2001.
Spectrum of Altair

University of Michigan news release detailing the CHARA array direct imaging of the stellar surface in 2007.
PIA04204: Altair
NASA. Image of Altair from the Palomar Testbed Interferometer.
Altair
''SolStation''.
Secrets of Sun-like star probed
''BBC News'', June 1, 2007.
Astronomers Capture First Images of the Surface Features of Altair
, ''Astromart.com''
Image of Altair
from Aladin. {{DEFAULTSORT:Altair A-type main-sequence stars Delta Scuti variables 4 G-Cloud Aquila (constellation) Aquilae, Alpha 7557 BD+08 4236 Aquilae, 53 0768 187642 097649

Altair Altair is the brightest star in the constellation of Aquila (constellation), Aquila and the list of brightest stars, twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinisation of name ...

TIC objects Astronomical objects known since antiquity