Altair is the brightest

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 origins of the e ...

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 ...

. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...

Alpha Aquilae, which is Latinised from α Aquilae and abbreviated Alpha Aql or α Aql. Altair is an A-type

main-sequence star In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

with an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

of 0.77 and is one of the vertices of the

Summer Triangle The Summer Triangle is an astronomical asterism in the northern celestial hemisphere. The defining vertices of this imaginary triangle are at Altair, Deneb, and Vega, each of which is the brightest star of its constellation ( Aquila, Cygnus, ...

asterism; the other two vertices are marked by

Deneb Deneb () is a first-magnitude star in the constellation of Cygnus, the swan. Deneb is one of the vertices of the asterism known as the Summer Triangle and the "head" of the Northern Cross. It is the brightest star in Cygnus and the ...

and

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, an ...

. It is located at a distance of from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. Altair is currently in the

G-cloud The G-Cloud (or G-Cloud complex) is an interstellar cloud located next to the Local Interstellar Cloud, within the Local Bubble. It is unknown whether the Solar System is embedded in the Local Interstellar Cloud or in the region where the two c ...

—a nearby

interstellar cloud An interstellar cloud is generally an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in t ...

, an accumulation of gas and dust. Altair rotates rapidly, with a velocity at the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

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 The Palomar Testbed Interferometer (PTI) was a near infrared, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County, California, United States. It was built by Caltech and the Jet Propulsion Laboratory and w ...

revealed that Altair is not spherical, but is flattened at the poles due to its high rate of rotation. Other

interferometric Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...

studies with multiple telescopes, operating in the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

, 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 Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...

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

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

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, Latinized from β Aquilae, is a triple star system in the equatorial constellation of Aquila. It is visible to the naked eye as a point-like source with an apparent visual magnitude of 3.87. Based on parallax measurements ...

and

γ Aquilae Gamma Aquilae, Latinized from γ Aquilae, and formally known as Tarazed , is a star in the constellation of Aquila. It has an apparent visual magnitude of 2.712, making it readily visible to the naked eye at night. Parallax measureme ...

, 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 A type or type A may refer to: * A-type asteroid, a type of relatively uncommon inner-belt asteroids * A type blood, a type in the ABO blood group system * A-type inclusion, a type of cell inclusion * A-type potassium channel, a type of voltage-g ...

with about 1.8 times the

mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

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, but 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 mass more than two and a half times that of all the other planets in the Solar System combined, but ...

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 nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

. Like those two planets, its rapid rotation forces the star to be

oblate In Christianity (especially in the Roman Catholic, Orthodox, Anglican and Methodist traditions), an oblate is a person who is specifically dedicated to God or to God's service. Oblates are individuals, either laypersons or clergy, normally livi ...

; its equatorial diameter is over 20 percent greater than its polar diameter. AlphaAqlLightCurve

Satellite measurements made in 1999 with the

Wide Field Infrared Explorer Wide-field Infrared Explorer (WIRE, also Explorer 75 and SMEX-5) was a NASA satellite launched on 5 March 1999, on the Pegasus XL launch vehicle into polar orbit between above the surface of Earth. WIRE was intended to be a four-month infrared ...

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 A Delta Scuti variable (sometimes termed dwarf cepheid when the V-band amplitude is larger than 0.3 mag.) is a subclass of young pulsating star. These variables as well as classical cepheids are important standard candles and have been used to es ...

star. Its

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

can be approximated by adding together a number of

sine wave A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...

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

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

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 fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

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 A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

. 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

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 The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...

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 Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where the poles of a star are brighter than the equator, due to rapid rotation and oblate shape. When a star is oblate, it has a larger radius at its equat ...

or the von Zeipel effect, was confirmed for Altair by measurements made by the

Navy Precision Optical Interferometer The Navy Precision Optical Interferometer (NPOI) is an American astronomical interferometer, with the world's largest baselines, operated by the Naval Observatory Flagstaff Station (NOFS) in collaboration with the Naval Research Laboratory (NRL) ...

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 The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across ...

. Altair is one of the few stars for which a direct 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 mIRC is an Internet Relay Chat (IRC) client for Windows, created in 1995. It is a fully functional chat utility and its integrated scripting language makes it extensible and versatile. mIRC has been described as "one of the most popular IRC cl ...

instrument on the

CHARA array The CHARA (Center for High Angular Resolution Astronomy) array is an optical interferometer, located on Mount Wilson, California. The array consists of six telescopes operating as an astronomical interferometer. Construction was completed in 20 ...

interferometer; this was the first time the surface of any

, 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 radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: :1\,R_ = 6.957\tim ...

, 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 Muḥammad al-Akhṣāṣī al-Muwaqqit ( ar, محمد الاخصاصي الموقت) was an Egyptian astronomer whose and catalogue of stars, ('Pearls of brilliance upon the solar operations'), was written at Cairo about 1650. Al-Akhsasi was a s ...

's catalogue, which was translated into

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

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

and

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 ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclin ...

s of England and Western Europe depicted Altair and Vega as birds. The

Koori Koori (also spelt koorie, goori or goorie) is a demonym for Aboriginal Australians from a region that approximately corresponds to southern New South Wales and Victoria. The word derives from the Indigenous language Awabakal. For some people an ...

people of

Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...

also knew Altair as ''Bunjil'', the

wedge-tailed eagle The wedge-tailed eagle (''Aquila audax'') 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. Adults of this species have lon ...

, and β and γ Aquilae are his two wives the

black swan The black swan (''Cygnus atratus'') is a large 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 upon c ...

s. The people of the

Murray River The Murray River (in South Australia: River Murray) (Ngarrindjeri: ''Millewa'', Yorta Yorta: ''Tongala'') is a river in Southeastern Australia. It is Australia's longest river at extent. Its tributaries include five of the next six longest r ...

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 Delphinus (Pronounced or ) is a small constellation in the Northern Celestial Hemisphere, close to the celestial equator. Its name is the Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-Europea ...

. In Chinese belief, the asterism consisting of Altair, β Aquilae and γ Aquilae is known as ''Hé Gǔ'' (; lit. "river drum"). The

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...

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 "The Cowherd and the Weaver Girl" are characters found in Chinese mythology and characters appearing eponymously in a romantic Chinese folk tale. The story tells of the romance between Zhinü (; the weaver girl, symbolizing the star Vega) and ...

'', in which Niulang (represented by Altair) and his two children (represented by

and

) are separated from respectively their wife and mother Zhinu (represented by Vega) by the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. They are only permitted to meet once a year, when magpies form a bridge to allow them to cross the Milky Way. The people of

Micronesia Micronesia (, ) is a subregion of Oceania, consisting of about 2,000 small islands in the western Pacific Ocean. It has a close shared cultural history with three other island regions: the Philippines to the west, Polynesia to the east, and ...

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

Māori people The Māori (, ) are the indigenous Polynesian people of mainland New Zealand (). Māori originated with settlers from East Polynesia, who arrived in New Zealand in several waves of canoe voyages between roughly 1320 and 1350. Over several ce ...

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 claim to discern information about human affairs and terrestrial events by studying the apparent positions of Celestial o ...

, the star was ill-omened, portending danger from

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s. This star is one of the asterisms used by

Bugis The Bugis people (pronounced ), also known as Buginese, are an ethnicity—the most numerous of the three major linguistic and ethnic groups of South Sulawesi (the others being Makassar and Toraja), in the south-western province of Sulawe ...

sailors for navigation, called ''bintoéng timoro'', meaning "eastern star". NASA announced ''Altair'' as the name of the

Lunar Surface Access Module The Altair spacecraft, previously known as the Lunar Surface Access Module or LSAM, was the planned lander spacecraft component of NASA's cancelled Constellation program. Astronauts would have used the spacecraft for landings on the Moon, wh ...

(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 A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...

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

.
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 Aquila (constellation) A-type main-sequence stars 4 Aquilae, 53 Aquilae, Alpha 187642 097649 7557 Delta Scuti variables

Altair Altair is the brightest star in the constellation of Aquila and the twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinised from α Aquilae and abbreviated Alpha Aql or ...

BD+08 4236 G-Cloud Astronomical objects known since antiquity 0768