Altair

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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 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.<ref name=sb0/><ref name=darlingaltair/><ref name="darlingsummer">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is located at a distance of Template:Convert from the Sun.<ref name="schaaf2008">Template:Cite book</ref>Template:Citation page Altair is currently in the G-cloud—a nearby interstellar cloud formed from an accumulation of gas and dust.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite news</ref>

Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s.<ref group=nb>From values of v sin i and i in the second column of Table 1, Monnier et al. 2007.</ref><ref name=monnier07/> This is a significant fraction of the star's estimated breakup speed of 400 km/s.<ref name=robrade2009/> 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.<ref name="pti2001">Template:Cite journal</ref> Other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon.<ref name="monnier07">Template:Cite journal See second column of Table 1 for stellar parameters.</ref>

NomenclatureEdit

File:AquilaCC.jpg

Altair is the brightest star in the constellation Aquila.

α Aquilae (Latinised to Alpha Aquilae) is the star's Bayer designation. The traditional name Altair has been used since medieval times. It is an abbreviation of the Arabic phrase {{#invoke:Lang|lang}} Al-Nisr Al-Ṭa'ir, "{{#invoke:Lang|lang}}".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name="WGSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name="WGSN1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="IAU-CSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Template:Clear left

Physical characteristicsEdit

File:Altair-Sun comparison.png

Altair in comparison with the Sun

Along with β Aquilae and γ Aquilae, Altair forms the well-known line of stars sometimes referred to as the Family of Aquila or Shaft of Aquila.<ref name="schaaf2008" />Template:Citation page

Altair is a type-A main-sequence star with about 1.8 times the mass of the Sun and 11 times its luminosity.<ref name=monnier07/><ref name=peterson06/> 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.<ref name=bouchaud2020/> Altair rotates rapidly, with a rotational period of under eight hours;<ref name=bouchaud2020/> for comparison, the equator of the Sun makes a complete rotation in a little more than 25 days. Altair's rotation is similar to, and slightly faster than, those of Jupiter and Saturn. 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.<ref name=monnier07/>

File:AlphaAqlLightCurve.png

A light curve for Altair, adapted from Buzasi et al. (2005)<ref name="Buzasi et al 2005"/>

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.<ref name="Buzasi et al 2005"/> As a result, it was identified in 2005 as a Delta Scuti variable star. Its light curve can be approximated by adding together a number of sine waves, with periods that range between 0.8 and 1.5 hours.<ref name="Buzasi et al 2005">Template:Cite journal</ref> It is a weak source of coronal X-ray emission, with the most active sources of emission being located near the star's equator. This activity may be due to convection cells forming at the cooler equator.<ref name=robrade2009/> Template:Clear left

Rotational effectsEdit

File:Altair PR image6 (white).jpg

Direct image of Altair, taken with the CHARA array

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 3Template:Nbsp milliarcseconds.<ref>Template:Cite journal</ref> Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Later, using infrared 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.<ref name=pti2001/>

Theory predicts that, owing to Altair's rapid rotation, its surface gravity and effective 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).<ref name=peterson06/><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>

Altair is one of the few stars for which a resolved image has been obtained.<ref name=nsf>Template:Cite press release</ref> 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.<ref name=nsf/> The false-color image was published in 2007. The equatorial radius of the star was estimated to be 2.03 solar radii, and the polar radius 1.63 solar radii—a 25% increase of the stellar radius from pole to equator.<ref name=monnier07/> The polar axis is inclined by about 60° to the line of sight from the Earth.<ref name=robrade2009/> Template:Clear left

Etymology, mythology and cultureEdit

File:Altair.jpg

Altair

The term Al Nesr Al Tair appeared in Al Achsasi al Mouakket's catalogue, which was translated into Latin as Vultur Volans.<ref>Template:Cite journal</ref> This name was applied by the Arabs to the asterism of Altair, β Aquilae and γ Aquilae and probably goes back to the ancient Babylonians and Sumerians, who called Altair "the eagle star".<ref name="Kunitzsch" />Template:Citation page The spelling Atair has also been used.<ref name="allen">Template:Cite book</ref> Medieval astrolabes of England and Western Europe depicted Altair and Vega as birds.<ref>Template:Cite journal</ref>

The Koori people of Victoria also knew Altair as Bunjil, the wedge-tailed eagle, and β and γ Aquilae are his two wives the black swans. The people of the Murray River knew the star as Totyerguil.<ref name="mudrooroo1994">Aboriginal mythology: an A-Z spanning the history of aboriginal mythology from the earliest legends to the present day, Mudrooroo, London: HarperCollins, 1994, Template:ISBN.</ref>Template:Citation page 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.<ref name="mudrooroo1994" />Template:Citation page

In Chinese astronomy, the asterism consisting of Altair, β Aquilae and γ Aquilae is known as Hé Gǔ ({{#invoke:Lang|lang}}; lit. "river drum").<ref name=allen/> The Chinese name for Altair is thus Hé Gǔ èr ({{#invoke:Lang|lang}}; lit. "river drum two", meaning the "second star of the drum at the river").<ref>Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed on line November 26, 2008.</ref> However, Altair is better known by its other names: Qiān Niú Xīng ({{#invoke:Lang|lang}} / {{#invoke:Lang|lang}}) or Niú Láng Xīng ({{#invoke:Lang|lang}}), translated as the cowherd star.<ref>Template:Cite book</ref><ref name=brown>p. 72, China, Japan, Korea Culture and Customs: Culture and Customs, Ju Brown and John Brown, 2006, Template:ISBN.</ref> 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 β Aquilae and γ Aquilae) are separated from respectively their wife and mother Zhinü (represented by Vega) by the Milky Way. They are only permitted to meet once a year, when magpies form a bridge to allow them to cross the Milky Way.<ref name=brown/><ref>pp. 105–107, Magic Lotus Lantern and Other Tales from the Han Chinese, Haiwang Yuan and Michael Ann Williams, Libraries Unlimited, 2006, Template:ISBN.</ref> In the Japanese version of this legend, celebrated in the Tanabata festival, Altair is known as Hikoboshi (彦星).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The people of Micronesia called Altair Mai-lapa, meaning "big/old breadfruit", while the Māori people called this star Poutu-te-rangi, meaning "pillar of heaven".<ref>Template:Cite book</ref>

In Western astrology, the star was ill-omened, portending danger from reptiles.<ref name=allen/>

This star is one of the asterisms used by Bugis sailors for navigation, called bintoéng timoro, meaning "eastern star".<ref name="kelley11">Template:Cite book</ref>

A group of Japanese scientists sent a radio signal to Altair in 1983 with the hopes of contacting extraterrestrial life.<ref>Template:Cite news</ref>

NASA announced Altair as the name of the Lunar Surface Access Module (LSAM) on December 13, 2007.<ref>Template:Cite news</ref> The Russian-made Beriev Be-200 Altair seaplane is also named after the star.<ref>Template:Cite press release</ref> Template:Clear left

Visual companionsEdit

The bright primary star has the multiple star designation WDS 19508+0852A and has several faint visual companion stars, WDS 19508+0852B, C, D, E, F and G.<ref name="wds" /> All are much more distant than Altair and not physically associated.<ref>Template:Cite DR2</ref>

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