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=                               Altair                               =
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                            Introduction
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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. It is located at a distance of 16.7 ly from the Sun. Altair is
currently in the G-cloud—a nearby interstellar cloud, an accumulation
of gas and dust.

Altair rotates rapidly, with a velocity at the equator of
approximately 286 km/s. 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, have
imaged and confirmed this phenomenon.


                            Nomenclature
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'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  'Al-Nisr
Al-Ṭa'ir', "".

In 2016, the International Astronomical Union organized a Working
Group on Star Names (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
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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'.

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 Sun 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 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.

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 can be
approximated by adding together a number of sine waves, with periods
that range between 0.8 and 1.5 hours. 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.


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 3milliarcseconds. 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.

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). 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 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 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,
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
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left
The term 'Al Nesr Al Tair' appeared in Al Achsasi al Mouakket's
catalogue, which was translated into Latin as 'Vultur Volans'. 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". The spelling 'Atair'
has also been used. Medieval astrolabes 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, and β and γ Aquilae are his two wives the black
swans. The people of the Murray River knew the star as 'Totyerguil'.
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 belief, 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'. 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
Zhinu (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.

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".

In Western astrology, the star was ill-omened, portending danger from
reptiles.

This star is one of the asterisms used by Bugis sailors for
navigation, called 'bintoéng timoro', meaning "eastern star".

A group of Japanese 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
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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. All are much more distant than Altair
and not physically associated.


                              See also
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* Lists of stars
* List of brightest stars
* List of nearest bright stars
* Historical brightest stars
* List of most luminous stars


                           External links
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*
[https://web.archive.org/web/20090414054427/http://origins.jpl.nasa.gov/library/story/072501-a.html
Star with Midriff Bulge Eyed by Astronomers], JPL press release, July
25, 2001.
* [https://sl.voxastro.org/library/UVES-POP/details/?star=Altair
Spectrum of Altair]
* [http://www.astro.lsa.umich.edu/~monnier/Altair2007/altair2007.html
Imaging the Surface of Altair], University of Michigan news release
detailing the CHARA array direct imaging of the stellar surface in
2007.
* [http://photojournal.jpl.nasa.gov/catalog/PIA04204 PIA04204:
Altair], NASA. Image of Altair from the Palomar Testbed
Interferometer.
* [http://www.solstation.com/stars/altair.htm Altair], 'SolStation'.
* [http://news.bbc.co.uk/2/hi/science/nature/6709345.stm Secrets of
Sun-like star probed], 'BBC News', June 1, 2007.
* [http://www.astromart.com/news/news.asp?news_id=697 Astronomers
Capture First Images of the Surface Features of Altair] ,
'Astromart.com'
*
[http://aladin.u-strasbg.fr/AladinPreview?-c=NAME+ALTAIR&ident=NAME+ALTAIR&submit=Aladin+previewer
Image of Altair] from Aladin.


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