Altair is the brightest star in the constellation Aquila and the twelfth brightest star in the
night sky. It is currently in the G-cloud. 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.
It is 16.7 light-years from Earth and is one of the closest stars visible to the naked
eye. Altair rotates rapidly, with a velocity at
the equator of approximately 286 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.
Characteristics
Altair is located 16.7 light-years from Earth and is one of the closest stars visible to
the naked eye. Along with Beta Aquilae and Gamma Aquilae, it 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 approximately 1.8 times the mass of the Sun
and 11 times its luminosity. Altair possesses an extremely rapid rate of rotation; it has
a rotational period of approximately 9 hours. For comparison, the equator of the Sun requires
a little more than 25 days for a complete rotation. This rapid rotation forces Altair
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 less than a thousandth of a magnitude.
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. Oblateness and surface temperature
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. Altair was later observed
to be flattened by infrared interferometric measurements made by the Palomar Testbed Interferometer
in 1999 and 2000. 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 Prototype
Optical Interferometer in 2001, and analyzed by Ohishi et al. and Peterson et al.. Also,
A. Domiciano de Souza et al. 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.
Etymology, mythology, and culture The name Altair has been used since medieval
times. It is an abbreviation of the Arabic phrase النسر الطائر, an-nasr aṭ-ṭā’ir.
The term Al Nesr Al Tair appeared in Al Achsasi Al Mouakket catalogue, which was translated
into Latin as Vultur Volans. This name was applied by the Arabs to the asterism of α,
β, and γ Aquilae and probably goes back to the ancient Babylonians and Sumerians,
who called α Aquilae 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, the asterism consisting of α, β, and γ Aquilae is known as hegu. Altair
is thus known as hegu er. However, Altair is better known by its other
names: qianniu xing or niulang xing, translated as the cowherd star. These names are an allusion
to a love story, The Weaver Girl and the Cowherd, in which Niulang and his two children are
separated from respectively their wife and mother Zhinu 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 as
Mai-lapa, "big/old breadfruit", while the Māori people called this star as Poutu-te-rangi,
"pillar of heaven". Japan Airlines's Starjet 777-200 JA8983 was
named Altair. In astrology, the star Altair was ill-omened,
portending danger from reptiles. The NASA Constellation Program announced Altair
as the name of the Lunar Surface Access Module on December 13, 2007. The Russian-made Beriev
Be-200 Altair seaplane is also named after the star.
The Altair 8800 was one of the first microcomputers intended for home use.
Altair is the name of three United States navy ships: USS Altair (AD-11), USS Altair (AK-257)
and USNS Altair. Altair is the name of a 1919 poem by Karle
Wilson Baker.
Visual companions The A-type main sequence star has the multiple
star designation WDS 19508+0852A and has three visual companion stars, WDS 19508+0852B,
C, and D. Component B is not physically close to A but merely appears close to it in the
sky.
See also Altair in fiction
Astronomy in medieval Islam Notes
References
External links Star with Midriff Bulge Eyed by Astronomers,
JPL press release, July 25, 2001. Imaging the Surface 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 at 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. Altair on Constellation Guide