Altair (
Alpha Aquilae /
Alpha Aql /
α Aquilae /
α Aql /
Atair) is the brightest
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
in the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
AquilaAquila is a constellation. Its name is Latin for 'eagle' and it is commonly represented as such. It lies roughly at the celestial equator. The alpha star, Altair, is a vertex of the Summer Triangle asterism.-History:...
and the
twelfth brightest star in the night sky. It is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the
Summer TriangleThe Summer Triangle is an astronomical asterism involving an imaginary triangle drawn on the northern hemisphere's celestial sphere, with its defining vertices at Altair, Deneb, and Vega, being the brightest stars in the three constellations of Aquila, Cygnus, and Lyra.The English term was...
; the other two are
DenebDeneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...
and
VegaVega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
.
Altair rotates rapidly, with a velocity at the
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
of around 286
kmThe kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second....
/
sThe second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units base unit of time...
.
[From values of v sin i and i in the second column of Table 1, Monnier et al. 2007.] A study with the
Palomar Testbed InterferometerThe Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was built by Caltech/JPL and is intended to serve as a testbed for developing interferometric techniques to be used at the Keck 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
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
, have imaged this phenomenon.
Star
Altair is located 16.8 light-years (5.14
pcThe parsec is a unit of length, equal to just under 31 million million kilometres , or about 3.26 light-years. The parsec measurement unit is used in astronomy and is defined as the length of the adjacent side of an imaginary right triangle in space...
) from Earth and is one of the closest stars visible to the
naked eyeThe naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
. Along with
Beta AquilaeBeta Aquilae is a star in the constellation Aquila. It has the traditional name Alshain from the Perso-Arabic term الشاهين aš-šāhīn "the falcon", perhaps by folk etymology from the Persian šāhīn tarāzū , the Persian name for the asterism α, β and γ Aquilae.Beta Aquilae has...
and
Gamma AquilaeGamma Aquilae is a star in the constellation Aquila. It has the traditional name Tarazed , which appears to derive from Persian شاهين ترازو šāhin tarāzu "the beam of the scale", referring to an asterism of the Scale, α, β and γ Aquillae...
, 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
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
of the Sun and 11 times its luminosity. It is notable for its extremely rapid rotation; it has a rotational period of approximately 9 hours. For comparison, the equator of the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
requires just over 25 days for a complete rotation. 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 a few thousandths of a magnitude. As a result, it was identified in 2005 as a δ Scuti variable star. Its
light curveIn astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band. Light curves can be periodic, as in the case of eclipsing binaries, cepheid variables and other variables, or...
can be approximated by adding together a number of
sine waveThe sine wave or sinusoid is a function that occurs often in mathematics, music, physics, signal processing, audition, electrical engineering, and many other fields...
s, 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 coworkers at
Narrabri ObservatoryThe Narrabri Stellar Intensity Interferometer was the first astronomical instrument to measure the diameters of a large number of stars at visible wavelengths. It was designed by Robert Hanbury Brown, who received the Hughes Medal in 1971 for this work...
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
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
interferometric measurements made by the
Palomar Testbed InterferometerThe Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was built by Caltech/JPL and is intended to serve as a testbed for developing interferometric techniques to be used at the Keck Interferometer...
in 1999 and 2000. This work was published by G. T. van Belle and his co-authors in 2001.
Theory predicts that, owing to Altair's rapid rotation, its
surface gravityThe surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
and
effective temperatureThe 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...
should be lower at the equator, making the equator less luminous than the poles. This phenomenon, known as
gravity darkeningGravity darkening, also referred to as Gravity Brightening, is an astronomical phenomenon where a star rotates so rapidly that it has a detectably oblate shape, such as in Regulus in the Leo constellation....
or the von Zeipel effect, was confirmed for Altair by measurements made by the
Navy Prototype Optical InterferometerThe Navy Prototype Optical Interferometer is an astronomical interferometer operated by the US Naval Observatory , the Naval Research Laboratory and The Lowell Observatory. The facility is located on Anderson Mesa about 15 miles southeast of Flagstaff, Arizona...
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 arrayThe CHARA Array is an optical astronomical interferometer operated by The Center for High Angular Resolution Astronomy of the Georgia State University . CHARA is the World's highest angular resolution telescope at near-infrared wavelengths...
interferometer; this was the first time the surface of any main-sequence star, apart from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, had been imaged. The image was published in 2007 and can be seen above and to the left. In this image, North (the direction towards the North Celestial Pole) is up and East is left, and the white line is the rotational axis of Altair. The black grid shows lines of latitude and longitude in an Altair-centric coordinate system. The image is false-color, with brighter regions shown in white and darker regions in blue. The von Zeipel effect can be observed in the image, which shows a white spot near the pole and a darker equator. The equatorial radius of the star was estimated to be 2.03 solar radii, and the polar radius 1.63 solar radii—a 20% 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
ArabicArabic is a Central Semitic language, thus related to and classified alongside other Semitic languages such as Hebrew and the Neo-Aramaic languages. In terms of speakers, the Arabic macrolanguage is the largest member of the Semitic language family. It is spoken by more than 280 million people as...
phrase ,
an-nasr aţ-ţā’ir (
EnglishEnglish is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
:
The flying eagle). This name was applied by the Arabs to the
asterismIn astronomy, an asterism is a pattern of stars seen in Earth's sky which is not an official constellation. Like constellations, they are composed of stars which, while they are in the same general direction, are not physically related, often being at significantly different distances from Earth....
of α,
βBeta Aquilae is a star in the constellation Aquila. It has the traditional name Alshain from the Perso-Arabic term الشاهين aš-šāhīn "the falcon", perhaps by folk etymology from the Persian šāhīn tarāzū , the Persian name for the asterism α, β and γ Aquilae.Beta Aquilae has...
, and
γ AquilaeGamma Aquilae is a star in the constellation Aquila. It has the traditional name Tarazed , which appears to derive from Persian شاهين ترازو šāhin tarāzu "the beam of the scale", referring to an asterism of the Scale, α, β and γ Aquillae...
and probably goes back to the ancient Babylonians and Sumerians, who called α Aquilae the eagle star. The spelling
Atair has also been used. The
KooriThe Koori are the indigenous Australians that traditionally occupied modern day New South Wales and Victoria.Many indigenous Australians object to the use of the terms 'Aborigine' and 'Aboriginal', as terms which had been forced on them. They prefer to use words from their own languages...
people of Victoria also knew Altair as
Bunjil, the
Wedge-tailed EagleThe Wedge-tailed Eagle or Eaglehawk is the largest raptor in Australia and is the most common of all the world's large eagles. It has long, fairly broad wings, fully feathered legs, and an unmistakable wedge-shaped tail...
, and β and γ Aquilae are his two wives the
Black SwanThe Black Swan is a large waterbird which breeds mainly in the southeast and southwest regions of Australia.-Taxonomy:The black swan was first described by English naturalist John Latham in 1790...
s. The people of the
Murray RiverThe Murray River, or River Murray and sometimes informally referred to as the "Mighty Murray", is Australia's largest river. At in length, the Murray rises in the Australian Alps, draining the western side of Australia's highest mountains and, for most of its length, meanders across Australia's...
knew the star as
Totyerguil. The Murray River was formed when
Totyerguil the hunter speared
Otjout, a giant
Murray CodThe Murray cod is a large Australian predatory freshwater fish of the Maccullochella genus and the Percichthyidae family. Although the species is a called cod in the vernacular, it is not related to the northern hemisphere marine cod species...
, who, when wounded, churned a channel across southern Australia before entering the sky as the constellation
DelphinusDelphinus is a constellation in the northern sky, close to the celestial equator. Its name is Latin for dolphin. It is one of the smaller constellation, ranked 69th in size out of 88...
.
In
ChineseChinese or the Sinitic language is a language family consisting of languages mutually unintelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the two branches of Sino-Tibetan family of languages...
, the asterism consisting of α, β, and γ Aquilae is known as ' onMouseout='HidePop("46222")' href="http://www.absoluteastronomy.com/topics/English_language">English
English is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
:
River Drum). Altair is thus known as ' onMouseout='HidePop("20908")' href="http://www.absoluteastronomy.com/topics/English_language">English
English is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
:
the Second Star of the Drum at the River, or more literally,
Riverdrum II). However, it is better known by its other names: or , or in
EnglishEnglish is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
,
Star of the Cowherd. These names are from the love story of ,
Qī XīQixi Festival , also known as Magpie Festival, falls on the seventh day of the seventh lunar month on the Chinese calendar; thus its name. It also inspired Tanabata in Japan, Chilseok in Korea, and That Tich in Vietnam...
, in which Niú Láng and his two children, β and γ Aquilae, are separated from their mother, ' onMouseout='HidePop("44965")' href="http://www.absoluteastronomy.com/topics/English_language">English
English is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
:
Weaver Girl), the star
VegaVega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
, by the
Milky WayThe Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies...
. They are only permitted to meet once a year, when the Milky Way is crossed by a bridge of magpies. The Japanese festival, in which Altair is known as , is based on this legend.
In
astrologyAstrology is a group of systems, traditions, and beliefs which hold that the relative positions of celestial bodies and related details can provide information about personality, human affairs, and other terrestrial matters. A practitioner of astrology is called an astrologer...
, the star Altair was ill-omened, portending danger from
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s.
The NASA Constellation Program announced
Altair as the name of the
Lunar Surface Access ModuleThe Altair spacecraft, previously known as the Lunar Surface Access Module or LSAM, is the planned lander spacecraft component of NASA's Project Constellation, which astronauts are to use for landings on the Moon intended to begin around 2019...
(LSAM) on December 13, 2007. The Russian-made
Beriev Be-200 AltairThe Beriev Be-200 Altair is a multipurpose amphibious aircraft designed by the Beriev Aircraft Company and manufactured by Irkut. Marketed as being designed for fire fighting, search and rescue, maritime patrol, cargo and passenger transportation, it has a capacity of 12 tonnes of water, or up to...
seaplane is also named after the star.
Visual companions
The A-type main sequence star has the
multiple starA multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...
designation
WDSThe Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for 102,387 pairs of double stars. The catalog also includes multiple stars...
19508+0852A and has three visual companion
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
s,
WDSThe Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for 102,387 pairs of double stars. The catalog also includes multiple stars...
19508+0852B, C, and D. Component B is not physically close to A but merely appears close to it in the sky.
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
The Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was built by Caltech/JPL and is intended to serve as a testbed for developing interferometric techniques to be used at the Keck 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 at Alycone Software's Star Data Pages.