Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||19h 50m 46.99855s|
|Declination||+08° 52′ 05.9563″|
|Apparent magnitude (V)||0.76|
|Evolutionary stage||Main sequence|
|Spectral type||A7 V|
|U−B color index||+0.09|
|B−V color index||+0.22|
|V−R color index||+0.14|
|R−I color index||+0.13|
|Variable type||Delta Scuti|
|Radial velocity (Rv)||−26.1 ± 0.9 km/s|
|Proper motion (μ)||RA: +536.23 mas/yr
Dec.: +385.29 mas/yr
|Parallax (π)||194.95 ± 0.57 mas|
|Distance||16.73 ± 0.05 ly
(5.13 ± 0.01 pc)
|Absolute magnitude (MV)||2.22|
|Mass||1.79 ± 0.018 M☉|
|Radius||1.63 to 2.03[nb 1] R☉|
|Surface gravity (log g)||4.29 cgs|
|Temperature||6,900 to 8,500[nb 1] K|
|Metallicity [Fe/H]||−0.2 dex|
|Rotational velocity (v sin i)||240 km/s|
Altair (/, , , /; α Aquilae, α Aql) is the brightest star in the constellation Aquila and the twelfth brightest star in the night sky. It is currently in the G-cloud—a nearby accumulation of gas and dust known as an interstellar 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 (the other two vertices are marked by Deneb and Vega). It is 16.7 light-years (5.13 parsecs) 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.[nb 2] 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.
Altair is located 16.7 light-years (5.13 parsecs) 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 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.
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. (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
The name Altair has been used since medieval times. It is an abbreviation of the Arabic phrase النسر الطائر, al-nesr al-ṭā’ir ("the flying eagle"). 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 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 (河鼓; lit. "river drum"). Altair is thus known as hegu er (河鼓二; lit. "river drum two", meaning the "second star of the drum at the river").
However, Altair is better known by its other names: qianniu xing (牵牛星) or niulang xing (牛郎星), translated as the cowherd star. These names are an allusion to a love story, The Weaver Girl and the Cowherd, in which Niulang (represented by Altair) and his two children (represented by β 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.
Japan Airlines's Starjet 777-200 JA8983 was named Altair.
Altair Airlines was a regional airline that operated out of Philadelphia from 1966 to 1982.
The NASA Constellation Program 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.
The Altair 8800 was one of the first microcomputers intended for home use.
Altair is the name of a 1919 poem by Karle Wilson Baker.
Three of them walk together-
She is the fairest of three;
And sweet as the heavenly weather
She maketh the heart of me!"
The bright primary star has the multiple star designation WDS 19508+0852A and has three faint 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.
|Multiple/double star designation: WDS 19508+0852|
|B||A||19h 50m 40.5s||+08° 52′ 13″||2007||192.1″||287°||9.82||Simbad|
|C||A||19h 51m 00.8s||+08° 50′ 58″||2007||189.6″||107°||10.3||Simbad|
- Owing to its rapid rotation, Altair's radius is larger at its equator than at its poles; it is also cooler at the equator than at the poles.
- From values of v sin i and i in the second column of Table 1, Monnier et al. 2007.
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- Resolving the Effects of Rotation in Altair with Long-Baseline Interferometry, D. M. Peterson et al., The Astrophysical Journal 636, #2 (January 2006), pp. 1087–1097, doi:10.1086/497981, Bibcode: 2006ApJ...636.1087P ; see Table 2 for stellar parameters.
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- HR 7557, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line November 25, 2008.
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- Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer, Naoko Ohishi, Tyler E. Nordgren, and Donald J. Hutter, The Astrophysical Journal 612, #1 (September 1, 2004), pp. 463–471, doi:10.1086/422422, Bibcode: 2004ApJ...612..463O .
- Gravitational-darkening of Altair from interferometry, A. Domiciano de Souza, P. Kervella, S. Jankov, F. Vakili, N. Ohishi, T. E. Nordgren, and L. Abe, Astronomy and Astrophysics 442, #2 (November 2005), pp. 567–578, doi:10.1051/0004-6361:20042476, Bibcode: 2005A&A...442..567D .
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|Wikimedia Commons has media related to Altair.|
- 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