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.
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 النسر الطائرAn-nisr Al-ta'ir "the flying eagle".
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.
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.
Altair is a type-Amain sequence star with about 1.8 times the mass of the Sun and 11 times its luminosity. Altair rotates rapidly, with a rotational period of about 9 hours; for comparison, the equator of the Sun makes a complete rotation in a little more than 25 days. Its 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 coronalX-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.
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.
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, the asterism consisting of Altair, β Aquilae, and γ Aquilae is known as Hé Gǔ (河鼓; lit. "river drum"). The Chinese name for Altair is thus Hé Gǔ èr (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: Qiān Niú Xīng (牵牛星) or Niú Láng Xīng (牛郎星), 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".
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
^ abcdeDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
^ abcdeResolving the Effects of Rotation in Altair with Long-Baseline Interferometry, D. M. Peterson et al., The Astrophysical Journal636, #2 (January 2006), pp. 1087–1097, doi:10.1086/497981, Bibcode: 2006ApJ...636.1087P; see Table 2 for stellar parameters.
^Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M
^Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer, Naoko Ohishi, Tyler E. Nordgren, and Donald J. Hutter, The Astrophysical Journal612, #1 (September 1, 2004), pp. 463–471, doi:10.1086/422422, Bibcode: 2004ApJ...612..463O.