Altair rotates rapidly, with a velocity at the
equator of approximately 286 km/s.[nb 2][11] This is a significant fraction of the star's estimated breakup speed of 400 km/s.[19] 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.[20] Other
interferometric studies with multiple telescopes, operating in the
infrared, have imaged and confirmed this phenomenon.[11]
Nomenclature
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 الطائرAl-Nisr Al-Ṭa'ir, "the flying eagle".[21]
In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[22] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[23] 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.[24]
Physical characteristics
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.[16]: 190
Altair is a
type-Amain-sequence star with about 1.8 times the
mass of the Sun and 11 times
its luminosity.[11][9] 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.[8] Altair rotates rapidly, with a rotational period of under eight hours;[8] 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.[11]
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.[6] 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.[6] 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.[19]
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).[9][26] 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.[27]
Altair is one of the few
stars for which a direct image has been obtained.[28] 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.[28] 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.[11] The polar axis is inclined by about 60° to the line of sight from the Earth.[19]
Etymology, mythology and culture
The term Al Nesr Al Tair appeared in
Al Achsasi al Mouakket's catalogue, which was translated into
Latin as Vultur Volans.[29] 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".[2]: 17–18 The spelling Atair has also been used.[30] Medieval
astrolabes of England and Western Europe depicted Altair and Vega as birds.[31]
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.[32]: 4 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.[32]: 115
In Chinese belief, the asterism consisting of Altair, β Aquilae and γ Aquilae is known as Hé Gǔ (河鼓; lit. "river drum").[30] 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").[33] However, Altair is better known by its other names: Qiān Niú Xīng (牵牛星 / 牽牛星) or Niú Láng Xīng (牛郎星), translated as the cowherd star.[34][35] 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.[35][36]
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".[37]
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.[13] All are much more distant than Altair and not physically associated.[42]
Multiple/double star designation:
WDS 19508+0852[13]
^
abKunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.
ISBN978-1-931559-44-7.
^
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.
^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
^
abcdefMonnier, J. D.; Zhao, M; Pedretti, E; Thureau, N; Ireland, M; Muirhead, P; Berger, J. P.; Millan-Gabet, R; Van Belle, G; Ten Brummelaar, T; McAlister, H; Ridgway, S; Turner, N; Sturmann, L; Sturmann, J; Berger, D (2007). "Imaging the surface of Altair". Science. 317 (5836): 342–345.
arXiv:0706.0867.
Bibcode:
2007Sci...317..342M.
doi:
10.1126/science.1143205.
PMID17540860.
S2CID4615273. See second column of Table 1 for stellar parameters.
^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.
^
abHoboken, Fred Schaaf (2008). The brightest stars : discovering the universe through the sky's most brilliant stars. New Jersey: John Wiley & Sons, Inc.
ISBN978-0-471-70410-2.
OCLC440257051.
^
abAboriginal mythology: an A-Z spanning the history of aboriginal mythology from the earliest legends to the present day, Mudrooroo, London: HarperCollins, 1994,
ISBN1-85538-306-3.
^
abp. 72, China, Japan, Korea Culture and Customs: Culture and Customs, Ju Brown and John Brown, 2006,
ISBN978-1-4196-4893-9.
^pp. 105–107, Magic Lotus Lantern and Other Tales from the Han Chinese, Haiwang Yuan and Michael Ann Williams, Libraries Unlimited, 2006,
ISBN978-1-59158-294-6.