Editing Altair (section)

==Physical characteristics==
[[File:Altair-Sun comparison.png|thumb|left|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''.<ref name="schaaf2008" />{{Citation page|page=190}} 

Altair is a [[Type-A star|type-A]] [[main-sequence star]] with about 1.8 times the [[mass of the Sun]] and 11 times [[Solar luminosity|its luminosity]].<ref name=monnier07/><ref name=peterson06/> 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.<ref name=bouchaud2020/>  Altair rotates rapidly, with a rotational period of under eight hours;<ref name=bouchaud2020/> for comparison, the equator of the [[Sun]] makes a complete rotation in a little more than 25 days. 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 spheroid|oblate]]; its equatorial diameter is over 20 percent greater than its polar diameter.<ref name=monnier07/>

[[File:AlphaAqlLightCurve.png|thumb|left|A [[light curve]] for Altair, adapted from Buzasi ''et al.'' (2005)<ref name="Buzasi et al 2005"/>]]
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.<ref name="Buzasi et al 2005"/> 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 wave]]s, with periods that range between 0.8 and 1.5 hours.<ref name="Buzasi et al 2005">{{Cite journal |last1=Buzasi |first1=D. L. |last2=Bruntt |first2=H. |last3=Bedding |first3=T. R. |last4=Retter |first4=A. |last5=Kjeldsen |first5=H. |last6=Preston |first6=H. L. |last7=Mandeville |first7=W. J. |last8=Suarez |first8=J. C. |last9=Catanzarite |first9=J. |date=February 2005 |title=Altair: The Brightest δ Scuti Star |journal=The Astrophysical Journal |language=en |volume=619 |issue=2 |pages=1072–1076 |arxiv=astro-ph/0405127 |bibcode=2005ApJ...619.1072B |doi=10.1086/426704 |s2cid=16524681 |issn=0004-637X}}</ref> It is a weak source of [[stellar corona|coronal]] [[X-ray astronomy|X-ray]] emission, with the most active sources of emission being located near the star's equator. This activity may be due to [[Convection zone|convection]] cells forming at the cooler equator.<ref name=robrade2009/>
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===Rotational effects===
[[Image:Altair_PR_image6_(white).jpg|thumb|left|Direct image of Altair, taken with the [[CHARA array]]]]
The angular diameter of Altair was measured [[interferometrically]] by [[R. Hanbury Brown]] and his co-workers at [[Narrabri Stellar Intensity Interferometer|Narrabri Observatory]] in the 1960s. They found a diameter of 3{{nbsp}}[[milliarcseconds]].<ref>{{cite journal |bibcode=1967MNRAS.137..393H |title=The stellar interferometer at Narrabri Observatory-II. The angular diameters of 15 stars |last1=Hanbury Brown |first1=R. |last2=Davis |first2=J. |last3=Allen |first3=L. R. |last4=Rome |first4=J. M. |journal=Monthly Notices of the Royal Astronomical Society |year=1967 |volume=137 |issue=4 |page=393 |doi=10.1093/mnras/137.4.393 |doi-access=free }}</ref> Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Later, using [[infrared]] interferometric measurements made by the [[Palomar Testbed Interferometer]] in 1999 and 2000, Altair was found to be flattened. This work was published by [[Gerard van Belle|G. T. van Belle]], [[David Ciardi|David R. Ciardi]] and their co-authors in 2001.<ref name=pti2001/>

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).<ref name=peterson06/><ref>{{cite journal | doi = 10.1086/422422| title = Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer| year = 2004| last1 = Ohishi| first1 = Naoko| last2 = Nordgren| first2 = Tyler E.| last3 = Hutter| first3 = Donald J.| journal = The Astrophysical Journal| volume = 612| issue = 1| pages = 463–471| arxiv = astro-ph/0405301| bibcode = 2004ApJ...612..463O| s2cid = 15857535}}</ref> 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]].<ref>{{cite journal | doi = 10.1051/0004-6361:20042476| title = Gravitational-darkening of Altair from interferometry| year = 2005| last1 = Domiciano de Souza| first1 = A. | last2 = Kervella| first2 = P.| last3 = Jankov| first3 = S.| last4 = Vakili| first4 = F.| last5 = Ohishi| first5 = N.| last6 = Nordgren| first6 = T. E.| last7 = Abe| first7 = L.| journal = Astronomy & Astrophysics| volume = 442| issue = 2| pages = 567–578| bibcode = 2005A&A...442..567D| doi-access = free}}</ref>

Altair is one of the few [[List of stars with resolved images|stars for which a resolved image]] has been obtained.<ref name=nsf>{{cite press release |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=109612 |title=Gazing up at the Man in the Star? |publisher=[[National Science Foundation]] |date=May 31, 2007 |access-date=2022-08-03 }}</ref> In 2006 and 2007, J. D. Monnier and his coworkers produced an image of Altair's surface from 2006 infrared observations made with the [[Michigan Infrared Combiner|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.<ref name=nsf/> 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.<ref name=monnier07/> The polar axis is inclined by about 60° to the line of sight from the Earth.<ref name=robrade2009/>
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