View source for Vega

{{Otheruses1|the star|Vega}}
{{Starbox begin
| name=Vega }}
{{Starbox image
| image = [[Image:vega in lyra.png|239px]]
| caption = Location of Vega in the constellation Lyra. }}
{{Starbox observe
| epoch=[[J2000.0]]
| constell=[[Lyra]]
| pronounce={{IPA-en|ˈviːɡə|}} or<br>{{IPA-en|ˈveɪɡə|}}
| ra={{RA|18|36|56.3364}}<ref name=SIMBAD>{{cite web
 | author=Staff | date=2007-10-30
 | url=http://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Ident=vega
 | title=SIMBAD query result: V* alf Lyr -- Variable Star 
 | publisher=Centre de Données astronomiques de Strasbourg
 | accessdate=2007-10-30
}}&mdash;use the "display all measurements" option to show additional parameters.</ref>
| dec={{DEC| +38|47|01.291}}<ref name=SIMBAD/>
| appmag_v=0.03<ref name=SIMBAD/> }}
{{Starbox character
| class=A0V<ref name=SIMBAD/>
| b-v=+0.00<ref name=SIMBAD/>
| u-b=−0.01<ref name=SIMBAD/>
| variable=Suspected [[Delta Scuti variable|Delta Scuti]]<ref name=asp93/> }}
{{Starbox astrometry
| radial_v=&minus;13.9<ref name=SIMBAD/>
| prop_mo_ra=201.03<ref name=SIMBAD/>
| prop_mo_dec=287.47<ref name=SIMBAD/>
| parallax=128.93
| p_error=0.55
| parallax_footnote=
<ref name=SIMBAD/>
| absmag_v=0.58
<ref group=note>For apparent magnitude ''m'' and parallax ''π'', the absolute magnitude ''M<sub>v</sub>'' is given by:
:<math>\begin{smallmatrix}M_v\ =\ m + 5 (\log_{10}{\pi} + 1)\ =\ 0.03 + 5 (\log_{10}{0.12893} + 1)\ =\ 0.58\end{smallmatrix}</math>
See: {{cite book 
 | first=Roger John | last=Tayler | year=1994
 | title=The Stars: Their Structure and Evolution
 | publisher=Cambridge University Press
 | pages=16 | isbn=0521458854 }}</ref> }}
{{Starbox detail
| age=3.86&ndash;5.72{{e|8}}<ref name=nature7086>{{cite journal
 | last=Peterson
 | first=D. M.
 | coauthors=Hummel, C. A.; Pauls, T. A.; Armstrong, J. T.; Benson, J. A.; Gilbreath, G. C.; Hindsley, R. B.; Hutter, D. J.; Johnston, K. J.; Mozurkewich, D.; Schmitt, H. R.
 | title=Vega is a rapidly rotating star
 | journal=Nature
 | year=1999
 | volume=440
 | issue=7086
 | pages=896–899
 | url=http://arxiv.org/abs/astro-ph/0603520v1
 | accessdate=2007-10-29
 | doi=10.1038/nature04661
}}</ref>
| metal=[M/H]&nbsp;=&nbsp;&minus;0.5<ref name=aaa391>{{cite journal
 | last=Kinman | first=T. | coauthors=Castelli, F.
 | title=The determination of T<sub>eff</sub> for metal-poor A-type stars using V and 2MASS J, H and K magnitudes
 | journal=Astronomy and Astrophysics
 | year=2002 | volume=391 | pages=1039–1052
 | url=http://adsabs.harvard.edu/abs/2002A&A...391.1039K
 | accessdate=2007-10-30
 | doi=10.1051/0004-6361:20020806 }}</ref>
| mass=2.11<ref name=nature7086/>
| radius=2.26&nbsp;×&nbsp;2.78<ref name=apj645>{{cite journal
 | last=Aufdenberg | first=J.P.
 | coauthors=Ridgway, S.T. ''et al.''
 | title=First results from the CHARA Array: VII. Long-Baseline Interferometric Measurements  of Vega Consistent with a Pole-On, Rapidly Rotating Star?
 | journal=Astrophysical Journal
 | year=2006 | volume=645
 | pages=664&ndash;675
 | url=http://www.chara.gsu.edu/CHARA/Papers/Paper6.pdf
 | format=PDF | accessdate=2007-11-09
 | doi=10.1086/504149 }}</ref>
| rotation=12.5&nbsp;[[hour|h]]
| luminosity=37&nbsp;±&nbsp;3<ref name=apj645/>
| temperature=9602&nbsp;±&nbsp;180<ref name=aaa391/>
| gravity=4.1&nbsp;±&nbsp;0.1<ref name=apj645/> }}
{{Starbox catalog
| names=Wega,<ref name=allen/> Lucida Lyrae,<ref name="kendall"/> Alpha Lyrae, α Lyrae, 3 Lyr, [[Gliese-Jahreiss catalogue|GJ 721]], [[Harvard Revised catalogue|HR 7001]], [[Bonner Durchmusterung|BD +38°3238]], [[Henry Draper catalogue|HD 172167]], [[General Catalogue of Trigonometric Parallaxes|GCTP 4293.00]], [[Luyten Two-Tenths catalogue|LTT 15486]], [[Smithsonian Astrophysical Observatory Star Catalog|SAO 67174]], [[Hipparcos catalogue|HIP 91262]].<ref name=SIMBAD/> }}
{{Starbox end}}
'''Vega''' (α Lyr / α Lyrae / [[alpha (letter)|Alpha]] Lyrae)   is the brightest [[star]] in the [[constellation]] [[Lyra]], the [[list of brightest stars|fifth brightest star]] in the night sky and the second brightest star in the northern [[Celestial sphere|celestial hemisphere]], after [[Arcturus]]. It is a relatively nearby star at only 25 [[light-years]] from [[Earth]], and, together with Arcturus and [[Sirius]], one of the most [[luminosity|luminous]] stars in the [[Sun]]'s neighborhood.

Vega has been extensively studied by astronomers, leading it to be termed, "arguably the next most important star in the sky after the Sun".<ref name=apj429/> Historically, Vega served as the [[North Star|northern]] [[pole star]] at about 12,000&nbsp;BC and will do so again at 13727 AD when the declination will be +86&deg;14'.
<ref>{{cite web
 | url=http://www.stellarium.org
 | title=Calculation by the ''Stellarium'' application version 0.10.2
 | accessdate=2009-07-28 }}</ref>
Vega was the first star, other than the Sun, to have its [[Astrophotography|photograph]] taken and the first to have its [[Astronomical spectroscopy|spectrum]] photographed. It was also one of the first stars to have its distance estimated through [[parallax]] measurements. Vega has served as the baseline for calibrating the [[Photometry (astronomy)|photometric]] brightness scale, and was one of the stars used to define the mean values for the [[UBV photometric system]].

In terms of years, Vega is only about a tenth the age of the Sun, but it is evolving so quickly that it has already approached the midpoint of its life expectancy, as has the Sun. It has an unusually low abundance of the elements with a higher [[atomic number]] than that of [[helium]].<ref name=aaa391/> Vega is also a suspected [[variable star]] that may vary slightly in magnitude in a periodic manner.<ref name=merezhin/> It is [[stellar rotation|rotating]] rapidly with a velocity of 274&nbsp;km/s at the [[equator]]. This is causing the equator to bulge outward because of [[centrifugal]] effects, and, as a result, there is a variation of temperature across the star's [[photosphere]] that reaches a maximum at the poles. From the [[Earth]], Vega is being observed from the direction of one of these poles.<ref name=nature7086/>

Based upon an excess emission of [[infrared]] radiation, Vega has a circumstellar disk of dust. This dust is likely the result of collisions between objects in an orbiting [[debris disk]], which is analogous to the [[Kuiper belt]] in the [[Solar System]].<ref name=apj628/> Stars that display an infrared excess because of dust emission are termed Vega-like stars.<ref name=apj124/> Irregularities in Vega's disk also suggest the presence of at least one planet, likely to be about the size of [[Jupiter]],<ref name=apj569/> in orbit around Vega.<ref name=apj598/>

==Observation history==

[[Astrophotography]], the [[photography]] of celestial objects, began in 1840 when [[John William Draper]] took an image of the Moon using the [[daguerreotype]] process. On July 17, 1850, Vega became the first star (other than the Sun) to be photographed, when it was imaged by [[William Bond]] and [[John Adams Whipple]] at the [[Harvard College Observatory]], also with a daguerreotype.<ref name=allen/><ref>{{cite book 
 | first=M. Susan | last=Barger
 | coauthors=White, William B. | year=2000
 | title=The Daguerreotype: Nineteenth-Century Technology and Modern Science
 | publisher=JHU Press | isbn=0801864585 }}
</ref><ref>{{cite journal
 | last=Holden | first=Edward S. | coauthors=Campbell, W. W.
 | title=Photographs of Venus, Mercury and Alpha Lyræ in Daylight.
 | journal=Publications of the Astronomical Society of the Pacific
 | year=1890 | volume=2 | issue=10 | pages=249–250
 | url=http://adsabs.harvard.edu/abs/1890PASP....2..249H
 | accessdate=2007-11-18
 | doi=10.1086/120156 }}</ref> [[Henry Draper]] took the first photograph of a star's [[Astronomical spectroscopy|spectrum]] in August 1872 when he took an image of Vega, and he also became the first person to show [[absorption line]]s in the spectrum of a star.<ref>{{cite journal
 | last=Barker | first=George F.
 | title=On the Henry Draper Memorial Photographs of Stellar Spectra
 | journal=Proceedings of the American Philosophical Society
 | year=1887 | volume=24 | pages=166&ndash;172 }}</ref> (Similar lines had already been identified in the spectrum of the Sun.)<ref>{{cite web
 | url=http://www.aip.org/history/cosmology/tools/tools-spectroscopy.htm
 | title=Spectroscopy and the Birth of Astrophysics
 | publisher=American Institute of Physics
 | accessdate=2007-11-15 }}</ref> In 1879, [[William Huggins]] used photographs of the spectra of Vega and similar stars to identify a set of twelve "very strong lines" that were common to this stellar category. These were later identified as lines from the [[Hydrogen]] [[Balmer series]].<ref>{{cite book 
 | first=Klaus | last=Hentschel | year=2002
 | title=Mapping the Spectrum: Techniques of Visual Representation in Research and Teaching
 | publisher=Oxford University Press | isbn=0198509537 }}</ref>

The distance to Vega can be determined by measuring its [[parallax]] shift against the background stars as the Earth orbits the Sun. The first person to publish a star's parallax was [[Friedrich Georg Wilhelm von Struve|Friedrich G. W. von Struve]], when he announced a value of 0.125&nbsp;[[arcsecond]]s (0.125&Prime;) for Vega.<ref>{{cite book 
 | first=Arthur | last=Berry | year=1899
 | title=A Short History of Astronomy
 | publisher=Charles Scribner's Sons
 | location=New York }}</ref> But [[Friedrich Bessel]] was
skeptical about Struve's data, and, when Bessel published a parallax of 0.314&Prime; for the star system [[61 Cygni]], Struve revised his value for Vega's parallax to nearly double the original estimate. This change cast further doubt on Struve's data. Thus most astronomers at the time, including Struve, credited Bessel with the first published parallax result. However, Struve's initial result was actually surprisingly close to the currently accepted value of 0.129&Prime;.<ref>{{cite book 
 | first=Suzanne | last=Débarbat | year=1988
 | chapter=The First Successful Attempts to Determine Stellar Parallaxes in the Light of the Bessel/Struve Correspondances
 | title=Mapping the Sky: Past Heritage and Future Directions 
 | publisher=Springer | isbn=9027728100 }}
</ref><ref>{{cite web
 | author=Anonymous | date=2007-06-28
 | url=http://astroprofspage.com/archives/1011
 | title=The First Parallax Measurements
 | publisher=Astroprof | accessdate=2007-11-12 }}</ref>

The brightness of a star, as seen from Earth, is measured with a standardized, [[logarithmic scale]]. This [[apparent magnitude]] is a numerical value that decreases in value with increasing brightness of the star. The faintest stars visible to the unaided eye are sixth magnitude, while the brightest, Sirius, is of magnitude −1.47. To standardize the magnitude scale, astronomers chose Vega to represent magnitude zero at all wavelengths. Thus, for many years, Vega was used as a baseline for the calibration of absolute [[photometry (astronomy)|photometric]] brightness scales.<ref>{{cite book 
 | first=Robert A. | last=Garfinkle | year=1997
 | title=Star-Hopping: Your Visa to Viewing the Universe
 | publisher=Cambridge University Press
 | isbn=0521598893 }}</ref> However, this is no longer the case as the apparent magnitude zero point is now commonly defined in terms of a particular numerically specified [[flux]]. This approach is more convenient for astronomers as Vega is not always available for calibration.<ref>{{cite journal
 | last=Cochran | first=A. L.
 | title=Spectrophotometry with a self-scanned silicon photodiode array. II - Secondary standard stars
 | journal=Astrophysical Journal Supplement Series
 | year=1981 | volume=45 | pages=83–96
 | url=http://adsabs.harvard.edu/abs/1981ApJS...45...83C
 | accessdate=2007-11-12
 | doi=10.1086/190708 }}</ref>

The [[UBV photometric system]] measures the magnitude of stars through [[ultraviolet]], blue and yellow filters, producing ''U'', ''B'' and ''V'' values, respectively. Vega is one of six A0V stars that were used to set the initial mean values for this photometric system when it was introduced in the 1950s. The mean magnitudes for these six stars were defined as: ''U''&nbsp;-&nbsp;''B'' = ''B''&nbsp;-&nbsp;''V'' =&nbsp;0. In effect, the magnitude of these stars is the same in the yellow, blue and ultraviolet parts of the [[electromagnetic spectrum]].<ref>{{cite journal
 | last=Johnson | first=H. L. | coauthors=Morgan, W. W.
 | title=Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas
 | journal=Astrophysical Journal
 | year=1953 | volume=117 | pages=313–352
 | url=http://adsabs.harvard.edu/abs/1953ApJ...117..313J
 | accessdate=2007-11-05
 | doi=10.1086/145697 }}</ref> Thus, Vega has a relatively flat electromagnetic spectrum in the visual region&mdash;wavelength range 350-850 [[nanometer]]s, most of which can be seen with the human eye&mdash;so the flux densities are roughly equal; 2000-4000 [[Jansky|Jy]].<ref>{{cite web
 | last=Walsh | first=J. | date=2002-03-06 | url=http://www.eso.org/observing/standards/spectra/hr7001.html
 | title=Alpha Lyrae (HR7001)
 | work=Optical and UV Spectrophotometric Standard Stars
 | publisher=ESO | accessdate=2007-11-15 }}&mdash;flux versus wavelength for Vega.</ref> However, the flux density of Vega drops rapidly in the [[infrared]], and is near 100 Jy at 5&nbsp;[[micrometre|micrometer]]s.<ref>{{cite web
 | last=McMahon | first=Richard G. | date=2005-11-23 | url=http://www.ast.cam.ac.uk/~rgm/magnitudes/vega_hl75.dat
 | format=Text | title=Notes on Vega and magnitudes
 | publisher=University of Cambridge
 | accessdate=2007-11-07 }}</ref>

Photometric measurements of Vega during the 1930s appeared to show that the star had a low-magnitude variability on the order of ±0.03 magnitudes. This range of variability was near the limits of observational capability for that time and so the subject of Vega's variability has been controversial. The magnitude of Vega was measured again in 1981 at the [[David Dunlap Observatory]] and showed some slight variability. Thus it was suggested that Vega showed occasional low-amplitude pulsations associated with a [[Delta Scuti variable]].<ref>{{cite journal
 | last=Fernie | first=J. D.
 | title=On the variability of VEGA
 | journal=Astronomical Society of the Pacific
 | year=1999 | volume=93 | pages=333–337
 | url=http://adsabs.harvard.edu/abs/1981PASP...93..333F
 | accessdate=2007-11-11
 | doi=10.1086/130834 }}</ref> This is a category of stars that oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.<ref name="araa33">{{cite journal
 | author=A. Gautschy, H. Saio
 | title=Stellar Pulsations Across The HR Diagram: Part 1
 | journal=Annual Review of Astronomy and Astrophysics
 | year=1995 | volume=33 | pages=75–114
 | url=http://adsabs.harvard.edu/abs/1995ARA&A..33...75G
 | accessdate=2007-05-14
 | doi=10.1146/annurev.aa.33.090195.000451 }}</ref> Although Vega fits the physical profile for this type of variable, other observers have found no such variation. Thus the variability may be the result of systematic errors in measurement.<ref name=merezhin>{{cite web
 | last=I.A. | first=Vasil'yev | date=1989-03-17
 | coauthors=Merezhin, V. P.; Nalimov, V. N.; Novosyolov, V. A.
 | url=http://www.konkoly.hu/pub/ibvs/3301/3308.txt
 | title=On the Variability of Vega
 | publisher=Commission 27 of the I.A.U.
 | accessdate=2007-10-30 }}</ref><ref>{{cite conference
 | first=D. S. | last=Hayes
 | title=Stellar absolute fluxes and energy distributions from 0.32 to 4.0 microns
 | booktitle=Proceedings of the Symposium, Calibration of fundamental stellar quantities
 | pages=pp. 225&ndash;252
 | publisher=Dordrecht, D. Reidel Publishing Co.
 | date=May 24-29, 1984 | location=Como, Italy
 | url=http://adsabs.harvard.edu/abs/1985IAUS..111..225H
 | accessdate = 2007-11-12 }}</ref>

In 1983, Vega became the first star found to have a disk of dust. The [[Infrared Astronomical Satellite]] (IRAS) discovered an excess of infrared radiation coming from the star, and this was attributed to energy emitted by the orbiting dust as it was heated by the star.<ref>{{cite journal
 | last=Harvey | first=Paul E.
 | coauthors=Wilking, Bruce A.; Joy, Marshall
 | title=On the far-infrared excess of Vega
 | journal=Nature | year=1984 | volume=307
 | pages=441–442 | url=http://www.nature.com/nature/journal/v307/n5950/abs/307441a0.html
 | accessdate=2007-11-12 | doi=10.1038/307441a0 }}</ref>

==Visibility==
Vega can often be seen near the [[zenith]] in the mid-northern [[latitude]]s during the evening in the [[Northern Hemisphere]] summer.<ref name=field_guide>{{cite book
 | first=Jay M. | last=Pasachoff | year=2000
 | title=A Field Guide to Stars and Planets
 | edition=Fourth
 | publisher=Houghton Mifflin Field Guides
 | isbn=0395934311 }}</ref> From mid-southern latitudes it can be seen low above the northern horizon during the [[Southern Hemisphere]] winter. With a [[declination]] of +38.78°, Vega can only be viewed at latitudes north of 51°&nbsp;S. Therefore, it does not rise at all anywhere in Antarctica, or in the southernmost part of South America, including [[Punta Arenas]], [[Chile]] (53°&nbsp;S). At latitudes to the north of +51°&nbsp;N Vega remains continually above the horizon as a [[circumpolar star]]. On about July 1, Vega reaches midnight [[culmination]] when it crosses the [[Meridian (astronomy)|meridian]] at that time.<ref name=burnham/>

[[Image:Summer triangle.png|left|thumb|280px|The [[summer triangle]].]]
This star lies at a [[Vertex (geometry)|vertex]] of a widely spaced [[Asterism (astronomy)|asterism]] called the [[Summer Triangle]], which consists of the zero-[[apparent magnitude|magnitude]] stars Vega in the constellation Lyra and [[Altair]] in [[Aquila (constellation)|Aquila]], plus the first magnitude star [[Deneb]] in [[Cygnus (constellation)|Cygnus]].<ref name=field_guide/> This formation is the approximate shape of a [[right triangle]], with Vega located at its [[right angle]]. The Summer Triangle is recognizable in the northern skies for there are few other bright stars in its vicinity.<ref>{{cite book
 | first=Arthur R. | last=Upgren | year=1998
 | title=Night Has a Thousand Eyes: A Naked-Eye Guide to the Sky, Its Science, and Lore 
 | publisher=Basic Books | isbn=0306457903 }}</ref>

The [[Lyrids]] are a strong [[meteor shower]] that peak each year during April 21&ndash;22. When a small [[meteor]] enters the Earth's atmosphere at a high velocity it produces a streak of light as the object is vaporized. During a shower, a multitude of meteors arrive from the same direction, and, from the perspective of an observer, their glowing trails appear to radiate from a single point in space. In the case of the Lyrids, the meteor trails radiate from the direction of Lyra, and hence are sometimes called the Alpha Lyrids. However, they actually originated from debris emitted by the [[comet]] [[Thatcher (Comet)|C/1861 G1 Thatcher]] and have nothing to do with the star.<ref>{{cite journal
 | last=Arter | first=T. R. | coauthors=Williams, I. P.
 | title=The mean orbit of the April Lyrids
 | journal=Monthly Notices of the Royal Astronomical Society
 | year=1997 | volume=289 | issue=3 | pages=721–728
 | url=http://adsabs.harvard.edu/abs/1997MNRAS.289..721A
 | accessdate=2007-11-02 }}</ref>

==Physical properties==

Vega's [[Stellar classification|spectral class]] is A0V, making it a blue-tinged white [[main sequence]] star that is [[nuclear fusion|fusing]] [[hydrogen]] to [[helium]] in its core. Since more massive stars use their fusion fuel more quickly than smaller ones, Vega's main sequence lifetime is only one billion years, a tenth of our Sun's.<ref>{{cite journal
 | last=Mengel | first=J. G.
 | coauthors=Demarque, P.; Sweigart, A. V.; Gross, P. G.
 | title=Stellar evolution from the zero-age main sequence
 | journal=Astrophysical Journal Supplement Series
 | year=1979 | volume=40 | pages=733–791
 | url=http://adsabs.harvard.edu/abs/1979ApJS...40..733M
 | accessdate=2007-11-05
 | doi=10.1086/190603 }}&mdash;From pages 769&ndash;778: for 
stars in the range 1.75<M<2.2, 0.2<Y<0.3 and 0.004<Z<0.01, stellar models give an age range of 0.43&ndash;1.64{{e|9}} years between a star joining the main sequence and turning off to the red giant branch. With a mass closer to 2.2, however, the interpolated age for Vega is less than a billion.</ref> The current age of this star is between 386 and 511&nbsp;million years, or up to about half its expected total main sequence life span. After leaving the main sequence, Vega will become a class-M [[red giant]] and shed much of its mass, finally becoming a [[white dwarf]]. At present Vega has more than twice the mass<ref name=nature7086/> of the Sun and its full luminosity is about 37 times the Sun's value. If Vega is variable, then it may be a [[Delta Scuti variable|Delta Scuti type]] with a period of about 0.107&nbsp;days.<ref name=asp93>{{cite journal
 | last=Fernie | first=J. D.
 | title=On the variability of VEGA
 | journal=Astronomical Society of the Pacific
 | year=1981 | volume=93 | issue=2 | pages=333&ndash;337
 | url=http://adsabs.harvard.edu/cgi-bin/bib_query?1981PASP...93..333F
 | accessdate=2007-10-30
 | doi=10.1086/130834 }}</ref>

Most of the energy produced at Vega's core is generated by the [[carbon]]-[[nitrogen]]-[[oxygen]] cycle ([[CNO cycle]]), a [[Stellar nucleosynthesis|nuclear fusion]] process that combines [[proton]]s to form [[helium]] nuclei through intermediary nuclei of carbon, nitrogen and oxygen. This process requires a temperature of about 15&nbsp;million&nbsp;K,<ref>{{cite book
 | author=Salaris, Maurizio; Cassisi, Santi | year=2005
 | title=Evolution of Stars and Stellar Populations
 | page=120 | publisher=John Wiley and Sons
 | isbn=0470092203 }}</ref> which is higher than the core temperature of the Sun, but is more efficient than the Sun's [[proton-proton chain reaction]]
fusion reaction. The CNO cycle is highly temperature sensitive, which results in a [[convection zone]] about the core<ref>{{cite journal
 | last=Browning | first=Matthew
 | coauthors=Brun, Allan Sacha; Toomre, Juri 
 | title=Simulations of core convection in rotating A-type stars: Differential rotation and overshooting
 | journal=Astrophysical Journal
 | year=2004 | volume=601 | pages=512–529
 | url=http://www.journals.uchicago.edu/doi/pdf/10.1086/380198
 | accessdate=2007-12-09 
 | doi=10.1086/380198 }}</ref> that evenly distributes the 'ash' from the fusion reaction within the core region. The overlying atmosphere is in [[Radiative transfer|radiative equilibrium]]. This is in contrast to the Sun, which has a [[radiation zone]] centered on the core with an overlying convection zone.<ref>{{cite book 
 | first=Thanu | last=Padmanabhan | year=2002
 | title=Theoretical Astrophysics 
 | publisher=Cambridge University Press
 | isbn=0521562414 }}
</ref><ref>{{cite web
 | last=Cheng | first=Kwong-Sang | year=2007
 | coauthors=Chau, Hoi-Fung; Lee, Kai-Ming | url=http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap14.html
 | title=Chapter 14: Birth of Stars
 | work=Nature of the Universe
 | publisher=Honk Kong Space Museum
 | accessdate=2007-11-26 }}</ref>

The energy flux from Vega has been precisely measured against standard light sources. At 5480&nbsp;Å, the flux is 3,650&nbsp;[[Jansky|Jy]] with an error margin of 2%.<ref>{{cite journal
 | last=Oke | first=J. B. | coauthors=Schild, R. E.
 | title=The Absolute Spectral Energy Distribution of Alpha Lyrae
 | journal=Astrophysical Journal
 | year=1970 | volume=161 | pages=1015–1023
 | url=http://adsabs.harvard.edu/abs/1970ApJ...161.1015O
 | accessdate=2007-11-15
 | doi=10.1086/150603
}}</ref> The visual spectrum of Vega is dominated by [[absorption line]]s of hydrogen; specifically by the hydrogen [[Balmer series]] with the [[electron]] at the n=2 [[principal quantum number]].<ref>{{cite web
 | last=Richmond | first=Michael | url=http://spiff.rit.edu/classes/phys440/lectures/boltz/boltz.html
 | title=The Boltzmann Equation
 | publisher=Rochester Institute of Technology
 | accessdate=2007-11-15
}}</ref><ref>{{cite book 
 | first=Donald D. | last=Clayton | year=1983
 | title=Principles of Stellar Evolution and Nucleosynthesis
 | publisher=University of Chicago Press
 | isbn=0226109534 }}</ref> The lines of other elements are relatively weak, with the strongest being ionized [[magnesium]], [[iron]] and [[chromium]].<ref>{{cite journal
 | last=Michelson | first=E.
 | title=The near ultraviolet stellar spectra of alpha Lyrae and beta Orionis
 | journal=Monthly Notices of the Royal Astronomical Society
 | year=1981 | volume=197 | pages=57–74
 | url=http://adsabs.harvard.edu/abs/1981MNRAS.197...57M
 | accessdate=2007-11-15 }}</ref> The [[X-ray]] emission from Vega is very low, demonstrating that the [[corona]] for this star must be very weak or non-existent.<ref>{{cite journal
 | last=Schmitt | first=J. H. M. M.
 | title=Coronae on solar-like stars.
 | journal=Astronomy and Astrophysics
 | year=1999 | volume=318 | pages=215–230
 | url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1997A%26A...318..215S
 | accessdate=2007-11-15 }}</ref>

Using [[spectropolarimetry]], a [[magnetic field]] has been detected on the surface of Vega by a team of astronomers at the [[Pic du Midi de Bigorre|Observatoire du Pic du Midi]]. This is the first such detection of a magnetic field on a spectral class A star that is not an [[Ap and Bp star|Ap]] chemically [[peculiar star]]. The average line of sight component of this field has a strength of {{nowrap|−0.6 ± 0.3 [[Gauss (unit)|G]]}}.<ref>{{cite journal
 | author=Lignières, F.; Petit, P.; Böhm, T.; Aurière, M. | year=2009
 | title=First evidence of a magnetic field on Vega
 | journal=Astronomy & Astrophysics | volume=500
 | pages=L41&ndash;L44 | doi=10.1051/0004-6361/200911996 }}</ref> This is comparable to the mean magnetic field on the Sun.<ref>{{cite news | author=Staff
 | title=Magnetic Field On Bright Star Vega
 | work=Science Daily | date=July 26, 2009
 | url=http://www.sciencedaily.com/releases/2009/06/090623111947.htm | accessdate=2009-07-30 }}</ref>

===Rotation===
When the radius of Vega was measured to high accuracy with an [[Astronomical interferometer|interferometer]], it resulted in an unexpectedly large estimated value of 2.73&nbsp;±&nbsp;0.01 times the [[solar radius|radius of the Sun]]. This is 60% larger than the radius of the star [[Sirius]], while stellar models indicated it should only be about 12% larger. However, this discrepancy can be explained if Vega is a rapidly rotating star that is being viewed from the direction of its pole of rotation. Observations by the [[CHARA array]] in 2005&ndash;06 confirmed this deduction.<ref name=apj645/>

[[Image:Size Vega.png|right|thumb|300px|Size comparison of Vega (left) to the Sun (right).]]
The pole of Vega&mdash;its axis of rotation&mdash;is inclined no more than five degrees from the line-of-sight to the Earth. The equator of Vega has a [[stellar rotation|rotation]] velocity of 274&nbsp;km/s (for a rotation period of about 12.5&nbsp;hours),<ref name=nature7086/> which is 93% of the speed that would cause the star to start breaking up from [[centrifugal]] effects. This rapid rotation of Vega produces a pronounced equatorial bulge, so the radius of the equator is 23% larger than the polar radius. (The estimated polar radius of this star is 2.26&nbsp;±&nbsp;0.02&nbsp;[[solar radius|solar radii]], while the equatorial radius is 2.78&nbsp;±&nbsp;0.02&nbsp;solar radii.<ref name=apj645/>) From the Earth, this bulge is being viewed from the direction of its pole, producing the overly large radius estimate.

The local gravitational acceleration at the poles is greater than at the equator, so, by the [[Von Zeipel theorem]], the local luminosity is also higher at the poles. This is seen as a variation in [[effective temperature]] over the star: the polar temperature is near 10,000&nbsp;[[Kelvin|K]], while the equatorial temperature is 7,600&nbsp;[[Kelvin|K]].<ref name=nature7086/> As a result, if Vega were viewed along the plane of its [[equator]], then the luminosity would be about half the apparent luminosity as viewed from the pole.<ref name=apj429>{{cite journal
 | last=Gulliver, Hill | first=Austin F.
 | coauthors=Graham; Adelman, Saul J.
 | title=Vega: A rapidly rotating pole-on star
 | journal=The Astrophysical Journal
 | year=1994 | volume=429 | issue=2 | pages=L81–L84
 | url=http://adsabs.harvard.edu/abs/1994ApJ...429L..81G
 | accessdate=2007-10-29
 | doi=10.1086/187418 }}</ref><ref group=note>From the poles, the star presents a [[circle|circular]] profile, while from the equator the star appears as an [[ellipse]]. The [[area of a disk|cross-sectional area]] of the star's elliptical profile is only about 81% of the cross-sectional area of the star's polar profile, so less energy is received along the plane of the equator. Any additional difference in luminosity is accounted for by the temperature distribution. From the [[Stefan–Boltzmann law]], the energy flux from Vega's equator will be about:
:<math>\begin{smallmatrix}\left( \frac{T_{eq}}{T_{pole}} \right)^4 = \left( \frac{7,600}{10,000} \right)^4 = 0.33\end{smallmatrix}</math>
or 33% of the flux from the pole.</ref> This large temperature difference between the poles and the equator produces a strong '[[gravity darkening]]' effect. As viewed from the poles, this results in a darker (lower intensity) limb than would normally be expected for a spherically symmetric star. The temperature gradient may also mean Vega has a [[convection zone]] around the equator,<ref name=apj645/><ref name=noao>{{cite news
 | author=Staff | date=2006-01-10
 | title=Rapidly Spinning Star Vega has Cool Dark Equator
 | publisher=National Optical Astronomy Observatory | url=http://www.noao.edu/outreach/press/pr06/pr0603.html
 | accessdate=2007-11-18
}}</ref> while the remainder of the atmosphere is likely to be in almost pure [[Radiation zone|radiative equilibrium]].<ref>{{cite conference
 | last=Adelman | first=Saul J.
 | title=The physical properties of normal A stars
 | booktitle=The A-Star Puzzle | pages=pp. 1-11
 | publisher=Cambridge University Press
 | date=July 8-13, 2004 | location=Poprad, Slovakia
 | url=http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=280601
 | format=PDF | accessdate=2007-11-22 }}</ref>

If Vega was actually a slowly rotating, spherically symmetric star and it was radiating the same energy as viewed from the Earth, then the luminosity of Vega would be 57 times the luminosity of the Sun. This value is much larger than the luminosity of a typical slowly rotating star with the same mass as Vega. Thus the discovery of fast rotation of Vega resolved this discrepancy. The true full luminosity of Vega is about 37 times the luminosity of the Sun.<ref name=apj645/>

As Vega had long been used as a standard star for calibrating telescopes, the discovery that it is rapidly rotating may challenge some of the underlying assumptions that were based on it being spherically symmetric. With the viewing angle and rotation rate of Vega now better known, this will allow for improved instrument calibrations.<ref>{{cite journal
 | last=Quirrenbach | first=Andreas
 | title=Seeing the Surfaces of Stars
 | journal=Science | year=2007 | volume=317
 | issue=5836 | pages=325–326
 | url=http://www.sciencemag.org/cgi/content/full/317/5836/325
 | accessdate=2007-11-19
 | doi=10.1126/science.1145599
 | pmid=17641185 }}</ref>

===Element abundance===

Astronomers term "metals" those elements with higher [[atomic number]]s than helium. The [[metallicity]] of Vega’s [[photosphere]] is only about 32% of the abundance of heavy elements in the Sun’s atmosphere.<ref group=note>For a metallicity of &minus;0.5, the proportion of metals relative to the Sun is given by:
:<math>\begin{smallmatrix}10^{-0.5}\ =\ 0.316\end{smallmatrix}</math>.</ref> (Compare this, for example, to a three-fold metallicity abundance in the similar star [[Sirius]] as compared to the Sun.) For comparison, the Sun has an abundance of elements heavier than helium of about Z<sub>Sol</sub>&nbsp;=&nbsp;0.0172&nbsp;±&nbsp;0.002.<ref>{{cite journal
 | last=Antia | first=H. M. | coauthors=Basu, Sarbani
 | title=Determining Solar Abundances Using Helioseismology
 | journal=The Astrophysical Journal
 | year=2006 | volume=644 | issue=2 | pages=1292–1298
 | url=http://adsabs.harvard.edu/abs/2006astro.ph..3001A
 | accessdate=2007-11-05
 | doi=10.1086/503707
}}</ref> Thus, in terms of abundances, only about 0.54% of Vega consists of elements heavier than Helium.

The unusually low metallicity of Vega makes it a weak [[Lambda Boötis]]-type star.<ref>{{cite journal
 | last=Renson | first=P.
 | coauthors=Faraggiana, R.; Boehm, C.
 | title=Catalogue of Lambda Bootis Candidates
 | journal=Bulletin d'Information Centre Donnees Stellaires
 | year=1990 | volume=38 | pages=137&ndash;149
 | url=http://adsabs.harvard.edu/abs/1990BICDS..38..137R
 | accessdate=2007-11-07 }}&mdash;Entry for HD 172167 on p. 144.</ref><ref>{{cite journal
 | last=Qiu | first=H. M.
 | coauthors=Zhao, G.; Chen, Y. Q.; Li, Z. W.
 | title=The Abundance Patterns of Sirius and Vega
 | journal=The Astrophysical Journal
 | year=2001 | volume=548
 | issue=2 | pages=77–115
 | url=http://adsabs.harvard.edu/abs/2001ApJ...548..953Q
 | accessdate=2007-10-30
 | doi=10.1086/319000 }}</ref> However, the reason for the existence of such chemically peculiar, [[Stellar classification|spectral class]] A0-F0 stars remains unclear. One possibility is that the chemical peculiarity may be the result of [[diffusion]] or mass loss, although stellar models show that this would normally only occur near the end of a star's hydrogen-burning lifespan. Another possibility is that the star formed from an [[interstellar medium]] of gas and dust that was unusually metal-poor.<ref>{{cite journal
 | last=Martinez | first=Peter
 | coauthors=Koen, C.; Handler, G.; Paunzen, E.
 | title=The pulsating lambda Bootis star HD 105759
 | journal=Monthly Notices of the Royal Astronomical Society
 | year=1998 | volume=301 | issue=4 | pages=1099–1103
 | url=http://adsabs.harvard.edu/abs/1998MNRAS.301.1099M
 | accessdate=2007-11-05
 | doi=10.1046/j.1365-8711.1998.02070.x }}</ref>

The observed helium to hydrogen ratio in Vega is 0.030&nbsp;±&nbsp;0.005, which is about 40% lower than the Sun. This may be caused by the disappearance of a helium [[convection zone]] near the surface. Energy transfer is instead performed by the [[radiation zone|radiative process]], which may be causing an abundance anomaly through diffusion.<ref>{{cite journal
 | last=Adelman | first=Saul J.
 | coauthor=Gulliver, Austin F.
 | title=An elemental abundance analysis of the superficially normal A star VEGA
 | journal=Astrophysical Journal, Part 1
 | year=1990 | volume=348 | pages=712–717
 | url=http://adsabs.harvard.edu/abs/1990ApJ...348..712A
 | accessdate=2007-11-07
 | doi=10.1086/168279 }}</ref>

===Kinematics===
The [[radial velocity]] of Vega is the component of this star's motion along the line-of-sight to the Earth. Movement away from the Earth will cause the light from Vega to shift to a lower [[frequency]] (toward the red), or to a higher frequency (toward the blue) if the motion is toward the Earth. Thus the velocity can be measured from the amount of [[redshift]] (or blueshift) of the star's spectrum. Precise measurements of this redshift give a value of &minus;13.9&nbsp;±&nbsp;0.9&nbsp;km/s.<ref>{{cite conference
 | first = D. S. | last = Evans
 | title = The Revision of the General Catalogue of Radial Velocities
 | booktitle = Proceedings from IAU Symposium no. 30
 | pages = 57 | publisher = Academic Press
 | date = June 20-24, 1966
 | location = London, England
 | url = http://adsabs.harvard.edu/abs/1967IAUS...30...57E
 | accessdate = 2007-11-09 }}</ref> The minus sign indicates a relative motion toward the Earth.

Motion transverse to the line of sight causes the position of Vega to shift with respect to the more distant background stars. Careful measurement of the star's position allows this angular movement, known as [[proper motion]], to be calculated. Vega's proper motion is 202.03&nbsp;±&nbsp;0.63&nbsp;milli-[[arcsecond]]s (mas) per year in [[Right Ascension]]&mdash;the celestial equivalent of [[longitude]]&mdash;and 287.47&nbsp;±&nbsp;0.54&nbsp;mas/y in [[Declination]], which is equivalent to a change in [[latitude]].<ref>{{cite journal
 | author=M. A. Perryman ''et al.''
 | title=The Hipparcos Catalogue.
 | journal=Astronomy and Astrophysics
 | year=1997 | volume=323 | pages=L49–L52
 | url=http://adsabs.harvard.edu/abs/1997A&A...323L..49P
 | accessdate=2007-11-09 }}</ref> The net proper motion of Vega is 327.78 mas/y,<ref>{{cite web
 | last = Majewski | first = Steven R. | year=2006
 | url = http://www.astro.virginia.edu/class/majewski/astr551/lectures/VELOCITIES/velocities.html
 | title =Stellar Motions | publisher =University of Virginia
 | accessdate = 2007-09-27 }}&mdash;The net proper motion is given by:
:<math>\begin{smallmatrix}\mu = \sqrt{ {\mu_\delta}^2 + {\mu_\alpha}^2 \cdot \cos^2 \delta }\ =\ 327.78\ \text{mas/y} \end{smallmatrix}</math>.
where <math>\mu_\alpha</math> and <math>\mu_\delta</math> are the components of proper motion in the R.A. and Declination, respectively, and <math>\delta</math> is the Declination.</ref> which results in angular movement of a degree every 11,000&nbsp;years.

In the [[Galactic coordinate system]], the [[Stellar kinematics#Space velocity|space velocity]] components of Vega are U&nbsp;=&nbsp;&minus;16.1&nbsp;±&nbsp;0.3, V&nbsp;=&nbsp;&minus;6.3&nbsp;±&nbsp;0.8 and W&nbsp;=&nbsp;&minus;7.7&nbsp;±&nbsp;0.3, for a net space velocity of 19&nbsp;km/s.<ref name=aaa339/> The radial component of this velocity&mdash;in the direction of the Sun&mdash;is &minus;13.9&nbsp;km/s, while the transverse velocity is 9.9&nbsp;km/s. Although Vega is at present only the fifth-brightest star in the sky, the star is slowly brightening as proper motion causes it to approach the Sun.<ref>{{cite book 
 | first=Forest Ray | last=Moulton | year=1906 | pages=502
 | title=An Introduction to Astronomy
 | publisher=The Macmillan company }}</ref> Vega will eventually become the brightest star in the sky in around 210,000 years, will attain a peak brightness of magnitude &ndash;0.81 in about 290,000 years and will be the brightest star in the sky for about 270,000 years.<ref>{{cite journal
 | last=Tomkin
 | first=Jocelyn
 | title=Once And Future Celestial Kings
 | journal=Sky and Telescope
 | month=April | year=1998
 | volume=95
 | issue=4
 | pages=59–63
}}</ref>

Based on this star's kinematic properties, it appears to belong to a stellar association called the [[Castor Moving Group]]. This group contains about 16 stars, including [[Alpha Librae]], [[Alpha Cephei]], [[Castor (star)|Castor]], [[Fomalhaut]] and Vega. All members of the group are moving in near parallel with similar [[space velocity|space velocities]]. Membership in a moving group implies a common origin for these stars in a [[open cluster]] that has since become gravitationally unbound.<ref>{{cite book 
 | first=Mike | last=Inglis | year=2003
 | title=Observer's Guide to Stellar Evolution: The Birth, Life, and Death of Stars | publisher=Springer
 | isbn=1852334657 }}</ref> The estimated age of this moving group is 200&nbsp;±&nbsp;100 million years, and they have an
average space velocity of 16.5&nbsp;km/s.<ref group=note>U&nbsp;=&nbsp;&minus;10.7&nbsp;±&nbsp;3.5, V&nbsp;=&nbsp;&minus;8.0&nbsp;±&nbsp;2.4, W&nbsp;=&nbsp;&minus;9.7&nbsp;±&nbsp;3.0&nbsp;km/s. The net velocity is:
:<math>\begin{smallmatrix}v_{\text{sp}} = \sqrt{10.7^2 + 8.0^2 + 9.7^2} = 16.5 \text{km/s}\end{smallmatrix}</math>
</ref><ref name=aaa339>{{cite journal
 | last=Barrado y Navascues | first=D.
 | title=The Castor moving group. The age of Fomalhaut and VEGA
 | journal=Astronomy and Astrophysics
 | year=1998 | volume=339 | pages=831–839
 | url=http://adsabs.harvard.edu/abs/1999astro.ph..5243B
 | accessdate=2007-10-31 }}</ref>

== Planetary system ==
=== Infrared excess ===

[[Image:Vega Spitzer.jpg|250px|right|thumb|A mid-infrared image of the [[debris disk]] around Vega.]]

One of the early results from the [[Infrared Astronomy Satellite]] (IRAS) was the discovery of excess [[infrared]] flux coming from Vega; beyond what would be expected from the star alone. This excess was measured at [[wavelength]]s of 25, 60 and 100&nbsp;[[Micrometre|μm]], and came from within an angular radius of 10&nbsp;[[arcsecond]]s (10&Prime;) centered on the star. At the measured distance of Vega, this corresponded to an actual radius of 80&nbsp;[[astronomical unit]]s (AU), where an AU is the average radius of the [[Earth]]'s orbit around the Sun. It was proposed that this radiation came from a field of orbiting particles with a dimension on the order of a [[millimeter]], as anything smaller would eventually be removed from the system by radiation pressure or drawn into the star by means of [[Poynting-Robertson drag]].<ref name=apj285>{{cite journal
 | last=Harper | first=D. A.
 | coauthors=Loewenstein, R. F.; Davidson, J. A.
 | title=On the nature of the material surrounding VEGA
 | journal=Astrophysical Journal, Part 1
 | year=1984 | volume=285 | pages=808–812
 | url=http://adsabs.harvard.edu/abs/1984ApJ...285..808H
 | accessdate=2007-11-02
 | doi=10.1086/162559 }}</ref> The latter is the result of radiation pressure creating an effective force that opposes the orbital motion of a dust particle, causing it to spiral inward. This effect is most pronounced for tiny particles that are closer to the star.<ref>{{cite journal | last=Robertson
 | first=H. P.
 | authorlink = Howard Percy Robertson
 | title=Dynamical effects of radiation in the solar system
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=97 | pages=423–438
 | publisher=Royal Astronomical Society
 | year=1937 | month=April
 | url=http://articles.adsabs.harvard.edu/full/1937MNRAS..97..423R
 | accessdate=2007-11-02 }}</ref>

Subsequent measurements of Vega at 193&nbsp;μm showed a lower than expected flux for the hypothesized particles, suggesting that they must instead be on the order of 100&nbsp;μm or less. To maintain this amount of dust in orbit around Vega, a continual source of replenishment would be required. A proposed mechanism for maintaining the dust was a disk of coalesced bodies that were in the process of collapsing to form a planet.<ref name=apj285/> Models fitted to the dust distribution around Vega indicate
that it is a 120&nbsp;AU-radius circular disk viewed from nearly pole-on. In addition, there is a hole in the center of the disk with a radius of no less than 80&nbsp;AU.<ref>{{cite journal
 | last=Dent | first=W. R. F.
 | coauthors=Walker, H. J.; Holland, W. S.; Greaves, J. S.
 | title=Models of the dust structures around Vega-excess stars
 | journal=Monthly Notices of the Royal Astronomical Society
 | year=2000 | volume=314 | issue=4 | pages=702–712
 | url=http://adsabs.harvard.edu/abs/2000MNRAS.314..702D
 | accessdate=2007-11-07
 | doi=10.1046/j.1365-8711.2000.03331.x }}</ref>

Following the discovery of an infrared excess around Vega, other stars have been found that display a similar anomaly that is attributable to dust emission. As of 2002, about 400 of these stars have been found, and they have come to be termed "Vega-like" or "Vega-excess" stars. It is believed that these may provide clues to the origin of the Solar System.<ref name=apj124>{{cite journal
 | last=Song | first=Inseok
 | coauthors=Weinberger, A. J.; Becklin, E. E.; Zuckerman, B.; Chen, C.
 | title=M-Type Vega-like Stars
 | journal=The Astronomical Journal
 | year=2002 | volume=124 | issue=1 | pages=514–518
 | url=http://adsabs.harvard.edu/abs/2002AJ....124..514S
 | accessdate=2007-11-10
 | doi=10.1086/341164 }}</ref>

=== Debris disk ===

By 2005, the [[Spitzer Space Telescope]] had produced high resolution infrared images of the dust around Vega. It was shown to extend out to 43&Prime; (330&nbsp;AU) at a wavelength of 24&nbsp;μm, 70&Prime; (543&nbsp;AU) at 70&nbsp;μm and 105&Prime; (815 AU) at 160&nbsp;μm. These much wider disks were found to be circular and free of clumps, with dust particles ranging from 1&ndash;50&nbsp;μm in size. The estimated total mass of this dust is 3{{e|-3}} times the mass of the Earth. Production of the dust would require collisions between asteroids in a population corresponding to the [[Kuiper Belt]] around the Sun. Thus the dust is more likely created by a [[debris disk]] around Vega, rather than from a [[protoplanetary disk]] as was earlier thought.<ref name=apj628>{{cite journal
 | author=Su, K. Y. L. ''et al.''
 | title=The Vega Debris Disk: A Surprise from ''Spitzer''
 | journal=The Astrophysical Journal
 | year=2005 | volume=628 | issue=1 | pages=487–500 | url=http://www.journals.uchicago.edu/doi/full/10.1086/430819
 | accessdate=2007-11-02
 | doi=10.1086/430819
 }}</ref>

[[Image:Ssc2005-01b.jpg|thumb|left|280px|Artist concept illustrates how a massive collision of objects may have smashed together to create the dust ring around the [[star]] Vega.]]

The inner boundary of the debris disk was estimated at 11&Prime;&nbsp;±&nbsp;2&Prime;, or 70&ndash;102&nbsp;AU. The disk of dust is produced as radiation pressure from Vega pushes debris from collisions of larger objects outward. However, continuous production of the amount of dust observed over the course of Vega's lifetime would require an enormous starting mass&mdash;estimated as hundreds of times the mass of [[Jupiter]]. Hence it is more likely to have been produced as the result of a relatively recent breakup of a moderate-sized (or larger) comet or asteroid, which then further fragmented as the result of collisions between the smaller components and other bodies. This dusty disk would be relatively young on the time scale of the star's age, and it will eventually be removed unless other collision events supply more dust.<ref name=apj628/>

Observations with the [[CHARA array]] at Mt. Wilson in 2006 revealed evidence for an inner dust band around Vega. Originating within 8&nbsp;AU of the star, this dust may be evidence of dynamical perturbations within the system.<ref>{{cite journal
 | author=Absil, O. ''et al.''
 | title=Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
 | journal=Astronomy and Astrophysics
 | year=2006 | volume=452 | issue=1 | pages=237–244
 | url=http://adsabs.harvard.edu/abs/2006A&A...452..237A
 | accessdate=2007-11-19
 | doi=10.1051/0004-6361:20054522 }}</ref> This may be caused by an intense bombardment of [[comet]]s or [[meteor]]s, and may be evidence for the existence of a planetary system.<ref>{{cite web
 | last=Girault-Rime | first=Marion | date=Summer 2006
 | url=http://www2.cnrs.fr/en/578.htm
 | title=Vega's Stardust
 | publisher=CNRS International Magazine
 | accessdate=2007-11-19 }}</ref>

=== Possible planets ===

Observations from the [[James Clerk Maxwell Telescope]] in 1997 revealed an "elongated bright central region" that peaked at 9&Prime; (70&nbsp;AU) to the northeast of Vega. This was hypothesized as either a perturbation of the dust disk by a [[extrasolar planet|planet]] or else an orbiting object that was surrounded by dust. However, images by the [[Keck telescope]] had ruled out a companion down to magnitude 16, which would correspond to a body with more than 12 times the mass of [[Jupiter]].<ref>{{cite journal
 | last=Holland | first=Wayne S.
 | coauthors=Greaves, Jane S.; Zuckerman, B.; Webb, R. A.; McCarthy, Chris; Coulson, Iain M.; Walther, D. M.; Dent, William R. F.; Gear, Walter K.; Robson, Ian
 | title=Submillimetre images of dusty debris around nearby stars
 | journal=Nature | year=1998 | volume=392
 | issue=6678 | pages=788–791
 | url=http://adsabs.harvard.edu/cgi-bin/bib_query?1998Natur.392..788H
 | accessdate=2007-11-10
 | doi=10.1038/33874 }}</ref> Astronomers at the [[Joint Astronomy Centre]] in Hawaii and at [[UCLA]] suggested that the image may indicate a planetary system still undergoing formation.<ref>{{cite news
 | author=Staff | date=1998-04-21 | title=Astronomers discover possible new Solar Systems in formation around the nearby stars Vega and Fomalhaut
 | publisher=Joint Astronomy Centre | url=http://outreach.jach.hawaii.edu/pressroom/1998_vega/
 | accessdate=2007-10-29 }}</ref>

Determining the nature of the planet has not been straightforward; a 2002 paper hypothesizes that the lumps are caused by a roughly [[Jupiter]]-mass planet on an eccentric orbit. Dust would collect in orbits that have [[Orbital resonance|mean-motion resonances]] with this planet&mdash;where their orbital periods form integer fractions with the period of the planet&mdash;producing the resulting clumpiness.<ref name=apj569>{{cite journal
 | last=Wilner| first=D.
 | coauthors=Holman, M.; Kuchner, M.; Ho, P.T.P.
 | title=Structure in the Dusty Debris around Vega
 | journal=The Astrophysical Journal
 | year=2002 | volume=569 | pages=L115–L119
 | url=http://adsabs.harvard.edu/abs/2002ApJ...569L.115W
 | accessdate=2007-10-30
 | doi=10.1086/340691 }}</ref>

In 2003 it was hypothesized that these lumps could be caused by a roughly [[Neptune]]-mass planet having [[planetary migration|migrated]] from 40 to 65&nbsp;[[Astronomical Units|AU]] over 56&nbsp;million&nbsp;[[year]]s,<ref name=apj598>{{cite journal
 | last=Wyatt | first=M.
 | title=Resonant Trapping of Planetesimals by Planet Migration: Debris Disk Clumps and Vega's Similarity to the Solar System
 | journal=The Astrophysical Journal
 | year=2002 | volume=598 | pages=1321–1340
 | url=http://adsabs.harvard.edu/abs/2003ApJ...598.1321W
 | accessdate=2007-10-30
 | doi=10.1086/379064 }}</ref> an orbit large enough to allow the formation of smaller [[Planet#Classification|rocky planet]]s closer to Vega. The migration of this planet would likely require gravitational interaction with a second, higher mass planet in a smaller orbit.<ref>{{cite news
 | last=Gilchrist | first=E. | date=2003-12-01
 | coauthors=Wyatt, M.; Holland, W.; Maddock, J.; Price, D. P.
 | title=New evidence for Solar-like planetary system around nearby star
 | publisher=Royal Observatory, Edinburgh | url=http://www.roe.ac.uk/roe/support/pr/pressreleases/vega.html
 | accessdate=2007-10-30 }}</ref>

Using a [[coronagraph]] on the [[Subaru (telescope)|Subaru telescope]] in Hawaii in 2005, astronomers were able to further constrain the size of a planet orbiting Vega to no more than 5&ndash;10 times the mass of Jupiter.<ref>{{cite journal
 | last=Itoh | first=Yoichi
 | title=Coronagraphic Search for Extrasolar Planets around &epsilon; Eri and Vega
 | journal=The Astrophysical Journal
 | year=2006 | volume=652 | issue=2 | pages=1729–1733
 | url=http://adsabs.harvard.edu/abs/2006ApJ...652.1729I
 | accessdate=2007-11-10
 | doi=10.1086/508420 }}</ref> Although a planet has yet to be directly observed around Vega, the presence of a planetary system can not yet be precluded. Thus there could be smaller, [[terrestrial planet]]s orbiting closer to the star. The [[inclination]] of planetary orbits around Vega is likely to be closely aligned to the [[equator]]ial plane of this star.<ref>{{cite journal
 | last=Campbell | first=B. | coauthors=Garrison, R. F.
 | title=On the inclination of extra-solar planetary orbits
 | journal=Publications of the Astronomical Society of the Pacific
 | year=1985 | volume=97 | pages=180–182
 | url=http://adsabs.harvard.edu/abs/1985PASP...97..180C
 | accessdate=2007-11-16
 | doi=10.1086/131516 }}</ref> From the perspective of an observer on a hypothetical planet around Vega, the Sun would appear as a faint 4.3 magnitude star in the [[Columba (constellation)|Columba]] constellation.<ref group=note>The Sun would appear at the diametrically opposite coordinates
from Vega at &alpha;={{RA|6|36|56.3364}}, &delta;={{DEC|&minus;38|47|01.291}}, which is in the western part of Columba. The visual magnitude is given by <math>\begin{smallmatrix}m\ =\ M_v - 5(\log_{10} \pi + 1)\ =\ 4.3.\end{smallmatrix}</math></ref>

==Etymology and cultural significance==

Each night the positions of the stars appear to change as the Earth rotates. However, when a star is located along the Earth's axis of rotation, it will remain in the same position and thus is called a [[pole star]]. The direction of the Earth's axis of rotation gradually changes over time in a process known as the [[Precession (astronomy)|precession of the equinoxes]]. A complete precession cycle requires 25,770&nbsp;years,<ref>{{cite book
 | first=Andrew L. | last=Chaikin
 | editors=Beatty, J. K.; Petersen, C. C.
 | year=1990 | title=The New Solar System
 | edition=4th
 | publisher=Cambridge University Press
 | location=Cambridge, England
 | isbn=0521645875 }}</ref> during which time
the pole of the Earth's rotation follows a circular path across the [[celestial sphere]] that passes near several prominent stars. At present the pole star is [[Polaris]], but around 12,000&nbsp;BC the pole was pointed only five degrees away from Vega. Through precession, the pole will again pass near Vega around 14,000&nbsp;AD.<ref>{{cite book 
 | first=Archie E. | last=Roy
 | coauthors=Clarke, David
 | year=2003 | title=Astronomy: Principles and Practice
 | publisher=CRC Press
 | isbn=0750309172 }}</ref> It is the brightest of the successive northern pole stars.<ref name=allen/>

Among the northern [[Polynesia]]n people, Vega was known as ''whetu o te tau'', the year star. For a period of history it marked the start of their new year when the ground would be prepared for planting. Eventually this function became denoted by the [[Pleiades]].<ref>{{cite journal
 | last=Smith | first=S. Percy
 | title=The Fatherland of the Polynesians &ndash; Aryan and Polynesian Points of Contact
 | journal=The Journal of the Polynesian Society
 | year=1919 | volume=28 | pages=18–20
 | url=http://books.google.com/books?id=0sh9-6UH644C&pg=PA27
 | accessdate=2008-08-08 }}</ref>

The [[Assyrian people|Assyrians]] named this pole star Dayan-same, the "Judge of Heaven", while in [[Akkadian language|Akkadian]] it was Tir-anna, "Life of Heaven".<!-- see Allen reference below --> In [[Babylon]]ian astronomy, Vega may have been one of the stars named Dilgan, "the Messenger of Light". To the [[Ancient Greece|ancient Greeks]], the constellation Lyra was formed from the harp of [[Orpheus]], with Vega as its handle.<ref name="kendall">{{cite book 
 | first=E. Otis | last=Kendall | year=1845
 | title=Uranography: Or, A Description of the Heavens; Designed for Academics and Schools; Accompanied by an Atlas of the Heavens
 | publisher=Oxford University Press
 | location=Philadelphia }}</ref> For the [[Roman Empire]], the start of autumn was based upon the hour at which Vega set below the horizon.<ref name=allen>{{cite book 
 | first=Richard Hinckley | last=Allen | year=1963
 | title=Star Names: Their Lore and Meaning
 | publisher=Courier Dover Publications
 | isbn=0486210790 }}</ref>

In [[Chinese mythology]], there is a love story of [[Qi Xi]] {{lang|zh|七夕}} in which Niu Lang {{lang|zh|牛郎}} ([[Altair]]) and his two children ([[Beta Aquilae|β]] and [[Gamma Aquilae|γ Aquilae]]) are separated from their mother Zhi Nü {{lang|zh|織女}} (Vega) who is on the far side of the river, the [[Milky Way|Milky Way 銀河]].<ref>{{cite book 
 | first=Liming | last=Wei | coauthors=Yue, L.; Lang Tao, L.
 | year=2005 | title=Chinese Festivals
 | publisher=Chinese Intercontinental Press
 | id=ISBN 750850836X }}</ref> However, one day per year on the seventh day of the seventh month of the Chinese lunisolar calendar, magpies make a bridge so that Niu Lang and Zhi Nü can be together again for a brief encounter. The Japanese [[Tanabata]] festival, in which Vega is known as [[Orihime]] (織姫), is also based on this legend.<ref>{{cite book 
 | first=John Robert
 | last=Kippax
 | year=1919
 | title=The Call of the Stars: A Popular Introduction to a Knowledge of the Starry Skies with their Romance and Legend
 | publisher=G. P. Putnam's Sons }}</ref> In [[Zoroastrianism]], Vega was sometimes associated with, Vanant, a minor divinity whose name means "conqueror".<ref>{{cite book 
 | first=Mary | last=Boyce  | year=1996
 | title=A History of Zoroastrianism, volume one: The Early Period
 | publisher=E. J. Brill | location=New York
 | isbn=9004088474 }}</ref>

The name Wega<ref name=allen/> (later Vega) comes from a loose transliteration of the [[Arabic language|Arabic]] word ''{{lang|ar-Latn|wāqi}}'' meaning "falling" or "landing", via the phrase '''{{lang|ar|النسر&nbsp;الواقع}}''' ''{{lang|ar-Latn|an-nasr al-wāqi‘}}'' "the alighting vulture".<ref>That is, a vulture on the ground with its wings folded. (Edward William Lane, ''Arabic-English Lexicon)''</ref> The constellation was represented as a vulture in [[ancient Egypt]],<ref>{{cite book 
 | first=Gerald | last=Massey | year=2001
 | title=Ancient Egypt: the Light of the World
 | publisher=Adamant Media Corporation
 | id=ISBN 140217442X }}</ref>
and as an eagle or vulture in [[History of India|ancient India]].<ref>{{cite book 
 | first=William Tyler | last=Olcott | year=1911
 | title=Star Lore of All Ages: A Collection of Myths, Legends, and Facts Concerning the Constellations of the Northern Hemisphere
 | publisher=G.P. Putnam's sons }}</ref><ref>{{cite web
 | last=Houlding | first=Deborah | month=December | year=2005
 | url=http://www.skyscript.co.uk/lyre.html
 | title=Lyra: The Lyre | publisher=Sktscript
 | accessdate=2007-11-04 }}</ref> The Arabic name then appeared in the [[western world]] in the [[Alfonsine Tables]],<ref name=allen/> which were drawn up between 1215&ndash;70 by order of [[Alfonso X]].<ref>{{cite book 
 | first=M. Th. | last=Houtsma
 | coauthors=Wensinck, A. J.; Gibb, H. A. R.; Heffening, W.; L&eacute;vi-Proven&ccedil;al
 | year=1987
 | title=E.J. Brill's First Encyclopaedia of Islam, 1913-1936
 | volume=VII | pages=292 | publisher=E.J. Brill }}</ref>

[[Medieval]] [[astrologers]] counted Vega as one of the [[Behenian fixed stars|Behenian stars]]<ref>{{cite book 
| first=Donald | last=Tyson | coauthors=Freake, James
| year=1993
| title=Three Books of Occult Philosophy
| publisher=Llewellyn Worldwide
| isbn=0875428320 }}</ref> and related it to [[chrysolite]] and [[winter savory]]. [[Cornelius Agrippa]] listed its [[kabbalistic]] sign [[Image:Agrippa1531 Vulturcadens.png]] under ''Vultur cadens'', a literal Latin translation of the  Arabic name.<ref>{{cite book 
 | first=Heinrich Cornelius | last=Agrippa | year=1533
 | title=De Occulta Philosophia }}</ref> Medieval star charts also listed the alternate names Waghi, Vagieh and Veka for this star.<ref name=burnham>{{cite book
 | first=Robert J. R. | last=Burnham | year=1978
 | title=Burnham's Celestial Handbook: An Observer's Guide to the Guide to the Universe Beyond the Solar System, vol. 2
 | publisher=Courier Dover Publications
 | isbn=0486235688 }}</ref>

Vega became the first star to have a car named after it when [[Chevrolet]] launched the [[Chevrolet Vega|Vega]] in 1971.<ref>{{cite web
 | last=Frommert | first=Hartmut
 | url=http://yuridrive.yurisnight.net/~spider/spider/Misc/alphaLyr.html
 | title=Vega, Alpha Lyrae | publisher=SEDS
 | accessdate=2007-11-02 }}</ref> Other vehicles named after Vega include the [[European Space Agency|ESA's]] [[Vega (launcher)|Vega]] launch system<ref>{{cite web
 | author=Staff | date=2005-05-20 | url=http://www.esa.int/SPECIALS/Launchers_Access_to_Space/SEMH3E67ESD_0.html
 | title=Launch vehicles - Vega
 | publisher=European Space Agency
 | accessdate=2007-11-12 }}</ref> and the [[Lockheed Vega]] aircraft.<ref>{{cite web
 | first=Judy | last=Rumerman | year=2003
 | url=http://www.centennialofflight.gov/essay/Aerospace/vega/Aero14.htm
 | title=The Lockheed Vega and Its Pilots
 | publisher=U.S. Centennial of Flight Commission
 | accessdate=2007-11-12 }}</ref>

==See also==
* [[Vega in fiction]]

==Notes==
<references group=note/>

==References==
{{reflist|2}}

==External links==
*{{cite web
 | title=Vega |  work=SolStation
 | url=http://www.solstation.com/stars/vega.htm
 | accessdate=2005-11-09 }}
*{{cite news
 | last=Gilchrist | first=Eleanor | date=2003-12-01
 | coauthors=Wyatt, Mark; Holland, Wayne; Price, Douglas Pierce; Maddock, Julia
 | title=New evidence for Solar-like planetary system around nearby star
 | publisher=Joint Astronomy Centre
 | url=http://outreach.jach.hawaii.edu/pressroom/2003_vegasolar/
 | accessdate=2007-11-10 }}
*{{cite news
 | author=Gay Yee Hill and Dolores Beasley
 | date=2005-01-10
 | title=Spitzer Sees Dusty Aftermath of Pluto-Sized Collision
 | publisher=NASA/Spitzer Space Telescope | url=http://www.spitzer.caltech.edu/Media/releases/ssc2005-01/release.shtml
 | accessdate=2007-11-02 }}
* [http://www.vega.org.uk/video/subseries/16 Sir Harry Kroto, NL presents 8 Astrophysical Lectures including discussion of Vega] Freeview videos provided by the Vega Science Trust.

{{featured article}}
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[[Category:Bayer objects|Lyrae, Alpha]]
[[Category:Delta Scuti variables]]
[[Category:Flamsteed objects|Lyrae, 3]]
[[Category:Lyra constellation]]
[[Category:A-type main sequence stars]]
[[Category:Circumstellar discs]]
[[Category:Northern pole stars]]
[[Category:Stars with proper names]]
[[Category:Castor Moving Group]]
[[Category:Hypothetical planetary systems]]
[[Category:Lambda Boötis stars]]

[[ar:النسر الواقع]]
[[az:Veqa]]
[[bn:অভিজিৎ (তারা)]]
[[be:Вега]]
[[bg:Вега]]
[[bs: Vega]]
[[ca:Vega]]
[[cs:Vega]]
[[de:Wega]]
[[el:Βέγας]]
[[es:Vega (estrella)]]
[[fa:کرکس نشسته]]
[[fr:Alpha Lyrae]]
[[ga:Vega (réalta)]]
[[ko:베가]]
[[hr:Vega]]
[[id:Vega]]
[[it:Vega]]
[[he:וגה]]
[[lv:Vega]]
[[lb:Vega (Stär)]]
[[lt:Vega]]
[[hu:Vega]]
[[mk:Вега]]
[[mg:Alpha Lyrae]]
[[nl:Wega]]
[[ja:ベガ]]
[[no:Vega (stjerne)]]
[[nn:Stjerna Vega]]
[[pl:Wega]]
[[pt:Vega (estrela)]]
[[ro:Vega]]
[[ru:Вега]]
[[sk:Vega]]
[[fi:Vega]]
[[sv:Vega (stjärna)]]
[[tr:Vega (yıldız)]]
[[uk:Вега]]
[[vi:Sao Chức Nữ]]
[[zh:織女一]]