# Vega

Starbox begin
name=Vega
Starbox image

caption = Location of Vega in the constellation Lyra.
Starbox observe
epoch=J2000.0
constell=Lyra
ra=RA|18|36|56.3364cite web
author=Staff
date=October 30, 2007
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.]
dec=DEC| +38|47|01.291
appmag_v=0.03
Starbox character
class=A0V
b-v=+0.00
u-b=−0.01
variable=Suspected Delta Scuti
Starbox astrometry
prop_mo_ra=201.03
prop_mo_dec=287.47
parallax=128.93
p_error=0.55
parallax_footnote=
absmag_v=0.58 [For apparent magnitude "m" and parallax "π", the absolute magnitude "Mv" is given by::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
]
Starbox detail
age=3.86&ndash;5.72e|8cite 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
]
metal= [M/H] = −0.5cite journal
last=Kinman | first=T. | coauthors=Castelli, F.
title=The determination of Teff 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
accessdate=2007-10-30
doi=10.1051/0004-6361:20020806
]
mass=2.11
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
]
rotation=12.5 h
luminosity=37 ± 3
temperature=9602 ± 180
gravity=4.1 ± 0.1
Starbox catalog
names=Wega, Lucida Lyrae, Alpha Lyrae, α Lyrae, 3 Lyr, GJ 721, HR 7001, BD +38°3238, HD 172167, GCTP 4293.00, LTT 15486, SAO 67174, HIP 91262.

Vega (α Lyr / α Lyrae / Alpha Lyrae) (pron-en|ˈviːɡə or IPAlink-en|ˈveɪɡə) is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus. It is a relatively nearby star at only convert|25.3|ly|pc|abbr=off|lk=on from Earth, and, together with Arcturus and Sirius, one of the most 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". Historically, Vega served as the northern pole star at about 12,000 BCE and will do so again at around 14,000 CE. Vega was the first star, other than the Sun, to have its photograph taken and the first to have its 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 photometric brightness scale, and was one of the stars used to define the mean values for the UBV photometric system.

This star is relatively young when compared to the Sun. It has an unusually low abundance of the elements with a higher atomic number than that of helium. Vega is also a suspected variable star that may vary slightly in magnitude in a periodic manner. It is rotating rapidly with a velocity of 274 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.

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. Stars that display an infrared excess because of dust emission are termed Vega-like stars. Irregularities in Vega's disk also suggest the presence of at least one planet, likely to be about the size of Jupiter, in orbit around Vega.

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 at the Harvard College Observatory, also with a daguerreotype. [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 | id=ISBN 0801864585
] [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
accessdate=2007-11-18
doi=10.1086/120156
] Draper took the first photograph of a star's spectrum in August 1872 when he took an image of Vega, and he also became the first person to show absorption lines in the spectrum of a star. [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
] (Similar lines had already been identified in the spectrum of the Sun.) [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
] 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. [cite book
first=Klaus | last=Hentschel | year=2002
title=Mapping the Spectrum: Techniques of Visual Representation in Research and Teaching
publisher=Oxford University Press | id=ISBN 0198509537
]

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 G. W. von Struve, when he announced a value of 0.125 arcseconds (0.125&Prime;) for Vega. [cite book
first=Arthur | last=Berry | year=1899
title=A Short History of Astronomy
publisher=Charles Scribner's Sons
location=New York
] But Friedrich Bessel wasskeptical 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;. [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 | id=ISBN 9027728100
] [cite web
author=Anonymous | date=June 28, 2007
url=http://astroprofspage.com/archives/1011
title=The First Parallax Measurements
publisher=Astroprof
accessdate=2007-11-12
]

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 with the unaided eye are sixth magnitude, while the brightest, Sirius, has 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 photometric brightness scales. [cite book
first=Robert A. | last=Garfinkle | year=1997
title=Star-Hopping: Your Visa to Viewing the Universe
publisher=Cambridge University Press
id=ISBN 0521598893
] 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. [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
accessdate=2007-11-12
doi=10.1086/190708
]

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" - "B" = "B" - "V" = 0. In effect, the magnitude of these stars is the same in the yellow, blue and ultraviolet parts of the electromagnetic spectrum. [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
accessdate=2007-11-05
doi=10.1086/145697
] Thus, Vega has a relatively flat electromagnetic spectrum in the visual region&mdash;wavelength range 350-850 nanometers, most of which can be seen with the human eye&mdash;so the flux densities are roughly equal; 2000-4000 Jy. [cite web
last=Walsh | first=J. | date=March 6, 2002
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.
] However, the flux density of Vega drops rapidly in the infrared, and is near 100 Jy at 5 micrometers. [cite web
last=McMahon | first=Richard G.
date=November 23, 2005
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
]

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. [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
accessdate=2007-11-11
doi=10.1086/130834
] This is a category of stars that oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.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
accessdate=2007-05-14
doi=10.1146/annurev.aa.33.090195.000451
] 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.cite web
last=I.A. | first=Vasil'yev
coauthors=Merezhin, V. P.; Nalimov, V. N.; Novosyolov, V. A.
date=March 17, 1989
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
] [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
accessdate = 2007-11-12
]

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. [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
]

Visibility

Vega can often be seen near the zenith in the mid-northern latitudes during the evening in the Northern Hemisphere summer.cite book
first=Jay M. | last=Pasachoff | year=2000
title=A Field Guide to Stars and Planets
edition=Fourth edition
publisher=Houghton Mifflin Field Guides
id=ISBN 0395934311
] 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° S. At latitudes to the north of +51° N Vega remains continually above the horizon as a circumpolar star. On about July 1, Vega reaches midnight culmination when it crosses the meridian at that time.

This star lies at a vertex of a widely-spaced asterism called the Summer Triangle, which consists of the zero-magnitude stars Vega in the constellation Lyra and Altair in Aquila, plus the first magnitude star Deneb in Cygnus. 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. [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 | id=ISBN 0306457903
]

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 C/1861 G1 Thatcher and have nothing to do with the star. [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
accessdate=2007-11-02
]

Physical properties

Vega's spectral class is A0V, making it a blue-tinged white main sequence star that is 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. [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
accessdate=2007-11-05
doi=10.1086/190603
&mdash;From pages 769&ndash;778: for stars in the range 1.75e|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.
] The current age of this star is between 386 and 511 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 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 type with a period of about 0.107 days.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
accessdate=2007-10-30
doi=10.1086/130834
]

Most of the energy produced at Vega's core is generated by the carbon-nitrogen-oxygen cycle (CNO cycle), a nuclear fusion process that combines protons to form helium nuclei through intermediary nuclei of carbon, nitrogen and oxygen. This process requires a temperature of 16 million K, which is higher than the core temperature of the Sun, but is more efficient than the Sun's proton-proton chain reactionfusion reaction. The CNO cycle is highly temperature sensitive, which results in a convection zone about the core [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
] that evenly distributes the 'ash' from the fusion reaction within the core region. The overlying atmosphere is in radiative equilibrium. This is in contrast to the Sun, which has a radiation zone centered on the core with an overlying convection zone. [cite book
title=Theoretical Astrophysics
publisher=Cambridge University Press
id=ISBN 0521562414
] [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
]

The energy flux from Vega has been precisely measured against standard light sources. At 5480 Å, the flux is 3,650 Jy with an error margin of 2%. [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
accessdate=2007-11-15
doi=10.1086/150603
] The visual spectrum of Vega is dominated by absorption lines of hydrogen; specifically by the hydrogen Balmer series with the electron at the n=2 principal quantum number. [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
] [cite book
first=Donald D. | last=Clayton | year=1983
title=Principles of Stellar Evolution and Nucleosynthesis
publisher=University of Chicago Press
id=ISBN 0226109534
] The lines of other elements are relatively weak, with the strongest being ionized magnesium, iron and chromium. [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
accessdate=2007-11-15
] The X-ray emission from Vega is very low, demonstrating that the corona for this star must be very weak or non-existent. [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
accessdate=2007-11-15
]

Rotation

When the radius of Vega was measured to high accuracy with an interferometer, it resulted in an unexpectedly large estimated value of 2.73 ± 0.01 times the 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.

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 rotation velocity of 274 km/s (for a rotation period of about 12.5 hours), 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 ± 0.02 solar radii, while the equatorial radius is 2.78 ± 0.02 solar radii.) 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 K, while the equatorial temperature is 7,600 K. 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.cite journal
last=Gulliver, Hill | first=Austin F.
title=Vega: A rapidly rotating pole-on star
journal=The Astrophysical Journal
year=1994 | volume=429 | issue=2 | pages=L81–L84
accessdate=2007-10-29
doi=10.1086/187418
] [From the poles, the star presents a circular profile, while from the equator the star appears as an ellipse. The 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::or 33% of the flux from the pole.] 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,cite news
author=Staff | date=January 10, 2006
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
] while the remainder of the atmosphere is likely to be in almost pure radiative equilibrium. [cite conference
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
]

If Vega was actually a slowly rotating, spherically-symmetric star and it was radiating the same energy as viewed from the Earth, then the apparent 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.

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. [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
]

Element abundance

Astronomers term "metals" those elements with higher atomic numbers than helium. The metallicity of Vega’s photosphere is only about 32% of the abundance of heavy elements in the Sun’s atmosphere. [For a metallicity of −0.5, the proportion of metals relative to the Sun is given by::.] (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 ZSol = 0.0172 ± 0.002. [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
accessdate=2007-11-05
doi=10.1086/503707
] 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. [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
accessdate=2007-11-07
&mdash;Entry for HD 172167 on p. 144.
] [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
accessdate=2007-10-30
doi=10.1086/319000
] However, the reason for the existence of such chemically-peculiar, 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. [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
accessdate=2007-11-05
doi=10.1046/j.1365-8711.1998.02070.x
]

The observed helium to hydrogen ratio in Vega is 0.030 ± 0.005, which is about 40% lower than for the Sun. This may be caused by the disappearance of a helium convection zone near the surface. Energy transfer is instead performed by the radiative process, which may be causing an abundance anomaly through diffusion. [cite journal
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
accessdate=2007-11-07
doi=10.1086/168279
]

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 −13.9 ± 0.9 km/s. [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
accessdate = 2007-11-09

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 ± 0.63 milli-arcseconds (mas) per year in Right Ascension&mdash;the celestial equivalent of longitude&mdash;and 287.47 ± 0.54 mas/y in Declination, which is equivalent to a change in latitude. [cite journal
author=M. A. Perryman "et al"
title=The Hipparcos Catalogue.
journal=Astronomy and Astrophysics
year=1997 | volume=323 | pages=L49–L52
accessdate=2007-11-09
] The net proper motion of Vega is 327.78 mas/y, [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::.where $mu_alpha$ and $mu_delta$ are the components of proper motion in the R.A. and Declination, respectively, and $delta$ is the Declination.
] which results in angular movement of a degree every 11,000 years.

In the Galactic coordinate system, the space velocity components of Vega are U = −13.9 ± 0.9, V = −6.3 ± 0.8 and W = −7.7 ± 0.3, for a net space velocity of 17 km/s. The radial component of this velocity&mdash;in the direction of the Sun&mdash;is −13.9 km/s, while the transverse velocity is 9.9 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. [cite book
first=Forest Ray | last=Moulton | year=1906 | pages=p. 502
title=An Introduction to Astronomy
publisher=The Macmillan company
] 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. [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
]

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, Fomalhaut and Vega. All members of the group are moving in near parallel with similar space velocities. Membership in a moving group implies a common origin for these stars in a open cluster that has since become gravitationally unbound. [cite book
first=Mike | last=Inglis | year=2003
title=Observer's Guide to Stellar Evolution: The Birth, Life, and Death of Stars | publisher=Springer
id=ISBN 1852334657
] The estimated age of this moving group is 200 ± 100 million years, and they have anaverage space velocity of 16.5 km/s. [U = −10.7 ± 3.5, V = −8.0 ± 2.4, W = −9.7 ± 3.0 km/s. The net velocity is::] cite journal
title=The Castor moving group. The age of Fomalhaut and VEGA
journal=Astronomy and Astrophysics
year=1998 | volume=339 | pages=831–839
accessdate=2007-10-31
]

Planetary system

Infrared excess

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 wavelengths of 25, 60 and 100 μm, and came from within an angular radius of 10 arcseconds (10&Prime;) centered on the star. At the measured distance of Vega, this corresponded to an actual radius of 80 astronomical units (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.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
accessdate=2007-11-02
doi=10.1086/162559
] 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. [cite journal | last=Robertson
first=H. P.
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
accessdate=2007-11-02
]

Subsequent measurements of Vega at 193 μm showed a lower than expected flux for the hypothesized particles, suggesting that they must instead be on the order of 100 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. Models fitted to the dust distribution around Vega indicatethat it is a 120 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 AU. [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
accessdate=2007-11-07
doi=10.1046/j.1365-8711.2000.03331.x
]

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.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
accessdate=2007-11-10
doi=10.1086/341164
]

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 AU) at a wavelength of 24 μm, 70&Prime; (543 AU) at 70 μm and 105&Prime; (815 AU) at 160 μm. These much wider disks were found to be circular and free of clumps, with dust particles ranging from 1&ndash;50 μm in size. The estimated total mass of this dust is 3e|-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.cite journal
author=K. Y. L. Su "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
]

The inner boundary of the debris disk was estimated at 11&Prime; ± 2&Prime;, or 70&ndash;102 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.

Observations with the CHARA array at Mt. Wilson in 2006 revealed evidence for an inner dust band around Vega. Originating within 8 AU of the star, this dust may be evidence of dynamical perturbations within the system. [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
accessdate=2007-11-19
doi=10.1051/0004-6361:20054522
] This may be caused by an intense bombardment of comets or meteors, and may be evidence for the existence of a planetary system. [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
]

Possible planets

Observations from the James Clerk Maxwell Telescope in 1997 revealed an "elongated bright central region" that peaked at 9&Prime; (70 AU) to the northeast of Vega. This was hypothesized as either a perturbation of the dust disk by a 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. [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
accessdate=2007-11-10
doi=10.1038/33874
] Astronomers at the Joint Astronomy Centre in Hawaii and at UCLA suggested that the image may indicate a planetary system still undergoing formation. [cite news
author=Staff | date=April 21, 1998
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
]

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 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.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
accessdate=2007-10-30
doi=10.1086/340691
]

In 2003 it was hypothesized that these lumps could be caused by a roughly Neptune-mass planet having migrated from 40 to 65 AU over 56 million years,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
accessdate=2007-10-30
doi=10.1086/379064
] an orbit large enough to allow the formation of smaller rocky planets closer to Vega. The migration of this planet would likely require gravitational interaction with a second, higher mass planet in a smaller orbit. [cite news
last=Gilchrist | first=E.
coauthors=Wyatt, M.; Holland, W.; Maddock, J.; Price, D. P.
date=December 1, 2003
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
]

Using a coronagraph on the 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. [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
accessdate=2007-11-10
doi=10.1086/508420
] 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 planets orbiting closer to the star. The inclination of planetary orbits around Vega is likely to be closely aligned to the equatorial plane of this star. [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
accessdate=2007-11-16
doi=10.1086/131516
] 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. [The Sun would appear at the diametrically opposite coordinatesfrom Vega at α=RA|6|36|56.3364, &delta;=DEC|−38|47|01.291, which is in the western part of Columba. The visual magnitude is given by ]

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 of the equinoxes. A complete precession cycle requires 25,770 years, [cite book
first=Andrew L. | last=Chaikin
editors=Beatty, J. K.; Petersen, C. C.
year=1990 | title=The New Solar System
edition=4th edition
publisher=Cambridge University Press
location=Cambridge, England
id=ISBN 0521645875
] during which timethe 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 BCE the pole was pointed only five degrees away from Vega. Through precession, the pole will again pass near Vega around 14,000 CE. [cite book
first=Archie E. | last=Roy
coauthors=Clarke, David
year=2003 | title=Astronomy: Principles and Practice
publisher=CRC Press
id=ISBN 0750309172
] It is the brightest of the successive northern pole stars.

In Inuit astronomy, Vega is known as the Old Woman.Fact|date=July 2008 Among the northern polynesian 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. [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
accessdate=2008-08-08
]

The Assyrians named this pole star Dayan-same, the "Judge of Heaven", while in Akkadian it was Tir-anna, "Life of Heaven". In Babylonian astronomy, Vega may have been one of the stars named Dilgan, "the Messenger of Light". To the ancient Greeks, the constellation Lyra was formed from the harp of Orpheus, with Vega as its handle.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
] For the Roman Empire, the start of autumn was based upon the hour at which Vega set below the horizon.cite book
first=Richard Hinckley | last=Allen | year=1963
title=Star Names: Their Lore and Meaning
publisher=Courier Dover Publications
id=ISBN 0486210790
]

In Chinese mythology, there is a love story of Qi Xi _zh. 七夕 in which Niu Lang _zh. 牛郎 (Altair) and his two children (β and γ Aquilae) are separated from their mother Zhi Nü _zh. 織女 (Vega) who is on the far side of the river, the Milky Way 銀河. [cite book
first=Liming | last=Wei | coauthors=Yue, L.; Lang Tao, L.
year=2005 | title=Chinese Festivals
publisher=Chinese Intercontinental Press
id=ISBN 750850836X
] 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. This is apparently a reference to the Perseids.Ref|date=August 2008 The Japanese Tanabata festival is also based on this legend. [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
] In Zoroastrianism, Vega was sometimes associated with, Vanant, a minor divinity whose name means "conqueror". [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
id=ISBN 9004088474
]

The name Wega (later Vega) comes from a loose transliteration of the Arabic word " _ar. waqi" meaning "falling", via the phrase _ar. النسر الواقع " _ar. an-nasr al-wāqi‘", which sources translate as "the falling eagle" [cite book
first=Cyril | last=Glasse | year=2001
title=The New Encyclopedia of Islam
publisher=Rowman Altamira
id=ISBN 0759101906
&mdash;"astronomy" entry.
] or "the swooping vulture", [cite web
last=Harper | first=Douglas | month=November | year=2001
url=http://www.etymonline.com/index.php?term=Vega
title=Vega | publisher=Online Etymology Dictionary
accessdate=2007-11-01
] as this constellation was represented as a vulture in ancient Egypt, [cite book
first=Gerald | last=Massey | year=2001
title=Ancient Egypt: the Light of the World
id=ISBN 140217442X
] and as an eagle or vulture in ancient India. [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
] [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
] The Arabic name then appeared in the western world in the Alfonsine Tables, which were drawn up between 1215&ndash;70 by order of Alfonso X. [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=p. 292 | publisher=E.J. Brill
]

Medieval astrologers counted Vega as one of the Behenian stars [cite book
first=Donald | last=Tyson | coauthors=Freake, James
year=1993
title=Three Books of Occult Philosophy
publisher=Llewellyn Worldwide
id=ISBN 0875428320
] and related it to chrysolite and winter savory. Cornelius Agrippa listed its kabbalistic sign under "Vultur cadens", a literal Latin translation of the Arabic name. [cite book
first=Heinrich Cornelius | last=Agrippa | year=1533
title=De Occulta Philosophia
] Medieval star charts also listed the alternate names Waghi, Vagieh and Veka for this star.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
id=ISBN 0486235688
]

Vega became the first star to have a car named after it when Chevrolet launched the Vega in 1971. [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
] Other vehicles named after Vega include the ESA's Vega launch system [cite web
author=Staff | date=May 20, 2005
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
] and the Lockheed Vega aircraft. [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
]

ee also

* Vega in fiction

Notes and references

*cite web
title=Vega | work=SolStation
url=http://www.solstation.com/stars/vega.htm
accessdate=2005-11-09

*cite news
last=Gilchrist | first=Eleanor
coauthors=Wyatt, Mark; Holland, Wayne; Price, Douglas Pierce; Maddock, Julia
date=December 1, 2003
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=January 10, 2005
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.

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