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New observations confirm that the star Vega, 25.3 light years away and the second brightest star in the northern sky, is far older and rotates far faster and in a different orientation, than once thought.
The findings are important because the star is widely used by astronomers as a calibration target when assessing other stars.
Vega was one of a few stars whose brightness has been calibrated against an absolute laboratory source.

The work may mean that other stars need to be reassessed, though it is unlikely to make any fundamental corrections.

"Vega has always been the star against which all other stars are compared. This work will send a ripple through the whole system, but it's not going to break anything" - Deane Peterson, astronomer at Stony Brook University in New York, US.


The dust disc around Vega was imaged by the Spitzer Space Telescope, which detected the heat from the dust.
Image: NASA/JPL-Caltech/K Su-U. Arizona


The 815 AU wide dusty debris disc around Vega is almost twice as old as previously thought, and this may have implications for planet formation theories.

"The problem with that is that we must then choose a star which does not have similar or greater. My guess is that once we understand Vega well enough, we will be able to continue using it as a fundamental calibrator." - Richard Gray, Appalachian State University in North Carolina, US

Peterson's team used three telescopes in the US Navy Prototype Optical Interferometer in Arizona to determine in detail the distribution of brightness around Vega's surface. Previously, it was thought to be uniform, and the star was believed to be oriented with its poles pointing "up" and "down" as seen from Earth.
But the brightness was not uniform, and the pattern suggests two things. First, that the star’s polar axis is pointing almost directly at Earth and, second, that the star is rotating much faster than thought, at more than 90% of the speed which would be needed to pull it apart. This means Vega’s poles must be more than 2000°C hotter than the equator.

A second team, led by Jason Aufdenberg, used paired telescopes at the Centre for High Angular Resolution Astronomy' on Mount Wilson in California and came up with similar results.
The uneven temperature and fast rotation means a simple model of the star can no longer be used as part of the calibration process, when looking at other stars.
The fast rotation speed means the observed composition of the star cannot be just for its surface layer, as the star must be well mixed. Since stellar age is calculated largely from its composition, Vega's age may be pushed from the 350 million years to about 570 million years.

Gray first suggested that Vega was a rapidly rotating star, nearly pole-on to the Earth, in the mid-1980s. This was to explain the star's abnormally high brightness as well as the apparent lack of spectral evidence for an equatorial view.

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Rapidly Spinning Star Vega has Cool Dark Equator

Strong darkening observed around the equator of Vega suggests that the fifth brightest star in Earth's sky has a huge temperature difference of 4,000 degrees Fahrenheit from its cool equatorial region to its hot poles.

Models of the star based on these observations suggest that Vega is rotating at 92 percent of the angular velocity that would cause it to physically break apart, an international team of astronomers announced today in Washington, DC, at the 207th meeting of the American Astronomical Society.
This result confirms the idea that very rapidly rotating stars are cooler at their equators and hotter at their poles, and it indicates that the dusty debris disk known to exist around Vega is significantly less illuminated by the star's light than previously recognized.

"These findings are significant because they resolve some confusing measurements of the star, and they should help us gain a much better understanding of Vega's circumstellar debris disk" - Jason P. Aufdenberg, the Michelson Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson, Arizona.

This debris disk arises mainly from the collision of rocky asteroid-like bodies.

"The spectrum of Vega as viewed from its equatorial plane, the same plane as the debris disk, should be about half as luminous as the spectrum viewed from the pole, based on these new results" - Jason P. Aufdenberg.

The team obtained high-precision interferometric measurements of the bright standard star Vega using the Centre for High Angular Resolution Astronomy (CHARA) Array, a collection of six 1-meter telescopes located on Mount Wilson, California, and operated by Georgia State University.
With a maximum baseline of 330 meters, the CHARA Array is capable of resolving details as small as 200 micro-arcseconds, equivalent to the angular size of a nickel seen from a distance of 10,000 miles. The CHARA Array fed the starlight of Vega to the Fiber Linked Unit for Optical Recombination (FLUOR) instrument, developed by the Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique of the Observatoire de Paris.

One major consequence of Vega's rapid rotation is a significant drop in the effective atmospheric temperature by approximately 2,300 Kelvin from the pole to the equator. This effect, known as "gravity darkening," was first predicted by theoretical astronomer E. Hugo von Zeipel in 1924.


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The CHARA/FLUOR measurements of the brightness distribution of Vega's surface also show it to be strongly "limb darkened." Limb darkening refers to the diminishing brightness in the image of a star from the centre of the image to the edge or "limb" of the image.
The new measurements are consistent with the "pole-on" model for Vega first proposed by Richard O. Gray of Appalachian State University, which proposes that Vega's pole of rotation points toward Earth. The pole-on view of Vega means that the relatively cool equator corresponds to the limb of the star, such that the gravity-darkening effect further enhances the limb-darkening effect.

The CHARA/FLUOR data support the pole-on, gravity darkened model for Vega by showing that Vega's limb darkening is 2.5 times stronger at a wavelength of 2.2 microns than expected for a star with a single effective atmosphere temperature. Archival observations from the International Ultraviolet Explorer indicate that this model for Vega is not complete. At far ultraviolet wavelengths, below 140 nanometers, the model is generally too bright.



Position(2000): RA 18 36 56.34 Dec +38 47 01.3
VEGA (Alpha Lyrae) a classic white main sequence star, lying 25.3 light years away in the constellation Lyra. Its name (more correctly "Wega") is derived from the Arabic for "Swooping Eagle" ("Al Nasr al Waki").

Located at a distance of 25.3 light-years from Earth in the constellation Lyra, Vega rotates about its axis once every 12.5 hours. For comparison, the Sun's average rotation period is approximately 27 Earth days. Vega is about 2.5 times more massive than the Sun, and 54 times brighter.

At Vega's rapid rate of rotation, the star's atmosphere is distorted, bulging 23 percent wider at its equator compared to its poles. This type of rotational distortion can be seen in images of the planet Saturn, where the planet's equatorial diameter is roughly 10 percent wider than the polar diameter. A direct measurement of Vega's rotational distortion is hidden by its pole-on appearance. However, the accurate angular diameter and darkening measured by CHARA/FLUOR are consistent with this distortion.

These resultsbuild upon recent measurements of Vega obtained by a team lead by Deane M. Peterson of the State University of New York, Stony Brook, using the Navy Prototype Optical Interferometer.

Co-authors of this result include Antoine Mérand, Vincent Coudé du Foresto, Emmanuel Di Folco, and Pierre Kervella of the Observatoire de Paris-Meudon, France; Olivier Absil of the University of Liège, Belgium; Stephen T. Ridgway of the National Optical Astronomy Observatory, Tucson, Arizona and NASA; Harold A. McAlister, Theo A. ten Brummelaar, Judit Sturmann, Laszlo Sturmann, and Nils H. Turner of the Centre for High Angular Resolution Astronomy, Georgia State University, Atlanta,
Georgia, and Mount Wilson Observatory, California; and David H. Berger of the University of Michigan, Ann Arbor, Michigan.


This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. The CHARA Array is operated by the Centre for High Angular Resolution Astronomy, Georgia State University, Atlanta, GA. Additional support comes from the National Science Foundation, the Keck Foundation and the Packard Foundation.

The National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the NSF.

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