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Rotation

Vega has a rotation period of 16.3 hours, much faster than the Sun's rotational period but similar to, and slightly slower than, those of Jupiter and Saturn. Because of that, Vega is significantly oblate like those two planets.

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 2005–06 confirmed this deduction.

The pole of Vega—its axis of rotation—is inclined no more than five degrees from the line-of-sight to the Earth. At the high end of estimates for the rotation velocity for Vega is 236.2 ± 3.7 km/s along the equator, much higher than the observed (i.e. projected) rotational velocity because Vega is seen almost pole-on. This is 88% 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 19% larger than the polar radius, compared to just under 11% for Saturn, the most oblate of the Solar System's planets. (The estimated polar radius of this star is 2.362 ± 0.012 solar radii, while the equatorial radius is 2.818 ± 0.013 solar radii.) From the Earth, this bulge is being viewed from the direction of its pole, producing the overly large radius estimate.

The local surface gravity at the poles is greater than at the equator, which produces a variation in effective temperature over the star: the polar temperature is near 10,000 K, while the equatorial temperature is about 8,152 K. 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 that Vega has a convection zone around the equator, while the remainder of the atmosphere is likely to be in almost pure radiative equilibrium. By the Von Zeipel theorem, the local luminosity is higher at the poles. As a result, if Vega were viewed along the plane of its equator instead of almost pole-on, then its overall brightness would be lower.

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 improved instrument calibrations.