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. (The estimated polar radius of this star is solar radii, while the equatorial radius is 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 , while the equatorial temperature is about . 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. Element abundance In astronomy, those elements with higher atomic numbers than helium are termed "metals". The metallicity of Vega's photosphere is only about 32% of the abundance of heavy elements in the Sun's atmosphere. (Compare this, for example, to a threefold 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 = . Thus, in terms of abundances, only about 0.54% of Vega consists of elements heavier than helium. Nitrogen is slightly more abundant, oxygen is only marginally less abundant and sulfur abundance is about 50% of solar. On the other hand, Vega has only 10% to 30% of the