  Vega, the second brightest star in the northern hemisphere, serves as a
primary spectral type standard. While its spectrum is dominated by broad
hydrogen lines, the narrower lines of the heavy elements suggested slow to
moderate rotation, giving confidence that the ground-based calibration of its
visibile spectrum could be safely extrapolated into the ultraviolet and
near-infrared (through atmosphere models), where it also serves as the primary
photometric calibrator. But there have been problems: the star is too bright
compared to its peers and it has unusually shaped absorption line profiles,
leading some to suggest that it is a distorted, rapidly rotating star seen
pole-on. Here we report optical interferometric observations of Vega which
detect the asymmetric brightness distribution of the bright, slightly offset
polar axis of a star rotating at 93% of breakup speed. In addition to
explaining the unusual brightness and line shape pecularities, this result
leads to the prediction of an excess of near-infrared emission compared to the
visible, in agreement with observations. The large temperature differences
predicted across its surface call into question composition determinations,
adding uncertainty to Vega's age and opening the possibility that its debris
disk could be substantially older than previously thought.
