Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard1. Although 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 visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models2), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers3 and it has unusually shaped absorption line profiles, leading some4,5 to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations6,7. 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 disk8 could be substantially older than previously thought9,10.
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We thank R. Kurucz for advice on calculating broadband fluxes using model atmospheres. The NPOI facility is a collaboration between the Naval Research Laboratory and the US Naval Observatory in association with Lowell Observatory, and is funded by the Office of Naval Research and the Oceanographer of the Navy. This research has made use of the SIMBAD literature database, operated at CDS, Strasbourg, France, and of NASA's Astrophysics Data System.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Contains Supplementary Information; Supplementary Figure S1; Supplementary Tables 1 and 2. (PDF 151 kb)
The full, calibrated closure phase data set for May 25, 2001 for Vega. Except for the first few lines, which give the format, this is machine readable. (TXT 14 kb)
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Peterson, D., Hummel, C., Pauls, T. et al. Vega is a rapidly rotating star. Nature 440, 896–899 (2006). https://doi.org/10.1038/nature04661
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