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Polarization in stars was first predicted by Chandrasekhar [1] who calculateda substantial linear polarization at the stellar limb for a pureelectron-scattering atmosphere. This polarization will average to zero whenintegrated over a spherical star but could be detected if the symmetry isbroken, for example by the eclipse of a binary companion. Nearly 50 years ago,Harrington and Collins [2] modeled another way of breaking the symmetry andproducing net polarization - the distortion of a rapidly rotating hot star.Here we report the first detection of this effect. Observations of the linearpolarization of Regulus, with two different high-precision polarimeters, rangefrom +42 parts-per-million (ppm) at a wavelength of 741 nm to -22 ppm at 395nm. The reversal from red to blue is a distinctive feature of rotation-inducedpolarization. Using a new set of models for the polarization of rapidlyrotating stars we find that Regulus is rotating at 96.5$\substack{+0.6-0.8}$%of its critical angular velocity for breakup, and has an inclination greaterthan 76.5 degrees. The rotation axis of the star is at a position angle of79.5$\pm$0.7 degrees. The conclusions are independent of, but in good agreementwith, the results of previously published interferometric observations ofRegulus [3]. The accurate measurement of rotation in early-type stars isimportant for understanding their stellar environments [4], and course of theirevolution [5].

Polarization due to rotational distortion in the bright star Regulus