The core rotation rate of a star - a key indicator of its evolutionary state - cannot be measured directly, because the core is inaccessible to observation. This paper presents a new method for calculating core rotation in an evolved star. The Fourier spectra of brightness variations of four stars, derived from data from the Kepler spacecraft, were used to determine the rotational frequency splitting of the recently identified 'mixed modes' caused by rotation in red-giant stars.
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