The ages of the most common stars - low-mass (cool) stars like the Sun, andsmaller - are difficult to derive because traditional dating methods usestellar properties that either change little as the stars age or are hard tomeasure. The rotation rates of all cool stars decrease substantially with timeas the stars steadily lose their angular momenta. If properly calibrated,rotation therefore can act as a reliable determinant of their ages based on themethod of gyrochronology. To calibrate gyrochronology, the relationship betweenrotation period and age must be determined for cool stars of different masses,which is best accomplished with rotation period measurements for stars inclusters with well-known ages. Hitherto, such measurements have been possibleonly in clusters with ages of less than about one billion years, andgyrochronology ages for older stars have been inferred from model predictions.Here we report rotation period measurements for 30 cool stars in the2.5-billion-year-old cluster NGC 6819. The periods reveal a well-definedrelationship between rotation period and stellar mass at the cluster age,suggesting that ages with a precision of order 10 per cent can be derived forlarge numbers of cool Galactic field stars.
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