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