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Main-sequence stars near one solar mass show an average level of Ca II H and K emission and rotation that decrease with age. Although the mechanism governing surface magnetism and rotation is not theoretically well-understood, the measurement of rotation or average activity level can yield an estimate of the age of a solar-mass star.
Several empirically-determined functions of the decay of rotation and activity have been developed over the last several decades, but more have concentrated on stars much younger than the Sun, whose Ca II activity and rotation have been relatively easy to measure. Observations of the Ca II H and K emission were obtained of solar-mass stars in the old open clusters NGC 752 and M 67 with the KPNO 4-m telescope and HYDRA spectrograph. Those spectra yield a large ($>50$) smaple of stars close to one solar mass and close to the age of the Sun. Those spectra have been calibrated to the system of measurement of Ca II H and K emission fluxes of nearly 1000 lower main sequence stars obtained at Mount Wilson Observatory.
The combined sample of Ca II fluxes yield: (1) a refined calibration of age as a function of activity, using rotation as an indicator of age; (2) the range of Ca II activity at a given age, caused by variations of surface magnetic activity over time scales of decades to centuries; and (3) an estimate of the uncertainty of age inferred from a measurement of the instantaneous activity level.
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