In addition, many observations need to be scheduled to follow rotation enough to calculate the rotation period, and this means that fewer stars can be observed with the telescope time available. The stellar age calibration diagram used by the HK Project. The small symbols are binaries of known age, and the larger letters correspond to the mean age determined for star clusters and moving groups. Models of stellar evolution show the progression of a star's luminosity and surface temperature with time, dependent on its mass. While measuring the luminosity and temperature of a single star without knowing the mass won't necessarily give an accurate measurement of its age, there are several ways we can improve our accruacy.
Star clusters have dozens of stars which all formed at nearly the same time.
Therefore, plotting their luminosity and temperatures will show a distribution that can be tied to a single age. We can then measure these stars' activity and rotation, thereby providing the link we need to measure the age of field stars. However, only the closest star clusters are close enough to have stars which are bright enough to be observable with the HK Project's instrument. Nonetheless, it has provided a good place to start. Another source of information is binary stars.