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. Even though they have only two stars instead of the dozens in star clusters, each star should have the same age. The figure to the right shows the age calibration currently used by the HK Project.
Activity is on the y axis and age is on the x axis.
Clusters with known age and some measured activity are shown with bold letters. The smaller symbols correspond to the age and activity of binaries of known age. Ages in the solar neighborhood Derived ages of dwarf stars in the solar neighborhood. Lines connect binaries. The series of filled circles show the Sun at different phases of its activity.