Strömgren uvby CCD photometry can be used in a variety of ways to constrain the absolute and relative ages of globular clusters .
The reddening corrected v - y,c _ { 1 } diagram offers the means to derive ages that are completely independent of distance .
Very precise differential ages for clusters of the same chemical composition may also be determined from such 2–color plots , or from measurements of the magnitude difference , \Delta u , between the subgiant and horizontal branches on the u - y,u plane ( where both of these features are flat and well-defined , even for clusters like M13 that have extremely blue HBs on the B - V,V diagram ) .
Based on high-quality photometry we find that : ( 1 ) M92 is \simeq 15 Gyr old , ( 2 ) M3 and M13 differ in age by < 1 Gyr , and ( 3 ) NGC 288 , NGC 362 , and NGC 1851 are coeval to within \sim 1.5 Gyr .
These results strongly suggest that age can not be the “ second parameter ” .
Finally , we suggest that the observed variations in c _ { 1 } among giant branch stars in all the metal–poor clusters that we have studied so far are likely due to star–to–star C and N abundance variations , and potentially indicate that most ( if not all ) globular clusters have “ primordial ” variations in at least these elements .