Time variation of the luminosity of stellar models calculated with the physical and chemical input described in Section 2. Stellar models representing the 1G and the 2G of the cluster are shown in the top and bottom panel, respectively. The dashed green line indicates the age of 12 Gyr. The tracks in green highlight the time evolution of the minimum mass supported by proton-proton chain luminosity at the age of 12 Gyr; namely, 0.074 M_⊙ (top panel, 1G) and 0.070 M_⊙ (bottom panel, 2G). The tracks of the stars of mass 0.2 M_⊙ (dotted lines), 0.1 M_⊙ (dash-dotted), and 0.08 M_⊙ (dashed) are highlighted.