Kippenhahn diagram showing the evolution of the internal structure of the non-rotating solar metallicity models of 0.3 (top), 0.5, 1.0 and 1.5 M_⊙ (bottom) from the PMS up to the end of the main sequence. The upper line represents the surface radius and hatched areas refer to convective regions. The green line displays the H-burning limit. The five pink vertical lines indicate the ages of open clusters used as markers of the evolution.