Kippenhahn diagrams showing mass (m) and radial (r) coordinates as a function of time (t) for a 55 M_⊙ star model with metallicity Z = 0.00042, undergoing chemically homogenous evolution. The total mass or radius is shown with a thick solid black curve. Convective regions are highlighted in purple, and regions with overshooting are marked in green. In addition, the apsidal motion constant (k) is shown as a solid red line, with its scale in the right side of the ordinate axis. Finally, we show a vertical dashed blue line at 5.12 Myr; we later use this model to assess stellar mergers shortly after the end of the main sequence.