Hertzsprung-Russell diagram for massive stars with ZAMS masses: 20 M_⊙, 32 M_⊙, 50 M_⊙, 80 M_⊙, and 100 M_⊙ at metallicity Z = 0.007; evolution of single star since main sequence to core helium burning. In the top panel, we marked stars cooler than log(T_eff/K) < 3.73 with the red line and with the blue line, those stars with log(T_eff/K) > 3.73. It is expected that the red line represents stars having convective envelopes, while the blue line is for radiative envelopes (Ivanova & Taam 2004; Belczynski et al. 2008). In the bottom panel, we marked with the blue line, the star main sequence and Hertzsprung gap phases and with the red line, the core helium burning phase. Evolutionary stages are defined as in formulas by Hurley et al. (2002) and Belczynski et al. (2008). With the grey lines, we marked the borders corresponding to stellar radii: 100 R_⊙, 500 R_⊙, and 1000 R_⊙.