Spectroscopic Hertzsprung-Russell diagrams showing the non-rotating and slowly rotating core-hydrogen burning models in the five grids corresponding to the different metallicities (see labels). The left Y-axis shows the quantity log (L/ L_⊙) with whereas the right Y-axis shows the corresponding values of log (Γ_e), in all the five panels. The Γ_e values are computed at the stellar surface considering electron-scattering opacity with a hydrogen abundance of X = 0.73 (completely ionised), and the black dotted line marks the location Γ_e = 1. Note that the assumption of completely ionised hydrogen breaks down for models with T_eff below ≈10 000 K. Black, blue, and red dots correspond to models with Γ_max< 0.9, 0.9 < Γ_max< 1, and Γ_max> 1, respectively. Only the models with log  (L/ L_⊙) > 3.8 have been shown. The black solid line is the ZAMS, and the masses of some representative models (in units of M_⊙) have been indicated.