  We provide an observational view of evolutionary models in the
Hertzsprung--Russell diagram, on the main sequence. For that we computed
evolutionary models with the code STAREVOL for 15 < M/Msun < 100. We
subsequently calculated atmosphere models at specific points along the
evolutionary tracks, using the code CMFGEN. Synthetic spectra obtained in this
way were classified as if they were observational data. We tested our spectral
classification by comparison to observed spectra of various stars. We also
compared our results with empirical data of a large number of OB stars. We
obtain spectroscopic sequences along evolutionary tracks. In our computations,
the earliest O stars (O2-3.5) appear only above ~50 Msun. For later spectral
types, a similar mass limit exists, but is lower. A luminosity class V does not
correspond to the entire main sequence. This only holds for the 15 Msun track.
As mass increases, a larger portion of the main sequence is spent in luminosity
class III. Above 50 Msun, supergiants appear before the end of core-hydrogen
burning. Dwarf stars do not occur on the zero-age main sequence above 80 Msun.
Consequently, the distribution of luminosity class V in the HR diagram cannot
be used to constrain the size of the convective core. The distribution of
dwarfs and giants in the HR diagram agrees well with the location of stars
analyzed by means of quantitative spectroscopy. For supergiants, there is a
slight discrepancy in the sense that luminosity class I is observed slightly
earlier than our predictions. This is mainly due to wind densities that affect
the luminosity class diagnostic lines. We predict an upper mass limit for dwarf
stars (~60 Msun) that is found consistent with the rarity of O2V stars in the
Galaxy. Stars with WNh spectral type are not predicted by our models. Stronger
winds are required to produce the characteristic emission lines of these
objects.
