  The distribution of stars in the Hertzsprung-Russell diagram narrates their
evolutionary history and directly assesses their properties. Placing stars in
this diagram however requires the knowledge of their distances and interstellar
extinctions, which are often poorly known for Galactic stars. The spectroscopic
Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is
independent of distance and extinction measurements. Based on spectroscopically
derived effective temperatures and gravities of almost 600 stars, we derive for
the first time the observational distribution of Galactic massive stars in the
sHRD. While biases and statistical limitations in the data prevent detailed
quantitative conclusions at this time, we see several clear qualitative trends.
By comparing the observational sHRD with different state-of-the-art stellar
evolutionary predictions, we conclude that convective core overshooting may be
mass-dependent and, at high mass ($\geq 15\,M_\odot$), stronger than previously
thought. Furthermore, we find evidence for an empirical upper limit in the sHRD
for stars with $T_{\rm{eff}}$ between 10000 and 32000 K and, a strikingly large
number of objects below this line. This over-density may be due to inflation
expanding envelopes in massive main-sequence stars near the Eddington limit.

Solar and Stellar Astrophysics