**3. The HR Diagram**

Fundamental data for the most luminous stars have application to numerous astrophysical questions specifically with respect to stellar evolution, the final stages of the most massive stars as the progenitors of supernovae, and the dependence of their basic parameters on the host galaxy. In the 1970s, there was also considerable interest in the luminosity calibration of the brightest stars and their potential as extragalactic distance indicators [14].

The massive star population in the Milky Way, although restricted to a relatively small volume, within ≈3 kpc of the Sun, is critical as a reference population and as representative of our local region of the Galaxy, despite uncertain distances and possible incompleteness. Humphreys [15] published an HR Diagram for the Galactic supergiants and O stars with spectral types and photometry in stellar associations and clusters with known distances to derive their luminosities. For a more complete population in the upper HR Diagram, the less luminous early B-type main sequence and giant stars were also included. Compared to previous work, numerous surveys, especially of the Southern sky, had added greatly to the number of confirmed supergiants and O stars [16–18] and red supergiants [19,20]. For comparison with evolutionary tracks, the derived absolute visual magnitudes and spectral types were transformed to absolute bolometric luminosities and effective temperatures based on the available calibrations.

Humphreys [21] subsequently published HR Diagrams for the LMC. The basic data for the confirmed supergiants and early-type stars came from the extensive catalogs of spectral types and photometry by Ardeberg et al. [22] and Brunet et al. [23], from Feast, Thackeray & Wesselink [5], and from Walborn [24] for the early O-type stars. The data for the M-type supergiants came from the spectroscopic survey in the same paper.

An empirical comparison of these HR Diagrams, for the massive stars in our region of the Milky Way and for the LMC, revealed comparable populations of massive stars based on the distribution of their spectral types and luminosities across the HR Diagrams. The most important result was the recognition by Humphreys & Davidson [25] of an empirical upper luminosity boundary or upper limit in the luminosity vs. temperature diagrams.
