Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way.

In this paper the fundamental properties of the brightest known stars in our Galaxy are determined for future comparison with results for the most luminous stars in other galaxies. The H-R diagrams (Mv versus spectral type and Mbol versus log Te), the luminosities of the brightest stars, and the ratios of blue to red supergiants are all discussed, and a catalog of over 1000 super- giants and 0 stars in associations and clusters is included at the end of the paper. Tile "theoretical" H-R diagram (Mbol versus log Te) reveals a group of superluminous 0 stars with Mbol between -10 and -12 mag, a lack of evolved supergiants at these very high luminosities, and an apparent upper limit to the luminosities of the later-type supergiants (>B5) near Mbol = -9.5 mag. The most luminous red supergiants have a maximum visual luminosity near Mv -8 mag, supporting the suggestion by Sandage and Tammann that they are good distance indicators. Excluding the superluminous star Cyg OB 2 No. 12 (Mv - 9.9 mag), the brightest blue stars are found at Mv -8.5 mag. The variation of the ratio of blue to red supergiants with luminosity is discussed. Tilere is also evidence for a gradient in this ratio with distance from the galactic center, although the results are limited by the incompleteness of the data. Subject headings: galaxies: Milky Way - galaxies: stellar content - stars: early-type - stars: late-type - stars: luminosities - stars: supergiants