A Hubble Space Telescope study of extragalactic OB associations

We present a study of OB associations, identified by objective means, in seven spiral galaxies observed with the Hubble Space Telescope as part of the Key Project on the distance scale. The properties of the associations, including their size distribution and average diameter, and the luminosity function of their brightest stars, do not vary significantly among galaxies, even for changes in Hubble type. These findings support previous suggestions about the existence of similarly sized elementary cells of star formation in galaxies, and the conclusion, obtained by others with more extensive data, that the upper stellar mass function does not vary significantly. The data suggest that the average number of stars in associations changes with morphological type, confirming previous studies of the integrated magnitudes of OB associations in H II regions.