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 p art of the Key Project on the distance scale. The properties of the as sociations, including their size distribution and average diameter, an d the luminosity function of their brightest stars, do not vary signif icantly among galaxies, even for changes in Hubble type. These finding s support previous suggestions about the existence of similarly sized elementary cells of star formation in galaxies, and the conclusion, ob tained by others with more extensive data, that the upper stellar mass function does not vary significantly. The data suggest that the avera ge number of stars in associations changes with morphological type, co nfirming previous studies of the integrated magnitudes of OB associati ons in H II regions.