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CCD Ha images were used to study the properties of H II regions in 29 normal Im galaxies and six blue compact dwarf/starburst irregulars. The Ha emission line fluxes were measured and used to construct luminosity functions for each galaxy. For most galaxies, the luminosity function is well represented by a power law. Only two of the 29 Im galaxies have supergiant H II regions, which is probably a result of the small size and small total numbers of H II regions in irregular galaxies. BCDs and starburst galaxies, on the other hand, do not always have supergiant H II regions but may have them even if only a few regions are present. Comparison of the cumulative composite H II luminosity function with that of spiral galaxies shows that the irregulars in our sample do not have an unusually large population of supergiant H II regions relative to the galactic luminosity. Thus, these galaxies will not suffer a disproportionate amount of disruption by the concentration of massive stars within such regions. However, in BCD/starburst galaxies there is an excess of H II regions with luminosities greater than 1038 ergs s-1 compared to Im and Sc galaxies relative to the luminosities of the host galaxies. Most of the H II region luminosity in normal Im galaxies comes from small regions and that in BCD/starburst galaxies from moderately large regions. Over 80% of the H II region luminosity in irregulars is found in complexes of H II regions typically with summed luminosities equivalent to [?]10 Orion nebulae. Two types of luminosity functions were observed: those that exhibit turnover and those that do not. Those that do exhibit turnover have an average slope of -1.5 +- 0.1, and those that do not exhibit turnover tend to be complete to fainter levels, have lower upper luminosity cutoffs, have fewer H II regions, and have shallower slopes of -1.0 +- 0.1. If the luminosity function is universal for a galaxy type, this difference suggests the possibility of a break in the luminosity function corresponding to H II regions ionized by single stars and H II regions ionized by clusters of stars. However, we cannot exclude the possibility that the two groups of galaxies are different in terms of their global star formation properties. Diameters of H II regions were also measured and diameter distributions were constructed for each of the galaxies. The diameter distribution was fitted with an exponential and the characteristic diameter D0 was measured for each galaxy, although this fit did not characterize the distribution very well in most galaxies. The weak correlation between D0 and the absolute blue magnitude of the galaxy suggested by others is confirmed to be very weak.

OB associations NGC 206 and A184 in M 31. The filters that were used are replicas of the F555W and F785LF filters that are with the WF/PC on the Hubble Space Telescope and which will be used frequently in the study of stellar populations with that instrument. These data were obtained in order to aid in the interpretation of the flight data of these fields and are presented here in order to lay a foundation for those observations. These observations serve as a basis for further explorations with the HST of the stellar luminosity function in different galactic environments.

The R136 cluster in 30 Doradus is the prototype "super star cluster," and the only example sufficiently close that its massive star content can be studied directly. We have used the HubbleSpaceTelescopeto obtain spectra of 65 of the bluest, most luminous stars in R136 and find that the majority of these stars are of type O3, the hottest, most luminous, and most massive stars known. The total number of O3 stars in this one cluster exceeds the total number known elsewhere in the Milky Way or Magellanic Clouds. The highest luminosity stars found are O3 If*, O4 If+, O3 If/WN6-A, and H-rich WN stars, with masses in excess of 120 ?, the highest masses for which appropriate evolutionary tracks are currently available. In accord with de Koter, Heap, & Hubeny, we conclude that these WN stars must be core H-burning stars whose spectra are WR-like because of high luminosity, and we find that their individual luminosities are a factor of 10 higher than is normal for WN stars of similar type but are like those found in the Galactic cluster NGC 3603, which they also resemble spectroscopically. Our spectroscopy does include stars as late as B0 V and samples most stars in the core of the R136 cluster with masses >50 M?. The spectroscopy has been combined with HSTphotometry to study the star formation history and initial mass function of the R136 cluster. The young age (<1-2 Myr) for the highest mass stars, combined with what was previously known for the intermediate-mass populations, suggests that the lower mass stars began forming 4-5 Myr ago and continued forming until the high mass stars formed, consistent with the paradigm in which the formation of massive stars shuts down further star formation in the molecular cloud. Despite the unique preponderance of the highest mass and luminosity stars ever seen, the initial mass function (IMF) is found to be completely normal, with a slope ? = -1.3 to -1.4. The number of high-mass stars is in good accord with that predicted by the IMF of the intermediate-mass stars, suggesting that a Salpeter-like IMF holds over the mass range 2.8-120 ?within the R136 cluster. The fact that the IMF slope in R136 is indistinguishable from those of Galactic and Magellanic Cloud OB associations suggests that star formation produces the same distribution of masses over a range of ~200 times in stellar density, from that of sparse OB associations to that typical of globular clusters. The large number of O3 stars in R136 is then simply a consequence of its youth (<1-2 Myr) and its richness, suggesting that the upper mass "cutoff" to the IMF seen in OB associations may simply be the result of their sparcity.