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R.C. Kennicutt (Steward Obs.)
The total rate of star formation in galaxy disks varies by orders of magnitude across the Hubble sequence, but are these changes reflected in the clustering of the star formation or in the stellar populations within young clusters? A rudimentary picture of the systematic trends in star clustering has come from a combination of direct observations of cluster luminosity functions measured from the ground or with HST, and from the statistics of the ionizing luminosities of young clusters, as traced by the HII region luminosity function. Both sets of data suggest that the dramatic changes in the total star formation rates along the Hubble sequence are due to changes both in the frequency of star formation events and in the characteristic masses of individual events. These are consistent with the broader trends in cluster populations observed between spiral, irregular, and starburst galaxies. There is little evidence to suggest that other properties of the stellar populations, such as the IMF, vary systematically in these clusters, but current observations do not impose very tight constraints.
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