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It is a common assumption in astronomy that star formation only occurs in star clusters. However, we have also known for decades that star clusters are often rather fragile, and internal mass loss or tidal forces can disrupt them. We have shown that on average only 20% of the UV light in starburst galaxies appears to be in tightly bound clusters, and that the fraction of light in such clusters correlates with surface brightness (Meurer et al. 1995). This latter result cannot be easily understood in terms of star formation being confined to star clusters that later dissolve. That result was obtained in the early days of the Hubble Space Telescope (HST), before optics were deployed on its instruments to correct for the spherical aberration in its primary mirror. In this project we will update the cluster fraction versus surface brightness correlation using UV images taken since HST was repaired. We will concentrate on galaxies with both Halpha and UV images from the SINGG and SUNGG surveys, because the Halpha/UV ratio is sensitive to the behaviour of the upper end of the Initial Mass Function (IMF), and there is some reason to believe that the IMF is sensitive to the fraction of star formation that occurs in star clusters.

Caption: HST image of the central starburst in NGC3310. The numerous point like sources are young star clusters.

Caption: HST image of the central starburst in NGC3310. The numerous point like sources are young star clusters.