ely in terms of a substantial steepening in the IMF above $40$ – $50$ .

We have therefore searched for other causes for this behavior. We believe an explanation can be found in an incorrect assumption in the standard synthesis models: that the full luminous output of newly–formed stars escapes into surrounding diffuse gas. This assumption justifies modelling starbursts as traditional low–density  regions. Instead, we suggest that the majority of massive stars in starbursts spend a substantial part of their main sequence lifetimes embedded within dense, highly–extincted regions—similar to the ultracompact  regions of the Milky Way—and are thus invisible to optical, near–infrared, and mid–infrared nebular line studies.

In the solar neighborhood, it appears that about $15\%$ of the main sequence life of a massive star is spent within an ultracompact  region (Kurtz, Churchwell, & Wood 1994). Hanson, Luhman, & Rieke (1996) have detected in the near–infrared about half of a sample of radio–selected ultracompact  regions. They conclude that the detected regions typically are obscured by A$_V$ $= 30$ – $50$. Since the undetected regions in their sample should be even more heavily obscured, we take a typical case to be A$_V$ $\sim 50$. Thus, these objects would not contribute to the optical or near–infrared emission–line spectra of the Milky Way. The heavy extinction would even diminish the fluxes of the mid–infrared fine structure lines such as \[\] $12.8$  and \[\] $15.6$ by a magnitude or more. More importantly, the densities in many ultracompact  regions exceed the critical densities for these lines (e.g., $2 \times 10^5$ cm$^{-3}$ for \[\] $15.6$ ). Thus, even in the solar neighborhood, the accuracy of traditional synthesis models would be improved by assuming that massive stars contribute their bolometric luminosity to the region for their entire main sequence lifetimes, but influence the usual  indicators in emission line spectra for only $85\%$ of their lives.

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