Annu. Rev. Astron. Astrophys. 1994. 32: 227-275 Copyright © 1994 by Annual Reviews. All rights reserved

6. CONCLUSIONS

Theoretical considerations and extensive modeling suggest that substantial distinctions in the evolutionary history of massive stars will arise because of the importance of mass loss and its strong dependence on metallicity. We have reviewed and discussed the properties, chemical abundances, and populations of individual OB stars, supergiants, and Wolf-Rayet stars in various galaxy environments and find order of magnitude differences from place to place. These are understood to depend on Z, along with the IMF and SFR, but not all these parameters are completely sorted out at the present time. From detailed star counts in stellar associations (Section 2), it does appear that there is no simple dependence of the IMF slope, , on Z, but there may be differences from place to place. There is no evidence that the M_upper limit depends on Z; data concerning potential differences in the M_lower limit with location is currently lacking. A major unresolved question is that concerning the SFR: What does this parameter depend upon? Clearly one needs sufficient molecular gas; future studies should address the relationship between this parameter and the actual numbers of massive stars in very quantitative terms.

We have lightly touched upon the connection between studies of nearby massive star formation regions where stellar statistics may be accomplished, and measurement and analyses of their integrated properties. These objects, such as 30 Dor in the LMC and NGC 604 in M33, can be used as "stepping stones" in our understanding of similar phenomena in more distant galaxies. It will be important to improve the "calibration" of these types of massive star groupings in various galaxy environments to better understand those even more energetic and less common starbursts found at larger distances.

In starbursts we are dealing nearly exclusively with integrated spectral properties. For those containing massive stars of O and W-R type we have an advantage that the lifetimes are less than 10 Myr, and the formation time scales appear to be only 1 Myr. The modeling can be thus simplified to that of a "burst." Probably SN will not have played much of a role, as yet, in the energetics of the phenomena we are observing; the excitation of the gas will be primarily due to stars. Multiwavelength studies of many galaxies have already been made but there has been little quantitative integration of all these data for the same galaxies and the same starbursts. In addition to the inferences concerning hot stars, one would like to know the cool star population, and the quantity of neutral and molecular gas. This requires observations at IR wavelengths and in the sub-mm and cm regimes. One certain caution is that of aperture; it is critically important that these are matched, independent of the wavelengths, so that the same volume elements are being examined in each case. Even more significant is the probability that as one goes to shorter and shorter wavelengths, differential internal galactic extinction might necessarily shield from our view the "back side" of a starburst episode.

With our new-found knowledge of the properties and evolution of massive stars, we now can begin to study and understand the appearances of ever more distant galaxies, and one would hope, to delineate their past history. "In the beginning" there were undoubtedly many massive stars in newly forming galaxies. Our improved comprehension of newly born local massive stars can help clarify our knowledge of some of the earliest stages in the evolution of our Universe.

ACKNOWLEDGMENTS

The authors are grateful to Lorraine Volsky and the JILA publications office for editorial assistance. PSC appreciates continuous support by the National Science Foundation. AM acknowledges support of the Ponds National Suisse de la Recherche Scientifique.