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

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1. INTRODUCTION

Massive stars are among the main drivers of the evolution of galaxies. These O type stars, along with their highly evolved descendants, the even more energetic Wolf-Rayet objects, are major contributors to the UV radiation and power the far-infrared luminosities through the heating of dust. Their stellar winds are important sources of mechanical power. As progenitors of supernovae, massive stars are agents of nucleosynthesis and may be intimately involved in the initiation of new star formation processes. Hence, massive star evolution is a key study in the exploration of the nearby and distant Universe.

The laws of physics are, so far as we know, the same throughout the Universe. Why should we study massive stars in other galaxies, which is certainly more difficult than studying these objects nearby? We do so because the initial compositions of those stars, in particular their modes of star formation and environments, may well differ from place to place. This leads to different evolutionary histories with a number of observable consequences. For example, it has been known for quite some time that the number ratio of blue to red supergiants shows a gradient in the Milky Way and seems to be different from the ratios found in the Magellanic Clouds. Also, the relative frequency of Wolf-Rayet (W-R) stars to their O-type progenitors appears to be much larger in inner Galactic regions compared to some low-metallicity galaxies. Similarly, the ratio of W-R stars of subtypes WN to those of type WC also changes by a factor of about 20 or more between metal-rich and metal-poor environments. Furthermore, the studies of starburst galaxies containing recently born massive stars show the existence of conspicuous differences in their massive star population statistics. Finally, spectroscopic abundance determinations in AGN and QSOs give us evidence of a very different chemical history among their constituent gaseous and stellar content. These few striking examples illustrate that targe differences may exist in massive star populations among galaxies. It is thus essential to present a good description of such differences and to have a proper understanding of them.

Until about 20 years ago, it was generally thought that the evolution of massive stars was fully understood. With an internal physics governed by electron scattering opacities and a simple equation of state, the stars were supposed to gently leave the main sequence (MS) and finally explode as red supergiants, giving rise to SN II. More recent years have demonstrated the major role of mass loss and initial metallicity, hi addition to the initial mass function (IMF), and the star formation rate (SFR) for shaping massive star evolution and population statistics. The color-magnitude diagrams of young clusters, the stellar abundances of He and CNO elements, and the studies about SN 1987A and its precursor have led to numerous additional investigations on the role of convection and mixing in massive star evolution.

In Section 2 we present some of the statistical properties of massive stars that can be studied individually, and consider what differences have been found between those in three relatively well-known galaxies (the Milky Way and the Magellanic Clouds). We examine the evolution models of OB stars and supergiants in Section 3, and compare them with the observations. We consider the properties of W-R stars, those highly evolved descendents of the most massive stars, in confrontation with the predictions of stellar evolution models in Section 4. Observations and models of even more distant galaxies containing starburst phenomena are considered in Section 5. In these cases, we are usually dealing only with integrated properties of stars in galaxies. We intimate some directions for the future in Section 6.

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