Annu. Rev. Astron. Astrophys. 1999. 37:311-362 Copyright © 1999 by Annual Reviews. All rights reserved

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Abstract: The physical conditions in molecular clouds control the nature and rate of star formation, with consequences for planet formation and galaxy evolution. The focus of this review is on the conditions that characterize regions of star formation in our Galaxy. A review of the tools and tracers for probing physical conditions includes summaries of generally applicable results. Further discussion distinguishes between the formation of low-mass stars in relative isolation and formation in a clustered environment. Evolutionary scenarios and theoretical predictions are more developed for isolated star formation, and observational tests are beginning to interact strongly with the theory. Observers have identified dense cores collapsing to form individual stars or binaries, and analysis of some of these support theoretical models of collapse. Stars of both low and high mass form in clustered environments, but massive stars form almost exclusively in clusters. The theoretical understanding of such regions is considerably less developed, but observations are providing the ground rules within which theory must operate. The most rich and massive star clusters form in massive, dense, turbulent cores, which provide models for star formation in other galaxies.

Keywords: star formation, interstellar molecules, molecular clouds

Table of Contents

• INTRODUCTION
• PHYSICAL CONDITIONS
• PROBES OF PHYSICAL CONDITIONS
• Tracers of Column Density, Size, and Mass
• Probes of Temperature and Density
• Kinematics
• Magnetic Field and Ionization
• Observational and Analytical Tools
• FORMATION OF ISOLATED LOW-MASS STARS
• Theoretical Issues
• Globules and Cores: Overall Properties
• Globules and Cores: Internal Conditions
• Classification of Sources and Evolutionary Scenarios
• Detailed Theories
• Tests of Evolutionary Hypotheses and Theory
• Collapse
• Summary of Isolated Star Formation
• CLUSTERED STAR FORMATION AND MASSIVE STARS
• Theoretical Issues
• Overall Cloud and Core Properties
• Evolutionary Scenarios and Detailed Theories
• Filaments, Clumps, Gradients, and Disks
• Kinematics
• Implications for Larger Scales
• Summary of Clustered Star Formation
• CONCLUSIONS AND FUTURE PROSPECTS
• REFERENCES