| Historical Introduction. Star Formation: the Early History | p. 1 |
| Pre-Main-Sequence Evolution of Stars and Young Clusters | p. 9 |
| Initial Conditions of Pre-Main-Sequence Evolution | p. 9 |
| The Classical Theory of Pre-Main-Sequence Evolution | p. 9 |
| Basic Results | p. 9 |
| The Impact of Star Formation | p. 12 |
| Protostellar Collapse: the Mass Accretion Rate | p. 13 |
| Classical Collapse Models | p. 14 |
| More Realistic Models | p. 17 |
| Protostellar Evolution | p. 20 |
| Central D-Burning | p. 22 |
| Radiative Barrier | p. 23 |
| The Onset of D-Shell Burning and Gravitational Contraction | p. 25 |
| The Stellar Birthline | p. 26 |
| Effects of the Accretion Flow | p. 26 |
| Pre-Main-Sequence Evolution | p. 32 |
| Evolutionary Models | p. 32 |
| Protostellar Initial Conditions | p. 32 |
| Stellar Structure Equations | p. 35 |
| Equation of State | p. 37 |
| Opacity | p. 38 |
| Treatment of Convection | p. 40 |
| Nuclear Reactions | p. 40 |
| The H-R Diagram | p. 41 |
| Low-Mass Stars: 0.1 <$$> | p. 43 |
| Intermediate-Mass Stars: 2 <$$> | p. 46 |
| Massive Stars: <$$> | p. 48 |
| Tests to PMS Evolutionary Diagrams | p. 51 |
| Comparison of Tracks | p. 51 |
| The Low-Mass M Dwarfs and the End of the Main Sequence | p. 54 |
| Derivation of Observable Quantities from Evolutionary Tracks | p. 57 |
| Pre-Main-Sequence Binaries | p. 60 |
| Physical Processes in PMS Stars | p. 70 |
| Lithium Depletion in Young Stars | p. 70 |
| Rotation and Depletion | p. 71 |
| Spectroscopic Classification Based on Lithium | p. 72 |
| Lithium and Brown Dwarfs | p. 72 |
| Mass Accretion in Young Stars | p. 74 |
| Disk Accretion | p. 74 |
| Effects on Evolutionary Tracks | p. 76 |
| Rotation and PMS Evolution | p. 80 |
| Rotational Properties of Young Stars | p. 80 |
| Structural Effects | p. 85 |
| Pulsational Instabilities in PMS Stars | p. 87 |
| The Input Models | p. 88 |
| The Instability Strip | p. 89 |
| PMS Evolution of Clusters and Associations | p. 97 |
| High Mass Star Formation | p. 98 |
| The Search for Massive Protostars | p. 99 |
| The Evolution of Ultracompact Hll Regions | p. 103 |
| Clustering of Young Stars | p. 105 |
| Clustering and Mass Segregation | p. 108 |
| Effects on the Environment | p. 109 |
| Clustering and Binary Formation | p. 112 |
| Clustering and Star Formation Efficiency | p. 113 |
| Clustering and the IMF | p. 115 |
| Stellar Populations in Clusters and Associations | p. 117 |
| H-R Diagrams | p. 118 |
| Star Formation Histories | p. 124 |
| The Post-T Tauri Problem | p. 128 |
| Observations of Young Stellar Objects (YSO) | p. 135 |
| Introduction | p. 135 |
| How to Find Young Stars and Protostars | p. 136 |
| Disks, Jets and Protostars | p. 140 |
| A VLT/ISAAC Study of Circumstellar Disks and Envelopes Around YSOs | p. 140 |
| Abstract | p. 140 |
| Scientific Rationale | p. 140 |
| Immediate Objective | p. 142 |
| Telescope Justification | p. 143 |
| Results | p. 143 |
| Infrared H(2) Imaging of the Highly Symmetric Proto-stellar Jets HH212 and HH111 | p. 146 |
| Abstract | p. 146 |
| Scientific Background: Star Formation and Herbig-Haro Jets | p. 147 |
| Scientific Goals | p. 150 |
| The Need for HST | p. 152 |
| Results | p. 153 |
| An Unbiased mm Continuum Survey for Protostellar Outflow Sources in the Orion A Cloud | p. 155 |
| Abstract | p. 155 |
| The Need for an Unbiased Millimetre Survey for Protostellar Outflow Sources | p. 155 |
| Framework: A Survey for Jets and Protostars in L1641 | p. 157 |
| Technical Details | p. 159 |
| Results | p. 159 |
| Mid-Infrared Imaging of Deeply Embedded Protostellar Sources in the Orion A Cloud | p. 161 |
| Abstract | p. 161 |
| Scientific Justification | p. 161 |
| Why Do We Need Mid-Infrared Observations at Keck? | p. 162 |
| Technical Remarks | p. 163 |
| Results | p. 164 |
| A Study of Faint Disks Around X-ray Selected Young Low-Mass Stars in an OB Association | p. 166 |
| Abstract | p. 166 |
| Description of the Proposed Programme | p. 167 |
| Results | p. 169 |
| Young Binaries | p. 172 |
| Statistics of Young Binary Stars: Implications for (Binary) Star Formation | p. 172 |
| Young Binary Stars: Why Care? | p. 172 |
| Observational Methods | p. 172 |
| Results | p. 175 |
| Implications | p. 177 |
| Future Prospects | p. 180 |
| Discussion | p. 181 |
| Low-Mass Binary Companions to Intermediate-Mass Stars in the Sco-Cen OB Association | p. 184 |
| Abstract | p. 184 |
| Description of the Proposed Programme | p. 184 |
| Results | p. 187 |
| Multiplicity of Massive Stars in the Orion Nebula Cluster and Implications on Their Formation Mechanism | p. 189 |
| Abstract | p. 189 |
| Introduction | p. 189 |
| Observations | p. 189 |
| Results | p. 190 |
| Conclusions | p. 195 |
| Low Mass Stellar Content of Young Star Clusters and Associations | p. 197 |
| The Low-Mass Initial Mass Function in the Upper Scorpius OB Association | p. 197 |
| Abstract | p. 197 |
| Scientific Justification | p. 197 |
| Results | p. 201 |
| The Full Population of T Tauri Stars of the Young Stellar Cluster IC 348 | p. 202 |
| Abstract | p. 202 |
| Scientific Justification | p. 202 |
| Technical Feasibility | p. 206 |
| Results | p. 206 |
| A Study of the Low-Mass Stellar Population in the Galactic Starburst Region NGC 3603 | p. 208 |
| Abstract | p. 208 |
| Description of the Proposed Programme | p. 208 |
| Required Observing Time | p. 211 |
| Results | p. 213 |
| The Low-Mass Pre-MS Stellar Content of the 30 Dor Starburst Cluster | p. 215 |
| Abstract | p. 215 |
| Scientific Background | p. 215 |
| Previous Work | p. 217 |
| Summary of the Scientific Goals and Method | p. 218 |
| The Need for NICMOS/HST | p. 218 |
| Description of the Observations | p. 219 |
| Results | p. 220 |
| Appendix: The Suite of Large Ground-Based Telescopes | p. 224 |
| Subject Index | p. 227 |
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