| Preface | 7 |
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| Contents | 9 |
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| List of Figures | 12 |
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| List of Tables | 15 |
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| Introduction | 16 |
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| 1.1 The Cosmos as a Laboratory | 16 |
| 1.2 Spectral Lines in Astronomy | 16 |
| 1.3 The Bohr Atom | 18 |
| 1.4 Spectral Lines in Radio Astronomy | 22 |
| RRLs and Atomic Physics | 37 |
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| 2.1 The First Surprising Results: The Absence of Stark Broadening | 37 |
| 2.2 The Broadening of Radio Recombination Lines | 39 |
| 2.3 Intensity of Radio Recombination Lines | 70 |
| 2.4 The Range of RRL Studies | 108 |
| 2.5 How Many Atomic Levels Can Exist? | 121 |
| 2.6 Summary | 127 |
| RRLs: Tools for Astronomers | 130 |
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| 3.1 Physical Conditions in HII Regions | 134 |
| 3.2 Ionized Hydrogen and Helium in the Galaxy | 148 |
| 3.3 Exploration of the Cold ISM by RRLs | 178 |
| 3.4 RRLs from Stars and Stellar Envelopes | 226 |
| 3.5 RRLs from Extragalactic Objects | 244 |
| Appendixes | 252 |
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| Constants | 253 |
| A.1 Miscellaneous Constants | 253 |
| A.2 Rydberg Constants | 253 |
| Tables of Line Frequencies | 256 |
| B.1 Frequencies Below 100GHz | 256 |
| B.2 Frequencies Above 100GHz | 269 |
| B.3 FORTRAN Code for Fine-Structure Frequencies | 285 |
| Supplemental Calculations | 292 |
| C.1 Early Estimates of Stark Broadening | 292 |
| C.2 Refinements to the Bohr Model | 294 |
| Hydrogen Oscillator Strengths | 296 |
| D.1 Population of Atomic Sublevels | 296 |
| D.2 Calculation of Oscillator Strengths | 297 |
| D.3 FORTRAN Code for Evaluating Radial Matrix Integrals | 299 |
| Departure Coefficients | 307 |
| E.1 FORTRAN Code for Calculating bn Values | 307 |
| Observational Units | 338 |
| F.1 What Radio Telescopes Measure | 338 |
| F.2 How Radio Telescopes Measure | 339 |
| References | 347 |
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| Author Index | 360 |
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| Subject Index | 365 |
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