Astrophysics of Gaseous Nebulae
and Active Galactic Nuclei

Second Edition

Donald E. Osterbrock
Lick Observatory

Gary Ferland
University of Kentucky


This book is a revised, updated, and expanded edition of Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (1989) by one of us (D. E. O.), and that book was based in turn on an earlier volume, Astrophysics of Gaseous Nebulae (W. H. Freeman, 1974) by the same author.  Over the thirty years between the publication of that first book and the completion of the manuscript of the present one the subject of ionized-gas astrophysics has grown phenomenally.  Astronomers have long studied gaseous nebulae, and the cooler, less intrinsically luminous dust and molecular clouds associated with them, for their intrinsic interest.  In addition, stars are almost completely opaque, and it is impossible to observe their interiors directly, or to measure their internal temperatures, pressures, densities or abundances of the elements within them except by indirect methods.  Many nebulae, on the other hand, are relatively transparent, and we can measure these quantities directly.

Since Walter Baade's discovery of the two stellar populations, which turned out to be young stars and old, and the working out of the main nuclear reaction chains that release energy on stars by Hans Bethe, C.F. von Weizsäcker and others, we have begun to understand the way in which most stars evolve.  Much remains to be filled in, both observationally and theoretically, and we have learned how to trace star formation and evolution not only from stars themselves, but from their effect on the gas around them as well.  We can estimate or measure the  star formation rate not only with nebulae in our Galaxy, but throughout distant galaxies in the universe, from emission lines emitted in nebulae, clouds, associations and spiral arms in galaxies, and even from entire galaxies.

Interpreting the measurements is not straightforward; the path between is through "nebular astrophysics", as explained in this book. Likewise we can measure abundances of the elements in all these objects with the same basic tools.  We can directly observe, in some planetary nebulae and supernova remnants, the changed abundances of the elements that resulted within the stars themselves, leading to slow mass loss, shells thrown off from the star, or thermonuclear reactions at the surfaces of highly evolved stars.

Active galactic nuclei and their close relatives, quasars and other types of quasistellar objects, are a field of research only forty years old, which has grown explosively.  Almost all we know about them came from study of their spectra, and especially their emission-line spectra, very similar to but not identical with those of gaseous nebulae.  Maarten Schmidt, J. Beverly Ole, and Jesse L Greenstein's giant leap in understanding the basic nature  of 3C 273 and 3C 48 depended on their knowledge of nebular emission lines,  and the subsequent study of all these objects has paralleled the study of gaseous nebulae.

Cosmology has advanced tremendously in the past decade or two, based on the availability of new, very large telescopes, either on  high mountains like Keck I and II on Mauna Kea, Hawaii and the VLT on Paranal, Chile, or in space, like the Hubble Space Telescope, and their fast, digital imaging systems and spectrometers.   All the most distant objects we know have been identified and measured by their "nebular" emission lines emitted within them, from H I La to [O II] l3727, [O III] ll4959, 5007 and Ha.  How can we be sure it is one of these lines that an astronomer has observed?  If it was, what other lines should be seen?  If they are not seen, should that purported identification be rejected? If the identification is correct, what physical information (besides redshift and hence distance and look-back time) can be drawn from it?  These are some of the questions which nebular astrophysics can help to answer.

In all these subjects, going beyond simply using a formula or canned interpretation from a previously published paper, is a necessity, in our opinion.  We have tried to explain and illustrate the methods used, but also to express reasonable doubts about some far-reaching conclusions drawn from minimal data, without examining possible alternative interpretation.  In the final chapter of the book we have outlined several questions about active galactic nuclei which shall await full explanations. The methods of the book will be useful in deciding the answers, we feel certain.

The fundamental outline and style of exposition of Astrophysics of  Gaseous Nebula and Active Galactic Nuclei, often referred to as  (AGN)2, has been preserved in this new  edition, but we have carefully gone through every chapter, weeding  out the old and including many new results, measurements, and ideas.  Each chapter was completely rewritten, and then revised several lines. Furthermore, since its publication of the first edition, results from two fast-developing observational techniques, infrared astronomy and X-ray astronomy, have multiplied many fold.  Hence we not only updated the material in  these two fields for this edition, which we call AGN3, but have added much more new material, so it now includes two more chapters, one on each  of these topics. In both these chapters, and in the rest of the book as well, we have emphasized the continuity of the physical ideas across the boundaries of "spectral regions" and the importance of observing and analyzing data over as wide a range of wavelengths as possible.

From comments of many recent students and current researchers who are using (AGN)2, we are aware that beginning graduate students of  astrophysics today generally know much more about applications of  quantum mechanics to nuclear physics than the previous generation,  but much less about its applications to atomic spectroscopy, so  necessary in nebular astrophysics. Hence we have added a completely  new, 24 page appendix on "nebular quantum mechanics", and also a shorter  one on "molecular quantum mechanics" (which neither generation of  astrophysics graduate students know very well) to help bring them  in to the fold.

Most of the figures are new, based on recent published measurements and interpretations, and on images obtained with the most advanced telescopes and detecting systems of today.  We are grateful to many colleagues who made them available to is gave us permission to use them in AGN3.

All the chapters, after the first five, which deal with basic material, have been reviewed for us by colleagues and friends who are active research experts in the various fields.  We are most grateful to the following for their efforts on our behalf, who either contributed material or reviewed chapters: Jack Baldwin, John Bally, Robert Bauman, Mark Bottorff, Eugene Capriotti, John Danziger, Kris Davidson, Reginald Dufour, Robert Fesen, Donald Garnett, William Henney, Richard B. Henry, Luis Ho, Roberta Humphries, George Jacoby, Sveneric Johansson, Steven Kahn, Kirk Korista, Steven Kraemer, Karen Kwitter, Sun Kwok, Xiaowei Liu,  John Mathis,  Jon Morse,  C. Robert O'Dell,  Manuel Peimbert,  Richard Shaw,  Gregory Shields,  Joseph Shields,  Lewis Snyder,  Phillip Stancil,  Barry Turner,  Sidney van den Bergh, Peter van Hoof, Sylvain Veilleux,  Nolan Walborn, Joseph Weingartner, Robin Williams, Robert E. Williams, Mark Wolfire, and Stan Woosley.  We also are extremely grateful to Nick Abel who carefully read the entire manuscript.  We thank Jeffrey Mallory and John Rickard for help in producing the final manuscript.  Any errors which may remain are our responsibly, not theirs.

                                                                                                                      D.E.O.                                                                                                                        G.J.F. 

Preface to the First Edition

Fifteen years ago I sent to the publisher my book on Astrophysics of Gaseous Nebulae. It was a graduate-level text and research monograph that evidently filled a need, for it soon became widely used and quoted. Over the years since then the book has found increasing use, not only in nebular research, but also in problems connected with quasars, Seyfert galaxies, quasistellar objects, and all the other fascinating types of active galactic nuclei whose emission-line spectra are similar, in general terms, to those of gaseous nebulae. My own research had turned in those directions since I came to Lick Observatory in 1973 and began obtaining data with its superbly instrumented 3-m Shane reflecting telescope, as it now is named.

Hence as AGN (for so my first book is often referred to) gradually became dated, particularly in its tables of observational results and theoretical calculations, it was natural for me to think of revising it, and of extending it to Astrophysics of Gaseous Nebulae and Active Galactic Nuclei at the same time. Many of my friends and colleagues urged me to do so.  Thus the present (AGN)2 came about.

Like the earlier AGN, it is both a graduate-level text and an introduction to nebular and AGN research. The first nine chapters are based upon the first nine chapters of the earlier book, but have been heavily revised and updated. The last three chapters are completely new, one on nova and supernova remnants, and the final two chapters on active galactic nuclei. The emphasis is very strongly on the ionized gas in AGNs and the emission-line spectra they emit; their X-ray and radio-frequency radiations are only briefly mentioned.

The book is based upon graduate courses that I have given often at the University of California, Santa Cruz. It represents the material I consider necessary to understand research papers that are now being published in its fields. So much is known today, and so many new results are pouring out, that it is probably impossible to go straight from studying any book to doing frontier research oneself. But I believe that this book will enable the reader to get up to speed, so that he or she will be able to read and understand current research, and then begin to add to it.

The reader for whom (AGN)2 was written is assumed to have a reasonably good preparation in physics, and some knowledge of astronomy and astrophysics. The simplest concepts of radiative transfer are used without explanation, since the reader almost invariably has studied stellar atmospheres before gaseous nebulae and active galactic nuclei. Physical parameters, such as collision cross sections, transition probabilities, and energy levels, are taken as known quantities; no attempt is made to derive them. When I teach this material I usually include some of these derivations, linking them to the quantum-mechanics textbooks with which the students are most familiar. Omitting this material from the book left room to include more interpretation and results on gaseous nebulae and active galactic nuclei.

References are given at the end of each chapter, in a separate section. They are not inserted in the text, partly so that they will not break up the continuity of the discussions, and partly because the text is a complicated amalgam of many papers, with no obvious single place at which to refer to many of them. Almost all the references are to the American, English, and European astronomical literature, with which I am most familiar; it is also the literature that will be most accessible to the readers of this book.

I would like to express my deep gratitude to my teachers at the University of Chicago, who introduced me to the study of gaseous nebulae: Thornton L. Page, S. Chandrasekhar, W. W. Morgan, and the late Bengt Strömgren. I am also very grateful to my colleagues and mentors at the Mount Wilson and Palomar Observatories, as it was then named, the late Walter Baade and the late Rudolph Minkowski, who encouraged me to apply what I knew of nebular astrophysics to the study of galaxies. I owe much to all these men, and I am grateful to them all for their continued encouragement, support, and stimulation.

I am extremely grateful to my colleagues and friends who read early drafts of various chapters in this book and sent me their suggestions, comments, and criticisms on them: Donald P. Cox, Gary J. Ferland, William G. Mathews, John S. Mathis, Manuel Peimbert, Richard A. Shaw, Gregory A. Shields, Sidney van den Bergh, Robert E. Williams, and Stanford E. Woosley. In addition, my two current graduate students, Richard W. Pogge and Sylvain Veilleux, carefully read the entire manuscript; their comments and corrections greatly improved it, as did those of Dieter Hartmann and Philip A. Pinto, both of whom carefully read the supernova material. I am most grateful to them all.

Though these readers found many misprints and errors, corrected many misstatements, and clarified many obscurities, the ultimate responsibility for the book is mine. I have tried very hard to find and remove all the errors, but some must surely remain, to be discovered only after publication. I can do no better than repeat once again the words of a great physicist, Richard P. Feynman, "Listen to what I mean, not to what I say." If the reader finds an error, I am sorry I did not catch it, but he or she will have proved his or her real understanding of the material, and I shall be very pleased to receive a correction.

I am greatly indebted to Gerri McLellan, who entered on the word processor the first drafts of all the chapters, and all the successive revisions of the manuscript, and to Pat Shand, who made the final editorial revisions and prepared the camera-ready copy for publication. I deeply appreciate the skill, accuracy, and dedication with which they worked on this book. I am also most grateful to my wife, Irene H. Osterbrock, who prepared the index for the book.

My research on gaseous nebulae and active galactic nuclei has been supported over the past fifteen years by the University of California, the John Simon Guggenheim Memorial Foundation, the University of Minnesota, the University of Chicago, the Institute for Advanced Study, the Ohio State University, and especially by the National Science Foundation. I am grateful to all of these organizations for their generous support. Much of my own research, and of the research of the graduate students and postdocs who have worked and are working with me, has gone into this book; I could never have written it without doing that research myself.

I am especially grateful to my friends George H. Herbig, Paul W. Hodge, Guido Munch, and Robert E. Williams, who provided original photographs included in this book. I am grateful to them and also to Palomar Observatory, Lick Observatory, and the National Optical Astronomy Observatories for permission to use the photographs (which are all credited individually) in this book. Publication of the photographs from NOAO does not imply the endorsement by NOAO, or by any NOAO employee, of this book! Many of the other figures are derived from published papers, and I am grateful to their authors for permission to modify and use their figures in this book.

Lastly, I wish to express my sincere thanks to my friends Bruce Armbruster, president of University Science Books, and Joseph S. Miller, my colleague, former student, and astronomy co-editor with me for USB, both of whom encouraged me time after time to go on with revising AGN and writing the additional new chapters for (AGN)2. Bruce was the astronomy editor for W. H. Freeman and Company when I wrote the earlier book, and he helped me greatly with it then, as he has helped me with (AGN)2 now. It was a great pleasure for me to work with him on both these books. I am also grateful to W. H. Freeman and Company for releasing me from my obligation to them, and allowing me to publish this book with USB.

Donald E. Osterbrock