An Introduction to Star Formation , Derek Ward-Thompson and Anthony P. Whitworth, Cambridge U. Press, New York, 2011. $65.00 (208 pp.). ISBN 978-0-521-63030-6
Principles of Star Formation, Peter H. Bodenheimer, Springer, New York, 2011. $124.00 (343 pp.). ISBN 978-3-642-15062-3
How stars form out of diffuse interstellar matter is a fundamental problem of astrophysics and one that has witnessed remarkable progress over the past few decades. Several textbooks at both the undergraduate and graduate levels already exist, including From Dust to Stars (Springer Praxis, 2005), by Norbert Shulz, and The Formation of Stars (Wiley-VCH, 2004), by me and Francesco Palla. Yet somehow the subject has not made its way into the standard curriculum, at least in US physics and astronomy departments. With the publication of two more texts, both written by active and respected researchers, that situation hopefully will change.
Two recent books, An Introduction to Star Formation by Derek Ward-Thompson and Anthony P. Whitworth and Principles of Star Formation by Peter H. Bodenheimer, are superficially very similar. Both are relatively slim volumes, meant to be digested in a single semester. Neither presumes any prior knowledge of astronomy; instead, each introduces the relevant concepts as needed. By now, both the choice of topics in a star-formation text and even their order are well established, and the authors follow that convention. There the similarity between the two books ends.
An Introduction to Star Formation is an informal survey of the main ideas in the field. As many of those ideas are quite beautiful, the book is a pleasure to read. Ward-Thompson and Whitworth, who both teach at the University of Cardiff, Wales, guide the reader step by step through the basics of interstellar clouds, the physics of gravitational collapse, and the connection of stellar birth to the formation of both planets and galaxies. They weave together observational results and relatively simple theoretical derivations, and they provide ample figures to illustrate both. The tone and content are generally most appropriate for an upper-level undergraduate course. However, the lecturer would need to supplement that course with additional readings, perhaps taken from the useful bibliographies at the end of each chapter. Also, undergraduates may have difficulty with the various forays into radiative transfer, which, somewhat incongruously, are presented at a higher technical level than the rest of the material.
Principles of Star Formation explores the subject more deeply. Bodenheimer, an emeritus professor at the University of California, Santa Cruz, has brought together representative observational findings and a seemingly exhaustive compilation of theories proposed in the past 30 years. His command of theoretical issues, based on long research experience, is evident. I especially enjoyed his chapter on binary-star formation, a frontier topic only briefly touched upon by Ward-Thompson and Whitworth. Bodenheimer’s text is a serious work that packs a lot of information, all scrupulously referenced, into a relatively short space. Its ideal reader is a graduate student with a strong physics background.
Ward-Thompson and Whitworth emphasize concepts, illuminated through derivations. By contrast, Bodenheimer avoids such derivations almost entirely and presents instead a broad range of calculated results from the literature. To my mind, the first approach is more successful pedagogically. A student entering the field from any level should be shown the main path through what is otherwise a bewildering landscape. In other words, Bodenheimer is too democratic; all ideas are not created equal. Many sections in Principles of Star Formation begin with the equivalent of “The following mechanisms have been proposed . . .” and then proceed to list those mechanisms in workmanlike fashion. That style is more suitable for a literature review than an introductory text.
A problem in both books is the persistence of outdated and confusing terminology. For example, a key concept is that of a protostar, a stellar object so young that its luminosity comes from the kinetic energy of the infalling cloud gas in which it is embedded. In Chapter 6 of An Introduction to Star Formation, the term is defined correctly. Figure 7.6 then shows evolutionary tracks in the Hertzsprung–Russell diagram of accreting and non-accreting “protostars.” Since accreting young stars have no optical photospheres, while non-accreting ones are actually contracting pre-main-sequence stars, it is not clear what we are seeing. Principles of Star Formation variously uses “protostar” to refer to the accreting star and to the cloud that is collapsing onto that star. The seasoned researcher will easily glide past the words to the underlying ideas, but the novice will stumble needlessly.
Both books present many images derived from the large-scale numerical simulations that dominate theoretical research in the field. Bodenheimer, in fact, was a pioneer simulator during the 1960s. Simulations are a marvelous tool, but they can be overused, as is unfortunately the case in star-formation studies today. In his chapter on massive star formation, Bodenheimer assesses future prospects with a statement that epitomizes the prevailing philosophy: “The key to solving the massive star formation problem is to perform fully three-dimensional numerical simulations with radiative transfer.” I respectfully disagree. The envisioned simulations, no matter how computationally intensive, will necessarily employ key physical assumptions and initial conditions that are neither trivial nor obvious.
For example, most collapse simulations, including those that purport to build up massive stars, begin with a cloud far out of force balance, in which the self-gravity of the gas overwhelms any pressure support. But the much simpler simulations of Bodenheimer and others from the 1960s taught us a valuable lesson. The state of a cloud just prior to collapse determines the whole character of its subsequent dynamical evolution. Given a cloud that is already collapsing, it is impossible to work backwards and its initial state, much less whether that initial state is physically plausible.
Newcomers to the field of star formation, including future readers of An Introduction to Star Formation and Principles of Star Formation, should understand that the purpose of simulations is not to generate realistic-looking movies, but to aid in physical understanding. Given the disparate length scales spanned in many problems and the complex processes involved, a direct, brute-force simulation is rarely the most fruitful strategy. The safer, if less flashy, approach practiced by many theorists is to divide the problem into regimes where the basic physics is well understood, find the solutions numerically or analytically in each regime, and then attempt to link those solutions together. The successful practice of theory in star formation, as in much of physics, is largely the art of successive approximation. But we knew that long before computers appeared.
