Stellar Spectral Classification, by R. O. Gray & C. J. Corbally, SJ (Princeton University Press, Woodstock), 2009. Pp. 592, 25"5 X 17'5 cm. Price $59'95/$100 (hardbound; ISBN 9780691 12510 7), $38'95/$65 (paperback; ISBN 978 0 691 125II 4). (Buy it on Amazon.com)
A Review by Ian D. Howarth
The Observatory: A Review of Astronomy. D.J. Strickland, R.W. Argyle, S.J. Fossey, Eds; vol. 129(1213), December 2009.
The foundations of stellar spectral classification can be traced back at least to Fr. Angelo Secchi, and the genesis of the present methodology lies in work carried out a century ago at the Harvard Observatory. However, these venerable antecedents don't negate its present-day significance, lying at the foundation of many more-obviously-quantitative studies, and continuing to develop hand in hand with the exploration of new physical domains (cl the extension to Land T dwarfs) and with new observational technologies (exemplified by comparative spectral morphology in the IR, UV, and even X-ray domains).
The proceedings of occasional specialized meetings have partly documented some of these developments, but there has long been a conspicuous absence of an authoritative, graduate-level monograph on the subject. The Jascheks' The Classification of Stars has done service in that role, but even if readers don't share the lack of enthusiasm for it expressed in the review published in this Magazine (108,29, 1988), its bias towards photographic techniques which were already dated at the time of its publication has rendered it of limited value for much of its life.
Enter Gray & Corbally, with a modern review of the field. An introductory chapter sets out not only the key historical developments leading to the MK system (today virtually synonymous with stellar spectral classification), but also the widely underappreciated philosophical principles which underpin it, and which have come to be identified with the 'MK process'. This chapter, and the following one, which provides an overview of both the basic two-dimensional classification scheme and the associated astrophysical processes, make for background reading that should engage anyone interested in these topics.
Subsequent chapters address in detail classification along the familiar OBAFGKM sequence, subtype by subtype, together with L- and T-type dwarfs, Wolf-Rayet stars, and 'Endpoints of stellar evolution'. The last is a bit of a potpourri, ranging from proto-planetary nebulae to supernovae, but otherwise discussion of subclasses (such as chemically peculiar stars, T Tauris, carbon stars, etc.) is integrated seamlessly into the main text at the appropriate points. While this core of the book is, arguably, destined principally for a specialist readership, or for reference, I was impressed that it could convey so much detail without descending into a mere catalogue of tedious minutiae. As one would expect, the pages are liberally illustrated with spectral sequences and other examples, the data for most of which the authors have made available in digital form through their web site - a valuable resource for reference, and for developing laboratory experiments.
The book concludes with discussions of "other classification systems" (essentially, just the BCD system, so pervasive and successful is MK classification), and classification of wide-field, low-dispersion spectra. It's salutary that the summary of automated methods of spectral classification leads inescapably to the impression that the most sophisticated algorithms in pattern recognition and correlation' analysis still aren't competitive with what the eye and brain of the practised human classifier achieve with relative ease.
This volume is elegantly written, comprehensive, and authoritative (the authors' very considerable personal expertise is augmented by specialist contributions from Adam Burgasser, Margaret Hanson, J. Davy Kirkpatrick, and Nolan Walborn). Its utility is enhanced by extensive tables of classification 2009 December Reviews 393 standards, scrupulous referencing of primary sources, and thorough general and object indexes. The book stands head and shoulders above anything comparable, and will stay within my easy reach both for reference and to browse. Highly recommended - and you don't have to take my word for it: Princeton University Press (who justifiably describe the work as "definitive and encyclopedic") have made Chapter I, The History and Philosophy of Stellar Spectral Classification, freely available on their web site (and have given Google Books permission to make pretty much the entire book accessible on-line). Do have a look. - IAND. HOWARTH.