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The Origin and Evolution of Planetary Nebulae

Egg Nebula: Hubble photograph of a planetary nebula.

The intricate morphologies and striking symmetries of the so-called planetary nebulae--rings, helixes, and butterflies--have long made them favorites of amateur and professional astronomers alike. Their popular appeal continues to the present: The remarkable images of these glowing gas clouds obtained by the Hubble Space Telescope (HST) are seen in textbooks, magazines, and newspapers (see http://oposite.stsci.edu/pubinfo/pictures.html).

The name "planetary nebula" is in fact a misnomer, arising from their superficial similarity (through early telescopes of poor optical quality) to the outer planets Uranus and Neptune, which gave the nebulae the appearance of small, greenish disks. Today we know that planetary nebulae represent a transitional stage in the life cycles of many stars, during which a formerly cool, distended red giant star sheds much of its mass, baring the hot stellar core, which in turn illuminates, ionizes, and heats the newly created circumstellar gas cloud. This phase lasts only 10 000-100 000 years, a mere blink of an eye in the typical lifetime of a star, which is measured in millions or billions of years.

Planetary nebulae lie at the crossroads of several disciplines (stellar, interstellar, and galactic astronomy, as well as several branches of physics). The physical state of the gas and the emerging spectrum of radiation (best described using the tools of interstellar-medium astrophysics--the study of diffuse gas in space). Yet, because planetary nebulae comprise material cast off by aging stars near the end of their lives, they carry not only the imprint of the star's initial composition but also the signatures of internal nuclear reactions and mixing processes that bring freshly-made heavy nuclei to the surface during its lifetime (stellar evolution). Planetary nebulae show us stars caught in the act of changing the composition of their environment by enriching it in stellar nuclear ashes, a one-way cosmic evolutionary process as inexorable as entropy (galactic evolution). Planetary nebulae have served as remote laboratories for matter under conditions that cannot be duplicated on Earth; from the mid-19th century to the mid-20th and beyond, they were premier sites for the study of atomic-level processes and photon-atom interactions (atomic physics). In the past two decades, they have taken on additional roles as laboratories for gas hydrodynamics (plasma physics) and as probes of the distances and masses of galaxies (extragalactic astronomy).

This slim volume by Sun Kwok, a major player in the field of planetary nebula research and the originator of the current structural paradigm (the "interacting stellar winds" scenario), represents a valuable contribution to the literature. It offers the most complete, accessible, and up-to-date entry to this subject for the newcomer with a strong general background in physics and astronomy at the advanced undergraduate level or above. It is more suitable for this purpose than the proceedings of symposia of the International Astronomical Union (the most recent is Planetary Nebulae, IAU Symposium 180, Kluwer, 1997), edited by H. J. Habing and H. J. G. L. Lamers, which are intended for the specialist. Kwok has wisely chosen not to attempt to fill the same niche as Donald Osterbrock's Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (University Science Books, 1989) or Lawrence Aller's Physics of Thermal Gaseous Nebulae (Reidel, 1984), which are comprehensive reference works on physical processes in nebulae.

Instead, The Origin and Evolution of Planetary Nebulae emphasizes recent developments and the broader astrophysical context, while still including enough nebular physics to orient the novice. The book's modular arrangement enables the reader to choose between two options: Those interested in the detailed nebular physics can progress sequentially through the book, while those more interested in origin and evolution can skip directly from the introductory chapter to chapter 10.

Some minor omissions are noticeable. I would have liked to have seen more material on the neutral atomic as opposed to molecular gas, on results from polarimetry, and on the properties of white dwarfs--the descendants of the central stars of planetary nebulae. More significant is the lack of coverage of the raging current controversy over whether the bipolar morphology of some planetary nebulae is caused by the originating star having evolved within a binary system. This topic deserves its own chapter, and has recently inspired a new series of conferences under the heading of "Asymmetrical Planetary Nebulae." Two volumes are out, a third conference is planned for some time in 2002. Kwok's book would also have benefited from more careful proofreading to eliminate minor grammatical and spelling errors, some of which appear in section headings or in the spelling of author names in the citations.

Overall, The Origin and Evolution of Planetary Nebulae is a significant achievement, drawing together both traditional and modern ideas about the nature of planetary nebulae and their place in stellar and galactic astronomy. The reader who is inspired by the images of planetary nebulae to go beyond aesthetic appreciation, to delve into the physics and astrophysics of these fascinating structures in space, cannot do better than to start with this book.

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