Cosmic Butterflies: The Colorful Mysteries of Planetary Nebulae
Sun Kwok
Cambridge University Press August 2001
Pp ix+179 Hardcover
ISBN 0-521-79135
Price £ 20.00 (US$ 29.95)
Despite the more than one hundred beautiful Hubble Space Telescope (HST) images, this is not just another "pretty" coffee-table book. The accompanying text is clearly and authoritatively written, giving a lucid account of the history of our understanding of planetary nebulae (PNe).
Cosmic Butterflies begins with a review of the early visual and subsequent spectroscopic observations of PNe. Kwok points out that at the turn of the 20th century, it was thought that PNe were young stars, newly formed. Two decades later, however, observations had shown that they had more in common with older stars than with younger ones.
By the 1960s, significant advances in our understanding of nuclear physics led to important changes in theories of stellar evolution - PNe were now considered to be old objects, and red giant stars their progenitors. According to theory, the fate of a star depends on its initial mass: stars lighter than 1.4 solar masses would become white dwarfs, while heavier stars would have their stellar envelopes ejected in supernovae explosions, and those between 2 and 3 solar masses would turn into neutron stars, whereas more massive stars would end up as black holes. This would imply that supernovae must occur very frequently - however, the observed rate of galactic supernovae is only a few per century, bringing theory and observation into conflict.
In 1970 it was shown that normal red giants were not PNe progenitors; a more luminous, and even redder type of star, the asymptotic giant branch stars (AGBs), was the precursor to the formation of PNe.
Over the next few years, this model of PNe development was elaborated, describing the evolution from a star burning hydrogen in its core into a hydrogen-shell burning red giant, then to a hydrogen- and helium-shell burning AGB star, then to a PNe, eventually ending up as a white dwarf. While in the PNe phase, the central star burns hydrogen in a thin layer above a core of carbon and oxygen. When the hydrogen stock depletes, the star fades over several tens of thousands of years.
With the nature of the central star understood, attention was turned to the formation of the nebula which formed from the circumstellar envelope.
Infrared observations in 1960 showed that red giant stars have dust (similar to beach sand) in their atmospheres, and that the AGB stars have the most dust of all. Further, PNe also seemed to be dusty.
In 1971, observations at millimetre wavelengths showed that the AGB stars, in addition to their strong infrared signal, also emit carbon dioxide, which must have been synthesised in their atmospheres and later ejected. This implied that large-scale mass loss is common during the AGB phase of evolution. In turn, this meant that, if a star could lose several solar masses in the AGB phase, stars with initial masses of between 3 and 8 solar masses could become white dwarfs, instead of neutron stars or black holes as previously thought. Thus approximately 95% of stars end their lives as white dwarfs, while supernovae and black holes are uncommon.
In 1978, the author and two other astronomers formulated the interacting winds theory (or two wind model), in which the slow wind from the AGB expansion is swept up by a later-developed fast wind, originating in the central star, and forming the dense planetary nebular shell.
Observations that year with the International Ultraviolet Explorer (IUE) satellite confirmed these fast stellar winds. The use of CCDs then revealed that many PNe have faint halos, which are the remnants of the circumstellar envelope, confirming the theoretical prediction that the shells of PNe are swept-up material of the AGB wind.
Another prediction of the interacting winds theory is that X-ray emission should be seen from PNe as the shock wave from the fast winds heat the nebular matter. This has been confirmed by two satellite missions - first by ROSAT (the Roentgen satellite) and then in 2000 by the Chandra space observatory.
Thus the mystery of PNe ejection was solved. There is no separate ejection as such, and the nebular shell is just re-arranged wind material.
The variety of shapes that PNe assume have fascinated deepsky observers and puzzled theoreticians. The author discusses the history of their classification and then goes on to summarise the current view. It is thought that these diverse forms result from one "basic" shape, seen from different angles. The brightest part has the shape of a torus; the outflow from the star is channelled through the open ends of the torus and fans outward, forming two lobes, giving the nebula a butterfly shape. Encompassing this is the extended, fainter, halo.
Kwok describes the issues relating to the development of the torus and lobes from the complex interacting winds, and discusses the role of slight density asymmetries in the AGB star envelopes that could lead to the development of bipolar lobes. One implication of this model is that the shape of PNe should be age-dependent, with young nebulae being rounder than older ones. The search for very young PNe - proto-planetary nebulae - was undertaken at several observatories. Follow-up studies by the author and other astronomers, using data from the Very Large Array (VLA) and the Infrared Astronomical Satellite (IRAS), yielded a list of suspect objects. Their optical counterparts were searched for using telescopes in Hawaii and Chile, and in 1986, the first proto-planetary nebula was found by the author and his colleagues; several other discoveries soon followed. Further ground-based images showed that morphological asymmetry develops very early in the life of PNe; this was subsequently confirmed by high-resolution HST images. PNe were thus shown to form when the cocoon around the central star is ruptured at opposite ends by a fast outflow, sweeping up gaseous material in the shell into an exquisite butterfly shape. The central star becomes hotter, its ultraviolet light ionising the surrounding material, and a planetary nebula is born.
Several unresolved ques-tions remain. Complex features such as multiple poles, jets, concentric arcs and cometary knots challenge our understanding. Knowledge of the number of PNe in the Galaxy, as well as an accurate method to determine their distance, still eludes us.
Despite these uncertainties, the study of PNe can also give insight into cosmological questions. Kwok describes how PNe can be used to determine the size and mass of the Universe, the value of the Hubble constant, and the presence of dark matter in other galaxies.
The closing chapters discuss the synthesis of complex molecules in PNe and the implication for the origin of life on Earth.
The last four chapters are end-matter. A glossary with 55 entries provides a handy reference, though a few terms are used without definition (e.g. "Chandrasekhar limit" and "Å"). A list of prominent planetary nebulae follows; curiously, several constellation names are given incorrectly ("Tarus", "Velorum", "Ursae Major"). A very useful and comprehensive "Further reading" section gives an overview of both the popular and technical literature. Finally, a good but brief discussion of the images, their acquisition with the HST and subsequent processing, rounds off this fine work.
The visual appeal of this book is enormous. The exquisite structure of PNe are beautifully illustrated by carefully selected and well-reproduced images. Simply browsing through the pictures is in itself a worthwhile experience.
Dr Sun Kwok is an acclaimed astronomer at the University of Calgary, Alberta, Canada, and an expert on PNe. He is chair of the Planetary Nebulae Working Group of the International Astronomical Union and has authored more than 200 scientific papers.
Readers interested in a more technical treatment of PNe can consult two other works by Kwok: "Planetary Nebula: A Modern View" (Publ. Astron. Soc. Pacific, 1994, 106, 344-355) and The Origin and Evolution of Planetary Nebulae (Cambridge University Press, 2000).
(Auke Slotegraaf)
[2002MNSSA..61...80S]
|
news > 