|Astronomy & Geophysics 42 (4) 4.31|
|What are galaxies?The Galaxies of the Local GroupCambridge Astrophysics Series; The Formation of Galactic Bulges,
Sidney van den BerghC M CorolloH C FergusonR F G Wyse, Cambridge Contemporary Astrophysics, .
Astronomers are everywhere working on origins: of the universe, of galaxies, of stars, and of the solar system. Probably none of these problems is harder than that of understanding how galaxies emerged from the tiny fluctuations in the cosmic density that were first seen nearly 10 years ago. Yet in setting us this physics exam, Nature provides two invaluable cribs. First, at moderate redshifts we can actually see galaxies forming. Second, galaxies such as our own are at least as richly packed with clues to how they formed as the surface of the Earth is with fossil evidence of the origins of life.
Sidney van den Berghs The Galaxies of the Local Group gives one insight into the magnitude of the problem by describing in some detail the 35 odd galaxies that are our nearest neighbours. There are several reasons why any serious student of galaxy formation should be familiar with the contents of this book. One is that most of the Local-Group galaxies are low-luminosity systems. According to the current picture of galaxy formation, in which larger galaxies formed by the coalescence of bits, which themselves formed by the coalescence of tinier bits, most baryons were once in a low-luminosity system, and it is clearly important to understand how they form and die. Moreover, while some low-luminosity galaxies appear to be more dark-matter dominated than giant galaxies, others are the most primeval in the sense that they have the largest fractions of their baryons in gaseous rather than stellar form.
The low-luminosity systems that we can study in the Local Group are hard or impossible to detect further away, so it is vital to study the local examples as carefully as possible. Van den Berghs book devotes a chapter each to the nine most luminous members of the Local Group, and then fits the remaining members into six further short chapters. He finishes with three short chapters on issues relating to the entire Local Group: its dynamics, interactions and intergalactic matter. The weakest part is the survey of the Milky Way, which occupies only 23 pages (compared to 47 pages for the LMC) and omits several important areas, such as 21 cm surveys of the disk and the kinematics of the solar neighbourhood. However, all chapters are packed with invaluable information on the morphology, photometry, kinematics and stellar content of each object. The text is accompanied by nice photographs and useful tables. Any good astronomy library should hold this book, and many research astronomers would benefit from returning to it again and again.
The Formation of Galactic Bulges is the proceedings of a meeting held in Baltimore in October 1998. It is divided into six parts, most of which open with a short overview of its contents from the editors. A galaxy can generally be considered to be made up of a disk and a bulge. The larger the fraction of its light that comes from the bulge, the earlier the galaxy lies in Hubbles fundamental classification galaxies in which the disk is so marginal that it can only be detected by quantitative surface photometry are called ellipticals, while galaxies like our neighbour M33 which possess virtually no bulge are late-type spirals. Thus bulges are absolutely central to our understanding of galaxies.
The articles in this book are organized into four sections: The Epoch of Bulge Formation, The Timescales of Bulge Formation, The Physical Processes of Bulge Formation, and Bulge Phenomenology. Each section is preceded by a useful editorial overview and there are also introductory and summary chapters for the whole book. As in all such proceedings the review articles are very much more worthwhile than the four-page contributed articles, which all too often are little more than trailers for a substantive paper in a journal. Whereas the section on timescales is entirely made up of four review articles, that on phenomenology has no review article.
How did bulges form? A good deal of evidence is presented in this volume that bulges are old. Yet, at least in more luminous galaxies, they are quite metal-rich. Hence, they synthesized significant amounts of heavy elements in the first gigayear or so of the universes existence, and they must have been about 30 times more luminous early on than they are now. The faint, unidentified objects that have recently been detected in large numbers by the SCUBA far-infrared bolometer array may be bulges seen at exactly this epoch.
For some time there has been compelling evidence that when disk galaxies of comparable mass collide at sufficiently low speed, they merge, and the product of this merger is an elliptical galaxy. About 15 Gyr from now the Milky Way will merge thus with M31. Recurring questions in this volume are were bulges formed in mergers of small disk galaxies? and are bulges small elliptical galaxies that have acquired disks?. It seems that both questions can be answered affirmatively, at least for some bulges. In particular, bulges share with elliptical galaxies a number of correlations between their observables, and from the structure of some anomalous S0 galaxies, which have more than one disk, we know that even fully formed galaxies can accrete fresh gas and form a new disk, so it seems likely that many small ellipticals that formed in the more gas-rich environment of z = 2 or 3 would by now have accreted a massive disk and become the bulges of disk galaxies.
A stellar disk is an inherently fragile structure, because all its stars are essentially confined to a line in the three-dimensional space of possible stellar orbits. Any disturbance will scatter those stars through a larger volume in orbit space, and then they are no more likely to gather again onto a line than spilt milk is to pick itself off the floor and plop back into the milk-jug. Entropy never decreases. This fragility suggests that bulges formed bare and were clothed with disks later, since it is hard to understand how a bulge could be forged at the centre of a galaxy without disrupting any fragile disk that lay around it.
Yet an interesting case can be made that disks have survived the formation of bulges at their centres. Numerical simulations show that as a disk accretes mass, a bar is liable to form at its centre. As the bar gains in strength it may buckle out of the plane of the disk, and become a thick bulge-bar. Seen from certain directions, these objects look boxy and they have kinematics that give rise to very characteristic positionvelocity diagrams. Some bulges display precisely these characteristics and, indeed, the bulge of the Milky Way is probably of this type. Again we are pulled up by the complexity of galaxy formation. The products can span such great ranges in scale and in morphology, and even when you confine yourself to objects of a given scale and morphology, it seems that they can have two radically different modes of formation. It is enough to make one despair, and take up a simpler problem, like understanding the origin of the universe. Those with stout hearts, who are determined to understand why galaxies are built the way they are, will find much to stimulate them in this useful book.
James Binney Oxford University.
The Hubble Space Telescopes view of a galactic bulge (NASA and John Trauger).
|Science in artSun symbolism and cosmology in Michelangelos Last Judgment,
Valerie Shrimplin, Truman State University Press, Kirksville, Missouri, 2000, ISBN ISBN 0-943549-65-5, xv375pp.
Michelangelos Last Judgment is one of those few works of art that continues deeply impressing people even when its religious meaning is not fully understood. Since its unveiling on 31 October 1541, the complexity of this large composition has prompted many interpretations of its representation. In this book Valerie Shrimplin uses an interdisciplinary approach to discover what was in Michelangelos mind when he conceived and executed this grandiose work of art. The book is based on Shrimplins doctoral thesis in art history in the University of the Witwatersrand, Johannesburg in 1991. The result is a beautiful book of large format with 127 illustrations (all black and white, maybe some colour would help), a large bibliography of 29 pages and a well organized text.
In successive chapters, the author examines the background of the Christian iconography representing the Last Judgementaccuse the Church of obscurantism. Since Bede the Venerable in the 8th century, all medieval authors, for , the work itself and the many interpretations given to it and the religious, literary, philosophical and scientific sources that may have influenced this painting. This is a very ambitious project that requires expertise in many subjects and the large number of footnotes is witness to the enormous amount of work put into this book. It is surprising to see how the author moves with ease in so many different fields, trying to find the central theme that inspired Michelangelo in organizing the composition of his large fresco.
The hypothesis of the book is that that Michelangelo represented Christ as the Sun in the centre of a circular composition that is taken from Copernican cosmology. This idea has been suggested already by Charles de Tolnay in 1960, but he rejected it because Copernicuss work was published in 1543, seven years after Michelangelo conceived his fresco. Accepting that the Last Judgment has a cosmic character, direct Copernican influence was difficult to explain. Shrimplin set herself to show that this is not so and that, though Copernicuss De Revolutionibus was published later and even the previous work by Rheticus, Narratio Prima, was published in 1540, still too late, Michelangelo could have known about the new heliocentric cosmology. The first notice of the heliocentric theory, the Commentariolus, was put in writing by Copernicus, though not published, before 1514 and was circulated among his friends. In 1533 Johann Albrecht Widmanstadt gave a lecture in Rome in front of Pope Clement VII about Copernicuss ideas and in 1536 Nikolau von Schônberg, Cardinal of Capua, wrote to Copernicus encouraging him to publish his work and asking him for a copy of his notes. Shrimplin makes it very clear that Copernicuss ideas were known in Rome years before their publication and that there were no negative reactions against them. On the Protestant side, however, Luther and Melanchton expressed their opposition to Copernicuss view as early as 1539. Problems with the ecclesiastic authorities of the Catholic Church began much later, about 1611, with Galileos public teaching. Michelangelo could, then, have known of them, at least at early as 1533, and thus they could have influenced his work.
The other important piece of evidence used by Shrimplin comes from the philosophical sources, and is the Neoplatonic ideas shared by both Copernicus and Michelangelo. Both Copernicus and Michelangelo were influenced by this philosophical school, especially through the work of Marsilio Ficino. In Neoplatonism, the symbolism of the Sun was specially important and it is recognized that Michelangelo represented Christ as the Sun in his painting. Thus Sun-symbolism and Copernican cosmology represent, for Shrimplin, the two major themes in the Last Judgment. Christ is represented as the Sun by the light that surrounds him, better appreciated after the recent restoration. Around him in clear circular patterns are distributed all the other figures. These patterns suggest the planetary orbits of the Copernican cosmology. Among literary sources, Shrimplin emphasizes that of Dantes Divina Commedia which Michelangelo was very fond of and where the Sun-symbolism for God is also very strong.
Shrimplin makes a strong case for the Copernican interpretation of Micheangelos fresco. However, without direct evidence from Michelangelos own writings, this must remain as an interpretation, although a very plausible one. it is possible that Michelangelo had early knowledge of Copernicuss theory. At that time Catholic ecclesiastic authorities had not yet shown any reserve against them. There is, then, nothing that would have hindered Michelangelo in adopting the Copernican cosmology. His Neoplatonic tendencies may have helped in this respect. The greatest difficulty in the acceptance of the Copernican ideas laid in the principles of Aristotelian physics which required geocentrism and had become the official doctrine of universities since the 13th century. Abandonment of Aristotelian doctrine in favour of Neoplatonist views predisposed people to accept Copernican cosmology. Aristotelian physics and the literal interpretation of the Scripture were the main reasons, in 1616, for the inclusion of Copernicuss book in the Index, until its correction.
However, it is still possible that, even if Michelangelo knew of Copernicuss work, his circular arrangements of the figures around the central figure of Christ, represented surrounded by light in a Sun-like manner, may not reflect necessarily a heliocentric cosmology. Michelangelo could have just wanted to show Christ as the centre of the new creation, based on pure religious ideas. We have to have in mind that Copernicuss book is highly technical and only professional astronomers could follow its arguments. Very few of Copernicuss contemporaries really accepted his work, although he was recognized as a great astronomer. Even 50 years after its publication, in the time of Kepler and Galileo, the number of followers of Copernican cosmology was very reduced. If Michelangelo was one of them much earlier, this shows an unusual perception that only a real genius could have had. Though Shrimplins book is not one of a historian of science, it is surprising that certain popular misconceptions are still present. One is the medieval flat-Earth cosmology that, in fact, never existed and it is a prejudiced invention of the 19th century, in order to example the influential Albert the Great and Thomas Aquinas in the 13th century, held the Earth to be spherical. The other is that Galileo proved that the Copernican theory was true. He tried hard, but not until Newton was the theory solidly founded.
|A big surpriseThe Encyclopedia of Astronomy and Astrophysics,
Paul Murdin, Nature Publishing/IoPP, 2001, ISBN ISBN 0333750888, 2197pp.
In 1996 I first heard of the plan to produce this four-volume work, and in 1997 I received an invitation, which I declined, to join the editorial team. At the time I was exceedingly sceptical about the feasibility of achieving a quality product on a reasonable timescale. Though I have reservations about the finished work, I am pleased to write that the editors, authors and joint publishers have done better than I then thought likely. They deserve congratulations.
It has been a huge undertaking. There are four volumes on paper and more in electronic form, spanning all aspects of astronomy and astrophysics. Material is presented as 2750 articles, 700 of them covering topics in depth, together with shorter pieces on people and institutions of note. The publishers describe it as an essential source of information for undergraduates, graduate students, researchers and seasoned professionals, as well as for committed amateurs, librarians and lay people.
A particularly relevant question concerns the motivation for undertaking such a work. Also in 1996, I was asked by the then president-elect of another major academic publisher, who was unaware of the IoPP/Macmillan plans, whether his organization should pursue a comparable endeavour. He explained that a wide range of librarians feel compelled, even in this age of budgetary pressure, to buy such a publication. Interaction with my own departments very able and exceedingly helpful contact librarian about the Encyclopedia has confirmed what he said. So a publisher gets involved with huge organizational problems because even schools and public libraries are possible purchasers. The number that I heard bandied about for the budget of this project is about £1m. Given that the total sale of an astronomical book is usually about several tens of thousands of pounds, relatively big money was invested in the tomes under review. By the way, the princely sum that I was offered was $100; obviously, the publishers consulted a government minister to get an idea of what an academics time is worth.
Though sales will be huge by astronomical publishing standards, who is likely to use the Encyclopedia? I certainly enjoy skimming articles by friends who, given the space limitation, have fulfilled their briefs remarkably well. However, I have learned nothing that I did not already know. So who can advance their knowledge through these articles? In many cases, no-one may be the disappointing answer. First-year university students writing end-of-year general essays may find some of the articles useful, but the most interesting will be far beyond them. More advanced students should be expected to go to articles in the Annual Review of Astronomy and Astrophysics, books presenting introductory but detailed, rigorous expositions, and a few articles in current refereed journals. A good introductory undergraduate-level overview of astronomy and astrophysics would probably be at least as useful as the Encyclopedia as a source for the broad background information that one might need in order to appreciate an Annual Review article.
I would like to see a successful realization of a rather different type of work. Rather sizable parts of astronomy must be described in a volume of a couple of hundred pages primarily aimed at the presentation of the study of cosmic phenomena from any of a variety of perspectives. For instance, any book on X in astronomy where X may signify molecules, winds, magnetic fields, spectroscopy, nuclei, or relativistic particles would cover a rich range of astronomical sources. In addition, such a book would illustrate that a good basic idea or a small set of interrelated mechanisms touches a lot of different lines of investigation and would, thus, communicate an appreciation of a perspective, something that is totally absent in the Encyclopedia. I would really like to see a well-written, well-edited, well-produced series of such books. However, I would pity any publisher who would try to find the authors for such a series, as, despite the number of people capable of writing fascinating overviews, they are all so busy that making a commitment to such a project would probably interfere with the completion of the tedious tasks that some administrators deem more valuable than scholarship.
Perhaps the idiosyncratic nature of this review is inspired by the rather amusing choice of people to whom small articles are devoted. As the Encyclopedias electronic version will be revised continuously, the publishers have the opportunity to try to recognize some great astronomers. Of course, some are already the subjects of short entries, but overall the list of the scientists so honoured is the consequence of an expert level of thoughtlessness. The decision to have articles on minor institutions with words like observatory in their names while departments with much more significant histories or current programmes are unmentioned is bewildering. Many omissions, such as the absence of a thorough article on the KleinmannLow Nebula, and redundancies, including an entry on the Inflationary Universe immediately following a well-written article on Inflation by A Linde, are surprising. Short pieces on each of the Messier objects compensate for some failings; they are particularly useful for a theoretician.
Despite my criticisms, the work represents quite an achievement of organization. Surely the better students who will see it will gain an impression of the richness of astronomy and astrophysics; I hope that they will be inspired.
Tom Hartquist Leeds University.
|A pinch of salt?Astronomy for GCSE,
Patrick MooreChris Lintott, Duckworth, London, 2001, ISBN ISBN 0 7156 2969 7, .
This compact book has been revised to accommodate the changes to the new GCSE astronomy syllabus which will be first examined in 2003. The 25 brief chapters are written in Patrick Moores immediately recognizable matter-of-fact style. These are generally well aligned with the requirements of the new syllabus and will provide useful background reading. Explanations of astronomical concepts such as mean sun and the leap year are clear and concise. However, the sections on seasons, black holes and cosmology are unnecessarily brief, with insufficient detail for higher grade candidates.
Each chapter ends with revision questions to check that the material has been read and understood. These are unlike the format of the GCSE papers so the candidate will need more practice than these afford. This is especially true of numerical work with Keplers Laws and Newtons Law of Gravitation, given only one example each.
The authors have unfortunately missed the opportunity to bring the units of measurement in line with the Systéme Internationale dUnités (SI units) convention used throughout the scientific world and also by GCSE. While inches are always followed by the equivalent in cm, this makes for confusion in calculations and for clumsy text. Also sec should be s and km per second should be km s1. Angstrom units are superfluous at this level.
The practical requirement of the new syllabus is well supported by guidelines on making a sundial or a simple refractor or projecting an image of the Sun to study sunspots. Some unguided photographic work would have been a useful addition.
A serious drawback to this new edition is the large number of errors both in the text and in the diagrams. Many of these arise from poor typesetting, such as the fifth root of 100 and the reversal of the conversion factors from metres to feet. Also there are errors in the answers to numerical questions, which will confuse many students wrestling with right ascension and hour angle. Mistakes in the diagrams include the wrong labelling of amplitude (in figure 34) and the labelling of the light curve of delta cephei in hours instead of days. The eclipse diagrams should have arrows on the light rays. Some of the other figures are poorly or ambiguously drawn. More worrying is the number of scientific mistakes in the text, for example RA and azimuth are confused (page 52) and the VLA is given 80 telescopes when it should have 27.
Despite this, the book has a place in a subject where there is very little reference material specifically for GCSE. As a supporting text it has much to offer student and teacher as long as both are wary of accepting details at face value.
Anne Cohen is an ex-Jodrell Bank radio astronomer who preceded Alan Pickwick as President of the Association for Astronomy Education and is currently teaching physics (and astronomy) at The Kings School, Macclesfield.
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