Dull star shows it's really hot stuff

The Cambridge Encyclopedia of the Sun - Nearest Star
September 20, 2002

Astronomy is generally regarded as the least precise of all sciences, if only because all the objects astronomers endeavour to study are so remote. Even the Moon is 250,000 miles away, yet it is by far our closest neighbour in space. The nearest star in the night sky is 100 million times as distant, yet it is extremely nearby compared with the inconceivable distances of most other stars in our Milky Way galaxy, let alone other galaxies.

Astronomers, then, have to put up with a huge handicap - unlike, say, botanists or particle physicists, whose objects of study can be observed at close quarters and can be subjected to experiments. Nevertheless, large astronomical telescopes with electronic cameras - and, in the past 50 years, dedicated space-based observatories - have opened up new avenues undreamt of 100 years ago, giving us new views of the universe around us.

The images that now abound in astronomy books and magazines are often of amazingly exotic objects whose description is challenging even in the arcane world of theoretical physics.

What, then, of our familiar, but perhaps a trifle dull-seeming, Sun? It is of vital importance to us here on Earth - practically all our activities ultimately depend on it - but do interesting things occur on, or maybe under or above, that blank yellow disc that we see in the sky? Open a copy of The Cambridge Encyclopedia of the Sun and you will find that the answer is a staggering "Yes".

Even small telescopes show sunspots - the transient, cooler areas that appear and disappear on the disc - with dimensions often exceeding by many times the diameter of the Earth. Much more detail is revealed by larger telescopes, but even more spectacular to the uninitiated are the extraordinary structures that make up what solar astronomers call the Sun's atmosphere (perhaps a misleading name since the Sun is gaseous right to its centre). If we had eyes that were sensitive to X-ray emission, and were to fly above the absorbing effects of the Earth's atmosphere, we would find strange loops and streamers extending out hundreds of thousands of miles from the solar surface or photosphere. The Japanese Yohkoh satellite obtained thousands of such images and many of the best of them, along with those from other satellites such as the Solar and Heliospheric Observatory (Soho), are contained in Kenneth Lang's lavishly illustrated and informative text.

These illustrations alone will ensure that the book has an enthusiastic readership, but those looking for explanations of what it all means will not be disappointed either. Throughout, there are texts that describe, in approachable and simple terms, the Sun's place in our solar system; the Sun regarded as a star, and its place in our galaxy; its life-history and ultimate destiny; its interior, its atmosphere and the influence of the magnetic field that gives rise to so much of the intricate structure visible in X-ray images; and the burgeoning subject of space weather that has come to play such an important part in satellite communications in the past ten years or so. Every concept is illustrated with some of the best line drawings I have seen in scientific books, clearly labelled and with excellent captions. There are plenty of the tabulated data that one would expect of an encyclopedia, with some mathematical description directed at more esoteric concepts confined to stand-alone boxes. And the book contains excellent descriptions of ground-based solar instruments, with, for instance, dramatic illustrations of the McMath solar telescope in Arizona. But I did wonder why Lang did not choose to show the workings of some of the space instrumentation, which are rather briefly described.

It is an unfortunate aspect of writing about such a fast-moving subject as solar astronomy that some text will become quickly dated. The preface was written in January 2001, but already developments have overtaken the section about the "mystery" of the solar neutrinos - the long-standing problem of "missing" neutrinos (particles with extremely small mass that are formed in the nuclear reactions occurring at the Sun's core). The problem has now almost certainly been explained by the oscillations that neutrinos undergo between three different "flavours". This is no reflection on the author; the reader, however, should be aware that such things may affect the text here and there. By and large, though, the encyclopedia should stand the test of time without serious revision for several years to come, notwithstanding the several future space missions described. All in all, a highly recommendable volume for a wide readership.

Leon Golub and Jay M. Pasachoff's book, not being an encyclopedia, has different aims. For example, its details about eclipses capture a lot of the excitement familiar to those of us caught up in the fervour surrounding the 1999 eclipse visible in the UK and parts of Europe and Asia. This is not surprising as the authors are among the world's leading experts in observing eclipses and have distinguished careers in studies of the solar atmosphere. Solar observing has a long history, partly because it is relatively easy to make telescopic observations, and the book reflects this with interesting diagrams from the past that help to fit the subject into historical perspective.

The book gives readable accounts of the Sun's evolution from its earliest state to its final fate and the spectrum of its visible light, again set in the context of past discoveries. The chapter "What we don't see" includes studies of the solar interior using the subtle oscillations at the solar surface - the geophysics analogy is the study of the Earth's interior from earthquakes. This "helioseismology" has given us a greatly increased understanding of the Sun's interior, such as rotation rates with depth, but on this topic the book does not make it easy for the reader to put together a coherent picture of what is going on by reading the text and relating it to illustrations in the plates.

Solar observations from space have altered our perceptions so dramatically that it must seem odd to young professionals coming into the subject how solar physicists could ever have managed without them. Of course, not only did they manage but they established a great deal using only ground-based telescopes. Even so, it is good to see an account, in the chapter on "Space missions", of rocket launches and all that goes into them. Golub has had much experience in this, and it must have been written from a personal point of view.

It is disappointing that this full account of rockets is followed by only a short section on satellites, from which much more has been learnt. Nevertheless, the detailed section on future space missions, which the reader can follow when they are launched, is useful, although a book is dated immediately one discusses the future, since what was "future" at the time of writing may be the past by the time the reader picks up a copy of the book; the Hessi (now renamed Rhessi) mission, to illustrate the point, is already an operating satellite, having been launched in February 2002.

Global warming is a daily topic of conversation. What is the influence of the Sun and what is the influence of human activities on the Earth's climate? This, too, is a hotly debated subject - much more so than ten years ago when I wrote my Guide to the Sun , although even then I felt impelled to give an account of the issues involved. Golub and Pasachoff's book rises to the challenge with some accurate and down-to-earth text about climate, as well as about attempts made by United Nations-sponsored meetings to limit the production of greenhouse gases.

There is no doubt that "space" weather is entirely caused by the Sun, but its implications for communications on Earth are enormous now that we rely so much on communications satellites. The concluding chapter goes into this in detail while maintaining a readable level.

If you are into illustrations, you might be less than enthralled by the book's relatively small number of coloured plates - just 16. However, the line and half-tone figures do a good job in explaining clearly the concepts mentioned in the text. I was impressed by the descriptive nature of the text, which has no mathematics or complex physics. This is a book for amateur astronomers and scientists who want to know what are the latest findings about the Sun and about some of the developments that led to them.

Ken Phillips is at the Nasa Goddard Space Flight Center, Greenbelt, Maryland, US.

The Cambridge Encyclopedia of the Sun

Author - Kenneth R. Lang
ISBN - 0 521 78093 4
Publisher - Cambridge University Press
Price - £29.95
Pages - 256

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