Star cast in a universal picture

Modern astronomy and cosmology have been in a state of ferment ever since the early 1950s when relativity was found to play a crucial role in our galaxy. It was discovered then that most of the diffuse radio waves emitted by the galaxy are produced in interstellar space by cosmic ray electrons moving so close to the speed of light that their properties must be described by Einstein's special theory of relativity. These ultrarelativistic electrons have their paths bent by an interstellar magnetic field, and thereby emit radiation - the so-called synchrotron radiation, named after the light emitted by electrons circulating in synchrotron accelerators in the laboratory. The resulting combination of relativistic physics, galaxy-wide phenomena and modern observing instruments - in this case radio telescopes - is entirely characteristic of the new era in astrophysics and cosmology, and gives it its special charm.

This era developed rapidly with a spectacular series of discoveries, including radio galaxies, quasars, pulsars, neutron stars, X-ray sources, neutrinos from the sun and supernova 1987A, gravitational lenses, (probably) black holes and culminating in the ultimate discovery - the thermal cosmic background radiation coming from the hot big bang origin of the universe. To obtain a complete picture of modern astronomy and cosmology one must graft these new discoveries on to more traditional topics such as the birth, evolution and death of stars, the interstellar medium, the galaxies and their distribution and the expansion of the universe.

Malcolm Longair has produced a superb and wonderfully illustrated book of this complete picture, so cunningly written that it can be used not only by the general reader interested in science, but also by undergraduates, graduate students and even professional scientists, nonastronomers and astronomers. Longair himself was plunged into the modern era as a research student in Martin Ryle's radio astronomy group at the Cavendish Laboratory in the early 1960s. Immediately he became heavily involved in the controversy that was then raging between Ryle and Fred Hoyle concerning the distribution of radio sources and the conflict between the evolving and steady state models of the universe. Now he is a professor at the Cavendish himself, and has built up a formidable reputation as a teacher, as well as carrying out his well-known researches in extragalactic astronomy.

The central role in his book is played by the physical processes that take place on a bewildering variety of length and time scales and in a bewildering variety of objects. These physical processes are beautifully expounded in just enough detail for genuine understanding to be possible, yet without becoming too technical for the general reader. Any such reader will leave this book with a remarkably up-to-date and accurate feeling for the formation of stars and galaxies, the operation of pulsars, quasars, X-ray sources, and black holes, the significance of the cosmic background radiation and the role of dark matter in the universe.

I have just one disagreement with the author. Although it concerns less than one page of text, it does involve a question of fundamental importance: the role of the so-called anthropic principle. This principle is often misrepresented by its opponents. This is perhaps not too surprising since it is also often misrepresented by its proponents. When understood correctly, I believe it to be a principle of great power. It concerns the undoubted fact that many of the laws and constants of nature need to be finely tuned for intelligent life, such as our own, to develop in the universe. According to the anthropic principle it is not a coincidence that the universe possesses these finely tuned properties. This idea is presented by the author (who does not like it) in the form "the presence of intelligent life on Earth influences the nature of the universe we live in I In other words, the universe is as it is because we are here to observe it". While these sentences give a formally correct statement of the idea involved, the use of the words "influences" and "because" convey an unfortunate suggestion of a mystical kind of backward causation. No such backward causation needs to be involved. If every logically self-consistent universe exists in the whole of reality, then it follows trivially that we will find ourselves in a suitably fine-tuned one, without the need to introduce either a coincidence or a backward causation. The author may not like this possibility, but I would have preferred to see the question more carefully discussed.

This cavil aside, Longair has written a wonderful book, which I urge everyone interested in astronomy and cosmology to read from cover to cover.

By contrast, the book by Carolyn Petersen and John Brandt on the Hubble space telescope has a narrower focus. The HST is a marvellous instrument and it has made a number of magnificent discoveries. Nevertheless, the account given here of the astrophysics flowing from these particular discoveries provides a rather lopsided picture of astronomy, especially when compared with Longair's very balanced discussion. The volume is better regarded as a sort of handbook of the HST for the general reader. It gives a good, well-written description of its origin in Lyman Spitzer's pioneering proposal of 1946, of the first serious committee meetings in the 1960s, of its building, and of the discovery and repairing of its famous spherical aberration.

This last episode was a very dramatic one. I well remember writing a paper at the time in which I was saying that a particular prediction of mine could be tested by HST if it were successfully refurbished. Even as I wrote those words I was watching on CNN live television pictures of the intrepid astronauts carrying out the repair. This was one of the most moving moments in my whole astronomical career. As the world knows, the repair was a great success and as a result HST has become a unique instrument for the study of astronomical objects, from the planets to the most distant galaxies and quasars. This book tells you how it was done.

D.W. Sciama is professor of astrophysics, International School of Advanced Studies, Trieste, and emeritus fellow, All Souls College, Oxford.