There has been a growing awareness in the scientific community – and to some degree among the reading public – that the Sun is a marvellously variable and mysterious object about which we have only a limited understanding. The standard theoretical model of the solar interior has now been verified through helioseismology to an accuracy of 0.2% – a major triumph. However, the internal convection generates magnetic fields that drive the Sun through a variety of regular and irregular cycles of activity. The accumulated records show that the various magnetic moods of the Sun drive the Earth’s climate into warm and cool phases from one century to the next by means that are not fully understood.

This growing appreciation of the Sun and of the Sun-Earth connection has fostered both ground- and space-based observations of our nearest star. Pushing towards ever better resolution of the Sun from radio wavelengths to gamma rays, our eyes have been opened to the underlying small-scale magnetic activity, and it appears that we may not have reached the bottom of the mysteries yet. It is therefore gratifying to see these two excellent new books that tell us what is known – and not yet known – about the Sun.

Nearest Star is the second collaboration between Leon Golub and Jay Pasachoff, following The Solar Corona (1997 Cambridge University Press), which remains the most up-to-date textbook available on the solar corona and its outward extension into the solar wind. In Nearest Star the authors stand back from the forest of details and take a descriptive look at the Sun, its variability, the effects of its magnetic activity on terrestrial climate, and the creation of the weather in space around the Earth.

Golub and Pasachoff begin the narrative with a brief discussion of the formation of the Sun and planets in the early universe, and of the life of the Sun in the present universe. They describe the fascinating historical development of major concepts such as the ice ages, global warming and the interior structure of the Sun – none of which became clear without extensive scientific investigation and lengthy debate. The presentation is well balanced and provides the relevant background physics in those areas that are clearly understood. There are mathematical equations in two or three places where they are deemed to be appropriate, but the accompanying text carries the narrative adequately.

The mystery of why the number of solar neutrinos that reach the Earth is so much less than the number predicted by theory is described clearly and assumes only that the reader knows that matter is composed of protons, neutrons and electrons. In the past year, while the book was in press, it has been determined experimentally that neutrinos do – as conjectured – ‘oscillate’ from one type (or ‘flavour’) to another as they travel from the Sun. This result, which explains the mystery, proves that the neutrino has a rest mass and opens up a whole new field of physics (Physics World July 2001 p5).

The authors then take the reader into the deep interior of the Sun, which has now been sounded with acoustic waves (helioseismology). This research has revealed the remarkable interior rotation of the Sun and, as already noted, accurately affirmed the theoretical models of the static Sun. The small-scale magnetic fields, which collectively produce the magnetic activity of the Sun, are part of the story, and the reader is provided with a well balanced review of the solar mysteries. The book concludes with a description of present and future space missions to study these mysteries, particularly the crucial fine structure of the magnetically active regions.

Nearest Star is authoritative, as one would expect from two such accomplished authors, and it is as free from typographical errors and misstatements as any book can be. One of the most ‘serious’ errors, however, is the statement on page 51 that it takes 10⁵ years for heat to leak out of the core to the surface of the Sun, whereas 10⁷ would be a better estimate. On page 64, meanwhile, the authors should have said that the abundance of helium in the Sun is determined from spectroscopic observations of prominences, rather than from the solar wind, in which the helium abundance is very low and variable.

Now if Nearest Star catches the reader’s interest, as I am sure it must, the reader will surely enjoy the more detailed Cambridge Encyclopedia of the Sun, and vice versa. The encyclopedia is a self-contained narrative of the Sun from beginning to end, commenting along the way on the place of the Sun in the universe and the eventual fate of the Sun and our planet.

The encyclopedia is well written, with a narrative that progresses through the different aspects of the active Sun. Relevant physics and maths are placed in ‘boxes’ set aside from the main text and there are an extensive index and glossary. I suppose that if one were to read the text of the encyclopedia, there would be no need to read Nearest Star, although that would deprive the reader of the pleasure of its somewhat different emphasis. I am happy to have had the chance to read both.

The encyclopedia carries the neutrino puzzle into the next generation of neutrino detectors, which are designed to attack some of the puzzles presented by the first generation of neutrino observations of the Sun. The new experiments can measure the energies of the individual neutrinos, thereby providing a statistical picture of their origins in the diverse nuclear interactions in the Sun. The experiments are sensitive in varying degrees to all three known flavours of neutrino so that they study neutrino oscillations directly.

The encyclopedia also covers the Sun’s control of our climate on Earth – from the little ice age to global warming – and includes the remarkable past climatic variations determined from ice cores from the polar regions. It illustrates the complexity and mystery of the present warming trend, which has arisen in response both to the more active and brighter Sun, and to the increase in carbon dioxide in the terrestrial atmosphere. It must be appreciated, however, that with the warming of the ocean surface water, the recent increase in carbon dioxide is not merely due to the burning of fossil fuels.

The final sections provide a concise review of ground- and space-based telescopes and instruments for studying the Sun. Throughout the encyclopedia there are pictures showing the crucial aspects of the observations and the puzzles. The great effort that the author has put into collecting so many diverse glimpses of the Sun and the universe is to be commended.

As with Nearest Star, I admire the high level of accuracy with which this encyclopedia has been written. There are only a few errors, one of which occurs on page 70, where it is stated that the 0.05 eV mass of the neutrino is a five-billionth – rather than five-billionths – of the mass of the proton.

Nevertheless, these two books should certainly go a long way towards introducing the reading public to the fascinating mysteries of the Sun.

Buy the books
Nearest Star: The Surprising Science of Our Sun: Amazon UK/Amazon US
The Cambridge Encyclopedia of the Sun: Amazon UK/Amazon US