The Sun is an object of great beauty and fascination that has been studied with interest for thousands of years. It was born from a contracting, rotating, interstellar cloud that spun up during the collapse. The protostar would have settled down into a state where gravity and a pressure gradient balance one another and where a continued slow contraction heats up the plasma and provides the luminosity. Eventually, the core temperature became high enough for fusion of hydrogen to helium to provide all the luminosity, so that the contraction ceased. The Sun then entered the main ten-billion year (1010 yr) phase of its life on the main sequence, during which essentially all of the hydrogen in the core is turned into helium. So far, the Sun is half-way through this stage. In about five billion years, when the core hydrogen is exhausted, fusion will continue to take place in a shell around the helium core, while the Sun will expand greatly into a red giant (see PROBLEM 1.1). (For PROBLEM 1.1 and all the other problems in this book, together with their solutions, see the web page at www.cambridge.org/9780521854719.) Eventually, the red giant will collapse to a white dwarf, containing most of its original mass in a size similar to the Earth.

During the twentieth century, it gradually became clear that much of the Sun's present observed structure and dynamic behaviour owe their existence to the magnetic field.