The Sun is a star that many astronomers assume is only worth studying because of its averageness; it’s middle-aged and middle-sized. Its convenient proximity to us means it can act as a testbed for physics research. But we’re too well-schooled in the Copernican principle to view it as ‘special’ in any way.
In contrast, Lucie Green’s huge enthusiasm for the Sun is apparent throughout her book. Her purpose is to convey the current state of knowledge about our neighbourhood star, and the story proper starts with sunspots. As a result of the invention of the telescope at the beginning of the 17th century, Galileo, and other early adopters of this technology, were able to observe dark patches on the surface of the Sun appearing and disappearing on timescales of just a few weeks. These apparent changeable ‘flaws’ on the Sun argued against the Church’s teaching that heavenly objects were both immutable and perfect, and thus inherently different to the Earth, helping to pave the way for the Copernican revolution.
We now know that sunspots are caused by variations in the Sun’s magnetic field. And it is this field which is the subject of Green’s professional research. The Sun is the only star to have its magnetic field so comprehensively studied, and Green explains how it’s responsible for many observed features; sunspots, solar flares, and the genuinely awesome coronal mass ejections during which billions of tons of material are expelled from the Sun’s outer atmosphere.
Green starts her story by explaining the nuclear fusion process in the Sun’s core that generates the light we see. This process also emits neutrinos — ghostly particles initially conjectured solely in order to ensure that energy is conserved during certain nuclear processes, a ‘balancing of the books’. Neutrinos often seem like an afterthought — they are only briefly covered in Green’s book — and yet they caused a crisis in physics in the 1970s when detections revealed that the Sun appeared to emit just one third of the predicted number. This discrepancy between theory and observation lasted nearly 40 years and was only solved when people worked out how neutrinos could oscillate between different types, work for which last year’s Nobel Prize in physics was awarded.
One beautifully serendipitous fact — that the Sun is the same angular size as the Moon when viewed from Earth even though their actual sizes are so very different — has been put to good use during solar eclipses, allowing us to see the corona, a tenuous aura that extends out for thousands of kilometres from the normally visible edge of the Sun. (The same phenomenon also allowed another test of fundamental physics: during the solar eclipse of 1919, the bending of distant starlight by the Sun was observed as predicted by Einstein’s general theory of relativity.)
Green explains how this corona is responsible for one of the greatest mysteries still to be solved in solar physics. As light is generated in the core and moves (very slowly) to the surface it dissipates energy, so the surface is cooler than the core. So far, so expected. But bafflingly, the corona — which is further out than the surface — is also much hotter. There are two competing theories for this: it’s either due to the magnetic fields being twisted and rearranged by the variable rotation of the Sun, or due to as-yet-undetected waves transmitting energy from the interior.
The book ends with a discussion of the Sun’s influence upon our climate, with Green scorning the widely reported claim that the Little Ice Age was due to a then-cooler Sun. It’s clear that the Sun’s variability at visible wavelengths isn’t large enough to cause global temperature changes on Earth, but she summarises intriguing work linking its output at ultraviolet and X-ray energies to regional variability in winter weather. Green is persuasive in her argument that we would do well to study the Sun in order to better understand our own climate.
There is so much material packed into this book that it feels a little rushed, particularly in the earlier chapters, but there’s a vitality to Green’s writing which stems from the fact that she’s writing about her research area, and she’s able to convey her passion for her work to us.