Our Sun is a fairly ordinary star, a bit brighter than most but not exceptionally so. There are many stars much bigger and brighter, while most stars are smaller and fainter. The Sun is not an especially variable or active star, and it has no enormous chemical or magnetic peculiarities. It is not a very young star, nor is it old and nearing the end of its life. It is, in short, truly exceptional in only one way: it is very close to the Earth – in fact, at just the right distance to make life as we know it possible.
Most of us do not worship the Sun as did many in ancient civilizations, but we certainly should not take for granted the light and heat that it provides. Left to itself, the Earth would be a fantastically frigid rock at near absolute-zero temperature. If the Sun had been slightly more massive, its high temperature would have made the Earth’s surface hot enough to melt lead. A smaller Sun would have left the Earth unbearably cold and possibly subject to high levels of radiation, since smaller stars tend to have higher levels of activity, giving off devastating ultraviolet and x-rays. Distance also matters. Had the Earth been closer, we might be as infernally hot as Venus; farther away and we might have been as cold and arid as Mars. We are in the position of Goldilocks, living at just the right distance from a just-right star.
Left to itself, the Earth would be a fantastically frigid rock at near absolute-zero temperature. If the Sun had been slightly more massive, its high temperature would have made the Earth’s surface hot enough to melt lead.
Does this mean that the planet Earth is unique and that we live in a providential “best of all possible worlds?” There are dangers with this way of thinking, flattering as it is to human sensibilities, because it may foster a certain complacency, a feeling that things could not be otherwise. Since indeed other planets in our Solar System do not so far appear to support life, this implies that life requires some fairly unlikely conditions in order to flourish. On the other hand, granted that the probability of finding Earth–like conditions is small, the number of planets in the Universe is very large (probably billions in our Galaxy alone). This obviously increases the statistical likelihood of habitable planets. On this view, the earth is not so much providentially unique as merely rare.
This in turn implies certain responsibilities for its inhabitants. Since life as we know it appears to be possible within only a narrow range of conditions, it would be prudent to know as much as we can about the star that provides the bedrock conditions on which our existence is founded. Moreover our new-found ability to alter the Earth’s state on a global scale brings this need into sharp focus. For example, it is not enough for the Earth to be at the right distance from the Sun, and reflect back the right percentage of the solar light it receives. The Earth’s atmosphere is also of major importance in determining the global temperature. Without it, the Earth would be colder by about 33◦C (roughly 60◦F), and therefore a frozen lump of ice. Right now, we are making small but significant changes to the composition of our atmosphere that are beginning to be large enough to produce major unpleasant effects. Do the natural variations in the Sun’s brightness enhance or diminish these man-made effects? How do changes in solar activity affect the formation of ozone and atmospheric circulation and weather patterns?
This book explores the Sun in a comprehensive way for the non-scientific reader who wants to gain a general idea of the range and significance of solar physics. We explain what is known about the Sun and how this knowledge is acquired, discuss the origin of the Sun’s light and heat, and explore how the Sun evolved and what it will become. We pay special attention to cutting-edge research on the Sun’s outer atmosphere – the part that we can see – and the effects of this atmosphere on the Earth and the space around Earth. Unlike other stars, which are mere points in the sky, the Sun is so close that we can see its surface. We see sunspots form and gigantic explosive events erupt out toward the Earth. Thanks to careful measurements of the Sun’s surface motions, we have recently even learned to “see” inside the Sun.
Read the full excerpt here.