The Sun

Leon Golub, Jay M. Pasachoff

• Publisher: Reaktion Books
• Year: 2017
• ISBN:
  9781780238029
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

It's a good story: we are made of matter like that we also find in the stars. Essential to our planet's existence, the Sun—our nearest star––is also the most fascinating object humans have ever adored, literally the difference between day and night. But getting beyond these basic perceptions requires scientific understanding. What, for instance, is the sun made of? Why does it burn so brightly? How long will it last? This book not only answers these questions but also tells the story of how we came to know—not merely behold—the grandest entity in our sky.Leon Golub and Jay M. Pasachoff offer an engaging and informative account of solar science and its history, drawing on centuries of study by solar astronomers who have looked to the Sun not only to learn about our own solar system but also about what lies in the distant wilderness of faintly glimmering stars. They skim along the surface of the Sun, which is decorated with sunspots, discussing these fascinating magnetic aberrations and the roughly eleven-year cycles they abide. They follow seismic waves into the interior of the Sun and its unending nuclear fusion. They show us what is unveiled in solar eclipses and what new views and knowledge our space exploration has afforded us. They brave solar weather, and they trace the arcs of radiation and particles whose effects we can see on earth in phenomena such as the northern and southern lights.Glowing with a wide assortment of astonishing images, this beautifully illustrated guide will delight everyone, from those who know what a coronagraph is to those who simply like to step out on a bright day, close their eyes, and feel the Sun's warmth upon their skin.

ONE

SUNSPOTS

Normally the Sun is too bright to look at directly without damaging the eyes, but when its brightness is diminished to a safe level by sunset haze or thin clouds, or if a special dark filter is used, we are able to see the round, yellowish circle of glowing light. At a quick glance it looks perfectly blank, but sometimes – for those with good vision – a small black dot or two can be seen marring the disc. With practice, groups of these dots can sometimes be seen. Repeated observation from one day to the next can show that the spots move across the disc, or grow or fade away from one day to the next. The obvious name to give these features is sunspots (illus. 2).¹

Recent archaeological investigations, primarily of inscriptions on ancient bones, indicate that observations of the Sun were important in China dating back to at least the Shang dynasty (c. 1500–1050 BCE). Systematic recording of sunspots is known to have begun during the Han dynasty (from 206 BCE), although the reason for this early interest is not known and there are far fewer records than there ought to be if spots were the main item of interest. But many of the descriptions are clear and allow us to know that the Sun had spots back then, as it does now, so we know that sunspots are a normal, enduring feature of the visible Sun.

In the West, there don’t seem to have been many sunspot records. Perhaps the ancient belief that the Sun was perfect and spotless discouraged any reporting of imperfections. But in the early 1600s Galileo changed things when he began using the recently invented telescope to observe celestial objects, including the Sun. He made careful, repeated observations (which may have contributed to his blindness) and drawings of sunspots as they grew, decayed and moved across the face of the Sun. From careful study of these sequences of drawings Galileo argued, from measurements of the path they follow across the disc, that sunspots are on the surface of the Sun and therefore are actual solar features. Fierce debates followed, arguing over whether the blemishes were indeed on the Sun, or passing in front of the Sun, or a type of cloud between us and the Sun.

3 As we shall see, Galileo and several competitors were the first to see and record details of sunspots. This drawing is from Galileo’s sunspot book of 1613, which shows a daily series of sunspots that reveals the Sun’s rotation.

Soon after these initial telescopic observations something strange happened: the spots vanished. For decades, from 1645 to 1715 and pretty much coinciding with the reign of Louis XIV (‘le Roi Soleil’ – the Sun King) in France, the Sun became nearly spotless. The absence of spots was well known at the time: the meticulous observer Johannes Hevelius noted in 1668 that ‘for a good many years now, ten or more, I am certain that absolutely nothing of great significance (apart from some rather unimportant and small spots) has been observed.’ The period of absent spots also coincides approximately with the deepest part of an extended spell of cold weather in Europe known as the Little Ice Age, leading to speculation that the absence of spots caused the Earth to cool somewhat. But the timing doesn’t quite work out for that, since the cooling seems to have begun about 1550 and lasted until 1850, and indirect evidence from glaciations, snowfall and ice core records provides only slight support to the idea that the rest of the world cooled the way Europe did. So the strength of the connection between sunspots and climate is still uncertain and is problematic at best.

But then in the eighteenth century the spots returned and they have appeared regularly on the Sun ever since. They do come and go in cyclical fashion: there are years when the Sun is peppered with large numbers of spots and other years when there are hardly any. The change from the low ‘solar minimum’ state to the high ‘maximum’ state takes a few years, and overall the time from one maximum to the next is about eleven years, give or take a few from one cycle to another. This is a surprisingly short time for a star to show such obvious changes, when we consider that the Sun is otherwise steady, needing many millions of years for any significant changes to happen.

The sunspot cycle is one of the big mysteries about the Sun that we are struggling to understand. For now, we ask a more basic question: what are these spots? The answer to that question took a few hundred years to discover and turned out to be surprising.

Sir William Herschel was born Wilhelm Friedrich Herschel in Hannover in 1738, the son of an oboist in the Hannover Military Band (he later himself became a first-rate musician). In 1757, at the age of nineteen, he was sent to England, where his musical activities continued. It was not until the 1770s that he and his sister Caroline (whom he had convinced to join him in England as a singer at his concerts) actively turned to astronomical investigations. With Caroline’s assistance, William discovered the planet Uranus in 1781, the first new planet found in over two thousand years. He named it Georgium Sidus – the Georgian planet – after King George III and both received lifetime pensions in return (£200 a year for William, £50 for Caroline), allowing them to devote themselves full-time to astronomy. Among his solar studies, William attempted to correlate the price of wheat in London with the variations in sunspot number, and he also discovered that the Sun emits vast amounts of radiation beyond the red end of the visible spectrum (now called ‘infrared’).² He proposed that sunspots are holes in the Sun through which one can see the dark interior, rather like the pupil of your eye. This followed a finding by the Scottish astronomer Alexander Wilson, Chair of Practical Astronomy at the University of Glasgow from 1760, who noticed that the dark spots appear to be slightly depressed below the visible surface of the Sun when they are viewed near the edge of the solar disc. It would be many years before the field of thermodynamics developed enough to show that Herschel’s theory was not tenable, and the true nature of sunspots remained a mystery into the twentieth century.

The Modern Age of Sunspot Studies

A most significant step in understanding sunspots was taken by the energetic American scientist George Ellery Hale at the end of the nineteenth century and into the early twentieth century. Hale was born in Chicago in 1868 into an engineering family (his father manufactured elevators, including the one in the Eiffel Tower) and studied at MIT. As a student there in 1889 he developed a new type of instrument for observing the Sun, the spectroheliograph, which is to this day one of the major tools for solar observation. Indeed, a version of this instrument was used to produce the sunspot image shown in illus. 4. Back in Chicago a few years later, he oversaw construction of the Yerkes Observatory, which included the largest refracting telescope (using lenses rather than mirrors) ever built. With support from Andrew Carnegie, Hale went on to build the Mount Wilson Solar Observatory in California, noting that ‘The prime object of the Solar Observatory is to apply new instruments and methods of research in a study of the physical elements of the problem of stellar evolution.’ To this end, he constructed laboratory facilities at his observatories to carry out experiments that helped to explain the astronomical observations, a field now known as laboratory astrophysics.

4 Spectroheliogram image of the Sun taken on New Year’s Day, 1 January 2013, near the maximum of the solar activity cycle. This image uses a method similar to that pioneered by George Ellery Hale, a very narrow waveband centred on the strong emission line of hydrogen at 656.3 nm wavelength. Only a few small, isolated sunspots are visible, indicating that this maximum was weak compared to those of the past fifty years.

After developing Mount Wilson, Hale went on to plan the Palomar Observatory, which was for many years the largest astronomical telescope in the world. But Hale did more than build observatories. As a young professor at the University of Chicago in 1895 he founded the American Astronomical Society, the leading professional organization in the U.S. for astronomers, and the Astrophysical Journal, which became and remains one of the premier professional journals in the world for publishing research in astrophysics. In 1904 he organized an international scientific group that later became the International Astronomical Union, arguably the leading professional organization for astronomers in the world. In 1907 he joined the board of the Throop Institute in Pasadena and led the effort that transformed it into the California Institute of Technology. In 1916 he led the founding of the National Research Council, the working arm of the U.S. National Academy of Sciences that carries out most of its scientific studies.³

Hale had the idea of trying to determine whether or not sunspots are magnetized. Of course, the fact that he made the measurement at all implies that he thought the spots might be magnetic features. He thought the fields were probably generated by electric currents, in what he described in a 1908 issue of the Astrophysical Journal as ‘vortices’ around sunspots (illus. 5), although his description seems to indicate that he saw the spots as being similar to terrestrial cyclones: ‘It seems evident, on mere inspection of these photographs, that sun-spots are centres of attraction, drawing towards them the hydrogen of the solar atmosphere.’

Whatever his motivation may have been, Hale decided to apply a recently discovered technique to measure magnetic fields merely by study of the light coming from a distant object. In 1896 the Dutch scientist Pieter Zeeman (Nobel Prize in Physics, 1902) had announced the discovery of a method for measuring magnetic fields in a hot gas by careful analysis of the light emitted by the gas. Zeeman showed that the energy levels in the emitting gas atoms would be slightly shifted due to the presence of a magnetic field, thus slightly altering the wavelength of the emitted light. He concluded his paper by suggesting that this method would be useful in astrophysics. Hale picked up on this idea by applying it to sunspots, and he struck gold.

5 Hale saw swirls and vortices around sunspots, convincing him that they were magnetic features. Referring to the spot shown here, he wrote: ‘the clearly defined whirls point to the existence of cyclonic storms or vortices.’ Note that the vertical stripes in the image are not present on the Sun, but are rather an artefact of the method used to create the spectroheliograms.

6 Top: The measurement that convinced Hale of the magnetic nature of sunspots is shown here, where the special pattern of wavelength splitting predicted by Zeeman is observed in the light coming from the sunspot. The vertical line on the left is the slit of the spectrograph, through which the light from the Sun passes on its way to the film. The image on the right shows the spectrum, the light that came through the slit having been spread out in wavelength. At the place where the slit crosses the sunspot, the image on the right shows the spectral line split into three components by the strong magnetic field. The horizontal lines are due to the construction of the filter used to make the measurements. Bottom: A modern version of the same measurement, from the McMath-Pierce Solar Facility on Kitt Peak, Arizona.

What did Hale see that he found to be such convincing evidence of a strong magnetic field? Illustration 6 shows an example, from Hale’s 1919 paper, of the so-called Zeeman Effect in a sunspot. A telescope focuses an image of the Sun onto a shiny plate that has a slit cut into it, and the slit is positioned across the location of a sunspot. This is shown in the left half of the figure, where the dark vertical strip is the slit through which some of the incoming light can pass, including light from the darker sunspot located near the centre of the image. The light coming through the slit is then analysed by spreading out the different wavelengths in the light, using a specially chosen wavelength that is sensitive to the presence of a magnetic field. The result is shown in the right half of the figure, where the wavelength-spreading is in the left–right direction. It is easy to see just by looking at it that the light is doing something strange in the area of the sunspot. Because the magnetic field causes shifted energy levels in the atoms of the solar gas, the wavelengths of the light emitted by the atoms are split into several components, exactly as described by Zeeman in his laboratory experiments. This splitting of the wavelengths, as well as other detailed properties of the light (the polarization, for instance, at different viewing angles), shows that the light coming from the spot is being produced in a place that has a strong magnetic field, several thousand times stronger than the average magnetic field of the Earth, for instance. Zeeman himself wrote a commentary on Hale’s article: ‘Professor Hale has given what appears to be decisive evidence that sunspots have strong magnetic fields, the direction of these fields being mainly perpendicular to the sun’s surface.’

So sunspots are indeed places where a very strong magnetic field pokes through the surface of the Sun. This single fact is at the heart of the sunspot phenomenon. Since most people find it difficult to visualize what a magnetic field is and what effects it produces, we need to digress a bit and talk about magnetic fields.

De Magnete

The year 1600 was an eventful one in Europe, starting with Giordano Bruno being burned at the stake for heresy and ending with the formation of the British East India Company, which became a major foundation of the expanding British Empire. The climate in Europe turned cold and damp, leading to an overpopulation of rats and the consequent spread of the plague. And a London physician named William Gilbert (whose patients included Elizabeth I) published a book, called De Magnete, that discussed magnetic bodies and claimed that the Earth itself is a giant magnet. Gilbert died in 1603, apparently from an outbreak of the plague.

Between the Black Sea, the Aegean and the Mediterranean lies Anatolia, or Asia Minor – now largely the modern Republic of Turkey – a crucial cultural crossroad between Asia and Europe going back to...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• CONTENTS
• PREFACE
• 1 SUNSPOTS
• 2 LOOKING INSIDE THE SUN
• 3 A SOLAR PULSE
• 4 A SPECTRUM AND WHAT IT TELLS US
• 5 THE SOLAR CHROMOSPHERE AND PROMINENCES
• 6 THE VISIBLE CORONA
• 7 THE INVISIBLE CORONA: A DISCUSSION MOSTLY ABOUT PHOTONS
• 8 STORMS FROM THE SUN: A DISCUSSION MOSTLY ABOUT PARTICLES AND FIELDS
• EPILOGUE: THE HELIOSPHERE
• APPENDIX I: OBSERVING THE SUN SAFELY
• APPENDIX II: AMATEUR SOLAR OBSERVING
• APPENDIX III: OBSERVING THE CORONA AT ECLIPSE
• APPENDIX IV: OBSERVING THE SUN FROM SPACE
• ENDNOTES
• FURTHER READING
• ACKNOWLEDGEMENTS
• PHOTO ACKNOWLEDGEMENTS
• INDEX