by Lucie Green
The simplicity of hydrogen, which has only one electron in orbit around one proton, means that the electron is not able to provide a good shield to the positive charge of the nucleus. Orphaned electrons in the vicinity feel this weak electrostatic charge and occasionally are captured to give a neutral hydrogen atom an extra electron. The two electrons provide two units of negative electric charge and this outbalances the single unit of positive charge in the nucleus, and so we have the negative hydrogen ion.
That loose bonus electron rattling around is the secret of the negative hydrogen’s continuous spectrum. The negative hydrogen ion is able to lose its extra electron through the absorption of a photon across a wide range of energies or wavelengths in the ultraviolet, visible and infrared ranges. The key is that instead of having to bump the electron into a new orbital, it knocks it off completely. This is a much more flexible process. When a new bonus electron is captured by a hydrogen atom, the process is reversed, leading to the production of a continuous spectrum of light across these wavelengths.
The negative hydrogen ion is responsible for 95 per cent of the light emitted by the Sun. It is easy for the hydrogen atom to gain or lose an extra electron. And when a photon is emitted it can be at a range of frequencies across the ultraviolet, visible and infrared parts of the spectrum. And this matches exactly what we observe.
You may find it bewildering to know that there is only one negative hydrogen ion to every one hundred million hydrogen atoms in the photosphere. Yet this is enough to control the flow of the light. That’s like saying in a country the size of the US with 300 million people only three of them control the entire flow of money through the economy. The Sun seems to be driven by some extremely unlikely processes: from the creation of light itself, which relies on two protons fusing to start the chain – something that only happens once in a hundred million collisions – to the release of that energy at the photosphere, which relies on a particle that is one in a hundred million. But, of course, the sheer scale of the Sun makes these things possible.
We did not understand the role of the negative hydrogen ion in facilitating the escape of photons from the photosphere until 1938, just one year before Bethe’s work on explaining the rate at which nuclear processes are happening to generate the Sun’s energy. The negative hydrogen ion was deemed theoretically able to exist in the 1920s, and at that time the energies involved in a hydrogen atom capturing and losing an extra electron were being investigated. Progress was slow, though, and the early results were inconclusive and sometimes even contradictory. It took the new field of quantum mechanics to provide a way to solve these uncertainties – again, the work of people like Schrödinger and his theory of describing particles as waves provided a mechanism by which to investigate them, and in 1929 and 1930 it was shown that a negative hydrogen ion could be formed and that it could be stable.
With the theoretical foundations laid, it was Rupert Wildt, at Princeton University Observatory, who suggested that the negative hydrogen ion might be responsible for controlling the flow of photons through the photosphere and hence the sunlight we receive. The founder of space science at University College London (UCL), Sir Harrie Massey, was also involved in this work. His contribution on the details of the absorption of photons by the negative hydrogen ion was key for gaining acceptance of the role of this particle in governing how light travels through the photosphere of the Sun, and, indeed, other similar stars. The Sun’s visible output was finally understood. The negative hydrogen ion is the particle that absorbs and emits photons in the photosphere, despite there being so very few of them.
What we now call the ‘standard model’ of the Sun was completed in 1957, after almost 100 years of development from the time of the early work of Lane. Which is not to say the story is over. The standard model of 1957 was lacking in one major aspect: it had no way to explain the presence of sunspots – dark ominous regions that regularly appear on the surface of the Sun.
5. Sunspots
As the Sun began to rise over London on 8 December 1610, there was frost on the ground and a thick mist was in the air. That morning a British astronomer and mathematician, Thomas Harriot, was poised and ready to do something that no one had done before: look at the Sun through a telescope. The telescope had been invented in Holland and was publicly known about by 1608. But the telescope had been used only for distant terrestrial objects, useful for the Dutch, no doubt, who were at war with Spain at that time.
Harriot’s problem was that the light from the Sun is too bright to look at directly at the best of times – using a telescope to gather even more sunlight was going to cause serious damage to anyone who put an eye up to the telescope. His solution was delightfully low-tech. But dangerously so. He waited for a very misty morning and looked at the Sun first thing, when it was just a few degrees above the horizon, so its light shone through as much mist as possible. He was using the mist as a kind of filter to dim the Sun’s light. This did reduce its brightness a lot, but not enough to be able to stare at it through the telescope. So Harriot flipped frequently between his left eye and his right so the light couldn’t dazzle, and cause too much damage to, either one of them. This method is not recommended!
We still have the notes he made about what he saw, and accompanying his writing is a drawing of how the Sun appeared to him that morning. It shows that on the circular disc of the Sun were three black spots: sunspots. We now know that the Sun is frequently covered in these black spots of changing size and position. In fact, observations across the centuries after Harriot have made us realize that sunspots are invaluable in our study of the Sun. They have a central importance because they allow us to scratch the surface of our star and reveal that it is not a perfect luminous orb and that it changes over time – its appearance varies.
In fact, who it was that first viewed the Sun with a telescope and saw sunspots is still debated. Galileo is the name that normally springs to mind but it seems he did not make any record of sunspots until April 1612 – two years after Harriot’s drawing. Galileo’s story is remembered because of the vast and important range of astronomical observations that he made but also because his story is so captivating. His ideas about the planets orbiting around the Sun, in a Sun-centred Universe, were at direct odds with the Roman Catholic Church, which still held the Aristotelian view that placed the Earth at the centre. The Church also proclaimed that the Sun was perfect, immaculate and, therefore, blemish-free. It was a discovery counter to doctrine and this culminated in Galileo’s trial in 1633 by the Roman Inquisition, who found him to support the idea of a Sun-centred Universe and ordered that he remain under house arrest for the rest of his life.
Slightly ahead of Galileo was Johannes Fabricius, in Germany, who first observed the spots in 1611 and made sure he told everyone about them, publishing what he saw in a pamphlet. Thomas Harriot in England, who drew spots in December 1610, would have predated Fabricius but Harriot never published his work. Instead, his manuscripts remained hidden for many years after his death in 1621, only coming to light in the late 1700s. Some people (myself included!) now consider him the first true observer of sunspots through a telescope, while others still back Fabricius. Interestingly though, Harriot’s 8 December 1610 diagram clearly shows sunspots, but he does not mention them directly in his text – perhaps because he did not have the words to describe what he saw or because he couldn’t realize their significance at the time he made that first drawing.
Whoever the first observer was, we know for sure that a German called Christoph Scheiner was the first person to take sunspots seriously. Starting in 1611, Scheiner made a comprehensive and dedicated survey of sunspots over the
following years, publishing them in the pioneering tome Rosa Ursina, sive Sol in 1630 (confusingly, much of his work was published under a pseudonym). This was a significant record of his careful study of sunspots’ shapes, lifetimes and motions that became the definitive work in this area for over 100 years. Scheiner also contributed to the development of telescopes to view the Sun, using coloured lenses instead of clear ones so that the Sun’s brightness could be diminished. He called this modified instrument a ‘heliotropii telioscopici’, which is normally translated as a ‘helioscope’. This method is the recommended one! Today many people enjoy back-garden solar astronomy by putting a specialist solar filter (much darker than sunglasses or even welding masks) over their telescope before pointing it at the Sun.
There are some observations of sunspots which predate telescopes. Naked-eye observations, although very dangerous, can be made when sunlight is diminished by fog or cloud. Given the curiosity of humans, with no other way to do it safely, people have been eyeballing the Sun for millennia. Evidence that the Sun was looked at directly can be found in records in Korea, Japan, Vietnam and, most famously, Ancient China which stretch at least as far back as 165 BC. The persistence of Chinese astronomers is to be admired as the records show that on average only one sunspot was seen every decade. What the ancients saw, though, gave crucial clues about the appearance of the photosphere and their records provide a valuable dataset that has allowed us to investigate how the Sun changes over time and is still used today, over 2000 years later.
The Ancient Greeks also saw spots. Theophrastus of Athens, a student of Aristotle, reported them in his work De Signis Tempestatum and discussed the occurrence of sunspots in relation to the weather, even using them to make weather forecasts – one of his less successful ventures. But he made up for it with pivotal works on biology, ethics and the most definitive book on stones ever written.
With the power of hindsight it even seems that there were some, albeit rare, European records of observations of sunspots before the development of the telescope. In 1128 an English monk, John of Worcester, made a drawing of the Sun with two black spheres against its disc, and in 1590 Henry Hudson was sailing on the ship Richard of Arundel just off the coast of West Africa, when he recorded spots on the Sun as he looked at it at sunset. But the spots weren’t recognized at the time as being sunspots or thought to be important. Even Johannes Kepler, whose work was central to the development of European astronomy, saw a black spot in 1607 when he projected an image of the Sun in Prague. Missing an opportunity to present the discovery of a new feature, he misinterpreted the black spot as being the planet Mercury transiting in front of the Sun.
The sunspots viewed through telescopes began to show certain consistencies which revealed information about the Sun that could not be known before. Galileo had mainly been using a projection technique to make fantastic observations of the Sun. Instead of looking down his telescope at it, Galileo let the light flood out of the eyepiece and project the image onto a piece of paper (much safer than projecting it on your retina!). A student of his had discovered the method and it meant that the Sun could be observed even at midday when it is at its highest point and the sky is more likely to be clear. And smaller spots could be made out using this technique than by looking directly through the telescope.
Galileo’s drawings showed that the sunspots consistently moved across the disc of the Sun from left to right. And when the spots were at the centre of the Sun they looked round, like circular plates viewed from above, but near the edge of the Sun the spots narrowed, like plates looked at side on, an effect known as ‘foreshortening’. This was the first conclusive evidence that the Sun was a sphere and that it was spinning. But it also showed that the sunspots were a feature of the solar surface, and not objects between the Sun and us. But what were they?
THE SHAPE OF THE SPOTS
Scientists made much progress in describing what sunspots looked like before they had any accurate interpretations of what those observations actually represented. Scheiner had shown that sunspots have a central black, round region that we call the ‘umbra’, which is surrounded by a slightly lighter region, named the ‘penumbra’. Sunspots formed by gradually increasing in size and decayed by reducing in size and fragmenting into smaller spots. And once a sunspot decayed, the area it had previously occupied looked exactly the same as the surrounding solar surface – as if nothing had ever been there. The spots tended to appear in two horizontal bands roughly 30–40 degrees above and below the equator and, most confusingly, bar a few lone cases, they always seemed to hang out in pairs.
The first insight into what sunspots were came in 1769 when an extremely large spot appeared and started to make its way across the Sun. Just as it does today, the appearance of the large sunspot sparked interest and excitement. Such sunspots are relatively rare – I will still get a flurry of emails from friends and colleagues when a new mammoth sunspot appears to make sure I don’t miss it. Knowledge of the extraordinary 1769 sunspot spread amongst astronomers and quickly reached the chair of astronomy at Glasgow University, Alexander Wilson, after a friend in London notified him. As Professor of Practical Astronomy, Wilson was keen to see the spot for himself and monitor it for any change in size.
On 22 November 1769 he turned his telescope to the spot and viewed it, magnified 112 times. It must have looked magnificent. When he first saw the sunspot the penumbra was equally broad all the way around the umbra. What he saw during his observations on subsequent days intrigued him. As the sunspot moved away from the centre of the Sun and towards the edge, Wilson saw that the penumbra on the side of the spot furthest from the edge of the Sun had contracted in width and was narrower than the penumbra on the other side of the spot. This observation was remarkable because it was exactly the opposite of the foreshortening that Wilson expected: the penumbra on the side closest to the Sun’s edge should contract the most because this side of the spot turns away from us first.
Wilson suspected that this sunspot represented a physical depression in the surface of the Sun. Sunspots might be sun-holes. He theorized that the umbra was the bottom of a deep excavation and the penumbra the downward-sloping sides – more like a bowl than a plate on the solar surface. Looking into this depression meant that you were able to see the sloping side closest to the edge of the Sun because it turned to face you. In contrast, the side furthest from the edge of the Sun becomes hidden from view. This was an elegantly simple interpretation of the observation and Wilson wanted to test it to see if it held up at other times and when sunspots were close to the other edge of the Sun. So he waited, hoping that the large spot would live long enough to survive its two-week journey around the far side and make another appearance on the face of the Sun. And on 11 December the spot returned.
5.1 The uneven contraction of the penumbra on the sides of sunspots when they are near to the limb led to sunspots being interpreted as depressions in the photosphere.
As it peaked around the left side of the Sun only three sides of the penumbra could be seen around the sunspot’s dark centre. The now centre-ward side of the penumbra, which had been visible two weeks earlier, was missing. The next day the missing penumbra had appeared but it was very narrow. These observations were exactly as Wilson had expected, based on his idea that sunspots are depressions in the surface. After 160 years of telescope observations a testable theory about the nature of sunspots had finally been proposed, tested and backed up by further observations.
Wilson then went on to estimate the depth of the umbra using his measurement of the breadth of the penumbra at full extent, the mathematics of triangles and the radius of th
e Sun. He proposed that they were at least 6000 kilometres deep, just a small notch compared to the radius of the Sun though, leading him to conclude that the Sun was a solid, dark sphere surrounded by a thin luminous substance – the idea being that sunspots are regions where you can see through the luminous layer to the sphere within. This view of the construction of the Sun prevailed for many decades, with a similar idea being proposed by Herschel almost three decades later. It was a while before Lane and Lord Kelvin were able to suggest otherwise.
Today, Wilson’s depression is understood as being a depression in the visible photosphere of the Sun. For some reason, in a sunspot the photons are able to escape the plasma sooner than in other regions. The plasma must be cooling down more in sunspot regions than it does elsewhere. The relative darkness of sunspots indicates that they contain plasma that is around 2000 Kelvin cooler than the surrounding photosphere and the transparency of this relatively cool plasma, controlled by the negative hydrogen ion, is heavily affected by its temperature. In the sunspot, the plasma is more transparent and we can see further into the Sun – hence the apparent depression.
THE MAGNETIC SUN
Understanding what caused sunspots fell to an American astronomer, George Hale, who was born into a wealthy family in Chicago in 1868. He grew up in an era when the developments that underpin modern science were being born and he was an insatiable scientist himself. Even as a young man his relentless drive led him to set up his own private observatory, funded by his wealthy father. The observatory was well equipped and was a first-rate facility in which Hale could do astrophysical research. Hale had a passion not only for science, but also for engineering. He could design and build the instruments that he needed for his investigations.
