Rebel Star: Our Quest to Solve the Great Mysteries of the Sun

Introduction

The sun is many things: beauty, beacon, battery, belligerent. It sustains us yet threatens, too. That it burns our skin from 150 million kilometres away is a hint at its raw power. It’s been worshipped as a god and feared as a demon, drives the weather and helps paint vivid aurorae across polar skies. Its magnetic mood swings flood the solar system with dangerous radiation. Birds sing to greet its daily arrival, only to flee for the safety of their nests when it departs for the night. Plants spring from the earth and unfurl their flowers in its presence. Its ancient energy is liberated from fossil fuels, as banks of solar panels attempt to usurp them by drinking in its light.

Yet for something so familiar to us, and so crucial to almost everything that happens on its third planet, the sun remains a confounding mystery. Our understanding tentatively improved when temples made way for telescopes. The last century has seen particular progress, starting with a solar observatory built in California by an astronomical visionary. The Space Age brought orbiting telescopes and since the 1990s we’ve dispatched an armada of dedicated solar space probes to surveil and scrutinize our nearest star like never before. Thanks to their work we’ve finally confirmed where all the sun’s energy comes from and how that light meanders towards the solar surface before making its way to Earth. Modern telescopes have witnessed star formation elsewhere in the universe, showing us how our own star came to be. But we have so much left to learn. We’re still largely in the dark when it comes to explaining the dizzying array of solar activity. For a century and a half we’ve watched the number of dark spots on the sun rise and fall every eleven years or so, but we still cannot forecast the timing or strength of the next cycle. Most pressing is the need to understand – and predict – the technology-crippling stellar explosions that could send us back centuries. They have the ferocity to fry satellite circuitry, throw power grids into meltdown and ground international air travel. The sun even nearly triggered a nuclear skirmish at the height of the Cold War. This threat is so serious that governments around the world now see the sun as a foe equal to earthquakes, hurricanes and terrorism. The sun could cause a trillion dollars’ worth of damage to our electrical infrastructure from which it would take months, if not years, to recover.

Astronomers are so desperate for answers that we now stand on the verge of another sea change in our understanding. Buoyed by these previous successes, the next generation of solar observatories is currently swinging into action. Giant mirrors are being hauled up the sides of enormous volcanoes to peer more closely at the solar surface. They’re powerful enough to spot a human at a distance equal to the Earth’s width. New space probes are being lofted into the solar system to swing closer to our star than ever before. In November 2018, the Parker Solar Probe broke the record for proximity to the sun. Eventually, it will inch within 6 million kilometres – far inside the orbit of Mercury – tolerating temperatures soaring above 1,000 degrees Celsius. Solar Orbiter will soon use Venus to gradually shuffle its orbit in order to climb high above the sun–Earth line to get an unprecedented peek at the solar poles – regions crucial in understanding the sun’s cyclical nature. In the decade ahead these intrepid travellers will send back more data on the sun than we’ve ever had before. There’s now so much of it that even an army of stargazers simply wouldn’t have time to trawl through it. Artificial-intelligence algorithms are being developed around the world to help us make sense of it all.

So now is the perfect time to look simultaneously backwards and forwards, returning to a time when the sun was no more than an orb in the sky, and ahead to how our understanding could be about to seismically shift. Charting our journey from monuments to mega-observatories, this book will showcase the stunning reality of life beside a stellar powerhouse, shining a light on its perplexing mysteries and exploring the feats of physics that have seen our knowledge grow. Yet this is more than just a story about a single star. It is also a very human tale; one of our ingenuity and unquenchable desire to learn more. Legions of astronomers have worked for centuries to further our understanding of the sun, with many overcoming significant personal and professional hurdles to add to what we know, such as the astronomers who escaped wars to witness eclipses, or those who smashed the patriarchy to make huge strides forward and fled political revolutions to kick-start astronomical ones.

After seeing how the sun arrived in this otherwise unremarkable corner of the Milky Way, we’ll plunge into the core to learn how our star’s energy is made, against all the odds, in a place of unimaginable hellfire. From there we’ll fly ever outwards, through the solar atmosphere, for a brief stop off at the Earth to see how astronomers are trying to tackle the threat of the space weather the sun unleashes upon us. Then it is on past the remaining planets, seeing how they too survive the solar onslaught, until we reach the edge of its influence way out beyond even Pluto, before heading into the wider Milky Way galaxy. Our odyssey will end with an account of how the sun – that great giver of life – will ultimately take it away.

But our journey starts by looking back at how our understanding evolved from the superstition of the past to the science of today.

1

From Superstition to Science

‘The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do.’

Galileo Galilei

The two hills overlooking the ancient citadel of Sacsayhuamán on the outskirts of the Peruvian city of Cusco are rammed with locals jostling for a view of the events below. Many arrive in the small hours, eager to secure a prime spot. Tourists often pay for grandstand seating in the heart of the action, with prices starting at $100. The surrounding streets are lined with crowds half a dozen people deep. It’s 24 June, and everyone has flocked here to celebrate the ancient Inca tradition of Inti Raymi – the Festival of the Sun. This venerable city – the old capital of the Inca Empire – grinds to a halt on this special day as colourful processions and celebrations replace the normal hubbub of traffic. Five hundred dancers, actors and musicians are brought in to entertain the crowds. The organizers claim it’s the largest festival in South America after the Rio Carnival.

Today’s events are a modern, pared-down reincarnation of a celebration that began in 1412 to honour the winter solstice – the shortest day of the year – in the southern hemisphere. It also marked the start of the Inca New Year. Of all their deities, the sun deity, Inti, was the most sacred to the Inca. The solstice was thought to mark his greatest distance from the Earth and the Inca did all they could to beckon him back again to illuminate and sustain them in the year ahead. The original festival lasted for nine days, with princes, priests and generals gathered in all their finery to parade and worship. The mummified remains of Inca ancestors were removed from their resting places inside nearby temples and shrines, wrapped in cloth and lofted through the streets on ornate chairs. Golden cups of chicha, an alcoholic drink often made from fermented maize, were raised in a toast to Inti. Hundreds of llamas were sacrificed in his honour at the Coricancha temple, with the surrounding streets running with rivers of their blood (streets that were deliberately built east to west to align with sunrise on the solstice). The animals’ internal organs were then picked over for omens of future events, much like the reading of tea leaves. The ritualistic killing of children was not unheard of. This mirrors a similar Aztec tradition where prisoners of war were slaughtered to ensure the sun retained its power and path across the sky. Today’s celebrations are somewhat more modest, with just a single llama offered up to the heavens, but the riot of colour remains.

The wonderful spectacle of Inti Raymi is just one example of the extent to which we humans have revered the sun throughout our history. In Hinduism, the sun is the deity Surya. She’s often depicted riding a chariot pulled by seven horses, one for each of the colours of the rainbow. The redness of sunrise and sunset is in fact Aruna, Surya’s charioteer. In Chinese mythology, the sun is the sole surviving sibling of ten brothers who once shone together in the sky. This proved too hot for the people of Earth, so the hero Hou Yi shot nine of them down with a bow and arrow. In England, the well-known Neolithic circle at Stonehenge was constructed around 2500 bce. Its giant stones are aligned with sunrise on the summer solstice and sunset on the winter solstice. Similar stone circles aligned with the sun can be found in Nabta and Karnak in Egypt. Likewise, the Temple of Ramses II at Abu Simbel was built so that the sun illuminates the face of the pharaoh’s statue on 22 February and 22 October each year – the dates of his coronation and birthday respectively. The famous pyramids at Giza are orientated almost perfectly to the points of the compass, with many researchers offering different explanations for how they managed it. Some have suggested the sun’s position on the autumn equinox helped the Egyptian engineers with the exact alignment. To the ancient Egyptians, the sun was the falcon-headed god Ra, who travelled across the sky in one of two boats from east to west each day. Every night Ra would plunge into the underworld, where the seditious serpent Apophis would try to halt his progress. The sun would rise the next day only if Ra successfully ran this gauntlet. He always did, of course.

Pyramids and serpents appear in other sun folklore around the world, too. Back in the Americas, over 4,000 kilometres north-west of Cusco, the ancient Mayan temples at Chichen Itza draw more than 2.5 million visitors a year to Mexico’s Yucatán Peninsula. At the centre of this architectural wonderland is the Temple of Kukulcan, also known as El Castillo (the Castle). It’s a step-pyramid completed between 900 and 1000 ce. Around its four sides it boasts a total of exactly 365 steps – one for each day of the year. On the spring and autumn equinoxes – when day and night are equal in length for everyone on Earth – the angle of the sun ensures that the shadows cast by the steps resemble the snake god Kukulcan descending the side of the pyramid. The spectacle lasts just forty-five minutes. Appropriately placed stone carvings of a serpent’s head help cement the illusion. It’s clear the Mayans were avid sky-watchers, with many of the site’s other temples dedicated to motions in the heavens. The nearby El Caracol structure – nicknamed the Observatory – was constructed so that the planet Venus appears through an opening every eight years.

Heliocentrism

The arrival of the Catholic Spanish conquistadors to Central and South America in the early sixteenth century put pay to much of the local sun worship. Inti Raymi was outlawed, with the last original Inca festival taking place in Cusco in 1535. It was revived in only 1944. Just three years before its abolition, another revolution was stirring back in the conquistadors’ home continent of Europe. Polish mathematician Nicolaus Copernicus had just finished his first draft of a highly controversial book that had taken him sixteen years to write. The contents were so inflammatory that he sat on it for years. On the Revolutions of the Heavenly Spheres would not see the light of day until just before Copernicus’s death in 1543. Its publication marked the beginning of our transition away from the superstition of the past and towards a modern, scientific view of the sun by suggesting it sits at the centre of the solar system.

That’s not to say humans hadn’t started to look at the sun in a critical way much earlier. Chinese astronomers were noting the presence of dark patches on the face of the sun as far back as 800 bce. They were recording these sunspots regularly by 2,000 years ago, but using them to foretell ominous events. In 807, a Benedictine monk named Adelmus spotted a similar blemish that persisted for more than a week. More sunspots appeared around the time of the death of the Holy Roman Emperor Charlemagne in 814. However, that it was also seen by some as an omen and linked to his demise in some way underscores the fact that we hadn’t yet moved away from a superstitious view of our nearest star. On Saturday, 8 December 1128, John of Worcester drew the oldest surviving sketch of a sunspot.

These observations were all made with the unaided eye, most likely when the glare of the sun was dimmed by clouds. The moon is also an occasional obscurer of the sun, during solar eclipses, and our ancestors wasted no time using them to get a closer look. The Greek philosopher Plutarch detailed one solar eclipse, noting that ‘a kind of light is visible about the rim which keeps the shadow from being profound and absolute’. On 22 December 968, there was a solar eclipse visible from what is now Istanbul in Turkey and the eighteen-year-old Leo Diaconus wrote: ‘It was possible to see the disc of the sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk.’ Both men were seeing the sun’s outermost layer – the corona – for the first time. Solar eclipses also afforded us our first look at prominences – flame-like eruptions seen on the edge of the sun. The Russian Chronicle of Novgorod describes the eclipse of 1 May 1185: ‘The sun became similar in appearance to the moon and from its horns came out somewhat like live embers.’

Despite these early observations, a persistent picture remained popular: that the sun was in motion around the Earth. It was this idea that Copernicus famously railed against and why he and his growing band of followers – known as Copernicans – often ended up in a significant amount of hot water. He argued the opposite – that the Earth is just another planet orbiting a central sun. This belief is called heliocentrism. Copernicus had no evidence that this was indeed true, but he saw it as a much simpler system than the one that had been in vogue since the days of ancient Greece. The Greek polymath Claudius Ptolemy was a passionate advocate of geocentrism – the Earth at the heart of everything, with the rest of the universe in tow. Ptolemy suggested that our planet is surrounded by a series of concentric spheres, each home to a celestial object, rotating around the Earth. Except that he encountered a problem with the other planets.

The geocentric (left) and heliocentric (right) explanations for why we often see a planet appear to change direction in the night sky

Many ancient civilizations, from the Egyptians to the Mayans at Chichen Itza, were keeping track of the planets as they wheeled across the sky. They noticed an oddity in their behaviour – often a planet would appear to stop in its tracks and begin moving back in the other direction. What could cause one of Ptolemy’s celestial spheres to suddenly grind to a halt and do an about-face? To get around this inconvenient truth he introduced the concept of an epicycle – a small circle nested on the bigger sphere. According to this picture, a planet moves around the small circle while being carried around the Earth by the larger sphere. For half of its journey around the small circle it would appear to be moving against the direction of the sphere, so that’s why we see a planet change direction. It was certainly a solution, but a far from elegant one. Centuries later, Copernicus realized you could do away with the whole messy business by simply having the Earth orbit the sun. That way, planets appear to change direction simply because we overtake them (or they overtake us) on our respective journeys around the sun.

While Copernicus often gets the lion’s share of the credit for this insight, others had mooted similar ideas much earlier. Aristarchus of Samos, an ancient Greek living five centuries before Ptolemy, had set out his own heliocentric solar system. These ideas failed to break through, however, because of the accuracy with which Ptolemy’s geocentric picture could predict the motion of the planets. It also suited the prevailing, largely Christian, doctrine in the West whose creation stories had Earth manufactured by God as a home for humanity. Muslim astronomers were starting to find fault with geocentrism by the tenth and eleventh centuries. The Arab astronomer Ibn al-Haytham wrote: ‘Ptolemy assumed an arrangement that cannot exist, and the fact that this arrangement produces in his imagination the motions that belong to the planets does not free him from the error.’ Harsh, but true. A truth that would become all too apparent with the invention of one of the most revolutionary pieces of technology in all of human history: the telescope. Arguably, no other device has so single-handedly changed the way we think about ourselves. No longer would we be betrayed by what our eyes alone can see.

Discovering sunspots

The first telescopic observations came thick and fast at the start of the seventeenth century. In Italy, the astronomer Galileo quickly found moons whizzing around Jupiter and vast mountains on the moon. Venus revealed itself to have phases just like the moon, a fact that put the first nail in the coffin of geocentrism. For the light Venus reflects towards Earth to be waxing and waning we must both be orbiting a central sun. If, as Ptolemy would have it, Venus lay between us and the sun, then most sunlight would fall on the back of the planet. We’d never see it fully illuminated. That we do proves that Venus ventures beyond the sun due to its quicker journey around it.

Word of Galileo’s work soon reached Germany, where a young Jesuit priest named Christoph Scheiner rushed to acquire himself a telescope. After looking at Jupiter and Venus for himself, Scheiner’s attention quickly turned to the sun, where he observed sunspots by using a telescope to safely project a solar image. Galileo had been doing the same, and soon a fierce row erupted between the pair about who had seen them first. Letters were fired back and forth across country borders as the international row escalated. One of their disagreements was on the nature of the spots themselves. Scheiner, being of a religious persuasion, subscribed to the scriptures’ view of heavenly bodies as pristine and perfect. So to him these dark blotches must be shadows cast on to the sun’s disc by orbiting objects. Galileo countered that they are part of the sun’s surface and appeared to change shape. Scheiner’s beliefs also made him a geocentrist, not a Copernican like Galileo. That Galileo would eventually be proved right on both counts is a lesson in learning with your eyes and not seeing what you want to see.

Scheiner soon became an advisor to Archduke Leopold V, the younger brother of the Holy Roman Emperor Ferdinand II. Leopold was also corresponding with Galileo, and Scheiner explicitly asked him not to tell the Italian about his own work on sunspots. Eventually, that work turned into a series of four books entitled Rosa Ursina sive Sol, which for centuries remained the pre-eminent texts on sunspots. The first book is used largely to bash Galileo. By the fourth book Scheiner was using the passage of sunspots across the solar disc to estimate that the sun rotates approximately once every twenty-seven days – a fact that modern astronomers agree with.

One of Scheiner’s sunspot sketches from the pages of Rosa Ursina sive Sol showing solar activity during April and May 1625

Much of Scheiner’s books were written during his time in Rome, where he’d been summoned to spend nearly a decade overseeing the construction of a new religious college between 1624 and 1633. The end of his sojourn in the Italian capital coincided with Galileo standing trial in 1633 for his allegedly heretic views on the Earth orbiting the sun. Two fierce rivals in the same city, one of them a priest with connections to the Church. Did Scheiner stick the boot in? History serves only to record that the notes of the trial contain a small memorandum stating that the German had opposed the Copernican viewpoint that Galileo was found guilty of spreading. The Italian would spend the rest of his life under house arrest and was not formally pardoned by the Church until 1992 – all for standing up for what was becoming increasingly obvious to those willing to back what their eyes were telling them. It wasn’t the first time an Italian had endured the wrath of the Church for their astronomical assertions. A few decades earlier, the Dominican friar Giordano Bruno had been stripped naked, hung upside down and burned at the stake in the Campo de’ Fiori in Rome. Among his perceived crimes was extending the Copernican viewpoint to suggest that all the stars in the night sky are simply faraway suns, perhaps with their own planets to boot. A refusal to recant this and other heretical beliefs cost him his life. Galileo was lucky to escape a similar fate, which he avoided by means of connections within the Church, deft political manoeuvring and even forging letters to cover his tracks.

Galileo and Scheiner needn’t have bickered. The benefit of hindsight shows us that neither astronomer was the first to see sunspots through a telescope. That honour fell to German father-and-son duo David and Johannes Fabricius. Johannes had studied in the Netherlands, where the telescope had been invented by a Dutch spectacle-maker. He returned home with his own instrument and set about observing the sun with his father. In 1611, Johannes produced a little-read, twenty-two-page pamphlet with the first published details of sunspots as seen through a telescope. Yet even their family ties could not prevent them from arguing about these blemishes. David wrote in a letter to a colleague that he did not believe the spots were a part of the sun. His son was of the opposite opinion. Both men were ultimately united in untimely deaths. First Johannes died in 1616 aged just twenty-nine; the cause of his premature end is unknown. David died the following year when his head was caved in with a shovel following an argument with a peasant he’d accused of stealing a goose.

Meanwhile, in England, mathematician and astronomer Thomas Harriot had also been observing the heavens. In the late sixteenth century he’d sailed with famed Elizabethan explorer Sir Walter Raleigh as an advisor on astronomical navigation. It was likely during this time spent in the loftiest circles of English society that he came across the first English translation of Copernicus’s ideas. During one voyage to North America he observed a solar eclipse from the deck of his ship and it ignited a lifelong fascination with the sun. By the time the telescope made it to England in the early seventeenth century, Harriot used it vociferously. He made over 200 observations