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The next time you have a clear night, with no clouds spoiling the view, stand with your eyes closed for a few minutes by the door to outside. Before you step out and look up, give your eyes time to adjust to the darkness. Even young children notice how when you first turn the bedside light off before sleep, the room plunges into pitch blackness. But wake in the middle of the night and you can see shapes and features again in even the lowest of lights.
So if you want to truly be awed by the night sky, let your eyes take a break from the bright lights of home first. Let your night vision develop and you won’t be disappointed. Only once your eyes are primed and ready can you then step outside and change your perspective on the world. Instead of looking down, or out, look up and watch thousands of stars burst into view. The longer you stand in the darkness, the better your night vision will be and the more stars will pepper the sky with tiny pinpricks of light.
As you gaze skyward, you might spot things you recognise, such as shapes in the patterns of stars that we call constellations, like Orion or the Plough.4 Then there’ll be things that aren’t familiar. But by just gazing at the sky and noting the brightness or perhaps the position of a star, you join an incredibly long list of humans from civilisations the world over, both ancient and new, that have done the very same and found themselves awed by the beauty of the sky. The stars and planets have long held an important cultural, religious or practical role in society. From navigation by land or sea, to helping people keep track of the seasons, leading to the development of the first calendars.
In the modern world, we have lost that innate connection with the night sky, with many of us not able to notice how the stars change with the seasons or pick out visiting comets because of the ever-present light pollution in cities drowning them all out. If you’re lucky enough to live somewhere you can see the stars, perhaps you might notice how the position of the Moon changes from night to night, or that one particularly bright ‘star’ wanders across the sky as the months go by. The Greeks also noticed these ‘wandering stars’ and dubbed them just that: planētai, meaning wanderer (the root of the modern English word, planet).
But not all of us can just look up and enjoy the view for what it is. Some of us want answers; an explanation of the things we see in the sky. It’s natural human curiosity. The very nature of what stars are and how they shine were questions that plagued humanity for centuries. In 1584, Italian philosopher Giordano Bruno was the first to suggest that the stars themselves might be distant Suns, even going so far as to suggest that they may also have planets of their own orbiting them. This was an idea that was incredibly controversial at the time, and came just forty-one years after the neat mathematical idea of the Sun, and not the Earth, being the centre of the Solar System was published by Polish mathematician and philosopher Nicholas Copernicus. Copernicus was a big fan of the simplicity and mathematical beauty of circles, and thought that if you arranged the Solar System with the Sun at the centre and the planets moving around it on circular paths, that would be the most mathematically beautiful way of arranging things. He wasn’t serious about it astronomically, necessarily, he just enjoyed the geometry of the whole idea.
But after a few more decades, there were those that started to support the idea astronomically, like Bruno and his fellow Italian astronomer Galileo Galilei, who would both eventually be punished for this supposed heresy against Catholic doctrine. It would take the combined efforts of Tycho Brahe, Johannes Kepler and Isaac Newton over the next century or so to compile overwhelming evidence in favour of the Sun being at the centre of the Solar System, and for the idea to finally be accepted both scientifically and publicly following the publication of Newton’s Principia in 1687. First, Newton determined the laws of gravity and the movements of the planets in their orbits. The same force that keeps us trapped here on the Earth’s surface is what causes the Moon to orbit the Earth and the Earth to orbit the Sun. These roughly circular orbits of planets around the Sun explained why the planets appeared to move backwards night after night in the sky for parts of the year, a phenomenon known as retrograde motion. Those planets closer to the Sun appear to be moving backwards in the sky when they were on the other side of the Sun (like cars on the opposite side of a circular racetrack),5 and those planets further out would appear to move backwards as Earth overtook them as it moved faster in its orbit.
Mercury in ‘retrograde’ appears to be moving backwards, but it’s just on the other side of the ‘racetrack’.
While Bruno was ahead of his time, his idea that the Sun was a star like any other, albeit a lot closer, still didn’t help to reveal how they shine. However, realising that the Sun was at the centre of the Solar System and governed by the same forces that we experience here on Earth removed the Sun’s God-like status, rendering it something more ordinary in people’s minds. Physicists of the 1700s started wondering whether the Sun and the stars could be powered by everyday processes like combustion, going as far as considering whether burning coal could account for the amount of energy outputted as light. Spoiler alert: it can’t. If the entire Sun was made of coal, it would burn through it at its present rate of energy production in just 5,000 years.6 Given that recorded history went back further than that – the Great Pyramid of Giza had been built over 4,000 years earlier – and that the Earth was then thought to be 6,000 years old, this idea was eventually dismissed.
So, if the Sun wasn’t made of coal, then what was it made of? Figuring out what the Sun was made of became a huge focus of physicists in the 1800s, but it was a Bavarian glassmaker who made the first breakthrough. Joseph Ritter von Fraunhofer was born in 1787, the youngest of eleven children in a family boasting many generations of glassmakers. His story has all the tropes of a good Disney movie: by the time he was a teenager, he was an orphan sent to apprentice with a master glassmaker in Munich who made decorative mirrors and glass for the royal court. His master was cruel to him though, depriving him of an education and a reading lamp to read his precious science books after dark. But one night his master’s house collapsed, burying Joseph alive inside. This was such huge news in the city of Munich that a Prince of Bavaria even came to the scene of the disaster and was there as Fraunhofer was pulled alive from the rubble. When the prince heard of Joseph’s plight, he set him up with a new master in the royal palace who supplied him with all the books on mathematics and optics that he could get his hands on. A true fairy tale story.
But the story doesn’t finish there, Fraunhofer ended up working at the Optical Institute in Benediktbeurern, where he was put in charge of all glassmaking, improving methods for grinding super-smooth glass for use as lenses in telescopes. The problem that Fraunhofer applied himself to was understanding the pesky refractions (a change in direction of the light) that would occur through the glass, scattering some of the light into the colours of the rainbow. This made his lenses imperfect. He was trying to measure how much the light was refracted, i.e. how much its direction was changed, through different types and shapes of glass. Isaac Newton had already shown in the 1600s that white light was made up of all the colours of the rainbow, showing how refraction occurs through a prism, changing the direction of red light less, and changing the direction of blue light more, to reveal the rainbow. If you’re picturing Pink Floyd’s Dark Side of the Moon album cover you’re on the right track.
The problem Fraunhofer had was that the colours of the rainbow aren’t clearly separated from each other. Next time you see a rainbow in the sky, see if you can pick out where the green ends and the blue begins: it’s impossible to tell. The colours blend into each other, making something really pleasing to the eye, but incredibly frustrating if you’re trying to measure how much the direction of each colour of light is changed by. So, Fraunhofer started experimenting with different sources of light. He noticed that when he used the light of a flame burning sulphur there was one section of the rainbow, of a yellow-orange colour, that was much brighter than the rest. He became curious whether the Sun also showed this bright yellow patch in the light as well, tweaking his experiment to change the path of the light more and more to get the rainbow to cover a larger area: he essentially managed to ‘zoom in’ on the rainbow to see more detail. By doing so, he invented the very first spectrograph; an instrument that is the cornerstone of modern astronomy and astrophysics.
Fraunhofer was shocked at what he then saw using his spectrograph with light from the Sun; instead of brighter patches of light, he noticed there were some colours of light from the Sun that were missing entirely. Dark lines in the rainbow, gaps that no one else had spotted before. He labelled the ten most obvious dark sections at first, eventually recording 574 gaps in the rainbow of light from the Sun. If you could zoom in on a rainbow in the sky, this is always what you would see.
Intrigued by this finding, Fraunhofer investigated further, finding that the gaps appeared in sunlight reflected off the Moon and the planets, and objects on Earth. He wasn’t certain though whether the gaps in the light were a true property of sunlight, or caused when the light passed through the Earth’s atmosphere. So he then used his spectrograph to look at the light from other stars, like the bright star Sirius, the ‘dog star’7 near the constellation of Orion (Orion is supposed to look like a hunter, with a smaller constellation next to him of his hunting dog, of which Sirius is the brightest star). Fraunhofer noticed that the gaps appeared once again in the light from Sirius, but they were in completely different locations, with a different pattern to sunlight. He concluded that it wasn’t the Earth’s atmosphere causing these gaps, but something in the very nature of stars themselves.
The Sun’s rainbow split by a spectrograph showing the missing colours that Fraunhofer spotted. Eventually Bunsen and Kirchoff showed that they were caused by elements in the Sun absorbing these colours, revealing what the Sun was made of.
With this discovery in 1814, Fraunhofer essentially kick-started modern astrophysics as we know it, and he lived happily ever after. Or at least, that’s how the Disney movie of Fraunhofer’s life would end. In reality, he died from tuberculosis at only thirty-nine years of age in 1826. The glass furnaces he worked with contained poisonous lead oxide and most likely contributed to his death.
Fraunhofer’s untimely death meant that he never lived to see these gaps in the Sun’s rainbow of light explained a few decades later, in 1859, by German physicist Gustav Kirchoff and chemist Robert Bunsen. Kirchoff and Bunsen didn’t set out to explain what Fraunhofer had seen, but were instead investigating something else using Bunsen’s new invention, which produced a very hot, sootless flame (that also wasn’t blindingly bright) for use in the laboratory. Today, every science lab around the world has one, from high-tech research institutes to school chemistry classrooms: a Bunsen burner.
Using a Bunsen burner, Kirchoff and Bunsen would burn various different elements in the flame and record what colour of light was given off. They even used a newly updated version of Fraunhofer’s spectrograph to split the light given off into its component colours. They found that each element burned with a very specific colour, or wavelength of light. For example, sodium burns a bright yellow colour, with a wavelength of exactly 589 nanometres (0.000000589 m), which is the colour of old-fashioned yellow street lamps which use sodium-powered bulbs. Kirchoff noticed that one of the missing gaps in the Sun’s rainbow of light that Fraunhofer had recorded was also at exactly 589 nanometres. Could it be that sodium was also present in the Sun, but instead of emitting light of that colour, it was absorbing it?
Kirchoff and Bunsen then cross-referenced all the wavelengths emitted by elements they had categorised in their lab with those recorded by Fraunhofer and found matches everywhere, suggesting the Sun contained sodium, oxygen, carbon, magnesium, calcium, hydrogen, and many other elements. This essentially confirmed that the Sun was indeed made of the same elements that we find on Earth. In his honour, Kirchoff and Bunsen dubbed the gaps in the Sun’s rainbow of light ‘Fraunhofer lines’.
So, in 1859, the problem of what the Sun was made of was solved, but the problem of how the Sun was powering itself with the same elements that made up Earth was still unsolved. There’s a wonderful Scientific American article from August 1863 entitled: ‘Experts Doubt the Sun is Actually Burning Coal’, which states:
The sun, in all probability, is not a burning, but an incandescent, body. Its light is rather that of a glowing molten metal than that of a burning furnace.
In other words, it’s something like Earth, but for some reason much hotter, so much so that it is glowing.
This article was based on the work of British physicist William Thompson – who would later be dubbed Lord Kelvin as he became the first scientist to be elevated to the House of Lords (the scientific unit of temperature, the kelvin, is named in his honour) – and German physicist Hermann von Helmholtz. Kelvin and Helmholtz are giants in the world of thermodynamics: they pioneered our understanding of heat and temperature. In 1856, Helmholtz published his idea that the Sun generated heat because it was being squished under gravity, essentially transferring huge amounts of energy from the crush of gravity inwards, into kinetic energy, which gives atoms (the building blocks of all the elements) more energy to move faster, heating up the Sun so that it glows like a hot piece of metal or molten glass.
In 1863, Kelvin used Helmholtz’s idea to calculate that the Sun would be able to power itself this way for at least 20 million years – far longer than the Earth’s supposed 6,000-year age that had stumped the ‘Sun is powered by coal’ calculation. The same year, Kelvin also applied the ideas of heat transfer to the Earth to calculate its age, by assuming that the Earth was once molten and has since been cooling for long enough to give us a solid crust of rock to stand on. Kelvin calculated that the Earth must also be around 20 million years old.8 The similarity of Kelvin’s two estimates, the age of the Sun and the age of the Earth, were interpreted as a success. If the Earth and the Sun formed at the same time, out of the same mix of elements, then this would finally explain the similarity of elements shared between the Earth and the Sun, and solve the problem of what powered the Sun, in one fell swoop.
The physicists, therefore, were happy, but the biologists and geologists were most definitely not. Because a few short years before Kelvin made his age estimates, in 1859, a biologist called Charles Darwin had published his book On the Origin of Species, detailing his new theory of evolution. In it, he said that all life on Earth had evolved from a common ancestor, branching through different mutations motivated by natural selection (what Herbert Spencer would call ‘survival of the fittest’ a few years later). By the 1870s, the majority of the scientific world – and members of the public who were paying attention – had accepted the idea of evolution. There was just one problem: this process of evolution took time, a lot of time. Darwin himself in his 1872 edition of On the Origin of Species commented that Kelvin’s 20 million-year estimate for the age of the Earth wouldn’t give enough time for evolution to occur. Evolution needs billions, not millions, of years.
Meanwhile, the geologists were attempting to use their own methods to calculate the age of the Earth. Either by working out the rate that rocks form and lay down sediment, or by considering the build-up of salt in the oceans. The chap who had this idea was Irish geologist and physicist John Joly; in 1899 he reasoned that salt (i.e. sodium chloride) dissolves out of rocks, into rivers, which then meet the sea. If the Earth’s oceans originally formed with no salt in them, then from the rate at which salt flows through rivers, you can work out how long it would have taken for salt to build up to the concentrations we measure in the sea today, and therefore get an estimate for the age of the Earth. In case you’re wondering, Joly estimated that there are 14,151 trillion tons of sodium in the ocean, whereas in rivers there are 24,106 tons of sodium per cubic mile of water. He also estimated that the total volume of water that leaves rivers and enters the ocean is 6,524 cubic miles per year. Running through the maths gives you an estimate that the build-up of salt in the ocean took almost 90 million years.9
This was closer to what the biologists were expecting – still not the billions of years that would be the boon for Darwin’s theory of evolution, but the death knell for Kelvin’s estimate of the age of the Sun. Another breakthrough was kick-started in 1895, when French physicist Henri Becquerel discovered that uranium atoms were unstable, and would spontaneously transform into more stable elements over time, giving off radiation in the process. His PhD student, French-Polish physicist and chemist Marie Skłodowska-Curie, decided to investigate this radiation for her PhD thesis, using a tool that her husband, Pierre Curie (studying crystals at the time), had invented fifteen years earlier to measure electric charge. She found that the radiation given off by the uranium atoms caused the air around them to conduct electricity, and hypothesised that the radiation must come from the atoms themselves (rather than be caused by an interaction with air molecules).
After the birth of her daughter Irène in 1897, Curie dedicated herself to finding yet more unstable elements, discovering thorium and finding it produced four times more radiation than uranium. By 1898, her husband Pierre had abandoned his own work on crystals for Marie’s far more interesting research on this unknown radiation. By the end of the year they had announced the discovery of two more unstable elements, which they dubbed polonium in honour of Marie’s homeland of Poland, and radium, after the Latin word for ‘ray’. In doing so they coined the phrase ‘radioactivity’. In 1903, Marie and Pierre Curie and Henri Becquerel were awarded the Nobel Prize in Physics for their discovery and characterisation of radioactivity.10
What’s so key about the discovery of radioactivity is that it establishes that the transformation (or ‘decay’) of unstable elements happens at a constant rate. If you can measure the amount of the unstable element and compare it to the stable element it decays into, then you can work out how long it has been decaying for. This was the breakthrough that revolutionised geology. By 1907, this method of ‘radioactive dating’ had been applied to the rocks of Earth, suggesting Earth (and therefore the Sun it orbited around) was at least a few billion years old.11
Finally, a value that made sense for all the biologists long convinced by Darwin’s theory of evolution. But it was a value that caused more pain for physicists trying to determine how the Sun could possibly be shining, finally scrapping Kelvin’s ideas. Although radioactivity produces heat (and is enough to explain the heat given off by Earth), it isn’t nearly enough to be the sole source of energy in the Sun. So, at the start of the twentieth century, we had a good idea for how old the Sun was (at least as old as the Earth), but had no idea how it could possibly have been shining for that long.
Enter stage left: German physicist Albert Einstein. Along with Stephen Hawking, Einstein’s name is perhaps most synonymous with black holes. He is perhaps the grandfather of black holes themselves, with his theories kick-starting decades of research into the nature of gravity, space and time. But for this part of the story, we need only his most famous equation (arguably the most famous equation ever), which he proposed in 1905: E = mc2. E stands for energy, m for mass and c for the speed of light – a whopping 299,792,458 metres per second. It means that energy and mass are equivalent – they are essentially the same thing and intrinsically linked. Mass can be converted into energy.12 Here, finally, was something that could explain where the huge amounts of energy produced in the Sun for billions of years was coming from; it was converting its enormous mass directly into energy. But how?
The first clue came in 1919 from French physicist Jean Baptiste Perrin, who would go on to win the 1926 Nobel Prize in Physics for showing that individual atoms could join together to make molecules. For example, O2 is formed of two oxygen atoms joined together. In his work studying atoms and molecules, he discovered that a helium atom, with four particles, weighs less than the total mass of four hydrogen nuclei, with one particle each. The mass difference was tiny, at just 0.07 per cent, but with E = mc2, a tiny mass can turn into a huge amount of energy. Perrin13 realised the significance of what he had found and suggested that this could be what is powering the Sun. If four atoms of hydrogen could be brought together to make helium, the leftover mass could become energy given off as light. The problem was that Perrin didn’t have a physical model for how this actually happened, pointing out that the central nuclei of hydrogen atoms were positively charged and would repel each other with a huge force (atoms have a central nucleus with positively charged particles, orbited by smaller negatively charged particles known as electrons).
It would take the stubbornness of English physicist Arthur Eddington in 1920 to convince the world that if this process of fusing four hydrogen nuclei together to make helium was going to happen anywhere, then it had to be happening in stars. Eddington was already somewhat of a household name by 1920, after writing a number of articles explaining Einstein’s newest theory of general relativity to the English-speaking world (more on that later). His own research, though, was on the nature of stars, and in 1920 Eddington reasoned a few things: first, using the same methods as Lord Kelvin himself, that the temperature at the centre of stars would be around 10 million degrees Celsius, and that at these temperatures our understanding of the interaction of nuclei and repulsive forces keeping positively charged hydrogen nuclei apart might break down. Second, that only 5 per cent of the mass of the Sun had to be hydrogen to produce enough energy to keep it burning for the billions of years that Earth had been around. These were all ideas that were proved correct over the next few decades, and further contributed to Eddington’s status as a BNIP (a Big Name in Physics).
In 1925, British-born American astronomer Cecilia Payne-Gaposchkin published her PhD thesis. Her research showed how Fraunhofer’s gaps in the rainbow of light from the Sun meant that hydrogen outweighed every other element in the Sun by a million times. Far more than just 5 per cent of the Sun was made of hydrogen. The final piece of the puzzle came in 1928, when American-Russian physicist George Gamow ran through the maths and realised there was a vanishingly small probability of a hydrogen nucleus outwitting the electric repulsion between it and another hydrogen nucleus to allow them to fuse together. The probability might be incredibly small, but crucially, it isn’t zero. So, if you have enough hydrogen squished into one place, like in the Sun, then theoretically this skipping of the repulsion can occur enough times to produce enough energy so that the Sun shines.
Finally, the problem was solved. Hydrogen was the fuel of the Sun and all the stars in the night sky: nuclear fusion was what made them shine. I can’t help but wonder how much of that story we’d even know if we couldn’t see the stars. Would we have even thought to ask questions like ‘what makes the stars shine?’ Would we have realised what the Sun actually was? Perhaps if Earth was in orbit around two stars, so that it was daytime on both sides of the planet, we would have had endless daytime and never have seen the night sky. What questions would we have never known to ask? What advancements in knowledge and technology would have eluded us?
I think we as humans have a lot to thank for the curiosity that staring up at the night sky has to offer. Not least for our knowledge of my favourite thing: black holes. Because once we figured out how the stars shine, this inevitably led us to another question: what happens when the fuel runs out? What happens when a star dies? And it is this simple question that eventually leads us to a black hole.