15 Million Degrees

by Lucie Green

  WHAT IS THE SUN MADE OF?

  The answer to the question, ‘What is the Sun made of?’, was finally cracked at Harvard College Observatory (in Cambridge, Massachusetts) in the 1800s and 1900s. Cambridge is still a focus for research, and the Observatory archives, part of the Harvard–Smithsonian Center for Astrophysics and which I visited while on a research trip, is an incredible place where almost 200 years of astronomical history are encapsulated – and it’s also a very significant place for women in astronomy.

  In the nineteenth century the staff at the Observatory were given the enormous task of categorizing the entire night sky. This wasn’t a simple matter of photographing every star – the light from each star was passed through a glass prism and the resulting spectrum recorded. It would take a huge amount of work. Worldwide, around 6000 stars are visible to the naked eye if the skies are dark enough. Using just a small telescope raises this number into the hundreds of thousands. To get through all of these stars, Harvard College Observatory hired a team of women.

  They became known as computers, well before a ‘computer’ changed from being a person to a machine. These women computers had the task of effectively processing the astronomical data and categorizing the stars according to any similarities they showed in their individual spectra. Sadly, they were not hired to do any actual research themselves; their job was to do the tedious work so that the real scientists didn’t have to. But these women were not going to be held back by a mere job title. They did some amazing work in their own right.

  During my visit I walked past cupboard after cupboard that together house over half a million photographic glass plates showing the light captured from hundreds of thousands of stars. These plates are the actual data that the computers would have worked with. I wanted to see the glass plates studied by one computer in particular, Annie Jump Cannon. Cannon was a phenomenally efficient and talented computer and categorized over 250,000 stars during her career, sometimes at a rate of three per minute. She would sit near a window and use a mirror to bounce the incoming light through the glass plate, decipher what she saw and enter the correct classification into a ledger before moving on to the next. (See plate 1.)

  Cannon’s classification of stars led to a system in which stars were organized by their temperature. This classification system is still used today: OBAFGKM. Stars range from ‘O’-type stars with surface temperatures from over 30,000 degrees Celsius (Kelvin, to be more specific) to ‘M’-type stars with surface temperatures of around 3000 Kelvin. Because of the erratic ordering of the letters, I was taught a mnemonic at university to help remember the scheme that Cannon had devised – ‘Oh! Be A Fine Girl – Kiss Me!’ Or, as I prefer to remember it, ‘Oh! Be A Fine Guy – Kiss Me!’ I was able to see the desk at which Cannon sat and the plates that she studied. In recognition of her outstanding talent, the American Astronomical Society now makes an annual award in her name to an early-career female astronomer who shows exceptional promise.

  Cannon was able to classify stars based on spectra because scientists had learnt how to decipher all sorts of information encoded into these little rainbows of colour. What Newton had seen as a range of colours was far from being that simple. By looking at a spectrum it is possible to tell not only how hot the source must have been, but also even what it was made of. The Sun and other stars could easily have remained for ever out of humankind’s reach, but it turns out that their light has brought all the information we need right to our doorstep.

  GAPS IN THE RAINBOW

  If you take the same rainbow that Newton saw and spread it out over a much greater distance you will start to see finer detail. It is not a continuous range of colours: there are gaps. If you look closely, there are dark lines within the Sun’s spectrum – thin slivers of colour are completely missing.

  These gaps were first seen by an English doctor, William Hyde Wollaston, in 1802. He was studying how light bends when it shines through various substances (the refractive index) and he happened to cast a very wide spectrum, and spotted the dark lines. He only saw a few and decided they must be the gaps between colours. In 1814 these lines were independently rediscovered by a German optician, Joseph von Fraunhofer, but with his superior optical equipment he found hundreds of them. Today these dark lines are known as the ‘Fraunhofer lines’ and thousands have been identified. Fantastically, these Fraunhofer lines are caused by the Sun and are not some new property of light. Understanding how they are formed goes as follows.

  If a solid or a gas is heated enough it will start to glow. This is a familiar concept and gives us the phrases ‘red-hot poker’ and ‘white heat’. It’s the same principle behind incandescent light bulbs too: a tungsten filament is heated to over 3000 degrees Celsius and it starts to glow. If you looked at the spectrum from a hot-glowing object, you would not see any missing lines though; the spectrum would be completely continuous.

  Another way to produce light is to energize a thin gas. This is the effect that is utilized in neon lights and some energy-saving light bulbs. In this case, a gas is given energy by passing an electric current through it, not by heating it. But if you looked at the spectrum of a neon light you would get a shock. It is the complete opposite of the nice continuous spectrum from an incandescent blub. It would not even be a ‘spectrum’ in some senses of the word; all you would see would be a few discrete bright lines, each of a single colour, with swathes of darkness in between. (See plate 2.)

  The light we see from the Sun looks as if a continuous incandescent spectrum had had these disjointed gas spectral lines subtracted from it. And it turns out this is exactly what has happened.

  In 1859, a German physicist, Gustav Kirchhoff, made a discovery that brought this unclear variety of spectra into focus. Kirchhoff carried out a series of experiments using very pure gases that were made only of single elements, and as he analysed the light of each one he realized that they all have their own unique disjointed rainbows, what we now call an ‘emission line spectrum’. The coloured lines produced by lithium are seen at different wavelengths to those of sodium, which are different to those of potassium and so on. It was no longer necessary to touch a substance, cut it up or do an experiment with it to find out what it is. As long as the substance you are curious about emits light, you can study its line spectrum and know exactly what it is. And the Sun is not short of light.

  Kirchhoff’s next discovery explained why these emission line spectra have been subtracted from the Sun’s continuous spectrum. He showed that if a thin (and relatively cool) gas was placed in front of a light source that produced a continuous spectrum, the emission line spectrum reversed itself and the previously bright lines became dark. The gas was now absorbing light at those wavelengths, not emitting them. The continuous spectrum (formed by the background incandescent object) with dark lines in it (formed by the intervening gas) is known as an ‘absorption spectrum’.

  Kirchhoff realized that if the wavelengths of the dark lines in the Sun’s spectrum were measured, they could be compared with the emission/absorption line wavelengths that he had found the elements in his lab produced, so that the chemical make-up of the solar atmosphere could be found. The absorption lines could be used as a cosmic barcode – a remarkable leap forward and one that totally changed the way in which the Sun could be investigated. Even without visiting the Sun, there was now a way to find out what it is made of.

  It was these absorption lines that Annie Jump Cannon and the computers in Harvard were looking at. Not only could the spectral absorption lines from distant stars be matched with terrestrial spectral lines to tell us what elements must be present in those distant stars
, but the particular lines that are present reveal what temperature those elements were at when they stole the photons from the spectrum. This was the vital work done by those women: they were exploring and sampling stars across our Galaxy.

  This technique conclusively showed that the Sun contained the same elements as the Earth, and acted as a kind of maternity test. But there was a problem – something which showed that the Earth and the Sun must have had very different upbringings. And this was discovered by another amazing female, who was also at the Harvard College Observatory. This woman did achieve the status of being able to do her own research.

  The discovery was made by a British astronomer, Cecilia Payne (her maiden name – she was later Payne-Gaposchkin), who began her scientific studies in 1919 at the University of Cambridge, where she had enrolled to read Natural Sciences, with a focus on botany. But then she attended a lecture by the director of the Cambridge Observatory, Arthur Eddington. He had just travelled to an island off the west coast of Africa to see a total solar eclipse and his lecture was on how his observations of stars close to the Sun in the sky had provided the first experimental proof of Einstein’s general theory of relativity. Einstein had only published his theory in 1915 and it was Eddington’s observations that thrust Einstein into the international limelight. The early twentieth century was an exciting time for physics, and after the lecture Payne abandoned botany to focus on astronomy.

  Unfortunately, Payne was studying at Cambridge several decades before women were awarded full university degrees, and there were certainly no opportunities for a research career in astronomy once she had completed her course. It was to fulfil her ambitions that she had to move across the Atlantic to Harvard College Observatory in 1923, to study for a Ph.D. in astronomy (literally moving from Cambridge to Cambridge). Going there gave Payne access to the observatory’s vast collection of stellar observations and placed her amongst the greatest astronomers of the era.

  Payne’s aim was to use the Harvard data to study stellar chemical composition. As she intricately examined the spectral lines in the light from the Sun and other stars she applied the latest theoretical ideas about how the light they emitted was affected by their surface temperature. In 1925 Payne discovered that even though the Sun has the same elements as the Earth, its composition isn’t at all similar to the Earth’s: the ratios of the elements are completely different.

  She found that the most abundant element in the Sun is hydrogen – the lightest and least complex of all elements in the periodic table. The next most abundant was helium. As for those elements commonly found on Earth – such as iron, oxygen and silicon – they accounted for less than 2 per cent of the Sun’s composition. This was a highly controversial result and put Payne up against the establishment, who had accepted the idea that the Sun should have the same proportion of elements as the Earth. Even Eddington thought that iron was the most abundant element in the Sun.

  Payne was forced to write off her findings as nothing more than a discrepancy in the data. The biased opinions of an older generation and their strong expectations of what should have been observed meant this remarkable discovery was totally overlooked. Payne’s gender and the lack of influence that women had in the astronomical patriarchy wouldn’t have helped either. In the years following Payne’s discovery, the work of established male astronomers confirmed her result and for a long time they took the credit. Payne received little acknowledgement for her discovery of the major constituent of the Sun, the stars and indeed the whole Universe.

  However, Payne’s role at the Harvard College Observatory did signal the start of a gradual change in both the role and the position of women at Harvard and in astronomy more generally. She carried out her own research and published her own ideas and pioneered the transition from women being employed only as computers to becoming researchers with ideas and leadership of their own. Even though she was silenced at first, she did eventually achieve recognition later in her career. She blazed a trail for generations of women who followed her, and her observations changed our view of the Universe, just like Eddington had done for her a few years earlier. It’s always surprising to me that her name is rarely mentioned.

  PULLING IT ALL TOGETHER

  Right, let’s put it all together and actually build our Solar System. We can deal with the differences in composition between the Sun and the Earth later. This is the theory first put forward by Kant:

  In the beginning there was a vast cloud of gas and dust, far enough from any other stars not to be significantly affected by their gravity. This explains the similarities across the Solar System; everything came from the same cloud of dust and gas. The cloud was simply drifting through our Galaxy, insubstantial and fragile, with only a few particles per cubic centimetre and stretching over a region millions of times bigger than the Solar System today. Then, for some reason, it started to collapse.

  There is still some speculation about how and why this happened. My favourite contemporary theory posits that it could have been the consequence of a nearby exploding star, a supernova. The pressure pulse from the shock wave created by the explosion could have compressed the cloud sufficiently for its own gravitational force to draw it in on itself. No matter how the process began, though, the particles moved closer and closer together, imperceptibly at first but then ever more rapidly as their own collective and self-perpetuating gravity pulled them inwards.

  As the cloud shrank it became denser, forming what is now known as the ‘solar nebula’ – literally, the cloud that became the Sun and the rest of the Solar System. But as the cloud collapsed down into a smaller and smaller region two strange things started to happen: it began to spin and it flattened out into a disc. Actually, it didn’t begin to spin – the cloud had always had a slight rotation, though it was just too subtle to notice. But any overall rotation of the initial gas cloud, no matter how slight, became amplified as the nebula contracted. As the nebula shrank, its rotation became faster and faster, just like an ice skater drawing in their arms to make them spin more rapidly. This occurs because the angular momentum of the nebula – the product of its size, mass and rate of spin – must remain the same. This is the law of conservation of momentum. So if one component, like the size, goes down, another component, that is the spin rate, will go up.

  This spin explains the flattened shape. As the material began to spin faster and faster, the material around the rapidly rotating Sun flattened into a disc. The reason for this is that the spinning nebula was experiencing another force – other than the inward force of gravity. It’s a force that we have all felt as children when playing on a roundabout or as adults driving a car round a sharp bend: we feel like we are going to be spun outwards – not up or down, but outwards from the centre. We know that spinning sends you in a very predictable direction and this happened to the nebula. As the nebula contracted, and started to spin more and more rapidly, the gas particles experienced a stronger and stronger outward force.

  What happens next depends on where the gas particles are in the nebula. The outward force is strongest at right angles to the rotation axis – in other words, in the plane the nebula is spinning in. So gas particles in this plane have a harder time being drawn in as the outward force works against gravity. This creates a flattened disc. Imagine a pizza chef. He or she starts with a ball of dough, and once they start whirling the dough above their head it creates a thin but wide spinning dough-disc.

  Within the swirling disc of the nebula, it’s thought that particles started to collide and within these collisions some particles were able to stick together because of the electrostatic forces of the electrically charged partic
les in the nebula – minuscule fragments at first. And, as fragments started to form, some collisions would break them apart whilst others led to fragments joining together so that they gradually grew in size. Fragments then became clumps and, as they got massive enough, the attractive force of gravity became strong enough for clumps to start to glue together too. Gradually, over a few million years, these clumps eventually became large enough to form the planets. And as more and more planetary material coalesced, the regions in between them started to empty and, around 10 million years after the solar nebula began to collapse, the formation of the Solar System was well under way.

  3.1 Cartoon of the formation of the Solar System from the collapse of a vast cloud of gas and dust known as a nebula. Not to scale!

  The formation of the Solar System from the collapsing and spinning nebula explains why we have a spinning Sun and spinning planets orbiting around it. They all came from a rotating cloud of material and so they all inherited that same sense of spin. But while some of the gas and dust was able to remain in orbit and form the planets, asteroids and other Solar System detritus, these are the outliers. A much larger amount of the dust became the Sun. Early on there would have been about an Earth’s worth of matter in a clump, and then a Jupiter’s worth. But more gas and dust kept raining down and the ball continued to grow. Eventually it became a sphere that was possibly 100 times wider than the Sun is today.