15 Million Degrees

by Lucie Green

  All this gravitational collapse had the effect of heating the matter up. In the same way in which an object dropped from a height to the earth accelerates as it falls, so the particles’ increasing ‘kinetic’ (i.e. moving) energy came from their decreasing ‘gravitational potential’ energy (i.e. the amount of energy inherent in any mass that has not yet succumbed to gravity). Put plainly, the closer each particle got to the centre of the nebula, the faster and faster it was moving. And increasing the kinetic energy of all the particles increased the temperature of the gas.

  Then this process of gravitational collapse carried on, as the massive proto-Sun started to crush itself down. The centre of this ball of matter was now starting to get extremely hot – so hot that it began to glow. This was the first stage of the Sun’s career producing light. But it was only the warm-up round. About 50 million years after the cloud collapse began, the temperature in the centre of the Sun reached the point where, thanks to Bethe and Gamow’s work, we know thermonuclear reactions would start. It was like billions of light switches had been flicked.

  Importantly, the thermonuclear reactions stopped an endless gravitational collapse of the Sun. The collapse had been important because it compressed and heated the centre until it was hot enough – and critically dense enough – for fusion to take place. But the nuclear reactions were now giving the Sun a way to push back, and this held up the rest of the Sun and it ceased collapsing.

  And this is the balance that keeps the nuclear reactor in the Sun under control to this very day. If the fusion reactions were to suddenly increase (for some unknown reason), the core would heat up, the pressure would increase and the gas would expand. This expansion would have the effect of reducing the core temperature and density, which would have the effect of bringing down the reaction rate. But if the fusion does ever start to ‘go out’, this drop will allow the Sun to continue collapsing, raising the density and temperature and increasing the reaction rate again. It’s this interplay and feedback between pressure, gravity and rate of fusion within the Sun that keeps it regulated, so that it doesn’t explode like a hydrogen bomb releasing its energy in a short but catastrophic burst. It works exactly like the ‘governor’ in a car engine that allows it to idle at a nice constant rate.

  But how do we explain the discrepancy between the chemical compositions of the Sun and the Earth? We still think that the solar nebula theory is a good description of what happened. The answer is that the Earth and all of the so-called ‘rocky’ planets that formed near the Sun – Mercury, Venus, the Earth and Mars – were too small and too hot because they are close to the Sun to hold on to the lightweight hydrogen and helium gas that would have been present in their atmospheres when they formed. At such temperatures, hydrogen and helium particles would have been able to escape the gravitational pull of these planets. Whereas the Sun has mostly maintained its original composition (except for the changes due to fusion, hidden from our sight in the core), the Earth has evolved. You can think of the Earth as having started the same as the Sun, but having since been distilled down to be almost only heavy elements.

  For all the successes of the solar nebula model it still makes some predictions that do not match today’s observations. According to the model, the Sun and the planets should all be spinning in the same direction. This nicely described the situation until the discovery that Uranus’s axis of spin is at 90 degrees to that of the other planets and that Venus spins backwards. Similarly, one would expect, owing to the laws of momentum, that the most massive object in the Solar System, the Sun, which is at the centre of the spinning disc that is the Solar System, would be spinning at a greater rate than the much less massive planets further out. Here we find a serious problem: the Sun is spinning 400 times more slowly than we would expect. At some point during its 4600-million-year life, it must have transferred some of that momentum out into the Solar System. A mystery we’ll revisit later.

  Even with these few open problems, the solar nebula model remains the most likely scenario for the formation of our Sun. The most compelling evidence is that we’ve seen other nebulae in action. Elsewhere in our Galaxy we can watch younger stellar system families growing up. The most famous nearby nebula in which stars are forming is easily observable with binoculars and is found in the constellation of Orion. This constellation shows Orion the hunter, who carries a club and a shield and has a sword hanging from his belt. In the sword is a fuzzy region of nebular gases that have been lit up by newly formed stars.

  The Orion nebula was observed as soon as telescopes were developed, with records dating back to 1610. Today it is known to be a vast cloud 40 light years across and around 1300 light years away. Many stars are forming in this vast nebula and the young ones have created strong winds that are blowing away the gas and dust from their surroundings, allowing us to peek into this stellar nursery and see what it is like to look back in time to the formation of our own Solar System.

  Finally, the elements that make up the Earth and the Sun give us a glimpse of what came before our current family – back to previous generations. Modern measurements of the composition of the Sun show that, by mass, the solar nebula must have been 73 per cent hydrogen and 25 per cent helium, with all the other elements that we are more familiar with on Earth contributing only 2 per cent of the mass. Hydrogen and helium were created during the formation of the Universe itself, so their presence is understandable.

  As for the other elements, the only way to explain their presence is if they were forged by nuclear fusion, something that can only happen in the centre of a star. Instead of reading this text as a book printed on paper, you might be reading it on an e-reader or a smartphone. In your hands now will be some very heavy elements that make up your electronic device, such as copper, aluminium and zinc. We know that these elements can only have been formed in stars far hotter than our own Sun is at the moment. And there are other elements such as tin or uranium that were formed when previous stars exploded as supernovae.

  We now know that our Solar System formed from the remains of previous stars. They lived their lives, burnt out and scattered their ashes into the Universe. Billions of years later, the dust and gas from these ancient stars formed our nebula. And not only does our Sun have parents – we now know that it has grandparents. We live with a third-generation star, with a heritage reaching far back into the Universe. This third-generation star makes its own light by cannibalizing its own material. Not only that, but the photons released in the proton–proton chain reaction are of an extreme and rare variety here on Earth: gamma rays, with the very smallest wavelengths and one of the very highest energies. They are well beyond the visible part of the spectrum and are very harmful. But by the time those same photons reach the Earth, they have become relatively harmless sunlight. Something else is going on. What happens to all those photons as they pass through the rest of the Sun and eventually escape from its surface? Something serious must be happening because they change character completely.

  4. The Secret Life of a Photon

  We have learnt that the Sun stays hot and produces electromagnetic radiation by the process of nuclear fusion, which means we now know what light is and where sunlight is born – except that the high-energy, dangerous gamma ray photons produced by nuclear fusion are very different to the almost harmless, garden-variety sunlight we all take for granted on Earth. The mystery is how this transformation takes place.

  They must change identity within the Sun itself, because the photons remain unaltered during their 8 minute and 20 second journey from the Sun to the Earth. Their transformation must occur as they travel from the centre of the Sun to the su
rface …

  … a journey that takes a single photon around 170,000 years.

  The light reaching you right now from the Sun began its journey when archaic Homo sapiens were still considering evolving into modern humans. The next time you are out in the sun, take a moment to appreciate that the light you are seeing is as old as our species.

  As we know, even though light moves extremely fast in a vacuum (300,000 kilometres per second) it slows down when it passes through other substances. And whatever is happening inside, the Sun is slowing the photons down by an incredible amount. But it does not stop them completely: the photons still creep their way across the 522,000 kilometres of gas above the core before they reach the surface, at an average of 3 kilometres a year – this is less than half a metre an hour. So while the inside of the Sun is not completely opaque, it does make the photons’ journey out into the Solar System a Herculean task.

  The first model of the interior of the Sun was constructed in 1869, around the time that Kirchhoff was carrying out his experiments but well before Payne’s discovery of the composition of the Sun and the realization that nuclear fusion was the power source. It was made at a time when even the physical nature of the Sun was still debated and the model was the work of Jonathan Homer Lane. Lane was an American scientist with a broad interest in science, engineering and experimentation. The best insight into the personality of Lane comes from the diary of a contemporary, Simon Newcomb.

  Newcomb was a Canadian-American astronomer and had recently read an article in an English weekly publication called the Reader. This article suggested a new theory that the Sun could actually be a mass of incandescent gas, rather than a molten liquid as was the popular view at the time. He decided to tell a few of his friends from the local scientific club about it, including one strange person he described as ‘an odd-looking and odd-mannered little man, rather intellectual in appearance … I did not even know his name, as there was nothing but his oddity to excite any interest in him.’ Newcomb soon realized he was making that classic scientist’s faux pas: accidentally describing a theory back to the person who came up with it in the first place.

  Lane was of the opinion that the Sun was made of gas – a controversial view at a time when there were even some who considered it possible for the inside of the Sun to contain cool material that had condensed into a solid. If it were a gas, though, then Lane knew from the work of Lord Kelvin that the surface of the Sun was likely to be much cooler and less dense than its interior. Newcomb, having later become one of Lane’s few friends, actually introduced him to Lord Kelvin and so began the wider acceptance of Lane’s theory.

  Lord Kelvin began his life as William Thomson, in Belfast, in 1824. But by the end of his life he had moved to Glasgow, made several landmark scientific discoveries, and been promoted to the title of 1st Baron Kelvin. Kelvin was the proponent of the theory that the energy source of the Sun was its own gravitational collapse. Occasionally, I have referred to ‘Kelvin’: units of the scientific scale of temperature named after him. These are exactly the same as degrees Celsius, except that they start 273.15 degrees lower. So a room at a toasty 30 degrees Celsius is at 303.15 Kelvin. Given that we round off temperatures in the Sun to the nearest thousand degrees, if not the nearest million, it actually does not matter if you use Celsius or Kelvin.

  Lord Kelvin had been developing a theory about the Sun based on his thinking about the Earth’s atmosphere – because physics is physics and the same laws apply across the Universe, so the Sun’s gaseous mass might be understood by considering what we see here on Earth. In the Earth’s atmosphere convection is common: warmer air rises while cooler air sinks. This is because the atmosphere is held in place by the gravitational pull of the Earth, so that cooler, more dense air has more mass to be pulled back down, displacing the warmer, less dense air. The same physics also applies in your kitchen and you will see an extreme version of this when you boil a saucepan of water: the low-density steam forming at the base of the saucepan rushes up to the surface.

  Kelvin considered that there would be a temperature gradient across the air in the Earth’s atmosphere. The convective motions do not disrupt this overall temperature structure; on the contrary, they maintain it. Any bubbles of air that are in motion will expand or contract until they reach the same density as their new surroundings. This alters the temperature of the gas bubble, and balances it with the surroundings.

  By making an analogy with the Earth’s atmosphere Kelvin had reasoned that the gaseous sphere of the Sun could also be in convective equilibrium, with layers of gas at different temperatures that get cooler the nearer they are to the Sun’s surface, where the energy is leaking into space. From this starting point, Lane brought to bear data based on experimental work: the measured value of the brightness of the Sun and how hot materials emit light. He calculated values for the temperature and density of the gas inside the Sun, from the centre outwards, and arrived at an estimate for the surface gas temperature. This value was remarkably close, within a factor of five, of what we now know it to be.

  These ideas developed by Lord Kelvin and Lane of the Sun being a ball of convecting gas, a massive roiling cosmic cooking pot, were in line with observations. The master of eighteenth-century astronomy, William Herschel, first noted a mottled appearance to the Sun in a paper he published in 1801. He wrote that ‘Corrugations change their Shape and Situation; they increase, diminish, divide, and vanish quickly.’

  The corrugations are now known as ‘granules’; if you zoom in on the Sun it looks almost exactly like a pot of boiling water. You can see the mottled appearance as plumes of hot gas reach the surface – only, instead of bubbles a few millimetres or centimetres wide as you see in boiling water, these granules are about a million metres across. The spacecraft I work with send back some amazing videos of the surface of the Sun in motion. (See plates 3 and 4.) If you get a chance to see them yourself, perhaps through an observing event at your local astronomical society or through pictures online, they are a sight to behold.

  Modern data show that there is more to the convection than just the granulation that Herschel saw. There are larger convection cells that engulf the smaller ones – like a series of Russian dolls. The largest convection cells, in features called ‘supergranulation’, stretch out over 30 million metres at the photosphere. Then, between the size scales of supergranulation and granulation, it has been proposed that an intermediate mesogranulation is circulating. But this has yet to be proved and the consensus at the moment seems to be that mesogranulation is nothing more than a misleading artefact in the data.

  This may feel like an open-and-shut case, but sadly Lord Kelvin’s and Lane’s model is an incomplete explanation of the Sun – really it just scratches the surface. We now know that the ‘convection layer’ of the Sun only extends about 18 per cent of the way to the centre of the Sun. It turns out that there is only a very tenuous shell in which convection is occurring, and below that in the Sun things are very different indeed.

  IN THE RADIATION ZONE

  School students are taught about the three ways in which energy can be transmitted: convection, conduction and radiation. The top 18 per cent of the Sun is all about convection, and nowhere in the Sun are conditions really suited to conduction (conduction normally plays a role in solids). The way to transmit energy through the remaining 82 per cent of the Sun is by means of radiation. This radiation is comprised of the photons themselves that are trying to get free. The first realistic model of the radiation within the Sun was developed by Eddington, not long after he had confirmed Einstein’s ideas about gravity and inspired Payne to switch
from botany to physics.

  ‘At first sight it would seem that the deep interior of the sun and stars is less accessible to scientific investigation than any other region of the universe’– this was the opening statement of Eddington’s 1926 publication The Internal Constitution of the Stars, one of the most important astronomical texts ever written. In it Eddington abandoned the earlier ideas of energy transport within stars being achieved solely by convection and considered the role of radiation.

  The reason why Eddington’s work became so influential, and why it is central to our story here, is that by making radiation the primary means of energy transfer within the Sun he came up with a picture of it that explains, at last, the true state of the hydrogen gas within it and how and why it is able to affect the photons on their journey from the core of the Sun to the surface. What Eddington realized is that the hydrogen gas in the Sun would be in the form of ‘plasma’, as it is known.

  Plasma is often referred to as the fourth state of matter, after the sequence: solid, liquid, gas. Take the water in your saucepan, for example. Below zero degrees Celsius all that water would be frozen solid; between zero and 100 degrees it is a liquid; above 100 degrees it boils as some starts to turn into a gas. But what happens if you trap the steam in your pan and continue to heat that gas? Would it go through another transition?

  Without any high-pressure cheating, at normal atmospheric pressure steam will change again at around 12,000 degrees Celsius, turning into the fourth state of matter: a plasma. This is the point where the energy of the atoms is so great that the electric force holding them together is overcome. The water molecules break up into single hydrogen and oxygen atoms. And then the electrons in the atoms themselves would start to break away from their nuclei. This process, whereby a neutral atom is changed into an electrically charged ion, is called ‘ionization’, and when this happens to many particles at once, it produces plasma.