Episode 4 – The Sun
Welcome to Untangling Science, a podcast about science that is for everyone, with me Darragh Ennis. You probably know me as The Menace from ITVs quiz show The Chase, but my day job is as a scientist at the University of Oxford. In this podcast I want to bring the world of science to people who think it’s too complicated to understand in a way that is fun and straightforward. We have a website, www.untanglingscience.com and you can follow us on Twitter @untanglings . I have a blog on the website that I leave useful information, links and diagrams from each episode in, so check that out. In this fourth episode we’ll be talking about the Sun. We will use a lot of concepts from episode 3 in this one so please give that a listen if you haven’t already.
First of all we will pose a series of questions that I will try to answer so that we can better understand the Sun. What exactly is the Sun? What does it do? How was it formed? What’s it like in the different parts of the Sun? What will happen to it in the future? We will then end with a little bit about some of the research missions that have studied the sun over the years.
When I was a student we learned about ecosystems and food webs where every living thing fed on something else that fed on something else until, eventually, you got to what was called the primary producers. These were green plants of some kind that turned water and some form of carbon into sugar by photosynthesis. Photosynthesis literally means to make something using light, and pretty much always this referred to sunlight. The sun is the energy source that ultimately almost all of life on earth relies on so in a way even the energy you are using to listen to this podcast is repurposed sunlight. Which is pretty amazing when you think about it. But what exactly is the sun, and how does it generate all of this energy? The first bit is easy, the sun is a star. The reason why it looks different to the twinkly little lights we see at night time is distance. In fact, as stars go ours isn’t all that big and some of the other stars we can see are immensely bigger, but to quote father Ted, this one is small but those are far away. There are other stars that are hundreds of times bigger and many times brighter than our sun as well as some that are much smaller and dimmer. Other than the fact that we revolve around it is actually quite ordinary as far as stars go. The sun is a little bit under 1.4 million km across, which is more than 100 times wider than the earth and is more than 300,000 times as heavy. You could fit more than 1.3 million of our planet inside the sun and it makes up 99.8% of the mass of the entire solar system. So even though in the galaxy it’s pretty bang average as stars go, in our solar system it is almost the whole thing. Well the hint is in the name, it’s a solar system after all.
Ok, so it’s really big and is the big cheese in our solar system but what does it and other stars actually do? Mostly it turns out that they burn. Stars are massive balls of plasma. It is tempting to say that they are on fire, but they’re not so I think we should explain the difference. Fire requires three things; heat, some fuel and oxygen. There is no oxygen in space so it’s not possible for fire to be sustained and stars are instead plasma. Plasma is thought to make up about 99% of the matter in the known universe, but it’s relatively rare on earth so I’ll explain what it is so we can get it straight. All stuff is in one of four forms and the first three are things we all know and understand; solids, liquids and gases. In general if you add energy to a substance, most of the time by heating it, you can change it’s form. Ice when heated will melt into water, and if you heat water enough it turns into steam. So that’s all fine and we have those concepts as easy enough to understand. If you take it further and add a lot of energy to a gas you can cause its electrons to break free and fly loose, which changes the charges of the atoms so they become ions. If that sounds mad to you, check out episode 3 which is all about atoms and electrons and will make what I’m about to say much easier to follow. This mixture of ions, electrons and lots of energy causes the gas to turn into a plasma. This burning plasma emits a very large amount of energy in the form of heat and light, and it’s this heat and light that we see and feel here on earth. One of the main laws of physics is that energy can’t be created it has to come from somewhere, so where does all of this energy come from? In the case of the Sun that is mostly down to something called nuclear fusion.
Nuclear fusion is what happened when the nuclei of atoms join together. The nuclei fuse together and that’s why it’s called fusion. Nuclear power that we use is caused by pretty much the opposite of this process, nuclear fission which as you may have guessed is when we split the nucleus of an atom. In the Sun about 75% of the plasma is made of hydrogen and when these hydrogen nuclei are fused together they will form Helium. Because of the different makeup of these two elements there is a slight difference in their mass, so a very small amount of mass is leftover. This mass has to go somewhere and it is released as a tiny amount of energy that radiates away as heat and light. For each helium atom formed the amount of energy is quite tiny, but the sun is made of a vast number of atoms and this happens a lot. In terms of numbers the sun produces almost 400 septillion watts of energy per second (that’s a 4 with 26 zeroes after it) which is about 1.8billion times more energy than the most powerful nuclear bomb ever made. And that’s in every single second. In terms of mass this means the Sun loses about 5 million tonnes of mass every second. Now that’s a ridiculous number so we will consider it in terms of elephants, and the sun loses the weight of about 1 million African elephants every second. So that’s where all of that energy comes from, but now we will have to ask how did it all start and how do the Sun and other stars form? But first a quick recap
- Pretty much all of the energy of all living things ultimately comes from the sun
- The Sun is absolutely huge in our solar system, about 99.8% of all the mass is in the Sun
- It is pretty much bang average in terms of other stars
- The Sun is made of mostly Hydrogen and Helium in the form of plasma
- Plasma is what you get when you add a lot of energy to a gas
- Hydrogen is turning into helium in the Sun by the process of nuclear fusion
- This fusion is what gives off the light and heat energy of the Sun
Right, back to the story of our favourite star. How did it form? To answer this we need to go back about four and a half billion years to when our solar system didn’t exist. Space is pretty empty, but it’s not totally empty. There are large clouds of gas and dust that can form called nebulae. If one or more of these large clouds starts to gather together due to gravitational forces they will eventually start to collapse in on themselves which causes them to spin. This spinning is the same type of force that causes the planets to rotate around the sun today. This spinning cloud of dust starts to flatten out into a sort of pancake shape with a lump in the centre. The centre lump then has a stronger gravitational force than the rest and begins to get bigger, giving it more gravity so it gets bigger and so on. This is now called a proto-star and the greatly increasing ball of gas and dust still has a small disc of material spinning around it that will eventually form the planets, but now the material in the protostar gets more dense and causes huge temperature and pressure to form which eventually kickstarts the fusion process and the protostar turns into a star proper. In the case of the Sun this process took about 50 million years, so it’s not quick to get to the point of releasing energy.
Now we know how the Sun formed let’s talk a bit about what the Sun is like when we look closer. Well other than the fact that it’s really hot of course. So like the Earth, the Sun is made up of different layers, and we’ll go through them one by one starting with the core at the centre. It’s in the core that the fusion happens to generate all of the energy the Sun releases into space. The core makes up about one quarter of the radius of the Sun, so it covers about one quarter way from the centre towards the edge. The temperature and pressure in the Sun’s core are absolutely immense, which is necessary to maintain the fusion reactions that fuel the Sun. It’s about 15 million degrees Celsius in there and the pressure is 250 million times more than at the bottom of the deepest part of the ocean. It’s this pressure cooker furnace situation that enables the Sun to keep its fusion going and about 99% of the Sun’s energy is produced in the core. Surrounding the core is an area called the radiative zone. In this area fusion is extremely rare. The heat and light from the core radiates out through this zone and the temperature and pressure drop sharply across this zone ending up around 2 million degrees by the time it reaches the next zone, the convective zone. Convection is a way to transfer energy that is different to radiation and the best way to think of this zone is like a pot of boiling water. When you put a pot of water on to boil the hot water rises up in little currents, moves to the surface where it reaches cooler air on top. The water then cools and falls back towards the bottom of the pot to be heated again. This causes a cycle of moving water as long as it’s heated from below. In the convective zone the same thing happens with the convective zone in the Sun. Heat from the core and radiative zone cause currents of plasma to rise toward the outer layers where they cool. This causes them to become more dense, and they sink back towards the core again. All of this seething mass of plasma causes changes in the magnetism of this layer and it is this that causes the sunspots that can be seen on the Sun using special equipment. It should be noted now that under no circumstances should you look directly at the sun, it is incredibly bad for your eyes and can cause permanent sight damage. Anyway, now on to the last zone of the sun, the surface layer which is called the photosphere. The photosphere is the visible surface of the sun and is a very thin layer on the surface only a few kilometres thick. At this point the temperature has dropped quite drastically to a mere 5,000 degrees. The energy of the Sun is emitted into space from the photosphere and it’s this layer that we see from earth. The light from the photosphere goes out into space and it takes about 8 minutes to reach earth, so where you see the sun in the sky is where it was about 8 minutes in the past.
Time for a recap:
- The Sun formed from a big cloud of gas about 4.5 billion years ago
- The planets formed at around the same time
- The cloud became a disc shape and eventually gravity collapsed it into a ball
- The temperature and pressure got so high that fusion started and it turned into the star we know today
- The Sun has different regions from the super hot core in the middle to the relatively cooler photosphere at the surface
Now that the energy is let loose into space and takes 8 minutes to reach us, what happens when it hits the earth. Well about 30% of it is reflected back into space. A lot of this 30% is reflected by different layers of our atmosphere. The rest is reflected back from the surface of the Earth. As white things reflect this type of energy best a lot of this reflection is done by clouds and the polar ice caps. And for those not paying attention shrinking polar ice caps due to climate change will mean less radiation reflected back into space, which means the earth will heat up quicker, which means the polar ice caps will shrink even faster, which means….. But what happens to the 70% that does reach the earth? The atmosphere absorbs quite a lot of the radiation, notably the ozone layer which absorbs a lot of the ultraviolet section of the energy. The earth and sea then absorb quite a lot of it as heat, a lot of which is then released back into space as heat energy. The atmosphere acts like a duvet which means that all of this heat energy doesn’t just vanish straight away, stabilising temperatures on Earth. Without the atmosphere we would have temperatures like the moon, baking hot during the day and plunging to freezing cold at night time. The particular makeup of the atmosphere is kind of like duvets with different tog values. Higher levels of some gases in the atmosphere are more able to trap this heat close to the Earth and this is how we are fuelling climate change. By trapping more heat and energy in the system more extreme climate trends become more common and all caused by trapping a small bit more of the energy from the Sun.
One cool thing that happens when the Sun’s interacts with the atmosphere are the polar lights. You may notice that I didn’t say the northern lights, also called the aurora borealis because of course there are similar lights in the southern hemisphere called the aurora Australis. Regardless of which half of the planet you are the polar lights are caused by what is known as solar wind. Solar wind is caused when the sun ejects plumes of plasma into space. This travels across the solar system and the intensity of it varies depending on conditions near the surface of the sun. These charged particles stream across space until they hit the earth’s atmosphere, and importantly they run into the earth’s magnetic field. The magnetic field deflects most of the particles back into space and a lot the others get pushed towards the north and south magnetic poles. Once they get close enough to hit the atmosphere at great speed. These charged particles collide with atoms in the atmosphere and some of their energy sort of rubs off on to the atmospheric atoms and this energy is released as light. The particles keep going further into the atmosphere, colliding with atoms and releasing light as they go. Depending on what atoms they particles hit and how high up they are the light emitted can be different colours and depending on how much solar wind is released by the Sun the brightness of them can vary quite a lot. But because of the magnetic field you always have a much better chance to see the lights the closer you are to the poles.
But how will the Sun end up? The formation of the Sun happened about 4.5 billion years ago and the current estimates think that the Sun is about half way through it’s lifespan. So what will happen in another 5 billion years or so? So, we’ve talked already about how the Sun gets it’s energy through fusion of Hydrogen into helium, but the thing is there is a limited amount of hydrogen. There’s a lot, don’t get me wrong but even that vast reserve of Hydrogen will be used up as the Sun is going through a million elephants worth every second. Well, when the sun runs out of hydrogen in its core the amount of energy produced is reduced and it collapses in on itself. This hugely increases the pressure and this allows the heavier elements to start to undergo fusion too. One of the main things that happens is that helium undergoes fusion to form things like carbon. This stops the star from collapsing by providing large amounts of energy and causes a huge increase in temperature, and this causes the star to expand massively. In the case of the Sun this will mean that the atmosphere of the Sun would go out far enough to engulf the Earth. At this point the star is called a red giant and it will stay like this for about a billion years. Then the fusion reactions will run out of fuel and the star collapses into a much smaller star that glows white with leftover heat and is called a white dwarf. So somewhere around 6 or 7 billion years from now that will be the end state our Sun. It should be noted that not all stars end this way. Much larger stars can explode in a supernova and will then collapse into something so dense and with so much gravity that it doesn’t even let light escape and these are black holes while other stars collapse into super dense stars called neutron stars. But our Sun is far too small to end up like those, so it’ll be a white dwarf instead.
Well time for a recap:
- It takes about 8 minutes for the energy from the Sun to reach the earth where about 30% is reflected back into space.
- The atmosphere absorbs some of the rest with a lot of what makes it to the surface heating up the planet. This heat is then released into space except what is trapped by the atmosphere which acts like a blanket.
- Human caused changes to the atmosphere are making this blanket better able to hold on to this heat, which is far from ideal
- The Sun releases the solar wind, a stream of particles that travel through space
- When this hits the earth it’s funnelled towards the poles by the magnetic field and when it hits the atmosphere energy is converted into light and we call this the polar lights
- In about 5 billion year the Sun will run out of Hydrogen fuel and it will collapse in on itself before expanding massively as far as the earth and it will be a red dwarf
- Eventually it will consume all of its fuel and collapse again into a white dwarf.
Now a quick chat about the latest NASA mission to study the Sun, The Parker Solar Probe. This is the only NASA spacecraft named after a living person, the eminent solar scientist Eugene Parker who coined the term ‘solar wind’. It was launched in August 2018 and cost $1.5 billion. In October 2018 the Parker probe became the closest manmade object in history to orbit the Sun when it got to within about 10 million kilometres of the Sun’s surface. It will get closer several times throughout the mission. The issues of dealing with the intense heat and gravitational forces of this part of space were a main concern when the probe was being constructed. A special heat shield to reflect some of the extreme temperatures means that the interior of the probe will be able remain at a heat that won’t fry the electronics. Those instruments will be able to tell us a lot more about the Sun’s magnetic field, electrical field, the origins of solar wind and an awful lot more. The Parker probe will enable scientists to much better understand the star that is central to our solar system and all life on earth.
Ok, that’s it for this episode. Thanks so much for listening. Please make sure to subscribe and share so we can get more listeners. As always comments and suggestions are very welcome, and you can find a transcript on the blog post for this episode on www.untanglingscience.com . Next episode will go back to earth and talk about what plants do with all of the energy from the sun, photosynthesis.