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Umeå University Department of Physics 21st October 2010 THE INTERIOR OF THE SUN Space Physics – Project on by Ursula Schlager Supervisor: Prof. Kjell Rönnmark CONTENT 1. Introduction ………………………………………………………. 3 2. The Structure of the interior of the Sun: ……………………….. 3 2.1 The Core …………………………………………………….. 3 2.2 The Radiation Zone ………………………………………….. 4 2.3 The Interface Layer (Tachocline) ………………………….... 5 2.4 The Convection Zone ………………………………………... 5 2.4.1 Granules …………………………………………… 6 2.4.2 Supergranules ……………………………………… 7 3. Helioseismology …………………………………………………... 7 4. References ………………………………………………………… 8 2 1. Introduction The Sun is our nearest star. It’s just a normal star but for us, it is very important. Without the Sun the Human race and all living creatures on earth wouldn’t exist. It gives us light and warmth and it holds our Solar System together. In the following abstract The structure of the interior of the Sun I will describe the interior of the Sun without talking about the atmosphere. It’s like you would talk about the interior of the earth: You wouldn’t pay attention to its atmosphere. But like in the case of the earth we cannot see inside the Sun. So there is still the question: How do we know what we know about the Sun? One answer to this question is Helioseismology which will be described in the same titled abstract. 2. The structure of the interior of the Sun 2.1 The Core The core is the central region of the Sun. It extends from the centre to one quarter of the radius of the Sun which is 696,128km. So the core has a radius of approximately 174·10³ km. It includes only 2% of the Sun’s volume but nevertheless it contains half of its mass. The Core is composed of approximately 72% hydrogen, 26% helium and 2% other elements. The temperature in this place is about 15·106 K and its density 150g/cm³. Because of this high temperature and density, all atoms are broken down into their constituent parts which means in the core of the Sun exist only electrons, protons and neutrons. Furthermore nuclear reactions take place which create helium from hydrogen. 3 These nuclear reactions generate also the energy which later will leave the Sun’s surface as visible light. About 85% of the nuclear reactions in the core follow the following scheme: 1 1 2 1 (1) ( 2) (3) 3 2 p +11p → d +11p → 2 1 d + e+ +ν 3 2 4 2 He + 23He → Each of this reaction is exothermic and the total thermonuclear energy which is released is 26.2MeV per 4 2 He nucleus formed. He + γ He + 211 p The remaining 15% of nuclear reactions take up at step (2): 2 1 d + 11p → 3 2 He + γ He + 24 He → 7 4 Be + γ ( 2) 3 2 e − + 47 Be 1 1 7 3 p + Li 7 3 → → 4 2 1 1 Li + ν 4 2 He + He p + 47 Be 8 5 B 8 4 Be * . Branch II [15%, released energy: 25.2MeV] 8 5 → → → 8 4 B+γ Be * + e + + ν 4 4 2 He + 2 He Branch III [0.02%, released energy: 19.1MeV] Whereas steps (1) to (3) make up Branch I, the ones above are the Branches II and III of the so called Proton-Proton Chain. Notice that a pre-existing 24 He acts as a catalyst (in branches II and III). During these reactions neutrinos ν are produced. These elementary particles leave the Sun very quickly because they have no charge and thus they don’t interact so much with the medium that surrounds it. When you move outside the centre, the density and temperature decreases. As a result the nuclear burning is almost completely shut off beyond the core. At the edge of the core the temperature is only half as high as in the centre and the density has decreased to 20g/cm³. 2.2 The Radiation Zone This Zone expands outward from the edge of the core to around 70% of the distance to the surface of the Sun. It includes 32% of the Sun’s volume and 48% of its mass. The density is 20g/cm³ at the bottom and it decreases to 0.2 g/cm³ on the top of the layer which is less than the density of water. The temperature also decreases with increasing radius from 7 million Kelvin to 2 million Kelvin. Because of the temperature being a little cooler than in the core, it is a region with highly ionized gas. As a result, every photon coming from the core and carrying energy is 4 absorbed. Thus energy can be stored for a while and emitted as new radiation later. The new created photon bounces again into another particle due to the high density and is absorbed and emitted later and so on. This is it what gives this layer its name Radiation Zone. Due to the jumping from particle to particle the light needs about a million years from the bottom to the top of this layer. If there weren’t any obstacles, light would need only two second for the same distance. 2.3 The Interface Layer (Tachocline) The next region is the so called Interface layer. It is a very thin layer which is around 200,000km beneath the surface of the Sun. It combines the turbulent outer region, the Convection Zone, with the orderly interior, the Radiation Zone. So fluid motions that are found at the bottom of the Convection Zone disappear from the top of the Interface Layer to its bottom where the Radiation Zone begins which is calmer. Consequently, the speed of gas within this layer changes abruptly and there is even a difference between how fast this fluid motion changes when you look at the equator in comparison to looking at the poles: At the equator the outer gas moves around the Sun’s axis of rotation faster than the inner gas whereas at mid-latitudes and near the poles the outer gas rotates slower. The changes in fluid flow velocities across the layer give this layer its alternative name Tachocline. They can stretch a magnetic field and they can make the magnetic field lines of force stronger. That is why it is currently thought that the Sun’s magnetic field is generated by a magnetic dynamo within this layer. In addition, it was discovered that the contrast in speed between the outer and inner layer of the Tachocline can change by 20% in six month. That means when the lower gas speeds up then the upper gas slows down and vice versa. Also very interesting is that there appear to be sudden changes in the chemical composition across this layer. With more details being discovered during the recent years like the ones mentioned above, the Interface Layer became more and more interesting for scientists and is still a topic of current research. 2.4 The Convection Zone This layer is the top layer of the solar interior, so it extends from the Interface Layer up to the visible surface of the Sun. The Convection Zone brings the missing 66% to the Sun’s volume and the 2% to its mass. The temperature at the base is “only” 2 million Kelvin, so it is cool enough for heavier ions like carbon, nitrogen, oxygen, calcium and ion to hold some of their electrons. This makes the material more opaque or non-transparent. Now it is more difficult for a photon to get outward with the help of radiation. The photon will be absorbed by an atom which doesn’t release it so readily again due to the cool temperature and the density which is 5 still very high. This traps heat what consequently makes the fluid unstable and convection starts. So, with the beginning of the Convection Zone, the transport of energy by radiation slows down significantly and it is replaced by transport with the help of convection. This kind of energy transport works much faster than the one by radiation. It takes only a little more than a week to transfer the energy from the bottom to the top of the Convection Zone. At the visible surface the temperature has dropped to 5,700 K which makes it possible that even neutral hydrogen exists. The density also dropped to 0.2·10-6 g/cm³ which corresponds to approximately 1/10000 of the density of the air at sea level. The convective motions are visible at the surface of the Sun in form of Granules and Supergranules. Just to have a better overview about the temperature and density in different layers, here some diagrams: 2.4.1 Granules Granules are the top of convection cells where hot gas rises up from the interior (bright areas), spreads out across the surface, cools down and sinks inward again (dark lines). They are about 400-1000km in diameter and move vertically a distance of the order of 200km. Granules cover the entire surface of the Sun except those areas covered by Sunspots. An individual Granule can last 15 to 20 minutes and it is continually evolving. That means old ones are pushed aside by new Granules emerging from the interior. The flow within the Granules can even reach supersonic speed of more than 7km/s. In this way it can produce sonic “booms” and other noise that causes waves on the surface of the Sun. 6 2.4.2 Supergranules Granules are grouped in Supergranules which expand about 35,000km across and extend 5000 to 8000km down into the Sun. Like the Granules, they cover the entire surface of the Sun and they are continually evolving. An individual Supergranule can last up to a day or two. Supergranules are best seen when measuring the “Doppler shift”, like you see in the picture. Red means that the material where the light comes from is moving away from us, whereas light from material moving towards us is shifted to the blue. 3. Helioseismology In the 1960’s it was discovered that sound waves are propagating in the Sun by Robert B. Leighton. About ten years later it was explained by Ulrich (1970) and Leibache and Stein in 1971. This has lead to the development of a new technique called helioseismology. The Sun is a ball of hot gas so its interior transmits sound waves very well which can be seen by the doppler shifting of light emitted at the Sun's surface. Helioseismology uses these sound waves to probe the interior of the Sun. It is the same like geologists use seismic waves from earthquakes to probe the inside of the earth. Some of these waves travel right through the centre of the Sun others are bent back towards the surface and stay in shallow depths. The lifetime of such waves can be as short as one or two days or as long as some months. Their frequencies depend on the thermodynamic, compositional and dynamic state of the material where the wave goes through. Consequently, with the help of these waves it is possible to construct extremely narrow probes of the temperature, chemical composition and motions throughout the interior of the Sun. In this way Helioseismology is a method to measure the internal structure and dynamics of the Sun directly. Some things which have been found out with the help of Helioseismology: The theory and application of Helioseismology has largely confirmed the main elements of the Standard Solar Model and it has ruled out solar models with low abundance of heavy elements or such in which Helium produced by fusion in the core is mixed with Hydrogen from outside the core. This method has fixed the bottom of the Convection Zone at a depth of about 200,000km and it has been found that the temperature at the bottom of this zone is higher than predicted by the Standard Solar Model. The Core on the other hand is cooler than expected, so the core and its nuclear reactor are maybe more complicated then thought. With the use of this theory it was also discovered that Superganules go 5000 to 8000 km deep. Before, theorists expected a depth of 15000 to 20000 km. Without Helioseismology some of the most important processes in astrophysics would remain only conclusions from theory and because our Sun is just a normal star, it is possible to generalize our results from the Sun to other stars. So Helioseismology offers a springboard to study the interiors of other stars as well. 7 4. References 1 “The Physics of Stars” Wiley, 2nd edition, by A.C.Phillips 2 http://solarscience.msfc.nasa.gov/interior.shtml 3 http://www.nasa.gov/worldbook/sun_worldbook.html 4 http://sohowww.nascom.nasa.gov/newsroom/oldesapr/042000pr/ 5 http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=30851 6 http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter1.html 7 http://solar.physics.montana.edu/YPOP/Spotlight/SunInfo/Structure.html 8