THE COLLECTION W from the Star to Domestic Energy

THE COLLECTION w From the star to domestic energy

1 w The atom 2 w Radioactivity 3 w Radiation and man 4 w Energy 5 w Nuclear energy: fusion and fission 6 w How a nuclear reactor works 7 w The nuclear fuel cycle FROM RESEARCH 8 w Microelectronics 9 w The laser: a concentrate of light TO INDUSTRY 10 w Medical imaging 11 w Nuclear astrophysics 12 w Hydrogen 13 w The Sun 14 w Radioactive waste 15 w The climate 16 w Numerical simulation 17 w Earthquakes 18 w The nanoworld 19 w Energies for the 21 stt century

13 wwTThhee SSuunn

OUR STAR VOYAGE TO THE CENTRE OF THE SUN THE SUN TAMED

IISSN 1637-5408.. 2 w CONTENTS w INTRODUCTION 3

From left to right: The satellite SOHO, a neutrino detector in Japan, a photovoltaic cell. A E C

R R C I

A S E

OUR STAR 4 has being providing us with light and

n A star is born 5 heat. Today it is man's ambition to What causes the Sun control this energy source.” u to shine? 6 What type of energy! 6 S The Sun, a self-regulating

nuclear fusion reactor 9 e VOYAGE TO THE CENTRE OF

h THE SUN 11 A

S onsidered as a star and a deity, for a long E / o

Solar topography: h

o period of time the Sun was though to be S

T the opaque part 12 C © another planet, whereas the word "star" was The visible part 14 Montage showing several views of solar flares. reserved for all the brilliant points of light in the seasons and provides us with heat and light. In the corona and beyond: the night sky. The Sun's status as a star in the But what exactly is the nature and origin of this the furi es of the Sun 16 sense of "an astral body producing and emit - prodigious energy source, with which man The solar plasma 17 ting energy" was firmly established only at the attempts to provide warmth and produce elec - An identity card for the Sun 18 beginning of the 20 th century. Today astrophy - tricity? How does this work? 21 sicists are revealing more and more secrets of What is happening in this gigantic ball of fire, Helioseismology and the fusion burning region located in its core. impossible to observe without protective glasses? simulation 23 It is thanks to the Sun that life has appeared And finally, how long will it continue to shine? Measuring activity from the and evolved on Earth; it controls the cycle of Questions such as these took many centuries core to the corona of the Sun 24 to be solved and will continue to be the sub - ject of research for a long time to come. THE SUN TAMED 26

Solar thermal energy 27 r a l o S

Photovoltaic solar energy 29 s n e m e i S

Design and productio n: Spécifique – www.specifique.fr – - Cover photo : © Soho (ESA/Nasa) - Illustrations : YUVANOE - Printing : Imprimerie Sénécaut - Summer 2011 From the star to domestic energy 13 w The Sun From the star to domestic energy 13 wThe Sun 4 w OUR STAR 5

ASTROPHYSICS HAS TAUGHT US THAT THE SUN IS A GIGANTIC NUCLEAR REACTOR . Our star A STAR IS BORN The Sun is a star; an enormous ball of hot gas which produces and radiates energy. The Sun is the closest star to the Earth (150 millions kilometers). It is one of the billions of stars which make up our galaxy, the Milky Way, which is several light-years away. In spite of the fact An astronomical unit of length, that it has a radius corresponding to the distance travelled by light in vacuum during one of 700,000 kilo - year. The speed of light in vacuum is meters, (more 300,000 kilometers per second and the light emitted by the Sun requires than 100 times only eight minutes to reach our planet, the radius of the whereas light from the next nearest star to us, Proxima Centauri (a Earth) and a mass c s distance of 4.2 light-years from the i D

330,000 times o t

Earth), takes more than four years. o h P greater than that © of the Earth (2 billion billion billion tons!), the “The temperature imbalance within the Sun creates radiation.” Sun is a relatively small star: for example, the radius of Betelgeuse, in the constellation of Orion, is 1,100 times greater than that of the gases: why doesn't this hot gas disperse into the Sun. inter-planetary vacuum? Its cohesion results from But how was our star born? the gravitational attraction which exists between The Sun was born 4.6 billion years ago, out of a particles of matter and tends to bring them clo - gigantic interstellar cloud composed of hydrogen, ser together. Therefore, a star is a self gravitating helium with less than 2% of heavier elements. body, with a spherical shape imposed by its own This cloud fragmented to give, among other celes - gravity. tial objects, a star surrounded by planets, comets But if gravity tends to bring particles closer toge - and dust: the solar system. The Sun has been the ther, why doesn't the Sun simply collapse in on subject of many studies in a scientific discipline itself? We need also to consider the stellar gas known as astrophysics, where the fields of astro - pressure, which is the result of incessant colli - nomy and physics (see the leaflet from this col - sion and rebounding of the particles with each lection entitled "Nuclear Astrophysics"). other. Overall, this pressure balances the action This science helps us to understand the pheno - of gravity. If this pressure was suddenly removed, mena occurring in the Universe. Hence, we know the Sun would completely collapse in on itself in r i o V

o that the Sun is in an enormous condensation of just three minutes! h P

“The temperature imbalance within The transformation of hydrogen into helium in the Sun the Sun creates radiation.” Step 1

Deuterium Positron: e+ Neutrino: n

Step 2

WHAT CAUSES THE SUN perature is around 15 million degrees. With a TO SHINE? surface temperature close to 6,000 °C, the Sun Helium-3 Gamma radiation It is well known that a compressed gas warms is subject to a large temperature difference ari - up: you only need to rapidly inflate a bicycle sing from its own gravity. This temperature Step 3 tire and then touch the end of the pump to rea - imbalance gives rise to a heat transfer which, lize this. Stellar material therefore becomes flowing primarily from the hot region to the cold hotter the deeper we go into the Sun, since it region, tries to make the temperatures more Proton Helium-4 is compressed by the mass of the layers which uniform. When it finally arrives at the surface, Neutron weigh down on it from above, acting rather like this flow of thermal energy escapes in the form a piston. Hence, the closer we get to the core of radiation which is then dispersed into inter - Like any other star, the Sun is a gigantic nuclear reactor. Nuclear fusion reactions transformed hydrogen into helium in of a star, the higher the pressure and the hot - planetary space: the Sun shines! its core, releasing energy. The temperature at the center of the Sun is fifteen million degrees and the density is one hundred and fifty times that of water (150 g/cm 3). ter it becomes: the pressure at the core of the The transformation of hydrogen into helium is complex; the following are the most common steps: Sun is 200 billion times atmospheric pressure WHAT TYPE OF ENERGY! • Step one: two protons interact to form a deuterium nucleus. During this process, one proton is changed into a neutron on the surface of the Earth and the core tem - The radiated energy must be coming from by emitting a positron (or positively charged electron) and a neutrino, a particle from the same family as the electron, somewhere: indeed " Nothing is created or which carries energy but has an unknown, very low mass. destroyed, it is all transformed ", as the great • Step two: one deuterium nucleus combines with a proton to form helium-3, releasing energy in the form of a gamma ray (or photon). chemist, Antoine de Lavoisier (1743-1794), • Step three: two helium-3 nuclei combine to form helium-4 by ejecting two protons. Temperatures and densities said 200 years ago. This assertion applies to within the Sun thermodynamics .

The branch of physics which How then, does the This idea was proposed again and developed and transported in the form of photons (mass - studies the phenomena involved Sun maintain its heat several years later by the German, Hans Bethe, less fundamental particles with no electric in heat exchange. Density (g/cm 3) 0 generation? Indeed, if who explicitly described the nuclear reactions charge) and neutrinos (very low mass particles 0,07 it were simply passively emitting its heat in produced in the core of the Sun. This physi - with no charge). 1,3 the form of radiated energy, it would cool down cist showed that, during the major part of its 2,0 inexorably and then die out. However, its lon - life, a star draws on its nuclear reserves to THE SUN, A SELF-REGULATING Temperature (K) gevity proves that it must have a way of repla - compensate for its constant loss of energy. NUCLEAR FUSION REACTOR 135 15 million 0 0 0 0 0 cing this lost energy; these days we know how In the densest and hottest central regions of A fusion reaction releases energy because the 0 0 0 0 0 0 0 0 7 million 0 5 4 0 this is achieved. the Sun, fusion reactions transform four mass of the nucleus produced is less than the 9 1 4 6 6 5 3 1 3 million In 1921, the physicist Jean Perrin provided Nuclear reactions in which hydrogen nuclei (protons) sum of the masses of the initial nuclei. This mass Depth (km) two lighter nuclei combine 1,5 million an explanation by pro - Jean Perrin (1870-1942) was to form one heavier into a helium nucleus, difference is transformed into energy according 6000 posing that nuclear one of the pioneers of atomic nucleus. Such reactions 4He, an element which is to Einstein's famous equation, E = Dm x c 2. theory and obtained the Nobel are accompanied by a reactions were the Prize for physics in 1926 for his large energy release. particularly stable, and These reactions The energy produced is equal to the mass diffe - rence multiplied by the speed of light squared. source of energy pro - work on the structure of matter. release energy which compensates for that lost cannot take place duction, i.e. reactions between atomic nuclei. at the Sun’s surface. This energy is emitted unless the temperature and pressure are suf -

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 w The Sun 8 w OUR STAR w OUR STAR 9

ficiently high that the two protons, stripped of • At higher concentrations, the fusion reaction their electron and thus positively charged, can be ignited by irradiation with very high fuse. This limits their field of action to the power laser beams: this subject will be stu - regions closest to the core of a star like the died by the Megajoule laser which is under Sun. The stars must also have sufficient mass, construction at the CEA Cesta centre near Bor - greater than one tenth of the Sun's mass, to deaux, as well as at the LIL prototype which compress the core sufficiently and ignite the is already in operation. This laser, assisted by nuclear reactions. In the core of the Sun, 620 the very short impulse laser, Petal, will pro - million tonnes of hydrogen are transformed duce plasmas similar to solar plasma but in into 615.7 million tonnes of helium every very small volumes and will enable certain cos - second; the difference is converted into energy mic processes to be studied in the laboratory. which radiates towards the outside. This We have seen that the Sun radiates energy: nuclear energy reserve enables us to calculate the earth only receives an extremely small part A

E the lifespan of the Sun as being around 10 C / a i

n billion years. Furthermore, thanks to measu - p a D Internal structure of the Sun

© rements of radioactivity in terrestrial rocks, we Optical module from the Antares neutrino detector. know that our planet, and hence the Sun, are 4.6 billion years old. The Sun will shine, the - NEUTRINOS refore, for another 5 billion years! Finally, we note that these fusion reactions are The core of the Sun is the region where hydrogen is Prize for physics in 2002. Having no electrical charge transformed to helium by nuclear fusion reactions. and an almost zero mass (less than 10 to the power of - self regulating. We know that the Sun, like all The process also produces ghostly particles called 33 grams), the neutrino interacts only very weakly with other stars, is: a nuclear reactor operating in neutrinos. At the central part of our star, the temperature matter. Every second, 65 billion neutrinos strike each the fusion mode. Core is sufficiently high to ignite fusion reactions between square centimeter of the Earth's surface and almost none of the star pairs of protons, the first step of a nuclear chain reaction are stopped! This is why large scale detectors have been Here on earth, man is attempting to master which leads to the formation of helium. This reaction constructed, and installed in mines (such as that at fusion reactions, to exploit this fantastic produces a deuterium nucleus (a hydrogen isotope), a Sudbury in Canada), underwater (the Antares project in energy source. Currently two types of pro - positron, the positive alter-ego of the negatively charged the Mediterranean Sea) and even in a tunnel (Gran Sasso, Neutrino electron, and of course energy. In 1930, the American under Mont-Blanc), to shield them from other types of cesses are being studied in the laboratory. physicist, Wolfgang Pauli (born in Austria in 1900 - dead cosmic radiation. A thick covering of earth or water acts • At low concentrations, fusion gas mixtures in Switzerland in 1958), in order to save the principle of like a sieve, only allowing the most penetrating particles conservation of energy, suggested the need for an to pass through. Scientifically these experiments have are contained by immaterial walls created by additional, unobserved particle with an assumed zero been impressive. They have allowed observation of magnetic fields: this is the subject that will mass. A Japanese team provided the experimental proof neutrinos emitted from the supernova which appeared in be investigated by the future international Photon trajectory (not to scale) of the existence of the neutrino mass in 1998; this the Large Magellanic Cloud in 1987 and it was possible Radiation flux discovery, a foundation stone in the edifice of particle to follow this remote explosion (170,000 years later!) experimental reactor ITER. Force of pressure (outwards) physics, brought its author, Masatishi Koshiba, the Nobel using the Hubble space telescope. International Thermonuclear Gravitational force (inwards) Experimental Reactor.

From the star to domestic energy 13 w The Sun From the star to domestic energy 13 w The Sun 10 w OUR STAR 11

PROMINENCES AND FILAMENTS , FLARES , “Helioseismology studies the surface EJECTIONS , GRANULES , SPOTS , FASTEN YOUR oscillations of the Sun and allows us to probe SEAT BELTS … the interior of our star.”

of this which, nevertheless, amounts to 178 mil - These waves propagate from the surface to the VVooyyaaggee ttoo tthhee lion billion watts! Thermonuclear fusion, the pro - core and require only one hour for this whole cess generating this energy, compensates for the travel, this providing almost live information on loss of energy at the surface and allows the Sun the internal microscopic and macroscopic pro - to burn sustainably; contraction causes an perties of the Sun. These waves generate per - centre of the Sun increase in heating providing the energy to remain iod motions at the Sun's surface and their study in balance from its gravitational capital. In rea - has given birth to a new discipline which allows lity, these two phenomena are linked: since the us to probe the interior of our star: helioseis - nuclear resources are limited, gravitational mology. contraction of the core will begin as they become exhausted. The resulting compression and hea - ting will make it possible to start a new fusion cycle which burns the "ashes" of the present cycle at a higher temperature. This is what happens in stars more massive than the Sun: hydrogen trans - forms into helium until it is exhausted, then the helium, in its turn, becomes the fuel. It will be transformed into carbon. Following these pro - cesses, known as "nucleosynthesis", a series of elements - carbon, neon, oxygen, silicon – is crea - ted, until finally iron is obtained.

We have seen that electromagnetic radiation is A E C

emitted from the surface of the Sun and that this © radiation takes eight minutes to arrive at the THE GOLF INSTRUMENT Earth. But, owing to the extreme density of the Sun, the light which is emitted by the core, takes The CEA made a large contribution to the production of the GOLF detector (Global several million years to reach the surface and Oscillations at Low Frequencies), an escape into the sidereal vacuum; it has therefore helioseismology instrument on board the SOHO lost all information concerning its origins in the satellite. With this tool, it is possible to make detailed studies of the internal structure of the Sun core of the star. using global acoustic wave measurements; This is the reason that astrophysicists prefer to These data have led to determination of the sound speed profile from the surface to the core, where a

study solar acoustic waves, which have the same s the nuclear reactions occur, and to the theoretical a N /

characteristics as seismic waves and carry infor - A predictions of the solar neutrino fluxes. S E / o h

mation on the internal structure of the star. o S

“The Sun is divided into two zones: the opaque part and the visible part.”

Solar topography: Radiative the opaque part transport zone

Today, it is possible to determine the fraction RADIATIVE ZONE: from 0 to 480,000 km which is opaque to light by calculation or Internal part of the Sun including the core . seismology.

It includes: THE CORE OF THE SUN: – A zone, called “radiative” , with includes the from 0 to 210,000 km core where nuclear reactions transform hydrogen It represents 50% of the mass of ions into helium; the core and radiative zone the Sun. This is the region where nuclear fusion reactions take represent 98% of the mass of the Sun; place. The temperature is 15 mil - – a zone, closer to the surface, which is called lion degrees at the centre, drop - “convective” . ping from the center to the extenal layers. When the plasma (see p.17) reaches 7 million degrees, The names of these zones are linked to the mode there is insufficient heat to sup - of transport of the energy produced in the port fusion reactions. central part and then carried to the exterior of the Sun. Transport is either by radiation In this zone, energy is transported by (propagating in the form of electromagnetic radiation. There are numerous waves) or by convection (electric heater principle: interactions between photons and the heating causes the motion of particles, and this various elements present. It results also absorption-reemission phenomena moving matter transports heat). which slow down and degradate their energy. Photons diffusion times are very long, on the order of a million years, whereas the CONVECTIVE ZONE: light emitted from the surface takes only from 480,000 to 690,000 km eight minutes to reach the Earth. The Convective temperature at its frontier is 2 million This zone represents 2% of the mass of the Sun. transport zone Turbulent plasma movements ensure transfer of degrees. energy towards the outside. Bubbles of hot matter rise, cool down and then descend back. This convective transport is analogous to that observed in a saucepan of hot water. These movements cause the granulation observed on the photosphere (see p. 14).

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 wThe Sun 14 w VOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 15

TRANSITION ZONE Between the chromosphere and the corona, there is

a transition zone, exhibiting an abrupt rise in n o i

THE CORONA s

The visible part i

temperature of up to 1 million degrees towards the V

l a t

The corona is the i

exterior. This temperature rise is linked to g i D This part of the Sun is directly observable. It consists of the outermost layer of

magnetic field effects. These complex mechanisms © photosphere (boundary with the opaque zone), chromosphere are being studied, today, using many satellites, the solar atmos - and the corona . These latter two are only visible during total including SOHO. phere. It has a tem - perature of around eclipses (natural or man-made) of the Sun. 1 million degrees and its density is 100 mil - lion times lower than that of the Earth. It THE PHOTOSPHERE extends to a distance The photosphere contributes 99% of solar of several solar radii and blends into the interpla - radiation. The surface, several hundred netary medium. The photosphere is so brilliant kilometers thick, is a virtual boundary that it prevents the corona being observed, except between the interior of the Sun (the radiative during total eclipses of the Sun. At other times, zone including the core and the convective coronagraphs must be used, devices which screen zone) and its atmosphere (chromosphere and the light from the disk of the photosphere. corona). Its temperature is around 6,000 °C.

Granules Observation of the photosphere Sunspots shows granules whose centers are Seen as wide dark areas, sunspots may be up to more brilliant than their circumfe - several hundred thousand kilometers in diameter.

L rence. These zones, with a diameter They appear and disappear in groups and return A S

M of around 1,000 km, have an indivi - regularly, with a period of around 11 years . Their L

© dual surface area comparable to that observation of France. Their lifespan is several minutes. This granular This cycle is today known as the solar cycle and makes it possible is linked to solar magnetism, the polarity of structure reflects the motions of the hot material which is to measure the which inverts every 11 years. bubbling in the convective zone; the darker edges of the superficial granules correspond to cooled matter. These are the des - rotation of the Sun; this fact allowed Galileo to cending streams which produce the acoustic waves which discover, in 1613, that the Sun rotated on its own axis. penetrate down to the core of the Sun. Today, we know that sunspots correspond to areas which are the seat of magnetic fields several Prominence thousand times larger than the Earth's magnetic field. These magnetic fields decelerate the motion of charged particles, the THE CHROMOSPHERE temperature drops (4,000 The chromosphere is the lower part of the solar atmosphere; °C) and the area becomes it contributes only slightly to solar radiation. It is a very non- darker. The diversion of uniform layer with a mean thickness between 2,000 and particles by these fields, ) a

3,000 km. Its temperature is several thousand degrees and s

associated with the a N increases towards the exterior where it reaches 20,000 °C. / creation of local magnetic A S E (

It is visible for short periods during total eclipses. It is a

fields, leads to extremely o h o

theatre of intense activity. It is here that solar flares are S

violent motions of solar born. These violent phenomena can spread over hundreds material. © of thousands of kilometers in several minutes.

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 w The Sun 16 w VOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 17

Prominences and filaments Prominences or filaments are dense, cold pockets of plasma Under the conditions of temperature and pres - (10,000 degrees nevertheless!) in the hot, dilute solar corona. Solar flares or coronal mass ejections sure found on the earth, matter is present in These gases, which are mostly hydrogen, leave the surface in Solar flares or coronal mass ejections occur three states: solid, liquid and gaseous. Pas -

the form of columns which stretch out into space or which a s a when the magnetic fields are sufficiently N sage from one state to another corresponds to return to the surface of the Sun, forming loops. An intense / A S strong to overcome gravitational attraction /E magnetic field lifts the dense matter and brakes its motion, T a reorganization of the molecules or atoms in I /E and eject the matter (plasma and neutral m leading to cooling. Prominences can rise to altitudes of several iu rt gas) out of the solar atmos - the matter. Take water as an example: in the so hundred thousand kilometers. on o C oh phere. Billions of tons of solid state, as ice, water has a very organized © S matter are transported structure, in which the molecules are strongly in this way. The flares bound to each other. Following application of are characterized by an abrupt release of heat, the molecules become agitated and move an enormous away from each other and the water moves into quantity of energy the liquid phase. At higher temperatures still, in the form of the structure becomes completely disorga - radiation (visible, IN THE CORONA AND BEYOND: THE FURIES OF THE SUN UV, X-ray and nized and the water molecules scatter in the In the upper, low-density layers of the solar atmosphere, charged particles radio waves). The form of a gas; this is boiling.

a (ions and electrons) are sensitive to electromagnetic forces. These forces may s a energy brought into N / A be stronger than gravity and can keep the matter in altitude, or even eject it out S play is considerable, /E THE SOLAR PLASMA m of the corona, sending it off into interplanetary space. The corona can therefore u ti on the order of several or ns Under conditions of extreme pressure and tem - presents very varied aspects, resulting from the magnetic activity of the Sun: Co million times the annual ho So perature a new state of matter appears in prominences, solar flares or coronal holes. © consumption of electrical energy in France. which the atomic structure itself becomes totally disorganized: a plasma . At the core of a star a great agitation This word first appeared in the reigns among the 1920s in the work of the American Irving Langley, Nobel atoms. These atoms Prize winner for chemistry in 1932, on ionized gases. Today normally exist as a the plasma is present in our daily nucleus, which itself is lives with plasma television Solar wind Coronal holes screens, plasma torches and made up of neutrons neon tubes. Beyond a certain distance, gravi - Coronal holes are less luminous, (with and neutral elec - tational forces are insufficient to cooler regions, where the hold on to the matter. This magnetic field enables matter to tric charge) and protons (with positive electric matter escapes along the escape. In these regions, the charge), accompanied by a stream of electrons magnetic field lines into inter - magnetic field lines do not (with negative electric charge). Overall, the ©

E planetary space. This is the solar I return to the Sun but project T atom is electrically neutral: it contains as many C o wind. Its mean velocity is around n radially away from the Sun. s o electrons as protons. At very high temperatures, rt iu 400 km/s. It is more rapid near m ( ES the atom can lose or gain one or more elec - the poles of the star than it is at A/ Na the equator. sa) trons, thus becoming a positively or neg atively

A ES charged ion, respectively. ©

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 w The Sun 18 w VOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 19

visible to the eye

10 -14 10 -12 10 -10 10 -8 10 -6 10 -4 10 -2 1 10 2 10 4 10 6 wavelength radiation from γ (radioactive and cosmic UV IR radio waves (m) sources ) How is this phenomenon explained? Very simply by the fact that the intensity of radiation X-rays microwaves (television, radar...) decreases as it crosses a material environment frequency 10 24 10 22 10 20 10 18 10 16 10 14 10 12 10 10 10 8 10 6 10 4 10 2 (Hz) to which it transfers part of its energy. However, an emission or absorption of energy results in extreme visible extreme visible a modification in the electronic structure of limit visible light limit the atoms concerned; this gives rise to a spec - 400 450 500 550 600 650 700 wavelength tral signature which is characteristic of a given (nm) element. To put this another way, the spec - ultra-violet infrared trum is a chemical identity card of the mate - rial being studied. Hence, the French astrono- O

A mer Jules Janssen (1824-1907) discovered, O

frequency N /

p by spectroscopy, an element that was unknown 14 7,6 65 r (10 Hz) a h S

l at that time on Earth, helium, whose name is e g Electromagnetic waves divided into various frequency and wavelength families. The distribution of energy from radiation is called i N derived from the Greek word "Helios" referring the electromagnetic spectrum and extends, in decreasing wavelength order, from low frequency radio waves to gamma rays, © passing through the microwave region, the infrared, visible and ultraviolet, and X-ray regions. The solar spectrum exhibits dark bands superimposed on a to the Sun. continuous spectrum. From all the wavelengths of radiation emitted by the Sun, the terrestrial atmosphere only In the core of the Sun, the energy is sufficiently AN IDENTITY CARD FOR THE SUN lengths: it provides an emission spectra consis - allows visible light to pass (with a small amount high to strip the electrons from the hydrogen For a long time, information about the Sun ting of a wide continuous band of frequencies. of infrared and UV) together with radio waves. atoms, which normally consist of one proton could only be gained from its light. We know In 1814, the German physicist Joseph von It is therefore necessary to go beyond the ter - and one electron. Hence, the hydrogen atoms from the work of Isaac Newton (1642-1727) Fraunhofer (1787-1826), the founder of spec - restrial atmosphere to measure other wave - become positively charged ions called protons. that light which appears to us as white is in troscopy, studied the spectrum from the Sun lengths of radiation (in particular X-rays and Stellar gas is in the form of dense plasma, made fact made up of all colors, or "of all wavelengths" and discovered the presence of dark bands gamma rays). We can use instruments on board up of ionized atoms. Almost all of the mass of as physicists would say. One just has to look superimposed on the continuous spectrum, satellites such as SOHO. the galaxy is found in the form of plasma. at a rainbow, or to observe a light source each corresponding to a specific wavelength. Production of light and heat is not the only From the core to the periphery, the solar plasma through a prism, to be convinced of this. Two At the time, he didn't have the means to inter - manifestation of the Sun: to these must be contains elements in various states of ioniza - centuries later, spectroscopy would be born pret this curious phenomenon. Later physicists added periodic vibration which can reveal tion, depending on their temperature and den - from this discovery (see illustration above): the noticed that when a gas is illuminated by white internal rotation and magnetism. sity. These variations stratify the Sun into several rainbow, "the scarf of Iris" according to the light source, it will absorb certain colors. The layers with very different behaviors. Various poets, is also a solar spectrum for physicists! appearance of dark lines superimposed on the HOW DOES THIS WORK? theoretical and experimental methods make it The tungsten filament in a light bulb is a good electromagnetic spectrum is the manifestation In 1960, regular, low amplitude motions of the possible to study the Sun, its composition, example of a body which, at high temperature, of this absorption: it is providing an absorp - solar surface were discovered. Ten years later, structure and properties. emits visible light over a broad range of wave - tion spectrum. theoretical work showed that in reality these

From the star to domestic energy 13 w The Sun From the star to domestic energy 13 w The Sun 20 w VOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 21

The evolution over time of the number of sunspots appearing on the solar photosphere. The approximately 11-year cycle is clearly visible but very variable.

motions were not simply superficial but reflected surface and are reflected by the various interior global motions which affected the whole of the layers, makes it possible to measure values Sun. These could be compared with a cavity such as the profile of the speed of sound or in which sound waves develop at very low fre - the profile of the rotation velocity.

A quencies, inaudible to the human ear. Sound moves at a velocity of around 340 m/s S E

© In fact, the sound waves produced by the Sun in the air that we breathe; in the Sun, this The SOHO satellite near to the Sun. are 17 octaves lower in frequency than the velocity is 500 km/s at the centre and around THE SOHO SATELLITE sound produced by a tuning fork with a fre - 7 km/s at the surface. quency set at 440 Hz: their frequency is of An acoustic wave penetrating to the core of SOHO (Solar and Heliospheric Observatory) is the fruit of conditions. SOHO is equipped with 12 observation an European and American collaboration. The satellite instruments: three of these are dedicated to seismology, the order of 3 mHz, corresponding to a period the Sun takes around one hour to traverse the was launched into space on 2nd December 1995, from the rest, to all the other atmospheric phenomena of the of around five minutes. star. This wave is very sensitive to the gaseous Florida by NASA. It is located 1.5 million kilometers Sun (solar wind, particle emissions, flares...). SOHO will from the Earth at a very special location known as the continue to observe the Sun for many years to come, Acoustic waves produced continuously by the matter that it encounters: investigating the Lagrange point. Here the gravitational pull of the Sun accompanied by other probes from Japan (HINODE), granulation at the surface, like rain on the sur - interior of the Sun by seismology makes pos - exactly matches that from the Earth. United States (STEREO, SDO) and by the CNES face of a drum, are a fantastic means to probe sible live and continuous monitoring of the state From this favored location, it can observe the Sun microsatellite, PICARD, to gain a better understanding of without interruption and in remarkably stable solar activity. the Sun throughout its opaque parts. The study of evolution of the Sun, in situ. of solar vibrations, which propagate from the Everything in the Universe rotates and the Sun

From the star to domestic energy 13 w The Sun From the star to domestic energy 13 w The Sun 22 wVOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 23

“Understanding the variation of the solar Profile of the internal rotation of the Sun obtai - magnetic activity ned by the Golf and MDI instruments on board will enable better SOHO: the red sections are rotating more rapidly forecasting, over the than the blue sections. The increase in rotation at coming decades, of its the core, in agreement with the signals from the impact on the first gravity modes, remains to be confirmed. a s terrestrial a N / A S E environment.” © The solar corona in a period of strong activity.

is also rotating; but it rotates in a differential Helioseismology, carried out by SOHO, has become a significant key in order to unders - motions. Today, it is associated with nume - way between the equator and the poles. We revealed that this differential rotation of the tand its effects on the human scale, as well rical simulations which try to reproduce the have seen that the surface of the Sun, the pho - surface is maintained over the entire convec - as the effective role of the Sun on the terres - observations in order to understand them. tosphere, is strewn with dark spots which one tive region (the low-density region represen - trial environment. In fact, the origin of external It has already been established that the dif - can follow over time: they provide a measure - ting 2% of the solar mass), then disappears magnetic phenomena located under the pho - ferential rotation of the convective region ment of the rotational speed of the Sun. It is abruptly in the transition region located at the tosphere, and its understanding, requires contributes to the presence of a magnetic possible to determine that the equator rotates boundary between the radiative and convec - knowledge of the internal motions of matter dynamo field which covers the whole convec - faster (with a period of around 25 days) than tive regions. Indeed, this region is the site of all the way to the centre of the Sun. Scien - tive zone and the transition region between the poles (period closer to 30 days). However, strong shear forces which contribute to rege - tists imagine meridional circulations for which radiation and convection, where this diffe- there is a relationship between the surface rota - nerate the magnetic field. Thus a large part of the velocity varies from tens of meters per rential rotation disappears, known as the tion and the magnetic phenomena. We can the solar mass (close to 50%) in fact rotates second (in the convective region), playing a tachocline . An understanding of this complex speak of an activity maximum when the number as a solid body. First observations of gravity determining role in explaining the 11 year Rapidly changing physics will need to describe of spots at the surface is at its highest: this is modes, by contrast, favor a more rapid rota - cycle, to fractions of microns per second in speed of rotation. how the magnetic field orga - the time period when magnetic activity of the tion of the nuclear core than of the rest of the the radiative region (hence suggesting one com - nizes and regenerates itself. It must also explain Sun is greatest, leading to strong solar flares. radiative region. If this is confirmed, it would plete rotation of this region requires a billion the duration and intensity of solar cycles, The SOHO satellite is well placed to monitor indicate that the Sun has retained at its core, years). which are not regular, having large maxima these activity phenomena, since it can conti - which holds nearly 50% of its mass, a rem - and large minima, as seen in the figure sho - nuously observe each region of the Sun. In nant of the rotation, which it acquired during HELIOSEISMOLOGY AND wing the evolution of cycles since they were periods of strong activity, large dark regions its formation, including probably a fossil SIMULATION first measured in the 17th century. These stu - and flares of matter are clearly visible at the magnetic field. Helioseismology is a necessary tool for provi - dies will, in the long term, make it possible to corona level (see above and p.16-17). Understanding this solar activity has today ding quantitative evidence of these large scale estimate changes in magnetic activity over the

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 wThe Sun 24 w VOYAGE TO THE CENTRE OF THE SUN w VOYAGE TO THE CENTRE OF THE SUN 25

coming decades. This has two important conse - quences: understanding the Sun enables us to understand other stars; but understanding the Sun also enables us to forecast pheno - mena likely to affect the Earth, such as magnetic storms which can cause many dis - turbances, notably to electrical networks and to high-frequency radio and satellite commu - nications. MEASURING ACTIVITY FROM THE CORE TO THE CORONA OF THE SUN A new discipline has appeared to estimate the R D impact of solar activity on the Earth's atmos - A E C The CNES microsatellite, PICARD, will observe the Sun at / n

phere. The PICARD satellite will help SOHO, u r

several wavelengths and will observe its superficial B . S the Japanese HINODE satellite and the Ame - . deformations. A

rican SDO satellite, to track and record all the © The magnetic field in a global non-linear simulation of the The radial velocity in the Sun's turbulent convective solar activity indicators from the Sun's nuclear Sun's convective dynamo. region. This convection is the source of heat transport to core to its corona. These measurements make the surface of the Sun. it possible to provide better estimations of the THE COAST PROJECT response of the stratosphere and the terres - trial atmosphere to solar perturbations, which After instrumentation and observation, simulation is the which have benefited from this program are in the develop from the visible domain to the UV and third channel of research in astrophysics. The most fields of cosmology, stellar physics, the study of proto- hard X-ray domains. Joint work on solar mode - beautiful images from astronomy only give a two- planetary discs and the study of the interstellar dimensional vision of the Universe. Thanks to the medium. ling and climatic modeling is in prospect, pro - power of new super-computers, today, scientists can vided that continuous observations are simulate the formulation and evolution of stars on a In stellar physics, stars are considered as large maintained over one or two decades. computer in sufficient detail to produce very high spheres of hot gas. They are turbulent objects, quality "virtual" images in three dimensions. exhibiting many convection phenomena; they rotate on This rapid growth in our knowledge of the Sun their own axis and are bathed in a self induced shows how such plasma can inspire a large The aim of the Coast program is to model complex magnetic field. The ASH computer program, supported astrophysical phenomena, in order to confirm current by the European commission via grant ERC-StG number of theoretical works, models and obser - theories on the physics of stars and to plan future STARS2, calculates the evolution of stellar fluids. vations with many enthralling implications for astronomical observations. The main investigations society.

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 w The Sun 26 w THE SUN TAMED 27

SOLAR ENERGY FOR HEATING AND ELECTRICITY PRODUCTION … The Sun tamed The Sun tamed The Sun constitutes an enormous source of The CEA took a particular interest in thermal energy in which we are permanently bathed. Man applications, at the time of the first oil crisis, has understood for a long time the importance at the start of the 1970s. Since 1978, it has of exploiting this source of heat and light. However, installed, in the Pacific, the first solar houses, in the past the problem has always been how to hospitals and hotels in the world. Since the recover this energy, transport it, store it and trans - 1980s, the CEA has pursued thermal solar solu - form it into electrical power. Exploitation of this tions for building applications and it has also energy source is recent. It is being developed, focused research on solar photovoltaics, an but it still remains very costly. Solar energy is area in which it has become a major player. one of the group of renewable energies . Currently, there are two Solar, wind, hydraulic, biomass and SOLAR THERMAL ENERGY geothermal energy sources . channels of fire Solar thermal energy is mainly used for water which solar energy is exploited, either directly or space heating. Thermal collectors are transforming radiation into heat or into electri - employed. There are various types, but the city, thermal and photovoltaic solar power, res - principle is always the same: solar collectors pectively. absorb photons from the Sun and transform their energy into heat. The collecting material must be strongly absorbing, such as chromium oxide, for example. The heat is then transfer - red to a liquid or gas (known as a "heat trans - fer fluid") which carries it to a thermal store. Four square meters of heat collectors are enough to supply a family of four with hot water, and 10 m² are enough to heat a 100-m² house at the latitudes found in France. This type of technique is, for example, used for "direct solar floors". In this application, the heat transfer fluid is transmitted directly into the floor of the building at a temperature of

n approximately 25 °C, providing comfortable o l u o C D

C thermal energy using thermodynamic tech - J

Polycrystalline silicon solar panels at the Cadarache research -

S centre. E niques. This requires high temperatures (of N I

PHOTOVOLTAIC SOLAR ENERGY beacons, roadside emergency telephones, The advantage of this technique is to convert domestic lighting, audio and visual applica - solar energy directly into electricity. This conver - tions and water pumps. But since the Sun is sion, known as the "photovoltaic effect", was dis - not present 24 hours a day, these applications covered in 1839 by Edmond Becquerel (1820- require the use of batteries which can insure 1891), but it was The father of Henri Becquerel, storage of energy for consumption outside of not until 1954 who was awarded the Nobel Prize daylight periods. for physics in 1903, jointly with that the first pho - Pierre and Marie Curie, for the tovoltaic cell with discovery of natural radioactivity. At the CEA, research is being carried out on high output (6%) appeared. Output for cur - lead batteries, derived from automotive accu - rent-day modules are of order 15-20 %. mulators. In particular, work is focused on Collections of photovoltaic Photovoltaic cells are extending the lifespan of batteries, which are cells in series, assembled made from semicon - currently much less than those of photovol - in a metallic framework e r i with their mechanical ductor materials, such as taic cells (on the order of 10 and 30 years res - W e

y structure. E silicon, produced from pectively). Recent work leads us to anticipate © A solar power plant in California for a thermodynamic utilization of solar radiation. very high purity raw materials. The "electronic" new technologies for storage, notably lithium quality is order 10 billionths of the impurity batteries, which will have lifespans equivalent ratio! to those of other components. order 1,000 °C) which are obtained by concen - thermal power plants. The power output from Currently, a third of the world's population does trating sunlight using mirrors. Indeed, which this type of facility is on the order of several not have access to electricity, but the Sun is of us has not tried, at some time or another, megawatts; for comparison a nuclear power everywhere. This is why this technique consti - INES: FRENCH NATIONAL to set light to a piece of paper using a mirror plant produces an electrical power output of tutes such a good solution for low energy SOLAR ENERGY INSTITUTE exposed to sunlight? This principle, known since 1,000 MW. A prototype power plant with tower consumption applications: examples include antiquity, is used today on a larger scale. There was constructed in France on the site at Tar - lighting, powering water pumps, refrigerators, INES was created in 2006 on the initiative of the public authorities, to promote and develop the use are thermodynamic units containing hundreds gasonne in the Pyrenees at the start of the televisions, etc., in rural regions. of solar energy in France and, in particular, to of mirrors (heliostats) which act by reflecting 1980s. It provided power from 1983 to 1986. There are two rapidly growing markets: auto - control energy use in buildings. solar radiation onto a boiler located on top of Power plants with troughs in parabolic collectors nomous applications, where the energy is pro - It was set up at Bourget du lac at the Savoie a tower. In this boiler, the heat exchange exist on the industrial scale; in this case, solar duced, stored and used at a single location, Technolac technology park, close to Chambéry. The Rhône Alpes region is a special area, with a liquids will store heat (they can reach tempe - radiation is concentrated on an axis where the and applications connected to a network often particularly dense concentration of leading players ratures of several hundred degrees), then trans - heat transfer fluid is heated by several hun - referred to as "photovoltaic roofs". in relevant fields: industrialists, researchers, local communities and associations. port the heat to a water reservoir where they dred degrees. This technology is employed in The earliest applications, and the most wides - A team of more than 150 researchers have exchange the heat with the water. Once heated, the world's largest, thermal solar-energy plant pread until 1990, appeared in the field of space developed solutions with industrial partners for the water turns to steam, then operates a tur - which is situated in California and has attained technologies for satellites and then for tele - solar, thermal and photovoltaic, power. bine to produce electricity as in conventional an electrical power of 150 MW. communications, for maritime and airborne

From the star to domestic energy 13 w The Sun From the star to domestic energy 13 w The Sun 30 w THE SUN TAMED w THE SUN TAMED 31

“Production of unlimited, "clean" energy that is available in all regions of the

world.” The photovoltaic cell contains electrical charges because of the doping: HOW DOES THIS negative in n-type (an excess of electrons), positive in p-type (lack of WORK? electrons). These charges create an electric field across the junction.

This technique profits from the optoelectronic properties of semiconductors which enable them, under certain conditions, to convert optical energy into electrical energy. The most advanced technologies use silicon as the base The second application, with connection to the material. network, consists of integrating photovoltaic

r A photovoltaic cell can be made up,

modules in the roofs and frontages of buil - a l

o Photons from sunlight strip the electrons

S for example, of two layers of silicon,

dings and houses, producing electricity which n from silicon atoms, creating positive and

e the transport properties of which m

e negative charges. will be either consumed on site or fed into the i have been modified by doping. S

network. © The addition of boron atoms favors An example of an autonomous application: conduction by positive charges. The telecommunications. silicon becomes p-type. In summary, the Sun constitutes a clean and Doping the silicone with phosphorus almost inexhaustible energy source, which pro - atoms improves conduction of mises a bright, but still expensive, future. negative charges. The silicon then No doubt we will learn to control this energy becomes n-type. Placing p-type silicon in contact source before it runs out in around 5 billion with n-type silicon creates a so years time... indeed the time will come when, called p-n junction. When this cell with its hydrogen running out, the Sun will The charges are made to move by the is exposed to solar illumination, electric field created by the junction; this electrons and holes are generated burn helium and transform it into carbon. But produces an electric current. at the p-n junction, which can be the process will not go any further: the tem - considered as the active area of the A

E cell. These electrons and holes then C

perature will not permit this stellar alchemy - e

u move, respectively, across the p and q i

to continue until the core was eventually n n zones, to reach the electrodes. h c e t reduced to iron. Rather, after the helium bur - r This series of processes, called the A /

n photovoltaic effect, leads to the o

ning phase, gravity will compress the Sun to h c i production of a current which can M .

the size of the Earth and the Sun will become D supply a piece of electrical

a white dwarf, a low luminosity star, which will © equipment. Silicon wafers are doped in this furnace, at a temperature fade away with a quiet death, unlike more mas - between 700 and 900 °C to create junctions which will become sive stars which explode into supernova. the core of photovoltaic cells.

From the star to domestic energy 13 wThe Sun From the star to domestic energy 13 w The Sun