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 © French Alltternattiive Energiies and Attomiic Energy Commiissiion,, 2011 Communiicattiion Diiviisiion Head Offffiice 91191 Giiff-sur-Yvetttte – www..cea..ffr 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 © “For the last 4.6 million years the Sun 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 © Solar energy powers a telephone kiosk. 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 © From the star to domestic energy 13 w The Sun From the star to domestic energy 13 w The Sun 6 w OUR STAR w OUR STAR 7 “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.