On the Edge of the Cosmos: A Century of Revolution in Astronomy 9782759827077

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Current Natural Sciences

Alain OMONT

On the Edge of the Cosmos A Century of Revolution in Astronomy

Cover illustration: The Antennae Galaxies (also known as NGC 4038 and 4039) are a pair of distorted colliding spiral galaxies about 70 million light-years away, in the constellation of Corvus (The Crow). This view combines ALMA observations, made in two different wavelength ranges during the observatory's early testing phase, with visible-light observations from the NASA/ESA Hubble Space Telescope. Source: https://www.eso.org/public/images/eso1137a/ (CC BY 4.0).

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EDP Sciences – ISBN(print): 978-2-7598-2706-0 – ISBN(ebook): 978-2-7598-2707-7 DOI: 10.1051/978-2-7598-2706-0 All rights relative to translation, adaptation and reproduction by any means whatsoever are reserved, worldwide. In accordance with the terms of paragraphs 2 and 3 of Article 41 of the French Act dated March 11, 1957, “copies or reproductions reserved strictly for private use and not intended for collective use” and, on the other hand, analyses and short quotations for example or illustrative purposes, are allowed. Otherwise, “any representation or reproduction – whether in full or in part – without the consent of the author or of his successors or assigns, is unlawful” (Article 40, paragraph 1). Any representation or reproduction, by any means whatsoever, will therefore be deemed an infringement of copyright punishable under Articles 425 and following of the French Penal Code. Ó Science Press, EDP Sciences, 2022

General Summary

Astronomy has been one of the fastest growing sciences in the recent decades, a field where a succession of major discoveries was followed by all audiences with passionate interest. The purpose of this book is to present the major achievements in astronomy in the last century in a succinct way, review the most recent advancements, and provide a sketch of the future. We hope that the reader, when introduced to the richness of the Universe, will appreciate how the recent discoveries have deeply changed our vision of the world. Astronomy has much more evolved in the last century than in the previous two centuries when it simply continued to progress among the other sciences, after having spearheaded the first scientific revolution in the 16th and 17th centuries with Copernicus, Galileo and Newton. Today, it seems surprising that, only a century ago, the vision of our ancestors was so short-sighted. In contrast, our Universe is incomparably richer after the continuous series of fundamental discoveries made in the 20th century. In a hundred years, the known Universe has gained an enormous factor in both its dimensions and history spanning 14 billion years since its origin. We have discovered that the observable universe is made of hundreds of billion galaxies, surrounded by vast empty spaces, each containing an exuberant variety of stars, exoplanets, black holes and many other objects with extreme physical conditions. Yet, we remain amazed by its surprising unity. As far as we look with our telescopes, reaching towards the edge of the Universe, we find in each galaxy exactly the same physics, the same atoms, practically the same classes of stars and galaxies comparable to our Milky Way and its neighbors at various stages of evolution. Today, most of the questions about the nature and the physics of the various stars and their history have been answered. However, despite these remarkable advances, our Universe is still rich with fundamental unsolved questions regarding its physics, its basic constituents and their nature, including the origin of life. This recent progress in our exploration of the Universe is directly linked to the accelerated path of knowledge in physics and the fast-evolving technical

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General Summary

development throughout the 20th century. Understanding the Universe and its constituents would not have been possible without the revolutions of quantum and relativistic physics. Astronomy has also fully benefited from the technological revolutionary progress in multiple domains, resulting in increasing sizes and performances of telescopes together with improved sensitivities and greater pixel numbers of the detectors. It is now possible to explore the Universe at wavelengths other than the visible (radio, millimeter, infrared, ultra-violet, X-rays and gamma-rays), as well as through non electromagnetic signals (cosmic rays, neutrinos, gravitational waves), enabling, by their combination, to trace the full complexity of celestial sources. Observing from space allows us to get free from atmospheric blockages and disturbances. The exponential growth in information technology opens up almost unlimited possibilities for data processing and analysis. Without aiming to be exhaustive, this book proposes an overview of the main advances in astronomy in the 20th century, highlighting the major breakthroughs in our discovery of the Universe and providing sufficient details to capture the overall progress of this science. It is divided into five main themes: stars, galaxies, cosmology, high energy astrophysics and planets. For each of them, the most revolutionary progress made in the last century will be highlighted. The observational knowledge of stars was already remarkably advanced in 1900. The 20th century revolution in this field consists in the deep understanding of the physics underlying stellar evolution. This provided the answer to two crucial questions: (i) their source of energy; (ii) the origin of the chemical elements of which we are made. On the other hand, it seems difficult to imagine that the concept of galaxies had not yet been accepted only a century ago, while the role of galaxies, including ours, is now part of our cultural background. The study of galaxies has since become one of the major areas of research in astronomy. Modern cosmology was actually born with the discovery of the recession motion of nearby galaxies and the expansion of the Universe that it implies. It has gradually flourished for almost a hundred years in the context of general relativity and particle physics. Cosmology, i.e. understanding the global properties of the Universe, is today one of the keystones of astrophysics, raising some of the fundamental questions of physics. Likewise, the most violent activity taking place in the Universe remained practically unsuspected a hundred years ago. Its gradual exploration has revealed a whole series of extraordinary objects, including supernovae, quasars and black holes, bringing enormous energies and extreme physics into play. Finally, the last third of the 20th century saw the fulfillment of two recurring dreams of humanity: first the beginning of direct exploration of the Solar System, then the discovery of planets around stars other than the Sun. Today, there are rich future prospects for the search for extraterrestrial life, and the study of exoplanets has become one of the most promising astronomical projects. Since there is no need for complicated mathematics to understand the deep reality of the astronomical world, mathematical formulas and complex notions of physics have been mostly avoided in this book. However, to provide precise benchmarks on astrophysical objects and phenomena, one cannot completely ignore a few basic notions such as the powers of ten, the constituents of the atom or

General Summary

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the scales of temperature and energy. A brief glossary aims to provide guidelines and, for curious readers, the key to explore more complex concepts. This book is intended for readers of any level albeit full of curiosity. It aims to give everyone the opportunity to wonder at the exuberant richness of the Cosmos, penetrating right into the fabulous world of astronomers. The aim is to give keys so as to understand some of the main questions that astronomy raises. To use Newton’s beautiful image, we are still on the shore of the Universe, facing the immensity of its mysteries. We begin to superficially penetrate its many secrets, playing with its unusual wonders, while still remaining at the edge of the unknown.

Acknowledgments

I am deeply indebted to my fellow astrophysicists and friends, Steve Shore and Pierre Cox, for the considerable time they spent at improving my poor English. Their insight and recommendations for the astronomical content were also of immense value. I warmly thank Michèle Leduc and Michel Le Bellac, editors of the “Introduction to” series, who made this book possible. I am grateful to them for constantly encouraging me and providing an extremely thorough proofreading and priceless advices for the two versions. Thanks also to the whole team of EDP Sciences for their high-quality editing work, especially to Mrs. Sophie Hosotte and Mrs. France Citrini. I address my thanks to all those who provided an important help for the French version, including James Lequeux, François Bouchet, Lionel Provost, Jean Mouette, Jean-Pierre Bibring, Benoît Mosser, Martin Harwit, Laurent Vigroux and Suzy Collin-Zahn. I would like to express my appreciation to several organizations whose copyright policy is particularly user-friendly and allowed EDP to use a rich iconography, in particular, ESA, NASA, ESO, CNES, JPL, GSFC, JAXA, NRAO, NOAA and Wikipedia.

To all readers around the world eager for astronomy. To young astronomers and students who will make the astronomy of the 21th century.

Contents General Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII Part I A Century of Revolution in Our Vision of the Universe . . . . . . . . . .

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CHAPTER 1 General View of 20th Century Astronomy and Its Starting Point . . . . . . . . . 1.1 Astronomy, the Key to Our Vision of the World . . . . . . . . . . . . . . . . . 1.2 Benchmarks on 1900 Astronomy and Its Shortcomings . . . . . . . . . . . .

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CHAPTER 2 Scientific and Technical Revolutions, Drivers of 20th Century Astronomy . 2.1 Physics Revolutions, Keys to Astrophysics . . . . . . . . . . . . . . . . . . . 2.2 Giant Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Overcoming the Disturbances of the Earth Atmosphere . . . . . . . . . . 2.4 Exploiting All Spectral Domains from Radio to X-ray and Gamma-ray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Visiting the Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 No Pause in the Progress of Signal Detection and Exploitation . . . .

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Part II Stars are Well Understood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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CHAPTER 3 How does a Star Work? . . . . . . . . . . . . . . 3.1 Understanding the Stars . . . . . . . . . 3.2 Solving the Mystery of the Origin of and the Stars . . . . . . . . . . . . . . . . . 3.3 The Life of the Stars . . . . . . . . . . . . 3.4 Our Atoms were Born in the Stars . 3.5 Stars also Die . . . . . . . . . . . . . . . .

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the Energy of the Sun . . . .

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Complexities of Star Birth and Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 General Star Formation Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.2 4.3 4.4 4.5 4.6 4.7

Young Infrared Stars: Born in Dusty Cocoons . . . . . . . . . . . . . Gravitational Contraction, Accretion and Discs . . . . . . . . . . . . Universality of Stellar Pairs – Complex Ending of Their Lives . Brown Dwarfs, Billions of Aborted Stars . . . . . . . . . . . . . . . . . Stars are Still at the Forefront of Current Astronomy . . . . . . . Stars and Ecology of Planets and Galaxies . . . . . . . . . . . . . . .

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59 59 61 63 65 68

The New World of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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CHAPTER 5 Discovery of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 The Appreciation of the Nature of Galaxies Dates Back Only to the Beginning of the 20th Century . . . . . . . . . . . . . . . . . . 5.2 First Steps in the World of Nearby Galaxies . . . . . . . . . . . . . 5.3 Architecture and Stellar Content of Galaxies . . . . . . . . . . . . .

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CHAPTER 6 Our Galaxy and Its Interstellar Medium . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Exploration of Our Galaxy, the Milky Way . . . . . . . . . . . . . . . . . . . . 6.2 An Ordinary Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Current Organization of Stars Resulting from the Milky Way History 6.4 The Interstellar Gas, a Key Player in the Evolution of Galaxies . . . . 6.5 Other Players in the Interstellar Medium . . . . . . . . . . . . . . . . . . . . . 6.6 Exotic Components of the Milky Way . . . . . . . . . . . . . . . . . . . . . . . .

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83 83 84 86 86 90 94

CHAPTER 7 Hundreds of Billions of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Galaxies at All Stages of Their Life . . . . . . . . . . . . . . . . . 7.2 The Turbulent Family Life of Galaxies . . . . . . . . . . . . . . 7.3 Understanding the Formation and Evolution of Galaxies .

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Part IV Cosmology, the Science of the Universe as a Whole . . . . . . . . . . . . 109 CHAPTER 8 Birth 8.1 8.2 8.3 8.4

of Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Universe of Galaxies is Expanding . . . . . . . . . . . . . . . . . . . . . . The Saga of the Big Bang Confirmation . . . . . . . . . . . . . . . . . . . . . The Very First Phase in the History of the Universe: Uncertain Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Well-Understood Second Phase: The Standard Big Bang Model . .

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CHAPTER 9 Content of the Universe and Structure Formation . . . . . . . . . . . . . . . . . . . . . 123 9.1 Formation of Galaxies and Structures of the Present Universe . . . . . . . 123

Contents

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Fundamental Parameters of the Universe are Better Known than Its Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Age of the Universe and Variations of the Determinations of the Hubble Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Overall Density Very Close to the Critical Density . . . . . . . . . . Need and Nature of Dark Matter . . . . . . . . . . . . . . . . . . . . . . . . . . A Last-Minute Surprise, the Re-Acceleration of the Expansion Involving an Unknown Source of Cosmic Energy . . . . . . . . . . . . . . . Summarizing: An Unexpected Universe Model Validated in Multiple Ways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Part V Singular Stars and Cataclysms in Extreme Physical Conditions . . . 135 CHAPTER 10 . . . . .

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Holes and Their Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Black Holes, General Relativity and the Cosmos . . . . . . . . . . . . . . . . Stellar Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gravitational Waves, Propagation of Spacetime-Curvature Disturbances . . . Quasars: New Stars a Thousand Times Brighter than Galaxies . . . . . Manifestations of Super-Massive Black Holes and Their Interpretation . . . Co-evolution of Galaxies and Their Black Hole . . . . . . . . . . . . . . . . . The Super-Massive Black Hole of Our Galaxy and Others . . . . . . . . .

155 155 157 158 162 164 170 171

Explosions of Stars and Their Singular Residues . . . . . . . . . . 10.1 Extreme Physics of Supernova Implosion/Explosion . 10.2 Neutron Stars, Hyper-Dense Supernova Residues . . . . 10.3 Gamma-Ray Bursts, Even More Powerful Bursts . . . . 10.4 Cosmic Rays, Messenger Particles of the High Energy

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CHAPTER 11 Black 11.1 11.2 11.3 11.4 11.5 11.6 11.7

Part VI

Planets, in the Solar System and Outside . . . . . . . . . . . . . . . . . . . 173

CHAPTER 12 Direct 12.1 12.2 12.3 12.4 12.5 12.6 12.7

Exploration of the Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Planets, Stars of Astronomy until the 19th Century . . . . . . . . . . . Half a Century Without Revolution for Planetology . . . . . . . . . . . Humans Went to the Moon! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . We Broadly Understand the Origin of the Moon and Its Importance for the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Very Rich Close-up Photos of All the Bodies of the Solar System . Summary of Planetary Expeditions . . . . . . . . . . . . . . . . . . . . . . . . Searching for Life in the Solar System: Where and When? . . . . . . .

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181 183 184 191

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CHAPTER 13 Entering the Dream World of Exoplanets . . . . . . . . . . . 13.1 Explosion of Discoveries of New Planets . . . . . . 13.2 The Majority of Stars have a Planetary System . 13.3 Surprising Variety of Exoplanets . . . . . . . . . . . . 13.4 The Search for Earth-Like Planets . . . . . . . . . . .

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Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 CHAPTER 14 A New Cosmos in the 21st Century? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 14.1 A New Cosmos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 14.2 Auguries for 21st Century Astronomy? . . . . . . . . . . . . . . . . . . . . . . . 212 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 Acronyms and Space Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

The asterisks * indicate words or acronyms whose meaning is explained in the “Glossary” or “Acronyms and space missions” sections at the end of the book.

Part I

A Century of Revolution in Our Vision of the Universe

Chapter 1 General View of 20th Century Astronomy and Its Starting Point 1.1

Astronomy, the Key to Our Vision of the World1,2,3,4,5

Astronomy has the immense advantage of immediately appealing to all audiences because the sky and the stars have always occupied a prominent place in all cultures around the world. Whatever its motivations – astrology, precision of the calendar, prediction of eclipses, navigation, simple curiosity or fundamental questioning on our nature – astronomy has constantly been at the forefront of the activity of the human mind in its deepest and most prodigious efforts to understand and represent our cosmic environment. What about the singular era we are living in after a century of exploding discoveries? How can each of us grasp the significance of these discoveries about the Cosmos that even reach the media headlines? How can we analyze the many factors that can explain this universal interest, or even the fascination, that astronomy exerts on us? The most immediate feeling is wonder. The scrolling of sumptuous images of galaxies, nebulae and planetary landscapes, the repeated revelation of new monstrous astronomical objects and the incessant announcement of fantastic discoveries give the impression of visiting a wonderland with the eyes of a child. Our hyper-mediatized generation is taking full advantage of the sparkling images of the Universe that giant telescopes, satellites and interplanetary probes offer us in abundance. Everyone can juggle with the billions of light years, the images of quasars and the magical concepts of Big Bang, supernovae, brown and white dwarfs, exoplanets, black holes, gravitational waves, even dark matter and dark energy… More deeply, astronomy undeniably touches the dream world. Since the dawn of our species, the night world of the stars has made people dream. The unchanging ballet of the Sun and the stars sets the rhythm of day and night and the seasons. The capricious tracks of the planets challenge the imagination. The magic of the Moon adds a monthly modulation of the depth of the nights. The realm of the stars, inaccessible but so present in the lives of our ancestors, has often been assimilated to the divine. Astrology, which asserts that the sky controls human destiny, has deeply impregnated most civilizations up to ours. From time immemorial, humans have DOI: 10.1051/978-2-7598-2706-0.c001 © Science Press, EDP Sciences, 2022

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dreamed of flying and entering the world of the stars. The myth of Icarus and of voyages to the Moon have been replaced by the crazy media focus on interstellar journeys and galactic empires.

FIG. 1.1 – Cosmic history. A pictorial diagram of the cosmic history of the Universe from the origin of the Big Bang to the present time, 13.8 billion years later. That the Universe has a history that can be told is probably the most important discovery of astronomy in the 20th century. The main stages of this history are shown here on an approximately logarithmic time scale. Credit: NASA/CXC/M.Weiss. In any case, it is certain that one of the main drivers behind the explosion of astronomical discoveries is the curiosity of scientists for the unknown, followed by that of the general public and above all fuelled by recent incredible advances in technology. Our curiosity is irresistibly triggered by the direct or intellectual exploration of the Universe. Half a century after the completion of Earth exploration, the landing on the Moon and interplanetary probes opened the direct exploration of the planets and satellites of the Solar System. From Galileo’s astronomical telescope 400 years ago, the constant progress of telescopes has allowed us to dive deeper and deeper into the Universe to discover new heavenly bodies, such as Jupiter satellites and Neptune, galaxies, quasars and their central black holes up to today’s exoplanets and merging black holes. Our giant telescopes now search the confines of the Universe and its galaxies as far as thirteen billion years ago (figure 1.1). Through modelling, we even travel upstream in the river of time up to tiny fractions of a second after the Big Bang.

General View of 20th Century Astronomy and Its Starting Point

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In this whirlwind of discoveries, it is important to consider the impact of astronomy on our vision of the world and of ourselves. By illuminating our cosmic origins, it profoundly influences our perception of our history, the environment and nature. Historically, it has shaped our representation of space and time. It initiated the emergence of scientific thought from the first accumulations of meticulous observations by all civilizations up to the foundations of modern physics. It has always inspired philosophy and deeply influenced religions as shown by the astronomical connections of the location and orientation of sacred places since the earliest megaliths and pyramids, or the impact of calendar problems and the most spectacular cosmic phenomena such as eclipses and comets. The unchanging Earth-centered starry sky was the corner stone of Aristotle’s philosophy. Copernicus, Kepler, Galileo, Newton and many others overthrew this antique world of an Earth-centered Universe that seemed so well founded on the immediate experience of our senses. They put the Earth in its proper place in the Universe and desacralized the heavens and stars. This has irreversibly thrown us into the infinite universe, as visionaries, such as Giordano Bruno intuited. It made the plurality of inhabited worlds plausible. In short, few fields besides astronomy have contributed as much to the birth of modern humanity, at the cost of an irreparable break from our cultural matrix based on the magical and coherent world of shamans and religions. The consequences of this revolution were extensively explored in the 18th and 19th centuries, constantly expanding the inventory of the Universe and beginning to disentangle it. Yet, our vision of the Universe has been much more transformed and enriched over the past century than in the previous two. The understanding of the Universe by astronomers in 1900 cannot compare with the present knowledge based on a long list of discoveries made during the 20th century, including, galaxies and their interstellar matter, the expansion of the Universe since the Big Bang, the structure and history of stars and how they synthesize the chemical elements we are made of, lunar and Martian landscapes and exoplanets, quasars and black holes, gravitational waves and other cosmic phenomena whose violence exceeds the imagination. The stars are so distant that they have practically no influence on our world and our individual lives. Venturing out the Solar System will long remain a utopia for humanity in the light of the urgent problems it faces for organizing itself in peace with a minimum of justice, managing the Planet and simply surviving in the turmoil of demographic, technological, environmental and cultural challenges. Yet astronomy is not a luxury. Beyond the dream, it turns out to be one of our fundamental windows on reality. Unravelling the Universe is just extending the long journey of the intellectual development of our human and pre-human ancestors for handling the reality of their environment. Becoming aware of the world of stars and galaxies, its dimensions and its temporal thickness is essential to fully grasp who we are, combining our insignificance in the infinite Universe with the almost limitless power of our intellect, yet burdened with illusions. Some of the most mysterious frontiers of our current knowledge of the world and matter lie in the field of astrophysics. Knowing the details of the functioning and evolution of the stars, including the Sun, remains a prerequisite for understanding our planet, its origin, history and future. Nor are the stars isolated. Our global environment already extends to our

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On the Edge of the Cosmos

Galaxy. Despite its immensity, we feel that the Milky Way may be within our reach in the distant future, if we survive. Moreover, we cannot think about the real world without the vast universe of galaxies and the surprising unity of its matter and stars. Finally, it is impossible to escape the question of the existence of extraterrestrial cosmic life. It has been asked with a new urgency since it was demonstrated that the Earth is only a planet among myriads of other ones. This question has stimulated passionate interest for more than four centuries, without being able to decide between the whole range of possible answers, not even excluding the most extreme ones: an (intelligent) life omnipresent on some planets in all galaxies, or, on the contrary, the uniqueness and solitude of our humanity in the whole Universe. Some reflection is needed to judge the importance and significance of a general and evolving science such as astronomy; despite its current acceleration, one century seems to be an appropriate time measure to consider its development: the 20th century, which has seen such dramatic changes for humanity and its history, especially since the First World War. The beginning of the last hundred years or so, which will be covered in this book, corresponds to a new era of physics with the revolutions of quantum microphysics and relativity. It is also a good starting point for astronomy; with the recognition of the concept of galaxies in the 1920s and the birth of astrophysics and cosmology, which occurred in the light of these revolutions of physics.

FIG. 1.2 – Popular Astronomy books of Agnes Clerke11 and Camille Flammarion12. Facsimile of the cover pages and photography of their authors. The immense success of these works, first published in 1885 and 1879, respectively, and reprinted several times, shows the public’s interest in astronomy at the time.

General View of 20th Century Astronomy and Its Starting Point

7

We will therefore begin with the state of astronomy at the end of the 19th century as described, for example, in Camille Flammarion’s or Agnes Clerke’s famous books about popular astronomy11,12 (figure 1.2). The immense success of these books is evidence of the public interest for astronomy during the Belle Époque, a period of time that seems simultaneously very close and very distant. With industrialization, colonialism, nationalism and the aspirations of socialism, this time lies at the crossroads of 19th century’s faith in science and the explosion of its consequences in the 20th century – from the transformation of our lives through medicine, technological innovation and computer science, to nuclear and environmental nightmares. The astronomy of 1900 seemed to be a mature science proud of its great successes in the two previous centuries. However, future would reveal the limits of its representation of the Universe.

1.2

Benchmarks on 1900 Astronomy and Its Shortcomings16,18

Heritage of three Copernican centuries At the end of the 19th century, astronomy was a well-established science. It has had thousands of years of history being the oldest science in all civilizations and had benefited from the many contributions of earlier cultures such as the Egyptian, Babylonian, Greek, Arabic, Persian, Indian, and Chinese ones1. The enormous amount of observations accumulated since the dawn of these civilizations, combined with the invention of geometry, trigonometry and elaborate empirical models, had led to fundamental astronomical results of surprising complexity and precision since Antiquity: in-depth knowledge of the motion of the Sun, Moon and planets; precise prediction of solar and lunar eclipses; catalogues of the positions of thousands of stars; early identification of the very slow apparent motion of stars in the sky caused by the displacement of the Earth’s axis (“equinox precession”); setting calendars with the remarkable accuracy required for agricultural and religious needs; measuring the Earth’s radius, the distance from the Earth to the Moon and, less accurately so, to the Sun2; archiving memorable events: comets, exploding and new stars, etc. However, the astronomy of 1900 is above all the product of the 16th and 17th century scientific revolutions that astronomy itself opened up, creating modern science and changing our vision of the world. As early as 1543, Copernicus showed that it is much more logical and economical to abandon the idea that the Earth is the center of the world, allowing it to rotate on its axis and turn around the Sun as just one planet among others. The power of this evidence was such that it was

1

It is known that Amerindian astronomy, especially Mayan astronomy, had reached a remarkable level, but the lack of communication prevented any contribution to later world astronomy. 2 The distance from Earth to the Sun was only determined with some precision in the 17th century by Cassini and Richer.

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gradually accepted in the ferment of the ideas at Renaissance, in the following century. It was supported by the formulation of the laws of planetary motion by Kepler (circa 1600) and the physics of Newton (1686) and by the revolution in the observation methods introduced by the astronomical telescopes from Galileo and Harriot (1609). In the worldview of the Copernican system, it was possible and even probable that the Sun is just another star among the multitude of stars. The astronomical distances deduced from the apparent fixity of the stars on the celestial vault imply that their luminous power is extremely great, inducing their possible similarity to the Sun. The consequences for humanity’s place in the Universe were immense. The impetus of this dual conceptual and technological revolution propelled astronomy from the 17th to the 19th century. Among other rapidly developing sciences, it has constantly kept a key position for the academies, the governments and the general public, with a primary focus on the movement of the planets and the cataloging and inventory of the sky. The increasingly refined mathematical theory of the movement of planets had an impact on the most sophisticated developments in mechanics126. It thus fuelled a significant part of the activities of the young science academies, their debates, controversies and competitions. It culminated for the general public and the media with the discovery of Neptune in 1846 (figure 1.3), whose international well-mediatized impact remains a triumph of 19th century astronomy. People had been similarly awed a century earlier by the accuracy of the calculation predicting the return of Halley’s Comet in 1757. The new astronomy had been impressive by rationalizing and capturing the passionate attention of the popular mind as usual anxious of bad events brought by comets. The impact of the passage of Halley’s Comet in 1910 is probably one of the best measures of the importance of astronomy at the beginning of the 20th century.

FIG. 1.3 – Astronomers involved in the discovery of Neptune. The planet was discovered on September 24, 1846 by Johan Gale at the Potsdam Observatory following published calculations by Urbain Le Verrier’s, while John Adams at Oxford had performed similar, but unpublished, earlier calculations. The stir of this discovery, accompanied by violent international controversy, is illustrated above by Le Verrier’s reception by King Louis-Philippe.

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Nevertheless, popular enthusiasm and curiosity were far from being the only, or even the main driving forces behind the development of astronomy in the 17th and 18th centuries. The main astronomical observatories of the time, in Paris and Greenwich, were royal creations (in 1667 and 1675) to provide logistical support to navigation and mapping as essential motivation. In particular, the determination of the longitude of a point on Earth is a difficult problem for which astronomy was fundamental for creating tables (“ephemerides”) giving the position of the Moon and Jupiter’s satellites. In the 18th century, this institutionalized astronomy took on a new dimension with the organization of coordinated measurement campaigns at various points on the globe, such as for the transits* of Venus in front of the Sun in 1761 and 1769 (with competition between English and French expeditions!) and the determination of the shape of the Earth and its flattening. Astronomers also played a decisive role in drawing up the first accurate maps, particularly of France, and then in measuring the length of the Earth’s meridian to produce the standard meter. This visibility of astronomy since the 18th century extended far beyond scientific circles and strategic interests. It strongly influenced philosophers, in particular E. Kant and the Enlightenment thinkers. The question of the plurality of inhabited worlds has constantly fascinated people’s minds, as witnessed by the success of the works of Fontenelle (1686) and Flammarion (1862), the tales of Voltaire at the beginning of the 18th century and the reports of illusory Martian channels at the end of the 19th century (figure 1.5). Interest in astronomy is also manifested in the generosity of sponsors who, together with the authorities, contributed to the financing of telescopes and other equipment. Some observatories, such as those in Nice and the Pic du Midi in France, were initially entirely financed by sponsors. Rich “amateurs” devoted a part of their fortune to the construction of telescopes and observatories. The most famous of them made significant contributions to the progress and discoveries of astronomy and are linked with well-known names in the history of observational astronomy, such as Tycho Brahe at the end of the 16th century and Herschel in the 18th century3. The impact of sponsors, such as Lick, Carnegie, Rockefeller and Yerkes, peaked in the United States at the end of the 19th century (extending to the present days throughout the 20th century). This had a decisive effect in making American astronomy the premier in the world at that time and now. The technical instruments of astronomy were constantly improved throughout this period: from Galileo’s telescope in 1609 to the 2.5 m telescope on Mount Wilson in 1917, a factor of one hundred thousand in collecting area and sensitivity compared to the naked eye! Other technical improvements were crucial to take advantage of such sensitivity gains, for instance the quality and inventiveness of telescope optics and mechanics, sensitive photographic plates and light detectors, spectrometers, etc., as well as the use of better quality observation sites (see § 2.2).

3

In fact, Tycho Brahe mainly took advantage of the King of Denmark whose supporting finances he exhausted building Uraniborg, which led to his exile to Prague. Herschel was substantially financed by the King of England.

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Classical physics also developed during the period following the time of Galileo and Newton, especially in the 19th century. Most of its branches have proven crucial for astronomy, as well as mechanics and mathematics. Optics is fundamental since almost all information about the stars comes from light. In addition to its essential role in the design of telescopes, it provided the astronomer with a key tool for analyzing the wavelength of starlight through spectroscopy. The detection of characteristic spectral lines of each chemical element made it possible to analyze its presence in each star and other astronomical bodies (figure 1.4). It should be noted that this implied that 19th century chemistry had previously identified almost all the elements (figure 1.4). Thermodynamics with its fundamental application to the physics of temperature, pressure and heat exchange in gases, provided astronomy with the tool for developing the first models of the interior of stars as hot gas balls.

FIG. 1.4 – Spectral analysis of the stars. Spectroscopy played a key role in the development of astronomy throughout the 19th century. As early as 1814, Fraunhofer inaugurated both laboratory and stellar spectroscopy with a remarkable spectrum of the main spectral lines of the Sun that he listed (from A to K and from a to h, 1st row above the spectrum). It was later discovered that some of these lines correspond to chemical elements such as sodium, calcium, magnesium and hydrogen (2nd row above the spectrum), and generally speaking that its line spectrum is the best way to characterize each element. However, most spectral element identifications by Kirchhoff and Bunsen were not achieved before 1860. Then, with the progress of laboratory spectroscopy and these identifications on the one hand, and astronomical observations (notably of Huggins and Secchi) on the other, the first spectra of the different stars and nebulae confirmed that the stars contain the same elements as the Earth. Conversely, the observation of an unidentified spectral line in the Sun in 1868 led, under the impulse of Lockyer and Janssen, to the identification of helium on Earth at the end of the century. With the progress of spectroscopy and telescopes, the richest information about stars comes from their spectroscopy. It was thus natural that the first star classifications were based on their line spectra at the end of the century (figure 1.7). In parallel, the interpretation of the spectrum of the solar corona led to the development of highly ionized plasma spectroscopy. Credit for Janssen’s laboratory photo: Annales de l’“Observatoire d’Astronomie Physique de Paris, Sis Parc de Meudon”, 1896.

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In return, astronomy made significant contributions to the progress of physics. In addition to the constant stimulus of its demand for increasingly sophisticated optics and advanced mathematical mechanics, we can note several astronomical discoveries of this period that were crucial for physics: the first measurement of the speed of light from the movement of Jupiter’s satellites in 1676 by Cassini and Rømer at Paris Observatory; the discovery of an unknown line in the Sun spectrum in 1868 by Janssen and Lockyer, leading to the identification of helium in the laboratory before the end of the century under Lockyer’s guidance (figure 1.4); and Pickering’s identification of the ionized helium spectrum which was central in Bohr’s quantum spectral analysis.

A snapshot of 1900 Astronomy – A rich set of achievements, and many gaps Two centuries after Newton, astronomers at the end of the 19th century could boast of immense progress and list many important achievements and discoveries. In the context of the triumphs of science in the late 19th century, Camille Flammarion’s and Agnes Clerke’s famous books about popular astronomy11,12 (figure 1.2), for example, provide fairly good popular syntheses of the state of knowledge in astronomy at that time. The planets had remained the topic of choice of astronomy. To the list of the five basic planets of the Solar System already known since ancient times (Mercury, Venus, Mars, Jupiter and Saturn) the two more distant planets had been added, Uranus and Neptune, discovered, respectively, in 1781 and 1846. In parallel, this inventory of the objects of the Solar System was enriched by the asteroids and important satellites of planets. The sustained effort to observe each planet, combining different methods (orbital motions, imaging, spectroscopy), had collected a considerable amount of information that made it possible to draw up a part of their identity card (see table 12.1). For each we already knew its radius, mass, density, rotation period (the duration of its day), and the inclination of its rotation axis (which, for instance, determines seasons on Mars as on Earth). The presence of a rocky surface had been identified on the “telluric” planets (Mercury, Venus, Earth and Mars), while the others are mainly gaseous4. The details of the images gave information about the visible outer layers of the planets: it was known that the Venus surface lies under a thick layer of clouds and that the very clear banded structure in Jupiter reflects strong upper atmosphere currents; Jupiter’s great red spot had been observed since the 17th century; seasonal variations in the image of Mars were also noted; but, if the presence of temporary polar ice caps had been correctly identified, many people had completely fooled themselves into believing for a time to see, in the images of Mars, the famous “canals” that were heralded as the signature of Martian civilizations (figure 1.5).

4

But the average temperature was only determined later (1924 for Venus, 1926 for Mars, Jupiter and Saturn), as well as the atmospheric pressure (very high on Venus, very low on Mars, compared to Earth), and the main constituents of the atmosphere – CO2 for Venus and Mars, hydrogen (H2) for gas planets.

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For the Sun, its proximity had led to its very detailed observation by successive generations of astronomers. We knew very well its complex and variable small-scale structure, the elements of which (spots5, etc.) had been cataloged and well documented. Their variations were recorded over very long periods of time and the existence of an 11-year solar cycle was established (figure 4.6). The observation of the eclipses had revealed the structure of the peripheral zone of the sun, called the corona*. It had provided a slightly better understanding of “solar meteorology” and linked it to the gigantic eruptions that project hot gas into spectacular arches at very high elevations (figure 4.5). Since the derivation of Stefan’s Law in 1879, which links the luminosity per unit area of a black body* to its temperature, there was a fairly accurate estimate of the average Sun surface temperature of about 5000 °C. Moreover, because of its exceptional intensity and availability, the Sun has always been a wonderful light source for spectroscopy. The ever more efficient observation of the Sun’s spectrum has therefore accompanied all developments in spectroscopy since the beginning of the 19th century (figures 1.4 and 1.7). By the end of this century, the Sun spectrum was already known to be extraordinarily rich, with more than 10 000 lines detected and many of the known chemical elements identified (figures 1.4 and 3.1); but the vast majority of the observed lines had not yet been identified. It can be said that, in the absence of an understanding of atoms and of their spectral lines and even of the notion of ionization of atoms, the origin of this spectrum and its relationship with the physics of the solar atmosphere remained essentially a mystery. However, it had been noted that some spectral lines were intensely emitted by very bright gas layers, probably warm, at altitudes above the surface of the Sun (chromosphere*, figure 4.5). But the nature of the farther solar corona*, visible during eclipses (figure 4.5), was not yet understood and most of its spectral lines were not identified6. This increasingly sophisticated probing of the Sun’s surface layers was obviously accompanied by questions about the nature and physical state of its interior and the source of the prodigious light energy it emits. By the end of the 19th century, physicists were already armed with mature sciences such as thermodynamics and hydrostatics. This physics had well established some of the fundamental characteristics of the Sun and, therefore, of the stars: they are huge balls of hot gas in equilibrium under the action of gravitational and pressure forces with central temperatures of several million degrees (§ 3.2). However, this theory was incomplete because it came up against the crucial question of the origin of the energy of the Sun and stars. It became increasingly clear that the known energy sources led to paradoxes about the age of the Sun (§ 3.2).

5

The first observations of sunspots by Chinese astronomers date back to several centuries before the modern era (reference19). 6 We know today that the corona is created by the solar wind* and that these lines come from highly ionized atoms, such as Fe24+, because of the temperature of several million degrees of the highly diluted gas of the corona.

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FIG. 1.5 – The canals of Mars. a. Canals drawn by Schiaparelli – b, c. Structures seen by Antoniadi – d. An image by Viking Orbiter – e. Canals illustrated by Flammarion – f. Lowell in his observatory – g. E. Antoniadi – h. “Grand Lunette” of Meudon. The illusion of the presence of canals on Mars was probably the most publicized part of astronomy over a period that extended roughly from 1880 to 1910 (it is at the origin of H. G. Wells’ novel, The War of the Worlds, published in 1898, and of much of the emerging science fiction). Although highly contested and based on fragile bases of very difficult observations, it was passionately defended by champions such as its initiator Schiaparelli (a), C. Flammarion (see figure 1.2) (e.g., image (e) from Terres du ciel, 1884), and the wealthy P. Lowell who founded a first-rate observatory in Arizona for this purpose (f). But it did not finally resist rigorous observations with the best instruments of the time, such as Meudon’s large telescope (h), which was best used by E. Antoniadi (g). The best images obtained by Antoniadi, such as the one drawn in b and illustrated in c, show the complete absence of straight “channels” and explain how they could be confused with the structures of image b. These structures, which result from variations in the appearance of the surface of Mars and dust storms, were confirmed 70 years later by images of space probes, such as the one shown in d. Credit for image d: NASA.

In fact, knowledge of the stars had progressed on several crucial points during the 19th century. The first distance measurement of a star other than the Sun was finally made in 1838 by Bessel on star 61 Cygni, which is only 10 light years away, thanks to an observational feat (figure 3.3). Although the order of magnitude found was not a surprise, these first stellar distances were crucial. On the one hand, they confirmed that the luminous power emitted by the stars is comparable to that of the Sun and, therefore, that they are objects of the same nature, but on the other, the results also showed that the range of star luminous powers was extremely wide, since stars up to 1000 times more powerful than the Sun were found quite quickly. However, at the end of the 19th century, only a few dozen of the stars closest to the

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FIG. 1.6 – The “Carte du Ciel”. Photograph by the well-known photograph, Nadar, of participants (all men!) at the launching convention of the “Carte du Ciel” (1887). This ambitious undertaking aimed to produce a map of the whole sky by standardized photographic plates and to provide the exact position of millions of stars, by federating the efforts of most observatories of Europe and of the southern hemisphere under the direction of the Observatory of Paris. This family portrait illustrates not only the ambition of the project and its universalism but also the complexity of its organization. Too slow and premature, it was never completed, partly because of the First World War, but mostly because the magnitude of the task of star position calculations had been greatly underestimated. Credit: Observatoire de Paris. Sun had seen their distance measured by this “parallax” method (figure 3.3). On the contrary, determining the surface temperature of stars is much easier by photometric measurement of their color. It developed around 1900 with the understanding of the blackbody* laws. In parallel, locating and cataloging the position of the myriad of stars in the sky made a leap forward using photographic plates. The sensitivity of these plates was already impressive and it did not much improve until they were abandoned at the end of the 20th century and replaced by CCD* detectors. This imaging of the sky had already proven so powerful at the end of the 19th century that it had allowed, for example, launching a very ambitious international project, the Carte du Ciel (Map of the Sky), aimed at creating a deep map of the whole sky (figure 1.6). This ambitious project was never completed, but the more than a century old images of the Carte du Ciel remain precious for detecting any change in the position or brightness of the stars over this period.

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FIG. 1.7 – Spectral classification of stars and chemical elements. Each spectral type corresponds to a given surface temperature of the star, decreasing from O to M, and to dominant spectral lines of hydrogen and other elements (helium, calcium, magnesium, etc.) or molecules (TiO, etc. for the coldest stars). The lines are noted I for neutral atoms and II for their first ion (e.g., Ca II = Ca+). Credit: Ulrike Heiter, Uppsala Universitet.

History will, however, remember that the main achievement of astronomy at that time was the monumental spectroscopic classification of stars. It was the result of a huge effort to record and measure the spectra of hundreds of thousands of stars, carried out in a remarkably consistent and persevering way by a very close team (mainly female, see figure 1.8) from Harvard Observatory in the United States under the direction of E.C. Pickering. The richness of the information contained in the hundreds of main stellar lines and the cleverness of this classification scheme made it possible to finally distinguish the vast majority of stars into a good hundred classes and subclasses that form a coherent sequence (figures 1.7 and 3.5). It was finally understood in the 20th century that the characteristics of the spectral lines, on which this classification is based, directly reflect the stellar surface temperature and its surface gravity. Half a century later, this classification proved to be an essential foundation for building the theory of the structure and evolution of stars, which remains one of the greatest scientific achievements of the 20th century (§ 3.3). It should also be noted that the study of binary stars was already remarkably advanced and that the measurement of stellar masses from their orbits provided the basis for understanding the scale of star masses.

A century later: an avalanche of discoveries in the different sectors of astronomy14,15,17 To set the scene in considering the pace of these advances in astronomy over the past century, we refer to Martin Harwit’s book13 on the most important “cosmic discoveries”, while remaining cautious about the subjective and somewhat arbitrary nature of such a concept. In this book published in 1981, Harwit estimated that nearly two-thirds of these discoveries at the time were to be attributed to the

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FIG. 1.8 – Harvard Star Classification Team. E.C. Pickering (1846–1919) and his team, including Annie Jump Cannon, Henrietta Swan Leavitt, Antonia Maury and Williamina Fleming, who built the universally used Harvard Star Spectral Classification between 1886 and 191217 (figure 1.7). H. Leavitt was also at the origin of the major discovery of the period-luminosity relationship of the cepheids, the basis for measuring distances that led to the discoveries of galaxies and the expansion of the Universe (§ 8.1).

20th century. In the update shown in figure 1.9, the percentage increases to 75% for discoveries made since 1900! Such figures dramatically illustrate the acceleration of knowledge about the Universe and its constituents over the past century. It should be noted that Harwit’s curve is roughly exponential (like the one of technological innovations whose frenzied pace we are experiencing, figure 2.13). It shows that the pace accelerated between 1950 and 1980 with about as many discoveries as between 1900 and 1950. Figure 1.9 (right) shows that the rate of discoveries made in the last third of the century (1982–2020) seems to maintain the same rate, about fifteen, as in the previous third (1950–1981). Even if it no longer seems to be accelerating, it remains very impressive despite the uncertainty of this counting. It is not really surprising that this exponential growth is eventually running out of steam and that astronomy may have experienced its golden age in the middle of the 20th century. The Nobel Prize in Physics awarded for astrophysical discoveries provides another illustration of the significance of these discoveries over the past half-century, although the comparison with the previous half-century is biased by the fact that astronomy was not then considered part of physics for the Nobel Prize. In total, 20 among the 104 Nobel Prize winners in physics in the last 43 years, 1978–2020, have made astrophysical discoveries. This testifies to the growing importance of astronomy for fundamental physics.

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FIG. 1.9 – Main astronomical discoveries. This list, which is indicative of their distribution

over time, illustrates their acceleration in the 20th century. The first part on the left is reproduced from the figure in Martin Harwit’s book “Cosmic Discovery”13 for discoveries made before 1980. The exponential pace of their progress is obvious. The part on the right tries to make an approximate list of more recent discoveries. It shows that the pace of progress has remained comparable. The asterisks indicate the phenomena listed – dark matter, dark energy and inflation – whose nature is not yet clear. Adapted from “Cosmic Discovery. The Search, Scope and Heritage of Astronomy,” Martin Harwit, Basic Books, New York, 1981, image courtesy of the author.

We hope to provide enough detail in this book to give a vision of the overall progress of this science. The presentation will be divided into five main themes corresponding to the traditional sectors of astronomy: stars, galaxies, cosmology, high-energy astrophysics and planets. It goes without saying that such a division is arbitrary, as is the order chosen for the presentation, which reflects the logic underpinning our understanding of these different themes and the order of appearance of the most sensational discoveries during the century. It should be noted that three of these five major themes were totally absent in the vision of the Universe in 1900 (galaxies, cosmology and high-energy astrophysics) and that the other two have now reached a stage that was only a dream at that time. Knowledge of the stellar world had just made decisive progress around 1900, so its inventory during the following century did not make as dramatic a leap as the other four sectors. The spectral classification, established at that time, is still in use

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today, with a few modifications. However, the subsequent revolution in this field lies in the deep understanding of the structure, source of energy, evolution and formation of the stars. We also learned two fundamental aspects of their importance in the Universe: stars are the source of most of the elements, and they play a key role in the evolution of galaxies. It seems difficult to imagine that the concept of the galaxy had not yet been identified and accepted only a century ago, as the role of galaxies as the essential bricks of the Universe is now part of our cultural heritage. Since their nature was discovered in the 1920s, they have remained one of the main fields of astronomy investigation (including that of our own system, the Milky Way). There was a constant growth in the knowledge of their contents and structure in stars and interstellar gas, as well as their mode of formation and evolution since the cosmic dawn. The discovery, around 1930, of the apparent receding motion of nearby galaxies which seem to fly away, immediately followed the discovery of their actual nature as vast, distant, independent systems. The expansion of the Universe that this motion revealed provided the observational basis to purely mathematical models of the Universe based on the young theory of general relativity. This may be considered as the birth date of cosmology, the science of the structure and evolution of the Universe in its totality. Nevertheless, it took another thirty years to discern the physical content of the initial evolution of the Universe in the Big Bang model. Since then, cosmology has acquired a central status in astrophysics through the background it provides for the formation and evolution of galaxies and of large structures in the Universe, as well as through the unresolved fundamental problems of physics it contains. An entire part of the Universe that was practically unsuspected a hundred years ago was only revealed in the second half of the 20th century: that of high-energy phenomena and of new astronomical objects with extreme physical conditions. This violent aspect of the Universe includes its major sources of energy production: thermonuclear reactions in stars, gravitational accretion and collapse, giant explosions (supernovae, etc.) that often accompany them; the incredible, hyper-compact and hyper-dense objects that are neutron stars and especially stellar-mass black holes, the presence of super-massive black holes in the centers of all massive galaxies, the hyper-luminous quasars that these black holes form in their active phase with the possible ejection of huge jets of matter, and the gravitational waves emitted during the fusion of binary black holes. We are here at the heart of the relativistic world for which astrophysics and its objects are the laboratory par excellence. Including the physics of the Big Bang with the fundamental question marks that remain in this field (cf. § 9) constitutes one of the major frontiers of current astronomy (and physics). The exploration of planets and exoplanets has always left minds awestruck with the greatest intensity. Central in the 19th century, this theme underwent a sort of eclipse and came back in the last third of the 20th century with the beginning of direct exploration of the Moon and the planets. The 20th century will remain the moment of the realization of the old and long mythical dream of humanity’s first steps on the Moon. The close-up views of the landscapes of Mars and other planets

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that the media daily broadcast are as impressive. The very end of the century also saw the realization of another dream, the discovery of planets around stars other than the Sun. The prospects for the search for extraterrestrial life appear immense, and the study of exoplanets has become in the last two decades one of the major projects of astronomy. The very beginning of the 20th century marked a turning point in the history of science with the double revolution of quantum physics and relativity. Astronomy has fully participated in the benefits of this revolution. Two engines, drawing from the same source of physics, have thus continuously fuelled astronomical discoveries throughout the century. The new physics was an essential prerequisite for understanding the Universe and all its stars. Astronomy is now dominated by astrophysics, based both on atomic, molecular and nuclear quantum microphysics, governing the production and circulation of energy and radiation in all stars, as well as on relativity, the natural framework of the global organization of the Universe and its most unique regions. Moreover, astronomers, constantly limited and constrained by the performance of their telescopes and therefore constantly in search of new instrumental improvements, have always been among the first to benefit from the powerful technological progress front. Initially based on the mechanical pursuit of the first industrial and technological revolution of the 19th century, this progress accelerated with the huge technological efforts that occurred during the two World Wars and the Cold War (radio, space, detectors, computers, etc.). It was boosted by the revolution of quantum microphysics in the middle of the century, and then, at the end of the century, by the resulting revolution of micro-devices and computer science. All these components are obviously at the base of the breakthroughs in astronomy which has surfed on this wave. Continuing to be driven by the unquenchable interest of post-Copernican humanity in the depths of the Cosmos, liberated from planetary and Aristotelian horizons, astronomy has in turn fuelled human dreams through its unceasing discoveries. Responding to the public’s and the media’s enthusiasm, from the tour of Einstein’s Parisian salons to the NASA* and ESO* communication departments, astronomy has substantially benefited from its symbiosis with physics, the proactive development of governmental research and the by-products of high-tech military and space developments. The result of this century of progress is a revolution in our vision of the Universe. However, its magnitude has not yet been fully appreciated for lack of perspective and because it has not really deeply penetrated the collective, and even philosophical, consciousness. Actually, these great changes in our perception of the Universe do not reach the same radical level as the Copernican revolution, that helped to destroy the edifice of the geocentric world of the west and the other cultures of the globe. However, this new vision of the Universe, is indeed an essential element in our contemporary representation of the world, of humanity and of the distant future. To review the achievements and astronomical discoveries of this century, we can arbitrarily distinguish two levels, covering more or less the esoteric distinction between astronomy and astrophysics: the first focuses on the description, inventory, exploration of the Universe and its objects; the second focuses on explaining,

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dissecting, understanding mechanisms and evolutions, based on physics. In the 20th century, both aspects exploded within the rich panorama of contemporary astronomy. The dimension of the world, the richness of its content and the depth of its history increased in a vertiginous way. In parallel, subtle descriptions and models of the stars and phenomena of the Cosmos are informing and provoking the most profound questions of physics. We will now take a detailed look at these two levels for the five sectors of this exploration of our universe in the 20th century. For each domain, the aim will be to explain the enormous progress of our knowledge and present the perspectives and questions it opens. We shall begin with a review of the physics and technology revolutions of the 20th century, analyzing their decisive impact on the progress of astronomy, both in terms of analysis power and the remarkable instruments they have made available to astronomers.

Chapter 2 Scientific and Technical Revolutions, Drivers of 20th Century Astronomy 2.1

Physics Revolutions, Keys to Astrophysics

Whereas at the end of the nineteenth century, physics appeared close to completion, the twentieth century opened with two revolutions that radically challenged its classical foundations: quantum mechanics that has dominated the physics of the twentieth century, from the structure of atoms and nuclei to elementary particles, from chemistry to the components of electronics and computers; and the theory of relativity that changed our conception of space, time, energy and gravitation. Each of these theories is an essential ingredient of the revolution that astronomy has experienced over the last century. Very quickly physics invaded the whole of astronomy, to the point that the semantical distinction between “astronomy” and “astrophysics” has practically disappeared. Moreover, we have witnessed for a few decades now such a strong interaction between physics and astrophysics that some of their most fundamental and difficult problems completely overlap.

Relativity, a natural framework for the Universe and its evolution22 Even in its “restricted” form (1905), the theory of relativity is essential when the velocities are no longer small compared to the speed of light, and for describing the high energies associated with such high speeds. This extreme situation is often encountered in high energy astrophysics (§ 10, § 11) from the stellar cores to the Big Bang and from pulsars to quasars. The theory of relativity first became a fundamental tool for high energy astrophysics with the discovery of cosmic rays in 1912, and, later, the basis for understanding the energy of stars (§ 3.2) explore the Big Bang (§ 8) and finally, to study X-rays, gamma rays and relativistic particles in the last third of the twentieth century. More fundamentally, the theory of general relativity (1915), as a unified theory of space–time and gravitation, was immediately employed as the essential framework of cosmology (§ 8). It is also at the heart of the problem of black holes (§ 11) and gravitational waves and lenses (§ 11.3, § 7.2).

DOI: 10.1051/978-2-7598-2706-0.c002 © Science Press, EDP Sciences, 2022

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Quantum physics, key for understanding matter and stars Classical physics had triumphed in the nineteenth century for understanding mechanics, thermodynamics, optics, and electricity. However, at the beginning of the twentieth century, it became more and more obvious that it was unable to penetrate the secrets of matter at the level of atoms and their constituents, such as the electron and proton that had just been discovered, as well as radioactivity. This mystery of the atomic world was solved in the first quarter of the twentieth century at the price of a conceptual revolution whose foundations still challenge our understanding. Quantum theory proved to be extraordinarily effective in analyzing not only the structure of atoms and their spectroscopy but also the entire organization of matter, in particular the chemical bonds that are the bases of biology, microelectronics and computers. Quantum physics is also required in almost all areas of astronomy where its first major impact was the theory of stars.7 Nuclear physics finally made it possible to understand the origin of their energy and the way in which they synthesize atoms (§ 3.4). Moreover, understanding spectral lines in terms of transitions between energy levels of atoms and molecules has provided astrophysics with its most important tool for remote sensing of stars and various cosmic bodies. Applying quantum theory to stellar spectra made it possible to pass from their empirical classification to the physical conditions of the superficial layers of the star. We thus have a precise diagnosis of these conditions and the abundances of the different elements in stellar atmospheres. It is the same for astrophysical environments as different as interstellar nebulae, comets, quasars, X-ray emitting hot plasmas, and radio-line emitting cold interstellar gas (§ 6.4). The physics of atoms, nuclei and elementary particles also made it possible to model various phases of the Big Bang (§ 8).

2.2

Giant Telescopes21,24

The power of a telescope depends first on its surface area since the number of photons it can collect from a celestial object is directly proportional to it. Compared to the naked eye, the ratio of the collecting surface was about 25 with the Galilean telescope. Since then, the total collecting area of telescopes has roughly doubled every 20 years (figure 2.13). It must be emphasized, however, that the collecting surface is very far from being the only factor controlling the performance of a telescope, as it also strongly depends on the quality of its optics and its mount. All these factors made substantial progress during the nineteenth century, accompanying those of metallurgy and industrial optics. Yet, until the 1990s, a limiting key factor remained the size of the main mirror.

7

Note, however, that the classical blackbody* theory was at the base of the determination of the temperature of the stars.

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FIG. 2.1 – Very large telescopes. (left, a). Mount Palomar. The 5 m diameter Hale telescope, built by Caltech University in California, was the world’s largest effective telescope from 1948 to 1993. Among other major achievements, it made significant progress in the observation of objects (galaxies and quasars) very far away with a very large redshift. (right, b). Mock-up of the European Extremely Large Telescope (E-ELT), which will be the largest telescope in the world (39 m in diameter, segmented mirror in 798 elements) when it will start operations around 2025. Since 2016, it is being built by ESO* to be installed in the Atacama Desert in northwestern Chile on Cerro Armazones near Paranal (figure 2.2). It will use sophisticated adaptive optics*. Its dome will measure 86 m in diameter and its instruments, cameras and spectrographs, will typically weigh more than 10 tons each. Credit: Caltech/Palomar Observatory; ESO.

The dawn of the twentieth century marked a turning point in astronomical observations with the development of a new generation of large telescopes of one- to two-meter diameter and a combination of various improvements. They were mostly paid for by wealthy American sponsors, such as the big refractive telescope (1 m) at Yerkes and especially the Mont–Wilson telescopes (1.8 m and 2.5 m). They dominated world astronomy for more than half a century. The other countries have more or less followed with national or multinational facilities to the extent of their means and their ambition. Toward the end of the twentieth century, Europe, uniting its forces through the ESO* organization, was able to match the United States. Thus, in the years 1960–1980, the 5 m telescope of Palomar (figure 2.1a) was accompanied by a good half-dozen telescopes of at least 4 m built by the main countries. The construction of the first 10 m Keck telescope in 1993 in Mauna Kea, Hawaii (figure 2.6), opened the way to a dozen 8–10 m class telescopes thanks to two technological advances: the production of large segmented mirrors in pieces of about 2 m (Keck) or the construction of single lightened 8 m mirrors (VLT*, figure 2.2). We are now living a new era of giant telescope competition since three 25–40 m segmented-mirror telescopes are to be commissioned in the 2020s (figure 2.1b).

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FIG. 2.2 – The VLT. The set of 4 telescopes (8 m), Very Large Telescope (VLT) of the European Southern Observatory (ESO) on Mount Paranal in northern Chile, is the most powerful telescope set in the world before the operation of the ELTs* in the mid-2020s. The photo on the right shows its desert site, near the Pacific Ocean, chosen for its excellent quality for astronomical observations (ESO’s giant E-ELT telescope, figure 2.1b, will be built nearby). We see that the observation platform was obtained by leveling the top of the mountain. The photo on the left shows the size of the telescopes whose 8.2 m mirrors are the largest ever made in a single piece (we see the massive gray support of one of them in the center). We also see, on the platform left in this photo, the dimensions of instruments, spectrographs and cameras, placed at the focus of the telescope to analyze the light of the stars. Credit: (left) ESO; (right) J.L. Dauvergne & G. Hüdepohl (atacamaphoto.com)/ESO.

2.3

Overcoming the Disturbances of the Earth Atmosphere

Choosing the best observation sites The Earth’s atmosphere is a major handicap to advanced astronomical observation. It limits the detection of astronomical signals in three main ways: (i) by a severe or total absorption of most of the electromagnetic spectrum outside the visible and radio domains (figure 2.4); (ii) because of an intense intrinsic emission and scattering at certain wavelengths that masks the weak astronomical signals; (iii) in blurring the images by the refraction of the turbulent atmosphere. Figure 2.4 shows the typical fraction of electromagnetic radiation absorbed by the atmosphere for all wavelengths. The narrowness of the window of visible radiation is striking, a window to which our eye is adapted because it contains most of the solar radiation. The electromagnetic spectrum is either fully absorbed, or only partially transmitted in some atmospheric “windows”. Absorption decreases with altitude and therefore, most of the observatories are now built on the top of mountains. In the submillimeter domain, where water vapor dominates the absorption, dry and high sites are critical, like the plateau of Chajnantor, located at 5000 m in northern Chile, where ALMA is located (figure 2.5), or even telescopes operating from the South Pole.

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FIG. 2.3 – Frequencies and electromagnetic wavelengths. Diagram of the frequencies ν and the wavelengths λ (λ = c/ν, where c = 300 000 km/s is the speed of light) of the different domains of the spectrum of the electromagnetic radiation: this radiation extends from the radio domain to high-energy gamma (γ) rays, passing through the microwaves (decimeter, centimeter, millimeter and submillimeter wavelengths), the far and near-infrared, the visible, the ultraviolet and X-rays. All these radiations obey the same equations (of Maxwell) and can be interpreted in terms of photons of energy hν = hc/λ, where h is the Planck constant. Modern astrophysics makes full use of this huge range of frequencies (and therefore of wavelength and energy) which varies by a factor of more than 1027 (one billionbillion billions). Credit: CEA. The adverse effects of atmospheric turbulence are felt at all wavelengths, but especially in the visible range. The random bubbles of turbulent gas act like lenses that deflect the light rays in a fluctuating way. There is no point to merely increasing the size of the telescopes to improve the accuracy of the images, because atmospheric turbulence produces a widening and distortion of the images at least equal to that of the diffraction*8 of a telescope of 30 cm (about 0.5 s arc, i.e. the angle where we distinguish 1 cm at a distance of 4 km, or a length of about 100 km on the surface of Mars). For the best performing telescopes, it is therefore vital to choose sites where atmospheric turbulence is minimal and to try to correct the effects of turbulence by adaptive optics*. Until the nineteenth century, most of the astronomical results had been obtained by observations in mediocre sites in or near major cities, but, especially after Hale’s pioneering observations at Yerkes Observatory at the very end of the 19th century, it became clear that the quality of the observation sites was a major factor to consider. The approximate limit imposed by light diffraction with a telescope of diameter D at wavelength λ is about α(arc second) = 1.2 × λ(µm)/D(m).

8

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On the Edge of the Cosmos

Starting in the United States with Mount Wilson, near Los Angeles, in the early 20th century, the search for the best sites intensified throughout the century to finally focus on the exceptional conditions of Hawaii (Mauna Kea) and Chile9 (figures 2.1b, 2.2, 2.5, and 2.6).

FIG. 2.4 – Atmospheric transmission. Opacity of the Earth’s atmosphere as a function of

wavelength λ. It can be seen that the opacity is total for the smallest10 and the largest values of λ as well as for far-infrared wavelengths, while it is very weak in the visible and radio domains. In the other areas, there are partial transmission “windows” whose transparency depends mainly on altitude and the absence of water vapor, explaining the predilection of astronomers for high altitude and very dry sites.

Adaptive optics The image distortion caused by atmospheric turbulence at the telescope can be artificially corrected by measuring at a given instant the distortion by the atmosphere of the propagation of the light waves emitted by a bright star, located close enough to the direction of the much weaker source to be observed. Knowing this distortion at every moment, it is possible in principle to correct for all sources close to this direction, by deforming almost instantly the surface of one of the mirrors of the telescope. In practice, it remains difficult to implement. In the absence of any nearby bright star, this disadvantage can be overcome by the use of an artificial star, simulated by the scattered light of a powerful laser beam on the stratospheric gas. Such adaptive optics methods have become increasingly routines on large telescopes and are vital for the extremely large 25–40 m diameter telescope projects (figure 2.1b). 9

Antarctic sites can be even better, but their quality is counterbalanced by the extreme technical and logistical difficulties. 10 Extremely energetic gamma-rays can be studied from the ground, especially through the observation of extensive showers of particles generated by their interaction with the upper atmosphere (§2.4).

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FIG. 2.5 – ALMA. The plateau of Chajnantor is the site where the Atacama Large Millimeter Array (ALMA) millimeter radio observatory is located, a network of 66 mobile antennas, mainly 12 m in diameter. The plateau is at an altitude of 5000 m on the Chilean Altiplano in the Atacama Desert close to Bolivian and Argentinian borders. The quality of the site allows ALMA to observe in the ten available atmospheric windows between λ = 1 cm (30 GHz) and λ = 0.3 mm (1000 GHz). Its interferometer operation (§ 2.4), where the antennas can be separated up to 16 km, can reach an angular resolution of 0.01″ (ten times better than the HST*). Credit: ALMA (ESO/NAOJ/NRAO).

FIG. 2.6 – Mauna Kea Observatory. This site, an extinct volcano at a 4200 m altitude on the island of Hawaii, hosts the largest concentration of large telescopes in the world, including four telescopes 10–8 m in diameter. The outstanding qualities of the site are due to the low water vapor content and especially the very low atmospheric turbulence which allows astronomers to obtain very high-quality images. Credit: Institute for Astronomy, University of Hawaii.

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FIG. 2.7 – HST and JWST, flagships in space telescopes (NASA, with ESA participation). On the right (a), Hubble Space Telescope (HST), launched in 1990, has been operating with remarkable success since its optics were repaired in 1993. Despite its relatively modest size (2.4 m diameter), the total absence of disturbance by atmospheric turbulence ensures perfect images with an angular resolution much better than on Earth. This allows not only the observation of very fine details in complex images such as those in figures 3.8, 7.2, and 7.4, but also the detection of the weakest and farthest galaxies (figure 7.1). Its orbit close to Earth allowed five expensive astronaut visits through the Space Shuttle to repair and install new instruments (cameras, etc.). Coupling with a powerful dedicated institute (Space Telescope Science Institute, STScI) has increased its effectiveness and enabled the development of a unique science dissemination program that has been a huge success among US schools. On the left (b), the John Webb Space Telescope (JWST) is the successor of the HST with a greatly improved sensitivity thanks to a collector surface five times larger, equivalent to a diameter of nearly 6 m. By extending its field of observation in the infrared, it aims in particular at very distant galaxies whose spectrum is very much shifted towards the infrared. The size of the telescope imposes its very precise deployment in space, which turned out to be much more difficult than initially planned. This resulted in a significant delay and cost, but its launch was successful on December 25, 2021. To limit the spurious effects, it will be located much further from Earth than the HST, which will probably prohibit any maintenance visit. Credit: NASA.

Going into space21 The ultimate way to overcome the disturbances of the Earth’s atmosphere is to get out of it. Astronomy has been one of the main beneficiaries of the space adventure for more than 50 years. This has allowed the opening of many spectral domains such

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as the far-infrared, the far-ultraviolet and the X-rays, where the atmospheric absorption from Earth is an insuperable obstacle (figure 2.4). Access to space is also crucial for visible light as it provides perfect images (figure 2.7) and enables incomparable precision astrometry.11 It has also opened a new key area of astronomy, the direct exploration of planets and other objects of the Solar System (§ 12). Yet, one can imagine the enormous difficulties to be overcome in order to put telescopes into space, making them bigger and bigger in an ultra-precise way. Sophisticated equipment must be perfectly stable and it is imperative to transmit large volumes of data and commands between the satellite and the Earth without any hitch. Space astronomy is therefore extremely expensive. Nevertheless, it has become a key part of contemporary astronomy.

2.4

Exploiting All Spectral Domains from Radio to X-ray and Gamma-Ray

The twentieth century has seen an enlargement in our ability to study the stars by exploiting all the signals they send outside of the domain of visible light, especially from environments that are either so cold or so hot that they emit most of their electromagnetic radiation outside the visible range. Astronomical observations thus gradually extended to the entire electromagnetic spectrum (figures 2.3 and 2.4). This was achieved first from the ground for the wavelengths that reach it: radio, near-infrared, gamma rays of very high energy; and then using space telescopes for those wavelengths that are blocked by the atmosphere: far-UV, X-rays, gamma-rays, medium and far-infrared. The contribution of each of these domains can be described by decreasing wavelengths as follows. Radio astronomy has emerged as a major new branch of astronomy. It burst in the middle of the twentieth century, relying mainly on the technological achievements during the Second World War. Covering a vast frequency range (roughly 10 MHz–1 000 000 MHz, or 0.3 mm–30 m in wavelength), it made in the years 1950–1970 an impressive series of discoveries: various radio sources, radio-galaxies and quasars, synchrotron radiation and interstellar magnetic field, the 21 cm line of neutral hydrogen, cosmic microwave radiation (§ 8.4), normal and binary pulsars, as well as interstellar masers and most identifications of interstellar molecules. Radio telescopes are similar to radar antennas, the most common form being a parabolic antenna similar to optical telescopes but larger (however, see alternative shapes, e.g., that will be used in the SKA* project, § 2.6). The constraints of surface precision, which must vary proportional to the wavelength, are indeed much easier to satisfy than in conventional optics. From the beginning, radio astronomers started building mastodons up to 100 m in diameter such as the Green Bank or the Effelsberg telescopes (figure 2.8a). However, as this limit proved difficult to

11

The satellites Hipparcos* and GAIA* thus measured the distance and movement of a multitude of stars (one billion for GAIA!).

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FIG. 2.8 – Very large single-dish radio telescopes (right, a) The Green Bank Telescope in West Virginia, US, 100 m diameter, is the world’s largest fully steerable radio telescope, together with the Effelsberg 100 m Radio Telescope near Bonn, Germany. In operation since 2001, it operates from meter to millimeter wavelengths. It is designed for flexible observations of various radio sources including: interstellar hydrogen in the Milky Way and other galaxies, pulsars, identification of interstellar molecules, etc. (left, b) FAST. The Five-hundred-meter Aperture Spherical radio Telescope (FAST, South-West China) has a 500 m diameter dish constructed in a natural depression using an active surface made of 4500 panels. It focuses decimeter and meter radio waves onto a mobile feed antenna suspended on cables above the dish. In full operation since 2020, the main goals of FAST include: neutral hydrogen surveys; pulsar observations; participation in international VLBI networks and pulsar timing arrays (§ 14) and detection of interstellar molecules. Credit: (left) CAST; (right) NRAO.

overcome for steerable telescopes,12 the majority of the largest radio astronomy observatories now use networks of many smaller antennas of up to 25 m at most (figures 2.5, 2.9, and 2.14). This makes it possible to build very large collecting surfaces and to achieve high angular resolution by means of techniques of interferometric image synthesis (see below). Another specificity of radio astronomy is the high spectral resolution thanks to heterodyne13 frequency techniques. Astronomers are constantly eager to increase their “visual acuity” to allow distinguishing finer details of the structure of cosmic sources. We have seen that atmospheric turbulence severely limits the resolution of optical telescopes, but it has little effect on radio images. The angular resolution of a radio telescope is therefore mainly limited by diffraction, with a radian value close to the ratio λ/D of the 12

There are, however, larger but less flexible radio telescopes at Arecibo (Puerto Rico, 305 m, commissioned in 1963) and in Guizhou Province, South China (FAST, 500 m, 2016, figure 2.8 left) whose mirrors are fixed to the ground, while the detector is moved in a nacelle above the mirror to change the direction of observation. 13 Frequency shift technique in radio, facilitating the analysis and amplification of high frequency signals.

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FIG. 2.9 – Radio interferometry. (Left, a) VLA. The VLA (Very Large Array) interferometer of the National Radio Astronomy Observatory (NRAO) was built from 1973 to 1980 in a desert site in New Mexico (USA). This array consists of 27 antennas of 25 m diameter operating in the centimeter and decimeter wavelength ranges. It extends over a length of up to 35 km, which allows to exceed 0.01ʺ in angular resolution. Improved in 2011, it remains the most powerful radio telescope in the world in its field. It produces spectacular images of radio sources (see figure 11.6), such as near Hercules A radio galaxy, shown in the box, or inventories radio sources in more or less extended fields. (Right, b) Event Horizon Telescope collaboration. World-wide Very-Long Base Interferometry (VLBI) network of the Event Horizon Telescope collaboration aimed at producing millimeter-wave images of the central supermassive black hole of the Milky Way and nearby galaxies (§ 11.7 and figure 11.9). Credit: (left) Dave Finley, courtesy of National Radio Astronomy Observatory and Associated Universities, Inc. (Inset) NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA).

wavelength to the diameter of the telescope2. Surprisingly, the radio domain achieves the best angular resolutions, although its wavelengths are at least a thousand to ten thousand times larger than in the visible. This is possible because one can easily combine the signals of different antennas networked in phase. With this interferometric procedure, a synthetic image is produced whose angular resolution is like that of a telescope of a dimension equivalent to the spatial extension of the network. It is thus possible to extend this distance between antennas up to intercontinental baselines of several thousand kilometers (VLBI*), usually reaching nearly one ten thousandth of a second of arc, soon a hundred thousandth (i.e. the thickness of a hair at 1000 km; figure 2.9b)! Of course, such incredible performance can only be achieved for very strong radio sources. Interferometry is much more difficult for visible or infrared radiation because of phase disturbances related to propagation in the atmosphere. However, it is starting to be used systematically, especially by combining the four ESO VLT* 8 m telescopes (figure 2.2), which should make it possible to reach an angular resolution close to that of the VLBI*. The last instrument on VLT Interferometer (GRAVITY) has made the first direct observation of an exoplanet using optical interferometry,

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revealing a complex exoplanetary atmosphere, and it has confirmed that a star orbiting the super-massive black hole at the center of the Milky Way moves just as predicted by the general theory of relativity. Infrared and submillimeter wavelengths which span the broad spectral range of 2.5 μm–800 μm are almost completely absorbed by the atmosphere except for a few low-quality atmospheric windows (figure 2.4). This radiation is emitted mainly in the near- to far-infrared by cold sources such as protostars, brown dwarfs and planets, dusty galaxies, or at submillimeter wavelengths from very cold dust or interstellar molecules. In fact, apart from the cosmic radiation (§ 8.4), the main source of infrared energy from the Universe comes from interstellar dust (§ 6.5) which absorbs a large part of the ultraviolet and visible radiation of the stars and reemits it in the far infrared. It is about half of all the energy emitted by all the stars in the Universe.

FIG. 2.10 – Space observatories for the infrared. IRAS (NASA) revealed the mid and far infrared sky in 1983. ISO (ESA), then Spitzer (NASA), in the next two decades, allowed to study thoroughly the average infrared sky. Thanks to more and more sophisticated detectors, they allowed methods of the same type as those of visible and near-infrared astrophysics. Finally, Herschel (ESA) obtained a comparable sensitivity in the far-infrared and submillimeter range, also extending the heterodyne techniques of radio13. All these observatories were dependent on the cooling of most of their receivers by a reserve of liquid helium, which limited their life. Yet Spitzer was able to continue to operate remarkably without helium for more than ten years with its only near-infrared detectors. Credit: NASA, ESA.

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Although William Herschel detected the infrared radiation of the Sun as early as the year 1800, the actual opening of the infrared window was delayed for a long time by the absence of effective photographic or electronic detectors. In fact, infrared astronomy really took off from the 1970s with the development of efficient detectors; first measurements were done from rockets and airplanes followed by the launch of large infrared space observatories equipped with multi-pixel detectors in the years 1980s–2000s (figure 2.10). The field of optical astronomy also benefited from significant progress by observing in space with the HST (figure 2.7a), the adaptive optics* (§ 2.2) and the extension to the atmospheric windows of the near-infrared (  1–2 μm, figure 2.4). Near-infrared astronomy is now as important as the visible because the dust absorption is lower and some important spectral lines are shifted from the visible and UV into this range for very distant extragalactic objects. It is therefore one of the favorite areas of today’s very large telescopes (figure 2.2) and the prime target for gigantic projects in construction (JWST* in space and 25–40 m telescopes on the ground, figures 2.7b and 2.1b). UV radiation carries much of the energy of the intrinsically brightest astronomical sources, such as the most massive stars and quasars. Despite its absorption by dust, it plays a key role in the physics of the interstellar medium by ionizing atoms and dissociating molecules (§ 6.4). As radiation at λ < 300 nm is completely absorbed by the Earth atmosphere, it was one of the first fields of implementation of

FIG. 2.11 – Major space observatories. For 50 years, all non-accessible ground spectral domains have benefited from major space missions represented here: UV with IUE* and HST* (figure 2.7a); X-rays with Chandra and XMM-Newton; gamma rays with GRO* and Fermi*; and infrared (figure 2.10). Similar benefits have been acquired from astrometric missions (Hipparcos* and GAIA*), solar observation (SOHO*) and planetary exploration (Cassini*, Rosetta* figure 12.10, etc.). Credit: NASA, ESA.

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space observatories in the years 1970–1980 and one of the main motivations of the Hubble Space Telescope (figure 2.7a). The atmosphere is also completely opaque to X-rays and therefore the history of X-ray astronomy coincides with that of the conquest of space. The first key discoveries were from small detectors launched on rockets for observations of a few minutes (X-ray detection of the Sun in 1949, the detection of the first X binary in 1962) or using balloons (nebula of the Crab supernova remnant in 1964, figures 6.5, 10.5, and 10.7). There have been about 30 X-ray space observatories since 1970. The two main X-ray observatories, Chandra (NASA) and XMM-Newton (ESA) (figure 2.11) have been flying since 1999. X-ray sources generally involve very hot plasmas, seats of violent phenomena (§ 10, § 11) such as stellar and solar eruptions and coronas, supernova remnants, compact-object binaries, the vicinity of neutron stars, active super-massive black holes and their gigantic relativistic jets, as well as clusters of galaxies. There is a strong connection between the X-ray and gamma-ray domains and thus in the astrophysical phenomena they involve, such as super-massive black holes and their jets, neutron stars and supernova remnants. More specific gamma-ray sources include massive-star explosions of gamma-ray bursts, cosmic-ray emission of interstellar gas, the bulge of the Milky Way, fusion of neutron stars, etc. (§ 11.3). Their observation makes full use of the techniques and detectors of nuclear physics. Aside from the Sun, a very few sources were detected in the Milky Way in the 1960s. During the years 1960s–1970s, US satellites looking for Soviet nuclear tests regularly detected mysterious “gamma-ray bursts” from the sky. These bursts turned out to come from astrophysical sources, mainly extragalactic (§ 10.3). A dozen even more successful gamma-ray observatories on satellites then took over.

Very high energy cosmic and gamma rays and neutrinos In addition to the visible and the radio, there are other windows of transparency of the atmosphere at extremely high energy. They allow the detection of astrophysical signals on the ground by techniques related to those of the physics of high energy particles. The end of the twentieth century saw the beginning of neutrino astronomy, which commonly detects solar neutrinos and less easily those from other sources (starting with supernova 1987A, figure 10.2a). Cosmic rays (§ 10.4) partially reach the ground where they have been studied since the beginning of the twentieth century (figure 10.9a). The most energetic as well as very rare high-energy cosmic and gamma rays cause extensive showers of particles through their interaction with the upper atmosphere, which are detected by ground-based telescope networks such as the large Auger and HESS installations (figure 10.9b). The atmosphere also does not affect the gravitational waves that have been detected since 2015 using large ground laser interferometers (§ 11.3, figures 11.1 and 11.2). However, placing the detectors in space is required to detect very low frequency gravitational waves because of seismic noise on Earth (figure 11.4).

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2.5

35

Visiting the Planets

In situ observation, even using cameras in close orbit around planets or robotic vehicles moving on their surface, gives a completely new dimension to our knowledge of the planets and other bodies that populate the Solar System. Chapter 12 provides a brief summary of the most important results of this exploration that followed the opening of the space age from Sputnik, the first Earth satellite (1957), and the first human in space (1961), to the landing on the Moon (1969) and the robotic exploration of Mars (figures 2.12, 12.3, 12.4 and 12.5).

FIG. 2.12 – Mars rovers (NASA, see also figure 12.4). (Right, a) Curiosity. Rock analysis on Mars through laser vaporization. The objectives of Curiosity were to analyze the mineral and geological composition of an area of Mars, to investigate whether an environment favorable to the appearance of life has existed in the past. Since its landing on Mars in 2012, Curiosity had traveled 27 km by February 2022, on various terrains whose composition has been analyzed daily by drilling and identifying rocks by laser vaporization. (Left, b) Perseverance. Landed onto Mars in 2021 as part of the Mars 2020 mission, Perseverance has a similar design to Curiosity. In addition, it carries a small helicopter and its main goal is collecting soil samples and storing them on the surface for later retrieval by the sample collection fetch rover of the Mars Sample Return mission (NASA–ESA) in the late 2020s. Credit: NASA, JPL-Caltech.

2.6

No Pause in the Progress of Signal Detection and Exploitation

Ultra-sensitive detectors While most of the twentieth century was dominated by photographic plates or film, especially for imaging and spectroscopy, detectors based on electronic tubes were developed in the 1950s–1970s for photometers and imaging systems. Since 1980, astronomy has moved into the era of multi-pixel CCD cameras that feature thousands of solid electronic micro-detectors as available today in cameras and smartphones. The size of these detector arrays has almost no limits (figure 2.13) and fits

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FIG. 2.13 – Evolution of astronomical-survey parameters. This figure, taken from the reference23, represents the last 50 year evolution of the three fundamental parameters that determine the efficiency of astronomical surveys (for example observing all over the sky): the surface of telescopes; the size of their receivers (number of pixels); the power of computers to process massive amounts of data. The overall area of the telescopes has grown by a significant factor,  30, but it remains very modest compared to increases of 105 or 106 of the other two parameters (they are close to follow a Moore’s law of doubling every two years!). As can be seen in the figure, it is significant that the fantastic increase in the performance of these astronomical tools has directly induced a similar exponential growth in the number of galaxies listed. Adapted from: Optical Synoptic Telescopes: New Science Frontiers, J. Anthony Tyson, Plenary talk at SPIE conference on Ground-based and Airborne Telescopes III, 28 June, 2010.

the largest field of view of current projects (10 square degrees, 3 billion pixels for the LSST-Vera Rubin project*). Infrared astronomy was able to exploit, soon after the visible, two-dimensional detectors almost as powerful as optical CCDs, but based on different principles and initially developed mainly for military purposes. They reach 65 million pixels in the case of the detectors of the future Euclid space mission (figure 9.4). The dimension remains smaller for the medium-infrared range (one million pixels for the MIRI imager of the JWST*, figure 2.7b), as well as for the far-infrared and the sub-millimeter because the angular resolution is limited by diffraction*. Here too, the progress of technology is impressive. It should be noted that in the whole range from the mid-infrared to the submillimeter, the detectors must be cooled by liquid helium or refrigeration systems to temperatures of at most a few degrees Kelvin. The need to embark large reservoirs of liquid helium was a heavy constraint for space observatories until the recent progress of other refrigeration systems.

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Other sectors of astronomy have also benefited from advanced instrumentation. This is the case of energetic-particle detection systems, cosmic rays, gamma or X rays, and also giant CCD cameras detecting atmospheric flashes triggered by cosmic or gamma rays of very high energy (figure 10.9b). In general, the stakes are such that astrophysics is often an immediate field of application, or even a test bed, of a huge variety of new and cutting-edge technologies based on the constant advances in technology and microphysics of the last century. The most striking example is the tour de force of the detection of the tiny signal of gravitational waves by ultra-precision laser interferometry (§ 11.3 and figures 11.1 and 11.2).

Exponentially growing data processing and storage Much of the capacity to build and operate giant telescopes and to succeed in launching increasingly complex space missions is based on the extraordinary advances made in electronics and computing, enabling the acquisition of complex data, remote control, command processes and information transfer. The detection of ever weaker signals with a quantity of information vastly multiplied by the number of effective pixels of images and spectra would soon be of limited use without parallel growth of computing speed and information processing. With each new advance in large astronomical instruments, which considerably multiplies the amount of information and data collected, there is also a new generation of computers which makes it possible to exploit most of the acquired information (figure 2.13). The prodigious increase in computing capabilities thus allows the signal to continue to be extracted from the noise in which it is drowned, to validate the information and to store it in ever larger databases that are conserved and carefully archived as a precious legacy for the astronomy of tomorrow, allowing possible verifications and future data mining. It is clear that a full exploitation of such enormous amounts of data produces major challenges, as will be the case for the SKA* project (figure 2.14). The example of the data processing of the major operations of particle physics, especially the LHC*, shows the way forward. This problem already sets the limits of certain projects, especially space projects, which cannot exploit all the data acquired because they cannot be transmitted down to Earth. In the same way, it is very difficult to exploit all the data acquired by all the ground and space-based observatories that are stored in gigantic databases. Considerable efforts are therefore invested in their standardization and their availability to the scientific community. The uninterrupted progress of computer performance is also driving and permitting ever more gigantic and sophisticated numerical simulations. These are becoming indispensable to the astrophysical theory. Using to-date largest and best performing computers, they are essential for understanding intrinsically extremely complex systems, such as the stars of a galaxy or the large structures of the Universe (figure 9.2b), or for exploring fundamental phenomena involved, such as turbulence, magneto-hydrodynamics or launching relativistic jets by rotating black holes. Artificial intelligence and machine learning are also more and more used to exploit ever-richer and more complete astronomical datasets.

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FIG. 2.14 – SKA. The Square Kilometer Array (SKA*) project aims to build the largest and most sensitive radio telescope in the world. Its total collecting surface, distributed between thousands of dishes and up to one million antennas, could reach 1 km2. At its completion, expected in the 2030s, the very important performance gain should allow breakthroughs in multiple fields such as the deep mapping of the entire sky in radio, cosmology (reionization of the universe, figure 9.1; “dark energy”, § 9.6), the formation of the first galaxies, their structure and evolution, the magnetic field of galaxies, etc. Its construction has already begun with complementary preliminary projects in deserts in Australia (right) and South Africa (left). The volume of data expected over time is enormous, since SKA could produce every day the same amount of data as the daily internet data traffic of all the world today!. Credit: SKA Organization.

Part II

Stars are Well Understood

Chapter 3 How does a Star Work? 3.1

Understanding the Stars31–35

Stars have always been the celestial bodies par excellence. They populate the night sky. The common nature of the Sun and the stars has been guessed for centuries, but it is only recently that their physical properties could be fully understood, shedding light on the past and future history of the Sun.

FIG. 3.1 – Spectrum of the Sun throughout the visible range, from violet to red. It contains thousands of more or less dark lines. Each line corresponds to the absorption of photons between two energy levels of an atom or ion (often iron) in the outer layers of the Sun’s atmosphere. Since these layers are a little colder than the innermost layers from which the photons of the continuous radiation between the lines originate, they appear darker on the background of the spectrum of this radiation. Credit: National Optical Astronomy Observatory (NOAO)/ESO.

DOI: 10.1051/978-2-7598-2706-0.c003 © Science Press, EDP Sciences, 2022

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As we have seen, the astronomy of 1900s was already largely stellar, having accumulated and classified a rich collection of spectra of the different types of stars (figures 3.1 and 1.7). The early years of the twentieth century also saw the identification of white dwarfs* and variable-star classes. However, this astronomy lacked the key to understanding stellar physics because the source of the energy capable of feeding stars for periods compatible with the geological age of the terrestrial rocks was still unknown. Revolutionary advances in physics in the beginning of the twentieth century suggested that this source of energy should be sought in the atomic nuclei (figure 3.2), but it was not until 1939 that the mechanism of these nuclear reactions was understood (§ 3.2). This paved the way for the theory of stellar structure and evolution, with a particular emphasis on the dramatic final phases of stellar evolution either as a planetary nebula14 followed by the formation of a white dwarf* or as a supernova* explosion and the birth of a neutron star* (box 3.1). With its extension into a detailed model explaining the nucleosynthesis* of all the elements, this theory is certainly one of the greatest scientific achievements of the twentieth century, most of which was put in place before the end of the 1950s. Since then, further progress in understanding how stars evolve has been achieved, with, in particular, substantial improvements in modeling the formation and childhood of the stars.

3.2

Solving the Mystery of the Origin of the Energy of the Sun and the Stars

Given its enormous distance to the Earth (150 million km), it is clear since Antiquity that the luminous power (“luminosity*”) emitted by the Sun is colossal. It is known today that in a year, the Sun radiates almost 20 trillion times all the energy consumed by humanity in the same period! We immediately ask ourselves what is the source of this energy and how long can it last? To try to answer these questions, physicists of the nineteenth century, armed with the laws of classical physics, were able to eliminate most of the known sources of energy (chemical combustion, electrical, mechanical, etc.), because they proved to be inadequate. They also found that although the thermal energy stored in this mass of gas at ten million degrees could supply the current luminosity of the Sun, it could do so for only a few tens of millions of years at most. The gravitational energy liberated in the formation of the Sun had been proposed as a possible source, since it was known that the Sun might have formed from the contraction of an immense gas nebula. During such a contraction, under the action of gravitational forces, the outer layers of the nebula “fall” onto the center, releasing a large energy similarly to water which falls from a dam. However, the energy thus released in the Sun could not ensure in the past a luminosity close to

Despite their name, these nebulae have nothing to do with planets. The name “planetary nebula” was coined by William Herschel in the 18th century because of their similitude with planet images produced by the telescopes of the time.

14

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its current value for a much longer duration than a few tens of millions of years (box 3.1). Therefore, the Sun must have been about that age at most. It is a practically infinite duration on the scale of human history; but geologists had increasingly convincing evidence that there were older terrestrial geological rocks. Since it seemed obvious that the Earth could not be older than the Sun, there followed serious controversies between physicists and geologists. The discovery of radioactivity allowed a more precise dating of rocks and confirmed in 1904 that the age of the Earth was at least 700 million years. This finding definitely required another source of energy for the Sun, but it remained unknown. Those years were precisely the time of the beginning of the great physics revolution (§ 2.1) with Einstein’s theory of relativity. This theory stipulates that every mass, M, contains a total energy Mc2. It was immediately noticed that the total mass of the Sun corresponded to a tremendous energy, which could theoretically allow it to radiate at its current rate for 15 000 billion years, if somehow that energy could be tapped. Less than a thousandth of this energy is enough to power the enormous luminosity of the Sun throughout its effective lifetime, estimated at around ten billion years (figure 3.6b). Still, it was necessary to find the mechanism to produce such energy in the physical conditions of the interior of the Sun. Although fission of radioactive nuclei could have been considered, as well as thermonuclear reactions other than that of hydrogen, we now know that the mechanism is the thermonuclear fusion of hydrogen into helium (as in a hydrogen bomb). The precise measurement of the mass of hydrogen and helium atoms shows that, if we imagine the reaction fusing four protons and two electrons into a helium nucleus (figure 3.2), the final mass is close to 1% lower than the sum of the masses of the initial particles. The conclusion that the mass energy thus released in this reaction is the source of solar energy was not convincingly proposed until 193915, following the development of the understanding of atoms, nuclei and nuclear reactions, thanks to the new quantum physics and, in particular, the discovery of the neutron in 1931. Once they had at their disposal the new physics of the atoms and the photons, astrophysicists found, after huge efforts based on spectroscopic measurements, that the Sun and the stars mainly consisted of hydrogen. They also managed to constantly improve the theoretical models of the stellar interiors, which remained inaccessible to observation. As a result, the key properties that determine the structure of the Sun were then recognized: its interior must be an ionized gas in equilibrium, the pressure compensating for the tendency to crush under the weight of the overlying mass, so that the pressure in the center must be huge; the temperature of the inner layers is between 10 and 20 million degrees, which implies that fusion of hydrogen into helium is possible (figure 3.2); the transport of energy from the central regions to the surface can be done by radiation (X-rays) or convection* (mixing layers) and it determines the luminosity by self-regulation of thermonuclear reactions (box 3.1).

15

The detailed thermonuclear model of the Sun was not definitely confirmed until the detection of solar neutrinos and its correct interpretation by the end of the century (§ 4.6).

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FIG. 3.2 – Thermonuclear reactions pp. Fusion of four protons into a helium nucleus (4He, made up of 2 protons + 2 neutrons) from which most of the energy radiated by the Sun comes. We see from top to bottom the three intermediate stages of this rather complex process: first deuterium nuclei are created (2H: 1 proton + 1 neutron), then, from these, nuclei of 3He (2 protons + 1 neutron), and, finally 4He. These reactions are accompanied by the emission of various particles: positrons (anti-electrons) allowing to conserve the charge in the transformation of protons into neutrons, neutrinos (ν) and photons (γ).

As early as the middle of the twentieth century, the fusion of hydrogen into helium was definitely established as the main energy source of the Sun as well as the stars. This is a major achievement for humanity to understand the ultimate origin of the solar light and energy that illuminate and rule life on Earth. The energy of the stars is also the source of most of the energy produced and radiated by galaxies throughout the Universe. Despite the enormity of this reserve of fusion energy, it is still finite, which limits the life of the stars, especially since they consume only a fraction of their hydrogen (  10%). We can thus very simply evaluate the approximate lifetime τ of a star from its mass M and its luminosity* L, τ  (Mc2/L)/1000. The approximate factor 1/1000 comes from the conjunction of the yield close to 1/100 of the conversion of mass to energy in the hydrogen fusion into helium (figure 3.2), and the fraction of about 1/10 of the initial mass of hydrogen actually used.

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FIG. 3.3 – Parallax. Principle of the distance measurement of stars by the method of parallax. If we observe a nearby star six months apart, when the Earth moves from A to B on its path around the Sun, its direction changes very slightly. If AB is perpendicular to the direction of the Sun at the star, the extremely small angle at which the direction of the star moves, AEB = 2θ, is twice the parallax of the star θ. One sees in the figure that tg(θ) = R/d. Knowing the parallax θ and the distance of the Earth to the Sun R, we deduce the value of the distance to the star d = R/tg(θ)  R/θ expressing the parallax in radian. Credit: F. Durret, IAP; G.B. Lima Nieto, São Paulo University, Brasil.

The dominance of hydrogen fusion in their cores does not prevent stars from also living on other energy sources, especially at the beginning and end of their lives. Just after being formed, it takes a while before the core of a star warms up enough to ignite hydrogen fusion. The brightness of young stars is first produced by gravitational contraction, then by the fusion of deuterium, easier to “burn” than hydrogen, but present only in minute quantities. Near the end of the star’s life, hydrogen is depleted in the central regions. As these layers heat up, the fusion of helium into carbon can ignite. More importantly, the hydrogen layers in a shell around the helium core also heat up. As a result, hydrogen fusion proceeds more efficiently and dominates the very bright period of the last phases of stellar evolution (red giant) (box 3.1, figures 3.5 and 3.6b). Finally, an even more enormous gravitational energy, approaching Mc2, is released in supernovae during the final implosion of the core of massive stars leading to a neutron star* or a black hole (§ 3.3, § 10 and § 11).

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3.3

The Life of the Stars

After the successful ending of the long quest to find the key of how the Sun produces its enormous energy, the way was wide open to seek a deeper understanding of the vast world of stars. Since the dawn of time, all we know about stars is transmitted to us by their light. When we look at the night sky, the stars appear to us as more or less luminous points. However, this weak apparent brightness, which is related to the amount of energy that enters our eye per second, can be misleading. It results from a combination of the intrinsic luminosity and the distance of the star (akin to the brightness of a lighthouse of the same power that seems weaker the farther away it is). In fact, if we look at a portion of the sky at random, the apparent grouping of the stars in a constellation is usually a fortuitous mixture of dim stars that are rather close and others that are brighter and more distant. We now know that the luminosity ratio can exceed one billion between the rare very luminous massive stars and the myriad of small stars (figure 3.5). However, in order to determine the stellar luminosities, it was first necessary to measure or estimate the distance to the stars. Measuring the distance to the stars long remained an extremely difficult challenge because of their enormous distances: more than 200 000 times the distance from the Earth to the Sun for the nearest stars. The basic method remains that of the parallax. It is similar to the triangulation methods used since ancient times by land surveyors and cartographers to measure distances. One needs to measure the tiny variation of the direction of a star when one observes it successively from two diametrically opposite points of the Earth’s orbit around the Sun, at six-month intervals (figure 3.3). To achieve this, one has to measure angles of a few tenths of a second of arc for the nearest stars (which roughly corresponds to the angular size of a marble at 2 km). In 1838, Bessel was able to accomplish such a tour de force for the first time, and, at the end of the nineteenth century, only a few dozen stellar parallaxes had been measured. In spite of their limited number, these direct measurements played a vital role in determining the stellar luminosity scale. As we know since Newton’s observations, the sunlight contains a very wide range of wavelengths, from violet to red, with extensions in the ultraviolet and infrared. As shown by measurements done in the mid nineteenth century, the same is true for stars. By measuring the distribution of the energy radiated by the stars as a function of wavelength, it was found that it corresponded roughly to that of a thermal radiation defined by a single temperature equal to the average temperature (Tsurf ) of the superficial layers of the star (figure 3.4). Tsurf is relatively easy to measure. It varies roughly from 3000 °C for the reddest stars to more than 10 000 °C for the bluest ones. Knowing the luminosity and Tsurf, one can place the star on the Hertzsprung–Russell (HR) diagram, universally used for nearly a century to determine the nature of stars (figure 3.5). In the HR diagram, the position of each star is defined by its luminosity and its surface temperature. The different stars are

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FIG. 3.4 – Blackbody* thermodynamic radiation and spectra of stars. (a) Emitted energy spectrum per unit area (arbitrary logarithmic units) by black bodies of various temperatures representative of star surface temperatures. Note the very rapid growth of the total power emitted with the temperature, proportional to T4, as well as the displacement with T of the wavelength of the maximum emission λmax towards the short wavelengths (gray line), according to the law λmax T = constant. (b) Low-resolution spectrum of the Sun showing that its average, excluding lines (figure 3.1), is close to the spectrum of a black body of 5 777 K, except in the violet and ultraviolet. (c) Low-resolution spectrum of a warmer star whose spectrum is shifted to the violet. The continuum visible part of the spectrum is fairly well represented by a black body with a temperature close to 9000 K (red curve), but the ultraviolet is very attenuated due to the increased ionization of the photosphere. (d) Typical medium resolution spectrum of a star a little cooler than the Sun. The lines of hydrogen are less intense than in the previous spectrum, but strong metal lines are present, including: sodium (Na), magnesium (Mg) and ionized calcium (Ca+). Credit: https://media4.obspm.fr/ public/AMC/pages_fdc/html_images/envimage5.html; SDSS.

far from occupying all the space in this diagram. Nearly 90% are gathered in a narrow band, known as the “Main Sequence*”. For these stars, the simple knowledge of Tsurf (or the spectral type) yields the intrinsic luminosity with a good approximation. Most of the stars that are outside the Main Sequence are clustered in the region of the Red Giants and a few are to be found in the regions of the Red Supergiants, the Blue Supergiants and the White Dwarfs* (figure 3.5).

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FIG. 3.5 – Hertzsprung–Russell (HR) diagram. It represents two fundamental parameters of the stars, namely their luminosity (their emitted power) and their surface temperature (which determines their color and their spectral class). The knowledge of these two quantities makes it possible to carry out a fundamental classification of the stars and to determine their stage of evolution and generally their mass. The most important regions where stars of different masses are located in the principal phases of their evolution are indicated: the Main Sequence where most of the stars derive their energy from the steady fusion of hydrogen for the most part from their life, from the smallest red dwarfs to the brightest and most massive stars O and B; the final well-populated stage of Red Giant corresponding to accelerated fusion of hydrogen and helium; the sparsely populated areas of Supergiants (because such stars are rare), and White Dwarfs (that are numerous, but almost invisible). Some of the brightest (nearby) stars of the different classes of mass and evolution, as well as the nearest star, Proxima Centauri, are identified in the diagram. The multitude of points represents all the stars of the sky whose distance was measured by the astronomical satellite Hipparcos (1989–1993) (the stars of the Gliese catalog, very close to the Sun, were added so as to have a significant number of stars with very low luminosity, especially White Dwarfs). In addition, a detailed analysis of stellar spectra reveals, as is the case for the Sun, a very large number of “spectral lines” (figures 3.1 and 1.7). The richness of the information contained in the stellar spectra, which constitutes a kind of star’s data sheet, has established itself as the basis of the powerful stellar classification system, which is based on a set of well-defined “spectral types” (figure 1.7). The presence and intensity of the various lines are characteristic of the atoms of the different elements and their different ionization states. These spectral types are mainly determined by Tsurf which determines the ionization state and the excitation of the atoms. Credit: Wikipedia, Creative Commons Attribution-Share Alike 2.5 Generic license. Richard Powell.

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FIG. 3.6 – Structure and evolution of the Sun. (left, a). (1) Sectional diagram of the internal structure showing: the nucleus, where the temperature approaches 15 million K, allowing the thermonuclear fusion of hydrogen (figure 3.2), then the zones of decreasing temperature where the energy produced by the fusion is transported to the surface, first radiatively by X-rays, then convectively by a large mixing of the outer layers, which are the seat of the differential rotation and complex magnetic processes; (2) The different superficial regions which are progressively hotter above the apparent surface of the Sun: photosphere (average temperature  5800 K, figure 3.4b), chromosphere (  6000 K to  100 000 K) and corona (up to 1 000 000 K), and various solar phenomena such as sunspots, prominences, eruptions (figure 4.5a), solar wind (figures 4.5b and 10.10), etc. (right, b) Evolution of the Sun. Evolutionary plot in the HR diagram (figure 3.5) of the states of a star of mass equal to that of the Sun, with the duration of the principal phases: the short phase of pre-Main Sequence; the slow evolution along the Main Sequence; the very bright final Red Giant phase that lasts for a significant amount of time; the blazing transition of the planetary nebula, and, to finish, the endless pale state of the white dwarf. Credit: (left) https://media4.obspm.fr/exoplanetes/pages_corot-approfondir/ interieur-etoiles.html; Observatoire de Paris/UFE. (right) adapted from http://chandra. harvard.edu/edu/formal/variable_stars/bg_info.html, credit: Observatoire de Paris/UFE. The brightness and, consequently, the lifetime of stars depend sensitively on their initial mass. If this mass increases, the pressure, density and temperature of the central regions increase. It follows that the rate of thermonuclear reactions increases enormously, because, like chemical reactions, their rate grows very rapidly with temperature. The production of energy, that is to say the luminosity, increases very quickly with the mass, roughly like the value of the mass raised to the power 3 or 4 (therefore the lifetime decreases quickly with the mass). A star 30 times more massive than the Sun is nearly 100 000 times more luminous and lives nearly 5000 times less (only a few million years). Conversely, the smallest stars with masses less than one-tenth that of the Sun, are nearly 10 000 times less luminous than the Sun (and they will live hundreds of times longer).

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Box 3.1 – Diagram of the major phases of stellar evolution (isolated stars, figures 3.5 and 3.6b) (1) Formation by gravitational collapse of an interstellar gas cloud (§ 4.2). (2) A brief phase of gravitational contraction whose release of energy supplies brightness for a few tens of millions of years and gradually raises the temperature of the star. (3) Ignition of hydrogen fusion in the center of the newly born star (figure 3.2) when the temperature exceeds ten million degrees. Temperature, pressure and thermonuclear reactions then stabilize by a self-regulating mechanism when the rate at which they produce energy becomes equal to that at which the energy can be lost to the outside. If the production of energy exceeds this value, the excess of energy causes the expansion of the core, which as a result cools; this greatly slows down the reactions and brings the system back to equilibrium. Conversely, as its hydrogen content decreases, the core contracts a little which slightly increases its temperature and therefore the rate of reactions. This maintains almost the same equilibrium values. This process works as long as there is hydrogen left in the core, maintaining the luminosity* fairly constant for a very long time (about 10 billion years for the Sun). This period typically occupies 90% of the total life of a star during which the star remains on the Main Sequence* of the HR* diagram (figures 3.5 and 3.6b). This explains why the vast majority of stars are in this sequence: they burn hydrogen in a controlled manner. (4) When the core hydrogen is exhausted, the pressure is no longer sufficient to prevent the contraction of the core. This much increases its temperature to  100 million degrees allowing the fusion of helium into carbon and producing a strong heating of the hydrogen shell surrounding the helium core. Hydrogen fusion can thus proceed violently in this shell. The star brightness then increases substantially, resulting in a much more unstable regime both in the central regions and in the outer layers. This envelope swells considerably, transforming the star into a red giant or supergiant. Its radius is dilated by a big factor, typically of a few tens, being able to exceed a hundred at the end of evolution. (5) At the end of the central combustion of helium, the star becomes unstable and expels a significant fraction of the mass of the star into the interstellar medium more or less violently. The process is very different depending on the mass of the star, so it is therefore necessary to distinguish: (a) For the small stars of mass lower than 6–8 M⨀ (solar mass), the carbon and oxygen core, fruit of the fusion of the helium, of mass lower than 1.4 M⨀, is stabilized by the quantum incompressibility of the degenerate gas of electrons. It becomes a white dwarf*, while the outer layers are expelled at low velocity into a circumstellar envelope (figure 3.9) and then a “planetary nebula” (figure 3.8). (b) The core of the stars more massive than 6–8 M⨀ implodes. This immediately provokes a violent supernova explosion, leaving generally a hyper-compact neutron star* residue (§ 10.2) or even a black hole for the most massive stars (§ 11.2).

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3.4

51

Our Atoms were Born in the Stars

The understanding of stellar structure and evolution has also allowed one of the most important discoveries of the twentieth century, namely to explain the origin of all the atoms of which we are made and of our environment. As already mentioned, the intensity of the spectral lines enables to measure the percentage by mass of each chemical element in the Sun and in the stars. We therefore know that in the Sun the composition in mass is about 73% for hydrogen, 25% for helium and 2% for the sum of all the other elements. Approximately similar percentages are found throughout the Universe: stars, galaxies, and interstellar gas (§ 6.4). The general decreasing trend of abundance with atomic mass (figure 3.7) suggests a progressive nucleosynthesis pattern from hydrogen. The existence of cosmic quasi-universal abundances further suggests that identical processes have been operating throughout the Universe to form the chemical elements. The mid-twentieth-century understanding of the structure of atomic nuclei and nuclear reactions opened the way for the discussion of the mode of cosmic formation of elements, without knowing at first

FIG. 3.7 – Relative cosmic abundances of the number of nuclei of chemical elements (relative to hydrogen) in the Solar System. They are close to those found in most cosmic environments. Note the logarithmic scale, the huge factor between the abundance of hydrogen and that of most elements, but also the significant presence of helium and the very low abundance of other very fragile light elements (Li, Be and B); the general decrease with the atomic number Z, and thus the mass of nuclei, from carbon and oxygen up to Z  20, reflecting the progressive nucleosynthesis of these nuclei; the systematic superabundance of the elements at Z even, whose nucleus is more stable than that of their neighbors at odd Z; the peak of abundance of iron and its neighbors whose nuclear binding energy is maximum; the general decrease in the heavier elements formed by successive captures of neutrons, with variations related to the different binding energies of their nuclei.

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whether they had been formed in the Big Bang or in stars. Soon it became apparent that the properties of possible nuclear reactions at the end of the Big Bang can explain the predominance of hydrogen and the synthesis of the bulk of helium and a few other light elements (§ 8.4), but they cannot form carbon and all the heavier elements. Understanding the synthesis of these latter elements went hand in hand with the development of the theory of stars and stellar evolution after the Main Sequence, since the temperature of the stars’ core, when they are on this sequence, is not sufficient to form the heavier elements from helium and hydrogen. The temperature rise expected next in the red giant phase made it possible to reach sufficient values (>  108 K), yet there had to be a nuclear reaction fast enough to form the carbon nucleus that is known to be particularly stable. The natural reaction to consider is the fusion of three helium nuclei into a carbon nucleus that releases substantial energy. It turns out that this reaction has a much higher chance of occurring than one would expect, because of the fortuitous presence of an excited level of the carbon nucleus with the right energy. The laboratory discovery of this level opened the way since the 1950s to understanding the synthesis of carbon, nitrogen, oxygen and all heavier elements. Note the crucial importance of the coincidence of this level of energy; without it, the synthesis of carbon and heavy atoms would have been infinitely less efficient, making rock formation and carbon-based life virtually impossible. Once carbon is formed, the synthesis of the heavier elements can continue step by step without difficulty provided that the temperature is sufficiently high. This holds for the core of the red giants where, from the fusion of helium, a good part of the carbon, nitrogen and oxygen observed in the cosmic* abundances are formed, but not the heavier atoms. Those are synthesized exclusively from stars massive enough to end their life as supernovae. As their cores contract, temperatures rise above one billion degrees, which allows continuing fusion to produce more and more highly bounded nuclei until we reach iron (26 protons) (figure 10.6b). The maximum binding energy of the iron nucleus among all the elements explains the peak observed for its cosmic abundance (and in fine the prevalence of iron in the Earth’s core). Things get complicated for all the atoms that are much more massive than iron because their synthesis by fusion consumes energy instead of releasing it, as for the lighter elements. This synthesis can, nevertheless, be achieved in the explosive phases, such as a supernova or the fusion of two neutron stars (§ 11.2), by the capture of neutrons that are liberated in abundance thanks to the enormous energy released in the collapse of the core of the star or in the neutron-star fusion.

3.5

Stars also Die

We noted the increasing instability of stars that have become very bright and extremely inhomogeneous at the end of the red giant stage. The star is more and more torn between a part of its mass, which swells excessively, and a dense contracting core that injects an increasing energy into the star. The radius of the red giant can finally reach the distance of the Earth to the Sun, 150 million km, while that of its nucleus barely exceeds a few times the radius of the Earth (6000 km).

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FIG. 3.8 – “Planetary and pre-planetary nebulae” (note 14) observed by the Hubble Space Telescope (HST, figure 2.7a). These are the last stages of evolution of medium-mass stars like the Sun, before becoming white dwarf. The extension of the nebulae (up to a light-year) is that of the gas of the circumstellar envelope ejected in the previous red-giant stage (figure 3.9a). This gas gets luminous when the envelope becomes transparent to UV radiation of the hot star that ionizes it. The characteristic coloring of the nebulae comes from the emission in spectral lines of atoms or ions that are emitted as a result of the recombination of the plasma ions with the electrons and collisions. Note the presence of many spectacular non-spherical nebulae, with central disks perpendicular to the symmetry axis. Most of them are thought to come from binary star systems. Some are in the pre-planetary nebula stage still barely ionized, but already transparent (figure 3.9b). Credit: NASA/ESA. Central helium eventually exhausts itself, leaving a core composed essentially of carbon and oxygen. The paroxysm of instability then very quickly leads to the loss of a large part of the mass of the star, which is ejected into the interstellar medium of the galaxy. At the same time, the small, very dense central core is exposed and will become a white dwarf (figure 3.6b). This red-giant white-dwarf scenario is, in fact, valid only for stars with masses not too different from that of the Sun. On the other hand, as discussed previously (box 3.1), massive stars, with a mass greater than 6 or 8 times that of the Sun, have a different, even more chaotic, end of life ending in the apotheosis of a supernova explosion. These different end-of-career versions, more or less apocalyptic, produce

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some of the most spectacular objects and beautiful astronomical events. For the observer, the stars remain essentially point objects for most of their life even with the best resolution of the telescopes. Their images are not extraordinary. On the other hand, the massive and prolonged ejection of matter during their terminal phases can give rise to quite extensive clouds of gas and dust, which, if properly illuminated, take on the appearance of remarkable nebulae (figure 3.8). However, in the case of the red giants, the first phases of this great mass ejection remain virtually invisible because the star then emits essentially in the infrared. These huge circumstellar envelopes around dying red giants are powerful infrared sources, with the closest being the brightest infrared lights in our sky (figure 3.9a). But a little later, when all of the matter that could be ejected from the star has been exhausted, the

FIG. 3.9 – Examples of circumstellar envelopes of molecular gas and expanding dust ejected

by red giants at the end of their life. On the left (a), IRC + 10216 (aka CW Leo) is the closest (about 400 light-years away) massive stellar envelope whose dust completely transforms the energy radiated by the bright red giant into infrared radiation. This makes it the brightest infrared source of the sky outside the Solar System and a unique object for the study of the physics of the envelopes and the search for new cosmic molecules. The intense emission of the CO molecule reveals the details in the very extensive structure (  0.1 light-year) of the envelope, with shells that reflect the history of the different ejection episodes of matter lasting for more than 1000 years. On the right (b), V838 Monocerotis is in a more specific phase of pre-nebula evolution. In this phase, the material ejection slowdown just begins to make the envelope transparent to visible radiation and UV light emitted by the central star, ionizing the envelope and transforming it into a planetary nebula. In the above images, obtained with the HST* in visible light, one can see the star in the center and follow its evolution, during nearly one year, through the successive luminous flashes that illuminate more and more distant layers of dust of the ejected envelope. The extension of the last image, 8 or 9 months after the flash, shows directly that the radius of the envelope approaches a light-year. Credit: (left) Astronomy and Astrophysics Cernicharo et al. 2014; (right) ESO/VLT Izan Leao (Universidade Federal do Rio Grande do Norte, Brazil)/NASA, ESA and H.E. Bond (STScI). STScI-PRC03-10.

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envelope continues to freely expand, gradually becoming less dense and less opaque. This allows the ultraviolet radiation of the hot residual core to penetrate and illuminate the gas of the envelope, which glows as splendid nebulae whose colors vary according to the spectral lines of the atoms that dominate the emission (figure 3.8). Supernovae are much rarer and more distant (§ 10.1). Their images are nevertheless striking if one visualizes the extent of the ravages in the region of the interstellar medium swept by their shock wave since they exploded a few centuries ago (figure 10.5). The only supernova observed in our galaxy and its suburbs since the telescope era is the 1987A supernova in the Large Magellanic Cloud, the main satellite of the Milky Way. In addition to the tour de force of the detection of its neutrinos (figure 10.2a), its image in visible light remains very impressive (figure 10.1a). These gigantic terminal stellar explosions, which volatilize the matter made in the stellar cores in the surrounding environment, are the key mechanism to inject into the interstellar medium elements freshly synthesized in stars such as oxygen, iron or silicon and a part of the carbon. Some of these atoms are later found in the formation of new generations of stars and also their planets. So we are the “children of supernovae”. Red giants and supernovae also play an important role in the formation of interstellar dust (§ 6.5). The initial formation of these solid particles requires the formation (“nucleation”) of first aggregates of atoms, which will then grow by aggregation of other atoms present in the gas. This nucleation is virtually impossible in normal interstellar gas because it is too dilute. However, it is very effective in the circumstellar envelopes of red giants. It should be noted that the fine analysis of the grains of matter included or agglutinated in meteorites* makes it possible to identify the presence of certain inclusions of material which originated in red giants, novae and supernovae. The totality of the star is not generally seen, volatilized in these explosions. They leave most often at the position of the center of the star an extremely compact object, with masses more or less comparable to that of the Sun and with luminosities that are always very weak compared to that of the initial star. The white dwarfs left behind by the red giants are essentially the bare carbon and oxygen core of a red giant when its outer layers have been stripped to form the planetary nebula. They are very dim because the nuclear reactions have stopped and their brightness is only produced by their slow cooling; they are corpses of stars that never really cease to cool. Their main characteristic is their enormous density: about one ton per cm3, because their mass, close to that of the Sun, is confined to a radius comparable to that of the Earth! This compactness comes from the compression by the great intensity of the gravitational forces to which these objects are subjected; but this contraction stops at this density because Pauli’s quantum exclusion principle renders the “degenerate” electron gas virtually incompressible (like our ordinary liquids and solids, but these are enormously less dense). However, the maintenance of this balance is only valid if the mass of the core does not exceed 1.4 times the mass of the Sun. Otherwise, the core’s compression continues to the point of a collapse that triggers a supernova (box 3.1). The discovery of this mass limit by Chandrasekhar, from the new quantum physics in 1930, was one of the great achievements of the theory of stars. Despite their high surface temperature, white dwarfs occupy a special position in the luminosity/surface temperature HR diagram

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due to their very low luminosity (figure 3.5). Their low brightness makes them difficult to detect. None are visible to the naked eye, although they currently represent more than 5% of the stars of our galaxy. In fact, the vast majority of stars will end up in white dwarfs. If the first white dwarf was observed in the late 18th century, it was only at the beginning of the twentieth century that we realized their enormous density and low luminosity (the name white dwarf was coined in 1922), and, it was only a few decades later, that the physical cause was understood. Although white dwarfs still have many stellar attributes despite the absence of nuclear reactions, neutron stars – with a one billion times higher density – are really objects of a different nature that only have stellar mass and origin (§ 10.2).

Chapter 4 Complexities of Star Birth and Physics 4.1

General Star Formation Scenario

New stars are constantly forming in our Galaxy (about one per year), as in most other galaxies. This has become clear through understanding the life course of stars and discovering their ages. It is an undeniably new key feature in our comprehension of the Universe: stars are not immutable, and new generations are constantly emerging. Some are being born before our eyes in regions of the Milky Way that are close to us. The birth and first steps of stars deeply mark their entire history and, more generally, the evolution of galaxies. The complex processes of star formation determine once and for all on the mass of a star and whether it will be born alone, with a companion or in a cluster. The general scenario of star formation by gravitational contraction of interstellar gas clouds and their flattening into discs had already been imagined in the 18th century, particularly by Laplace (after Descartes and Kant). However, around 1900, this scenario was no longer popular because it seemed impossible to find a solution to the removal of the rotational angular momentum (see § 4.2 and note2) of the initial cloud. The origin of stars thus remained mysterious and controversial. There was a certain predisposition toward catastrophic theories of formation of the Solar System that dated back to Buffon in the 18th century, involving a collision between the Sun and another star. However, there is now no doubt today that stars are formed by the gravitational contraction of the gas in interstellar clouds16. Thanks to the new ground- and space-based telescopes that give access to different spectral domains, we have recently been able to follow the different stages of stellar formation in detail (box 4.1). Admittedly, these are extremely complex processes, particularly in their magneto-hydrodynamic* aspects. Achieving a detailed understanding involves various observations and theoretical simulations. It is only in the last few decades that we have obtained a clear view of the origin of the stars, around which galaxies and planets are organized throughout the Universe. Understanding the origin of our Sun

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Although collisions and mergers are thought to also play a minor role in dense clusters of young stars.

DOI: 10.1051/978-2-7598-2706-0.c004 © Science Press, EDP Sciences, 2022

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and the detailed phases of its evolution during the simultaneous formation of the planets is rapidly becoming a reality. Some of these episodes are critical for the emergence of planets (box 4.1), including the circumstellar disc in which the planets formed (figures 4.1a and 4.2), and the T Tauri* phase of the Sun during which violent stellar winds ravaged the inner Solar System and probably emptied it of its primitive gas, which is absent in the telluric* planets.

FIG. 4.1 – Young stars. (left, a). Protoplanetary disc observed in millimeter radio wavelength with the ALMA radio telescope array (figure 2.5), around the young star HL Tau. This star, with a comparable mass to the Sun, is in the pre-Main Sequence phase T Tauri*, through which the Sun also passed at the beginning of its existence. Such discs (figures 4.1a, 4.2 and 13.4), which remain after the gas in their central part has been accreted to form the star, are where the planetary systems form (box 12.1) and are now known to exist around the majority of very young stars. The remarkable annular structure, reminiscent of music CD discs, is probably caused by the instabilities of the gas disc. It can be enhanced by the presence of young planets that increase the amount of disc gas in the vicinity of their orbit. (right, b). Nursery of young stars. Cluster of young stars (R136a) in the Tarantula nebula of the Large Magellanic Cloud (Milky Way satellite galaxy, figure 5.6). Home to a gigantic star formation outbreak, it is one of the most active stellar formation regions of the Milky Way and its satellites. We notice the nebulous environment that corresponds to remaining fragments of the giant molecular cloud from which these thousands of young stars were formed (in blue). This cluster contains the most massive stars known (>100 M⨀), which will quickly end their career as supernovae. It is no coincidence that the only supernova (SN1987A) observed by modern astronomy in three centuries in the Milky Way environment appeared in this nebula (SN198A is located slightly out of this figure below the bottom right corner, see figure 10.1). Credit: (left) ALMA (ESO/NAOJ/NRAO); (right) NASA, ESA, P Crowther (University of Sheffield).

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Young Infrared Stars: Born in Dusty Cocoons

When they first shine during their formation period, young stars are infrared objects. They form in the densest regions of the interstellar gas, deep inside dark clouds, where dust makes them opaque to visible radiation (§ 6.5). It is therefore impossible to study the state of the very young stars and even to detect their existence with optical telescopes. On the other hand, all the energy radiated by the star is completely absorbed by the dust and re-emitted in the far infrared (λ > 30 μm). Since this far-infrared radiation is blocked by the Earth’s atmosphere (figure 2.4), infrared space telescopes are essential for detecting and studying young infrared stars. Those proto-stars have been the main targets of the series of major far-infrared space missions that were successively launched over an interval of about thirty years with increasingly powerful telescopes, from IRAS (1983) to ISO (1995), Spitzer (2004), and Herschel (2009) (see § 2.4 and figure 2.10). However, space astronomy has severe limitations due to the cost and size of the telescopes (§ 2). Without going to space, besides the near-infrared windows on large ground-based telescopes (figure 2.4), it is mainly millimeter radio astronomy that has allowed us for half a century to explore this world both at high angular and spectral resolutions. The power of these successive generations of radio telescopes, culminating with ALMA (figure 2.5), provides us with a profusion of detailed images and dynamical tracers of the very complex regions where families of stars are formed (see figure 4.1b).

4.3

Gravitational Contraction, Accretion and Discs

While there is no doubt about the “big picture” view that stars form by gravitational contraction of the densest lumps in interstellar clouds, it has been a challenge to understand how to overcome the many physical difficulties of this contraction.

Box 4.1 – Steps in the formation and early life of stars (Single star of low mass; figures 3.6b and 4.2). – Formation of giant molecular clouds in the interstellar medium (§ 6.4). – Contraction and fragmentation of these clouds into dense clouds. – Contraction and collapse of a dense cloud to form a disc with a “proto-star” growing in the center. – Ejection of material (and angular momentum) by a bipolar wind on the axis of the disc. – Accretion of disc material by the proto-star through complex gas movements in the disc. The gravitational energy released increases the brightness and central temperature TC of the star (T Tauri phase). – When TC is large enough, the nuclear reactions light up and the star switches to the Main Sequence. It is still surrounded by a disc. – Planets form in the disc at the end of the previous step (see box 12.1), leaving debris in the outer disc (these debris are the components of most asteroids, comet nuclei and the dust responsible for zodiacal light in the Solar System).

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The core problem is explaining how to gather this matter, initially dispersed in an enormous nebula, a fraction of a light-year in radius, into the very small space of the star: a reduction factor of ten million in radius (or 1021 in volume). The fundamental laws of physics require the proto-star to overcome the formidable obstacles resulting from the different pressure forces, the loss of gravitational energy and, above all, the conservation of the rotational angular momentum. First of all, as we have already seen, a contraction of the gas releases gravitational energy that heats the gas, thereby increasing its pressure and henceforth slowing or halting the contraction. For this process to continue, it is necessary to lose the energy quickly to avoid excessive heating of the gas. This is achieved by the far infrared radiation emitted by the dust and interstellar molecules.

FIG. 4.2 – Stellar formation. Diagram of the final phases of formation of a star of about the mass of the Sun (box 4.1): final collapse of the nebula; formation of a luminous proto-star surrounded by a proto-planetary disc with ejection of a bipolar stellar wind; T Tauri phase; formation of planets; reaching the Main Sequence. Credit: universe-review.ca. However, the gas pressure is not the only agent acting to prevent the contraction of the interstellar cloud. Turbulence* or magnetic fields could also block star formation. But we now understand how the proto-stellar nebula can overcome these impediments under certain conditions. The main obstacle to an easy contraction of proto-stellar clouds into stars is certainly the need to preserve the angular momentum* of rotation – i.e. roughly the product M × V × R of the mass of the nebula by its rotation speed and radius. Although all these clouds initially have low rotational speeds, when the radius decreases during the contraction, the speed increases17 and there comes a time when 17

A well-known way to visualize the notion of angular momentum and its conservation is to imagine a skater rotating with weights at the end of her or his extended arms. If the skater bends its arms, reducing the distance R from the masses M to the axis of rotation, the velocity V of rotation increases to keep the angular momentum, M × V × R, and thus the product, V × R.

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the centrifugal force equals the gravitational attraction exerted by the central layers. This blocks the contraction. However, this only applies to contraction perpendicular to the axis of rotation and the nebula can therefore flatten to form a disc rotating fairly rapidly around this axis. Thus, stars can only be formed from such discs (box 4.1) through complex magneto-hydrodynamic* phenomena that conserve the total angular momentum. The disc organizes its structure to store a large part of its angular momentum in its outer regions where large planets orbit around the central condensation that can more easily collapse and form a star. Thus, for the Solar System, while almost all of the mass is concentrated in the small radius of the Sun, most of the angular momentum is contained in the orbital motion of Jupiter and Saturn around the Sun, but not at all in the modest spinning of the Sun on itself.

4.4

Universality of Stellar Pairs – Complex Ending of Their Lives

Forming in groups by fragmentation of interstellar clouds, young stars remain bound by gravitational attraction (figure 4.1b). Yet these “clusters” of young stars are short lived, quickly dispersed by the movements of their own members and interactions with other stars in the galaxy. Nevertheless, more than half of today’s stars are part of a binary or multiple star system bound by gravitational attraction throughout their lives. This can have crucial effects on the evolution of the two stars. More generally, the very existence of binary stars has far reaching consequences for our knowledge of the Universe, as they are particularly useful to measure stellar masses. Binary stars are also at the origin of some of most extraordinary celestial phenomena and objects. In most binary systems, the distance between stars remains large and greater than the size of the Solar System, with very long orbital periods (exceeding centuries) of one star around the other. They evolve almost independently, as if they were isolated. Their coupled life is limited to travelling together across the Galaxy, with the interesting feature of having the same age, but a small fraction of binary systems has a much smaller separation between the stars. Usually nothing notable happens as long as the stars remain on the Main Sequence with a modest radius compared to their separation. However, when the more massive of the two stars reaches the red giant (or supergiant) phase at the end of its life and expands more and more, there may come a time when its gas is drawn toward the other star by its gravitational attraction, resulting in a huge transfer of matter. A very rich variety of complex phenomena can then occur. One of the most frequent results is the formation of a “close binary star” (“close binary”) where a white dwarf, produced by the evolution of the more massive star, orbits close to the other star with which it interacts. This is the case for “cataclysmic variables” (figure 4.3a) with a period of a few hours and sporadic variations. Massive stars can give rise to similar phenomena and result in tight binary systems, one member of which is a neutron star or even a black hole. The subsequent interaction

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FIG. 4.3 – Massive stars. (left, a). Cataclysmic binary. In a binary system, the more massive star evolves more quickly and finally transforms itself into a compact object, a white dwarf, a neutron star or a black hole (§ 10, § 11). Later on, when the less massive star enters a phase of expansion (giant), the material of its outer layers can be attracted and even sucked in by the gravitational attraction of the compact object. This artist’s view thus represents a blue giant whose gas forms an accretion disc around a white dwarf. This gas heats up as it swirls through the disc, accelerating when it approaches the center and eventually falling onto the surface of the white dwarf. The extreme conditions created in these processes often lead to the emission of X-ray energy or even cataclysmic nuclear explosions of novae (§ 10). (right, b). Eta Carinae. Eta Carinae is one of the very brightest stars in the Milky Way and the closest to the Sun among them. It is located in the Carina southern constellation, invisible from Europe, at a distance of nearly 8000 light years (more than three times closer than the Galactic Center). It is a system of two hyper-massive stars (  100 and 40 M⨀; total luminosity  5 106 L⨀ [solar luminosity]), with a period of 5.5 years. This power is mainly radiated in the infrared, because the UV photons are almost completely absorbed by the dust of the surrounding nebula. The above image was observed in visible light by the Hubble Space Telescope (figure 2.7a); the stars are invisible, but one can see the spectacular Homunculus nebula surrounding the central binary, which was produced by a material ejection in 1843. Eta Carinae seems close to explosion in a very bright supernova (§ 10) within a few tens of thousands of years. Credit: (left) STScI/NASA; (right) NASA/CXC/GSFC/STScI.

between the members of the various tightly packed binaries leads to a wide variety of more or less violent and fascinating astrophysical objects. The result is a zoo of subclasses, including: – Nova: a violent thermonuclear explosion in the accreted material on the white dwarf of a cataclysmic variable (figure 4.3a). – Type-Ia Supernova: a more violent thermonuclear explosion affects the entire core of the white dwarf when its mass increased by accretion (or perhaps by fusion with another white dwarf) reaches the stability limit of the white dwarfs. These objects are used as “standard candles” to measure cosmological distances and have thus led to the discovery of dark energy* (§ 9.6).

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– X-ray binary: a tight binary of a neutron star or black hole (which led to the discovery of stellar black holes) and a “normal” star. X-rays come mainly from the enormous gravitational energy released by the accretion of the star’s matter onto the compact object (figure 4.3a). – Massive X-ray binary: consisting of a black hole and a very massive star, which will lead to a system of two black holes (double black hole, see below). – Micro-quasar: an X-ray binary whose accretion disc emits an axial jet of relativistic matter. – Millisecond pulsar: a neutron star whose rotation has been accelerated by accretion of matter from the companion in a low mass X-ray binary (box 10.2). – Binary pulsar: a tight binary of two neutron stars, one or both of which is a pulsar. The slow braking of their revolution around each other by the emission of gravitational waves was the first observational manifestation of these waves (figure 10.8). – Double black hole: a tight binary of two stellar black holes. The fusion of such objects produced most of recent direct gravitational wave detections (figure 11.2).

4.5

Brown Dwarfs, Billions of Aborted Stars

The standard model of stellar structure has remarkably put limits to the possible mass range for stars. After its formulation, mostly in the 1950s, its elaboration over the past 50 years, while refining the properties of the initial mass distribution of stars at the time of their formation, has seen significant progress in exploring the two frontiers of this domain. On the side of very large masses, greater than 100 M⨀, infrared observations have uncovered some of the very rare examples of hypermassive stars in the Milky Way and its very close neighbors (figure 4.3b). This type of object has generated a vast renewal of interest in theoretical modelling. They are thought to be at the heart of three frontier questions of current astronomy. Their final explosion in a hypernova* seems to explain the most powerful gamma-ray bursts* commonly detected up to the far limits of the Universe (§ 10.3, figure 10.6c). They could also have played a crucial role in the primordial generations of stars and their impact on the reionization of the Universe and the formation of the seeds of the first super-massive black holes (§ 11). Finally, in pairs, they might give birth to the double black-hole systems whose fusion has recently produced the first direct detections of gravitational waves (figure 11.2). At the other end of the stellar mass spectrum, rarity is no longer the problem, because the smallest ones abound. Yet, their low luminosity and surface temperature make them very pale in visible light. In fact, the stars of lower mass, red dwarfs*, have long been well documented down to the lower mass limit whose value, close to 0.08 M⨀, was already well established 50 years ago. But it was the discovery and analysis of pseudo-stellar objects below this limit, the brown dwarfs, that has been one of the most important advances in stellar astronomy in the last 30 years. Brown dwarfs (figure 4.4) are objects that resemble stars of smaller mass, especially at the time of their formation, but their mass is too small for their core temperature to

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FIG. 4.4 – Brown dwarfs. Radius of brown dwarfs of different types, ages and temperatures, compared to the Sun, a red dwarf star (Gliese 229A) and Jupiter, a massive planet. The old and cold brown dwarfs are quite comparable in size to Jupiter, while the younger and warmer ones are a little larger, but remain much smaller than any star. Credit: Wikipedia Creative Commons Attribution 3.0 Unported license. MPIA/V. Joergens.

reach the value of about 10 million degrees at which hydrogen fuses into helium. Therefore, they remain deprived of the main source of energy for the stars throughout their lives. This makes the 0.08 M⨀ limit that separates them from the stars very sharp. This difference appears particularly clear-cut after a few million years when they have exhausted their other sources of energy (gravitational and deuterium fusion) and have no other possibility than living increasingly sparingly on their stored heat reserve, cooling indefinitely and becoming increasingly obscure. That is where their name, brown-dwarf, comes from. Their effective surface temperature reflects this process and depends on their mass and age; it is always lower than red dwarfs and is roughly between 2000 and 500 K for those currently known, with luminosities ranging from about 10−4 to 10−6 L⨀. The maximum emission is thus in the infrared, shifting at progressively longer wavelengths as their temperature decreases. Their very low surface temperature produces special characteristics for their spectra, dominated by molecular bands, for which new spectral types have been defined that extend the classification of stellar spectra. Despite their large number, it does not seem that brown dwarfs play an important role in the evolution of galaxies. Their environment does not appear to be a favourable site for the formation of the most interesting planets. However, we must be careful not to jump to conclusions. They constitute a new phenomenon whose exploration has only just begun.

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4.6

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Stars are Still at the Forefront of Current Astronomy

Solar magnetic field and stellar oscillations Despite its extraordinary explanatory power, the “standard” theory of stars and the Sun remains full of approximations. Among other things, it assumes perfect spherical symmetry, does not include two major elements, rotation and magnetism, and only uses a simplified treatment of convection*. Recent decades have seen considerable progress in modelling these four key issues, which are strongly linked. They focused mainly on the Sun, for which observational information is vastly more developed. This modelling has enormously benefited from the fantastic progress in computer power. Yet, these are extremely difficult questions, as the very example of the Sun shows, for which a problem as fundamental as that of the 11-year solar cycle, linked to differential rotation (see below), is not yet well understood. All stars rotate on themselves with a speed that is measured by the broadening of the photospheric spectral lines by the Doppler* effect. However, stars do not rotate as solid bodies, but with different rotation periods depending on the distance to the

FIG. 4.5 – Solar chromosphere and corona. (left, a). Particularly spectacular solar flares above the Sun’s surface. The charged particles that cause the light emission are channelled along the magnetic field lines. Some form huge arches anchored at both ends in the Sun’s surface (photosphere), while others remain open to the outside, generating the solar wind of charged particles. (right, b). Total solar eclipse of June 21, 2001. The central part of the black and white image shows the filament structure of the near solar corona observed from the ground (Angola). Since it is difficult to observe the distant corona from the ground, this montage reveals its outer structure using the image (in false reddish colors) observed almost simultaneously from space by the SOHO* solar space observatory (figure 2.11). Such observations help to better understand and monitor the Sun’s magnetic activity and predict its influence on the Earth’s ionosphere, for example for the activation of aurorae borealis (figure 10.10). Credit: (left) NASA; (right) Image: J. Vilinga (Angola, IAP), LASCO, NRL, SOHO, ESA, NASA; Processing: R. Wittich; Composition & Copyright: S. Koutchmy (IAP, CNRS).

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FIG. 4.6 – Solar cycle. The Sun’s activity varies almost periodically over cycles of about 11 years. It can be measured by the number of sunspots on its surface, as shown here in the figure on the left (in white) for the last three cycles, 22 to 24 (the cycles are numbered by choosing the first measurement in 1755 as N° 1). We see in this figure that activity varies considerably between the minimum and maximum of a cycle, up to 20 times, and that the average activity can differ significantly from one cycle to another. The last cycle, 24, was the quietest for 100 or 200 years. Figure on the right. The intensity of the Sun’s UV emission is another fundamental aspect of solar activity. This intensity, linked with that of solar flares (figure 4.5a), has been measured continuously since 1996 by the SOHO* satellite (figure 2.11) and more recently by the STEREO* and SDO* missions. The figure on the right shows the variation in the UV image of the Sun during Cycle 23 (1996–2007). We see that the eruptions, almost absent at the minimum of the cycle, occupy almost the entire surface at the maximum of the cycle (2001, as in the red background of the figure on the left). Credit: (left) Hathaway NASA/ARC; (right) SOHO (ESA & NASA).

center and latitude. This has a decisive influence on their magnetic field, which is an essential ingredient in stellar physics. In the Sun, the magnetic field controls the 11-year solar cycle, with the variation in sunspot intensity and gigantic eruptions in the “chromosphere*”, above the “surface” of the Sun (figure 4.5a). The solar coronal wind gusts (figure 4.5b) that these eruptions trigger disturb the entire interplanetary environment even beyond the Earth. During the solar cycle, the differential rotation twists and coils the magnetic field lines, storing enormous torsional energy as in a gigantic elastic belt. The torsion reverses at some point, producing the inversion of the solar magnetic field every 11 years (figure 4.6). Another problem related to the magnetic field is the heating of the solar corona. It has long been known that the Sun’s magnetic field makes its corona much hotter than its surface, but how this field transports and deposits the energy is still a mystery. It is hoped that the many data expected in the next decade from specialized solar telescopes and space probes will help solving the problem. Such a complexity and the absence of direct information on the inside of stars, including the inside of the Sun, give an enormous value to observational information on solar and stellar oscillations in helping to improve theoretical models. The gas spheres that are stars behave like resonators in which waves, mainly acoustic

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(that is pressure waves), can propagate. Like a sound pipe, they have resonance frequencies. As with the big musical instruments, because the Sun is huge, we expect very low frequencies, super infra-sounds, which is exactly what we observe. The waves show a multitude of oscillation periods close to 5 min. It is clear that the characteristics of these resonant solar oscillations directly depend on its internal structure and they have indeed proven to be the best tool we have at our disposal to probe it. These oscillations have been detected in the Sun for about fifty years by the very small periodic variations they produce in the velocity of the surface layers and therefore in the Doppler* displacement of the spectral lines. Their study is called helioseismology, by analogy with seismology for the Earth. Its major contribution is the determination of the rotation speed in the different regions of the Sun’s interior. In particular, it highlighted a transition radius, at two-thirds of the solar radius, between the central region that rotates as a rigid body and the convective outer zone, the site of the differential rotation. It now seems well established that it is from a thin layer around this radius that most of the complex magnetic processes governing the solar cycle take their origin.

Validation of thermonuclear models of the Sun by neutrinos The other important probe from inside the Sun is the analysis of neutrinos* emitted in the nuclear reactions of its core. Neutrinos are key particles in the theory of radioactivity and nuclear reactions (figure 3.2). With almost no mass, they interact extremely weakly with matter so that they easily pass through the thickness of the Earth and even the Sun without attenuation. They are abundantly emitted in the hydrogen fusion reactions of the Sun’s core (figure 3.2). We therefore continually receive a huge flow of neutrinos from the Sun at the Earth. Every second, our bodies are crossed by nearly a million billion of these neutrinos. Despite the extreme weakness of their interactions, their detection has been possible since 1967 using detectors most often consisting of a huge water18 or ice reservoir buried deep underground (figure 10.2b). However, for decades, astronomers and particle physicists were puzzled by the too small number of solar neutrinos detected compared to expectations: about three times fewer neutrinos were detected than predicted in models of the Sun and its nuclear reactions. After years of intense activity to check and improve solar models, it turned out that the problem resided in the neutrino theory. It is now established that the observed deficit in solar neutrinos is mainly due to the transformation during their propagation of the initial “electron” neutrinos into another variety of neutrinos, mainly “muon” neutrinos, which are much more difficult to detect. The number of neutrinos detected is thus in perfect agreement with the predictions of the best models of the Sun. This conclusion provides further confirmation of the remarkable solidity of the models of the Sun and therefore of the other stars.

18

More modern detectors are based on heavy water or liquid scintillators.

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On the Edge of the Cosmos

Stars and Ecology of Planets and Galaxies

We have seen that stars occupy only a tiny portion of the galactic volume. Therefore, they lead an essentially solitary life and their chances of contact by collision are practically zero (except with their possible partner or in star clusters). But each star has a “sphere of influence” that extends far beyond its surface. Its action is exerted depending on the distance and its power. It can be said that its authority is undivided over its close suburbs, most often constituted by its planetary system. Beyond that, its gravitational attraction prevails over that of other stars up to a fraction of the distance to the nearest star; it can keep in orbit more or less large bodies such as the comet nuclei of the Oort cloud (§ 12), but this space is already affected by interstellar influences: cosmic rays and UV, magnetic field, gas, etc., not to mention massive stars that can make their presence felt quite far away by their UV radiation, stellar winds and terminal explosions. However, it is above all the planets and their acolytes (satellites, asteroids*, comets and their nuclei, etc.) that live in symbiosis with their star, as we do experience in the Solar System. In fact, there is a common destiny of the star and the inhabitants of the planetary system since their almost simultaneous birth in the accretion disc that fed the star, whose remains formed the planets (figure 4.2). The planets live under the wing of their star, completely dependent on its light, which warms them, determines their external temperature and dominates their climate and meteorology. We also know that almost all of our energy (except nuclear and geothermal energy) ultimately comes from the Sun: not only “solar” energy, but fossil fuels (coal, oil, gas) and current biomass which are photosynthesis products, and also hydro and wind energy, etc. However, the proximity and absolute dependence of such a monstrous source of energy can also have disastrous effects on its neighborhood. We have seen the devastation that the Sun was able to exert during its pre-Main Sequence T Tauri phase. The damage caused by massive stars is certainly worse and can even prevent the formation of planets. Even with a quiet Main Sequence star like the Sun cruising on the Main Sequence, the effects of its magnetic eruptions are noticeably felt on Earth. It should be noted that this nourishing or calamitous dependence is almost one-way. The planets bring practically nothing to their star, which only swallows a comet from time to time and can engulf its internal planets during its maximum expansion into a red giant. Stars are a key component of galaxy ecology. Their rate of formation and the impact of their terminal phases are crucial factors in the evolution of galaxies. Conversely, the properties and evolution of galaxies determine the successive generations of its stars. Each generation of stars is thus strongly influenced by the previous ones. The stellar formation rate (mass of interstellar gas transformed into young stars per unit of time) is considered one of the essential parameters describing the state and evolution of a galaxy. This is the main factor for the decline of interstellar gas, but this rate also determines the number of massive stars and

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therefore the brightness of the galaxy, the rate of formation of its atoms heavier than carbon, the rate of supernovae, etc. The action of massive stars is exerted over distance by their UV radiation, their winds and especially their explosions in supernovae. These processes can control the fate of the surrounding giant molecular clouds and go so far as to shape the entire interstellar medium of the galaxy and even partially expel it from the galactic disc (§ 6.5).

Part III

The New World of Galaxies

Chapter 5 Discovery of Galaxies 5.1

The Appreciation of the Nature of Galaxies Dates Back Only to the Beginning of the 20th Century

The understanding of the nature of galaxies is certainly one of the most important discoveries in the history of astronomy. It is the first discovery on our list of those achieved in the twentieth century (1924) and is closely linked to that of the expansion of the Universe, which immediately followed. It is really amazing that it is so recent, as it is hard to imagine that the word “galaxy” was not coined until 1925, while it has been, since then, so popularized by science fiction that it pervades our language. The emergence of the galaxy concept has its roots, however, in previous centuries. It can be followed in a pair of questions and intuitions about the Universe revealed by Galileo’s telescope and his followers. First of all, what could we say about the world of stars around us, the Milky Way, which ultimately proved to be our Galaxy? Although its disc shape was recognized as early as the eighteenth century, its dimensions and our position within it were still completely unknown in 1900. At that time, many large spiral nebulae were already widely known, since their first catalogues dated back to the eighteenth century, but around 1900, many astronomers were convinced that these nebulae were part of the Milky Way.

Our position in the Milky Way The luminous circular strip of the Milky Way studded with large dark areas, which runs across our summer northern skies (figure 5.1 right), is the trace of the multitude of stars that compose it. This is one of the first revelations of Galileo’s modest telescope in 1609, leaving no doubt about its decomposition into a vast aggregation of stars. The band of stars suggests that their distribution around us is not uniform in all directions, but is flattened in the shape of a thin disc, like a pancake, in which the Solar system is immersed.

DOI: 10.1051/978-2-7598-2706-0.c005 © Science Press, EDP Sciences, 2022

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FIG. 5.1 – The Milky Way. Right: view from the Northern Hemisphere (Nevada, United States) in excellent conditions. Left: spectacular view from the Southern Hemisphere (ALMA Observatory site in Chile, figure 2.5). The south aspect is much more impressive because the center of the Milky Way is in the southern part of the sky, so that the central regions richer in bright stars can be seen in their entirety and very high in the sky. The large dark patches of interstellar dust absorption are also much more visible, which led the populations of the Altiplano to see in them a series of remarkable figures in the shape of animals, like our constellations. Credit: (left) Teruomi Tsuno/NAOJ; (right) Wikipedia Creative Commons Blaise Thirard. This was recognized very quickly in the eighteenth century when William Herschel estimated that the number of stars was of the order of several hundred millions and the diameter of their complex distribution19 several thousand light years, assuming that the Sun was almost in the center. In fact, these estimates were much lower than reality, because one is quickly blinded by the extinction of interstellar dust that absorbs most of the visible light from the distant stars. These first estimates of the size of the Milky Way and the position of the Sun did not change much throughout the nineteenth century. This problem and that of the nature of spiral nebulae were addressed with new vigour and more powerful telescopes in the first decades of the 20th century. At the same time, various efforts to observe stars and their spectra had generated new astrophysical knowledge and tools to improve their distance measurements. In particular, the Cepheids quickly proved to be the most reliable indicators for determining distances (box 5.1). However, it is only in 1908 that Henrietta Swan Leavitt discovered the period-luminosity relation of the Cepheid variables, when the sensitivity of observations made it possible to detect them at the large distance of the Large Magellanic Cloud (figure 5.6). This relation then proved decisive for the approximate estimation of the distances of the Andromeda galaxy and other nearby galaxies. At the same time, the period-luminosity relationship of RR Lyrae was used 19

Herschel’s reconstructed map of stars inferred a very irregular structure because of the patchy distribution of interstellar absorption.

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Box 5.1 – Astronomical distance measurements The determination of the distances of the stars is a major problem in astronomy. It was addressed since ancient times for the distance of the Earth from the Sun and the Moon. It was essential to evaluate the luminosity* of the stars (§ 3.3), the distance from spiral nebulae (galaxies, § 5.1) and to reveal the expansion of the Universe (§ 8.1). It remains fundamental in cosmology to calibrate this expansion (value of the Hubble constant, reacceleration of the Universe by “dark energy”, § 9.6). We can distinguish three main methods to measure our distance to the stars: Parallax method It is the direct measurement of the distance from the Sun to nearby stars by triangulation (§ 3.3 and figure 3.3). As it requires extreme accuracy in angle measurements, it has long been limited to the nearest stars, but after the Hipparcos* space mission, the GAIA* satellite is in the process of extending it to a large part of the Milky Way Galaxy (§ 6.3 and figure 6.1). From the luminosity* There is a direct relationship between the brightness E of a star (received light power per unit area) and its luminosity L (light power), provided that the emission is isotropic, which distributes the received light energy at a distance d over the entire sphere of radius d: E = L/(4πd2). If we have a way to knowing L, we immediately deduce d from the measurement of E. The types of light sources whose luminosity is sufficiently stable and well defined for this purpose are often referred to as astronomical “standard candles”. The most used are: – some periodic stars, such as Cepheids or RR Lyrae stars, where the luminosity is related to the period by a well-defined relation, so that the easy measurement of the variability period is sufficient to determine the luminosity; – the stars on the Main Sequence whose luminosity can be approximated from the spectral type (figure 3.5); – supernovae of type Ia whose luminosity is well enough defined to determine the distance from the distant galaxies in which they explode (§ 9.6). From galaxy redshifts Provided that the cosmological parameters are well known (box 9.1), there is a well-defined relation between distance and spectral redshift z = Δλ/λ. It is simple (z = H0 d/c, Hubble–Lemaître law § 8.1) for nearby galaxies, more complex, but well established for distant galaxies. Since z is easy to measure from the spectrum of galaxies, it is the simplest parameter to describe their distance.

to determine the distance of globular clusters*. It was first realized that their apparent distribution in the sky was concentrated in the direction of the Sagittarius constellation. It seemed logical that this strange distribution should result from the location of these clusters centered on the center of the Milky Way. This point of

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view, which was consolidated by other arguments, was definitively established in the 1920s. However, their distance had yet to be determined. It was not until the absorption of interstellar dust was better understood (§ 6.5) that reliable estimate of the distance from the Sun to the Galactic center was finally available around 1930, approaching the current estimate, about 27 000 light years. The fact that the Sun is a small, ordinary star, lost at the edge of an immense ordinary galaxy, among some 200 billion others is now such a part of our culture that it is hard to imagine the shock and resistance caused by dropping the conviction that we could only be at the center of the Milky Way, let alone the world!

FIG. 5.2 – The Andromeda Galaxy. Together with the Milky Way, Andromeda is the main galaxy in our small “local” group of galaxies (figure 6.8b). It is also a large spiral galaxy, a little peculiar because of its interaction with another smaller neighboring galaxy. Its relative proximity, 2.4 million light years, makes it just visible to the naked eye, with a size of 3 degrees on the sky, six times greater than that of the Moon. Credit: NASA/JPL-Caltech.

The discovery of the nature of other galaxies It is no coincidence that the discovery of the nature of other galaxies occurred almost at the same time as that of ours. The improvement in astronomical methods for measuring distance that made it possible to survey the Milky Way, naturally extended to greater distances thanks to advances in telescope sensitivity. Moreover, the new knowledge of the anatomy of our Galaxy and its dimensions made it possible to immediately conclude that the hundreds of nebulae like Andromeda (figure 5.2), at least ten times further away than the diameter of the Milky Way, must be external to the Milky Way, but the “discovery” of galaxies was a long process that generated fierce controversy until its final resolution in 1924.

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As early as the eighteenth century, visionary and logical minds, such as the philosopher Kant, had intuited that gigantic groups of stars similar to the Milky Way, placed at very great distances, could look exactly like the numerous spiral nebulae that sky observers were already discovering. However, the study did not progress much throughout the nineteenth century and it was not until mid-1920s that a correct order of magnitude was established for the distance from the Andromeda galaxy first, and then very quickly for the nearest other galaxies. The controversy raged for years over the distance and the nature of the Andromeda Nebula and reached even the general public. It led to articles in the American press and organized public debates between astronomers arguing whether or not the spiral nebulae belong to the Milky Way. Without going into the details of the argumentations, it was once again the recognition of stars bright enough to be individually distinguished in the Andromeda galaxy and the estimation of their luminosity (box 5.1) that provided the true distance of this nebula and demonstrated its nature as a galaxy distinct from the Milky Way. The identification of the first Cepheid in this galaxy, by Hubble in 1924, at Mount Wilson with the power of the new telescope, definitively settled the issue. The recognition of the true nature of the Andromeda “nebula” was immediately extended to the myriads of galaxies that were known or could be guessed at that time. Our world thus appeared with a new face. Certainly, since Galileo, we knew that it was made up of an almost infinite number of stars. Then it became clear that this infinity itself was nested in an almost infinite number of galaxies. Our terrestrial world and even the Sun appeared even more insignificant, microscopic dusts on the scale of the infinite world of galaxies. The dimensions of our Universe were suddenly expanded by a factor of nearly 10 000, soon to be one million.

5.2

First Steps in the World of Nearby Galaxies

For almost a century, we know that we live in a universe of galaxies. Since astronomers understood it, observing and studying galaxies has become one of the main fields of astronomy. It is through the study of our close neighbors, for example, the Magellanic Clouds and Andromeda, that we know today the details of the morphologies of galaxies and how they are organized. Due to their proximity, all the large telescopes have examined them in detail through all available spectral intervals. We can thus draw up a complete census, starting with the majestic Andromeda galaxy, sister of the Milky Way, and ending with the smallest dwarf galaxies. The Milky Way itself offers us unique close-up views of the interior of a typical galaxy. However, hampered by a view of the galactic disc obscured by interstellar dust, we lack a global view of our own system (figure 6.1). Galaxies are huge objects and vastly more complex than stars. There is an enormous difference in scale, akin to the size of the Earth compared to a simple house and its garden. In addition, galaxies are composite structures. They are systems, not monolithic objects. This allows a variety of galactic landscapes much richer than the stereotypical classification of stars. Like an individual or a tree, each galaxy is unique with its own features and history. However, as with any complex

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system (think of botany), classification, even empirical, is an essential step in understanding them. In fact, beyond all exceptions and particularities, the basic classification of galaxies is extremely simple, with two fundamental types: spiral and elliptical galaxies.

FIG. 5.3 – Spiral galaxies. Examples of spiral galaxies seen from extreme angles – face on for M 101 (left) and edge on for NGC 891 (right) – highlighting the characteristic properties of spiral galaxies: extreme thinness of the disc and more or less regular structure of the spiral arms. These two examples could fairly well represent our Galaxy, the Milky Way (figure 6.2), except that M 101 has no central bar and the bulge of the Milky Way is more pronounced. This is seen in the right bottom inset which shows a view of the Milky Way in the near-infrared, where its emission is dominated by red giant stars. Dust absorption (dark areas) is lower than in visible light (see figures 5.1 and 6.1), but still well marked. The prominent infrared bulge in the center is brighter than in visible light (figure 6.1) because of the predominance of the red giants. Credit: (left) NGC 1232 APOD. Image Credit: FORS, 8.2 m VLT Antu, ESO; (right) NGC 891 J. C. Barentine (PSI) et al., KPNO, NOAO; (insert) SDSS, Apogee.

We are all familiar with the iconic image of a galaxy whose emblem is the more or less schematic spiral structure. In fact, this represents well the population of galaxies since nearly 70% of the nearby large galaxies are spirals. Most of their stars are situated in a very thin disc and they display, more or less marked bright spiral arms (figure 5.3). However, nearly a third of the large galaxies do not show this structure, most of them belonging to the other main class of galaxies, ellipticals. Elliptical galaxies show no trace of spiral structure or extreme flattening and their shape appears swollen and oval (figures 5.4a and 5.5). They have the shape of a more or less crushed sphere, which mathematicians call a flattened ellipsoid.

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FIG. 5.4 – Galaxies. (left, a). Giant elliptical galaxy, M 87, in the center of the Virgo galaxy cluster (the cluster closest to the Milky Way). The exceptional mass of such galaxies in the center of clusters (the total mass of stars equals nearly 1013 solar masses, almost 100 times that of the Milky Way) results in part from the absorption of many galaxies from the central regions of the cluster that they have engulfed. They also contain the largest known black holes (cf. § 11; 6 billion solar masses, 6 109 M⨀) for M 87; figure 11.9b). (right, b). Barred spiral galaxy (NGC 1300), with a remarkably prominent bar (see figure 5.5b for other examples of barred spiral galaxies). Credit: (left) J.-C. Cuillandre (CFHT); (right) ESO.

It should be noted that these two main classes, spiral and elliptical galaxies, do not encompass the full diversity of galaxies. First, a multitude of spirals or ellipticals have various characteristics such as rings, shells, traces of tidal interaction or even merger activity with another galaxy, different types of active nuclei* (for about 10% of galaxies, § 11), etc., which justify the definition of as many particular subclasses. In addition, a few percent of galaxies, especially among the smallest, have such an irregular structure that they are traditionally assigned a third class, known precisely as “irregular” galaxies. The basic classification into ellipticals and spirals may be naturally refined into subtypes according to a scheme proposed from the beginning by Hubble himself,

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when he reviewed the “nebulae” which he had just shown to be galaxies. This scheme, now somewhat extended, is shown in figure 5.5. The sequence begins on the left with the ellipticals that appear on the sky as more or less flattened ellipses but without a disc. They are almost exclusively old stars distributed very regularly, with very little gas and young stars. They are mostly more massive and brighter than spirals. Lenticular galaxies make the transition between ellipticals and spirals. For spirals, the sequence of subtypes takes into account the relative size of the bulge and the disc, the visibility of the spiral arms, and the importance of interstellar gas (§ 6.4) and young stars. An obvious difficulty comes from the fact that the spiral structure only appears fully to us for galaxies seen almost face on. Those seen by the edge show how extremely thin their discs can be (figure 5.3), because the ratio between their diameter and thickness can go up to nearly a hundred, as in the Milky Way. About two-thirds of the spirals have a bar structure in the center connected to the spiral arms (figures 5.4b and 5.5); these are called barred spiral galaxies, our Galaxy is one of them. At the end of the spiral sequence, the Sm galaxies, whose prototypes are the Small and Large Magellanic Cloud (figure 5.6), make the transition to the irregular ones. The latter do not have a well-defined structure and often contain a lot of gas (and dark matter*).

FIG. 5.5 – Galaxy classification scheme. The two figures, on the left in a very schematic form,

on the right based on images of real galaxies, illustrate the continuous progression of galaxy types (the so-called Hubble sequence). They are going from quasi spherical, often massive, elliptical galaxies (figure 5.4a), on the far left, to spiral galaxies, the most irregular ones, on the far right, via increasingly marked disc structures and the branching of barred and unbarred spirals. Irregular galaxies that escape any classification by definition are not represented here. It should be noted that this schematic sequence classification does not imply any evolutionary sequence. Credit: astro.wisconsin.edu.

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FIG. 5.6 – The Magellanic Clouds. The images of the Large Magellanic Cloud (left) and the

Small Magellanic Cloud (right) show the irregular structure of these two grouped galaxies, which are probably satellites of the Milky Way. The two Magellanic Clouds are clearly visible to the naked eye in the southern sky (see figure 6.1 for an illustration of the position of the Magellanic Clouds in relation to the Milky Way). Both are close to the Milky Way (about 150 000–200 000 light-years) and relatively massive (  1–2 billion stars). Although the Magellanic Clouds are relatively poor in chemical elements heavier than helium, some areas of the Large Cloud, in particular, have extremely active star formation (see figure 4.1b). Right: Credit ESO, Bruno Dias.

In addition to the crucial information on the shape of the galaxies, which is behind their classification, the data collected on galaxies also provide a great variety of additional and essential information, which enables to characterize them. We will discuss later (§ 5.3) their colors, which mainly reflect the age distribution of their stars, and the other details of their spectra. The radio domain characterizes their interstellar medium (§ 6.4). As with stars, it is first of all their total luminosity that makes it possible to locate galaxies in the hierarchy. With their distance, the overall luminous power determines their apparent brightness on the sky. As it results from the addition of the luminosity of all the stars in the galaxy, its value reflects the total number of stars modulated by their age and also the total mass of the stars in the galaxy. If, as usual, we take the luminosity or mass of the Sun (represented by the symbol ⨀) as units of measurement, the luminosities and masses of galaxies typically range from a few billion to a few hundred billion luminosities or solar masses (L⨀ or M⨀).

5.3

Architecture and Stellar Content of Galaxies

Stability of the ensemble and movement of individual stars Galaxies are first and foremost gigantic groups of billions of stars bound together by their mutual gravitational attraction. The stars constitute the main part of galaxies’ “normal” (baryonic) mass of matter. A priori nothing is simpler than the principles that govern the arrangement of stars in galaxies, since they obey the universal gravitational attraction law between stars (and dark matter, see § 9.5). It is this law

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alone that determines the movement of stars within the galaxy, like that of the planets around the Sun. Yet, there is one notable difference: the force exerted on each star results from the combination of the attraction of all the other stars and dark matter in the galaxy. Each star feels an average force directed roughly towards the center of the galaxy. Its movement thus shows some analogy with that of the planets, because it approximately describes an elliptical or even almost circular trajectory around the galactic center. However, beyond this approximation, reality is more complex. The precise trajectories are far from being simple ellipses, but above all, the main difficulty is that the stars influence each other and that we cannot treat the movement of each star as if the others were fixed. This fact explains the emergence and evolution of collective structures of stars, such as spiral arms and galactic bars. It took decades after the discovery of galaxies to fully understand their nature and that such structures are not necessarily permanent but transient and caused by instabilities in the ensemble of stars in the galaxy. The spiral arms are thus gigantic waves of over-density in the distribution of stars, somewhat similar to the waves propagating on the sea surface. The arms rotate in galactic discs at a speed different from that of the stars. This angular velocity varies with the distance to the galactic center, which produces the spiral winding of the arms. Bars are more fundamental structures of star distribution in the central regions of most spirals. They can persist for most of the galaxy’s life and are often difficult to distinguish from the bulge as in the Milky Way. The overall rotation of the galaxy dominates the motion of the stars with typical rotation speeds of 100–200 km/s or more for spirals. On the one hand, it does not rotate as a solid: the external parts rotate at a slower angle velocity than the central regions, which produces the winding of the arms. On the other hand, each star has an individual motion with respect to this rotating reference frame (such as the motion of an aircraft with respect to the Earth in rotation). All this gives rise to complex phenomena for the movement of stars that require elaborate modelling.

Various generations of stars revealed by the spectrum of each galaxy A galaxy contains stars of various ages whose age distribution reflects its history of the stellar formation. This history has spanned billions of years with a variable rate and occasional peaks. The age distribution results in a characteristic signature in the spectrum of the light emitted by each galaxy. Indeed, the integrated spectrum of a galaxy results from the combined spectra of its billions of stars. The history of the stellar formation plays a decisive role in producing the global spectrum of a galaxy. As their lifetime is very short, the young very bright stars, i.e. the most massive and hottest ones, disappear very quickly. A galactic spectrum rich in UV and blue lines characteristic of young stars therefore attests to a recent or ongoing star formation. Otherwise, it can be concluded that the galaxy has not hosted any significant stellar formation for a few tens or hundreds of millions of years. The spectra of such galaxies are dominated by red giants, and they appear red like these. Using spectra to characterize galaxies has been constantly refined throughout the century. This has led to highly accurate galaxy spectrum modelling in recent decades, allowing us to extract an impressive amount of information about each galaxy and its history.

Chapter 6 Our Galaxy and Its Interstellar Medium 6.1

Exploration of Our Galaxy, the Milky Way51

It is not an exaggeration to say that one of the landmark undertakings of the twentieth century was starting the systematic exploration of our own galaxy, our home, the Milky Way, “the Galaxy” as the astronomers say. Actually, humanity is still very far from the dream of science fiction journeys to explore our galaxy. Since humans began to realize the immensity of the world of stars, a few centuries ago, they have continually overcome the frustration of not being able to go in situ by scrutinizing it more deeply from Earth. In fact, the vision of the Milky Way is marvelous even with the naked eye, especially in the southern hemisphere where the bright regions of the Galactic Center (in the Sagittarius constellation) shine high in the sky (figure 5.1). A century ago, the majority of astronomers were convinced that

FIG. 6.1 – Milky Way seen by GAIA. First general view in visible light of the Milky Way provided in 2016 by the GAIA satellite (launched at the end of 2013). The absorption by dust (dark areas) is rendered with great precision; even the central bulge is partially obscured. The two Magellanic Clouds (figure 5.6) are seen in the lower right. Credit: ESA/Gaia/DPAC. DOI: 10.1051/978-2-7598-2706-0.c006 © Science Press, EDP Sciences, 2022

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the Universe was just this flattened system of stars in the Milky Way. Its inventory was begun with the most powerful telescopes of the time, including drawing more and more detailed maps of the sky. The emergence of the galaxy as a concept comes from this undertaking. When, in the 1920s, the Milky Way was identified with a galaxy, the research focused on understanding the organization of its stars and their movement in the galactic system. In less than a decade, we learned the splendour of this edifice of hundreds of billions of stars, driven by rotation and governed by Newton’s laws of gravitational attraction and dynamics. Since then, i.e. for almost one century, the Milky Way has remained at the center of our exploration of the Universe in many ways. Despite the enormous distances, it is for us the most accessible part of the Universe where we can find as close as possible a representative sample of most stars and other objects that populate our Galaxy and the Universe in numbers of billions of billions. We can analyze here the essential physical processes that are repeated throughout the Universe. Moreover, we have very good reasons to think that the Milky Way is a prototype of the large spiral galaxies which constitute the most important class of galaxies. Having a detailed knowledge of our own system provides therefore a good model for this type of galaxy throughout the Universe. The observations at various scales of the different regions of the local interstellar medium of the Milky Way are a fundamental reference for understanding the interstellar medium of all galaxies. Moreover, it is by understanding our galaxy, the Milky Way, that we infer our place in the Universe, as it is our cosmic habitat in which the Solar System was formed and is embedded. Our origins are anchored here. The Galactic context is the starting point for any further reflection on issues such as exobiology, possible extraterrestrial civilizations or humans leaving the Solar System.

6.2

An Ordinary Galaxy

The Milky Way is approximately disc-shaped. It is actually very thin, about a hundred thousand light-years in diameter but only a little more than a thousand light-years thick (figure 6.2), except for the central bulge (figures 5.3 and 6.2). It is composed of some two hundred billion stars, but the matter of this fantastic mass of stars occupies only a tiny part of the whole galactic volume that is a billion trillion (1021) times bigger! If we ignore the dark matter (§ 9.5), our Galaxy is above all empty space sprinkled with stars, fantastic sources of luminous energy. However, the volume of the space between the stars, the interstellar medium, is not empty: it is filled with extremely tenuous gas. We know that the paths of the stars through space are approximately elliptical orbits, often close to circular, tracing the overall rotation of the Galaxy. The Sun takes about 200 million years to perform a complete revolution around the Galactic Center at the large speed of 220 km/s. The Earth and the Sun have made about twenty galactic merry-go-round rides since their birth 4.5 billion years ago. But the Galaxy does not rotate in a block; its internal parts perform a complete turn in a much shorter time than the outer regions.

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Nevertheless, we have a major difficulty in understanding the structure of the Milky Way, because we are irremediably immersed in it. We thus lack a vantage point from which we can capture an overall view. However, the multiple images of neighboring sister galaxies help our imagination to construct panoramic models of the Milky Way as if viewed from above. We know that it would appear as a splendid spiral galaxy (figures 5.3 and 6.2). Thanks to the accumulation of observations at different wavelengths, including at radio wavelengths, we now have a fairly good understanding of the structure and properties of its spiral arms (§ 5.3, figure 6.2). However, we remain hampered both by the interstellar absorption and by seeing all the structures aligned (figure 6.1).

FIG. 6.2 – Schemes of the Milky Way. These fictitious diagrams show the Milky Way as one

could see it: (1) face-on (left) with its central bar, its main spiral arms and the eccentric position of the Sun; (2) edge-on (right) showing again the disc and the position of the Sun, and emphasizing the bulge, the galactic halo and its globular clusters. Credit: (left) NASA; (right) Société d’Astronomie Populaire de Limoges.

If the inner distribution of stars and interstellar gas is completely dominated by the disc structure, the Galaxy also includes peripheral regions that play an essential role. The gas extends far into the outside regions of the disc. In addition, a substantial number of stars is distributed above and below the disc populating a volume that has the same radius as the disc. This nearly spherical region, the galactic “halo” (figure 6.2 right), is important for its peculiar stellar population, one of the oldest in the Galaxy, and also for the presence of the mysterious dark matter that dominates the mass of the galaxy (§ 9.5). Finally, the central regions of our galaxy have a well-marked structure, a bar-like concentration of stars and gas, also observed in

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many other galaxies (figures 5.4b and 5.5). The bar dominates the structure of the central disc, governing the motion of its gas supply, a condition for its exceptional stellar formation activity. The galactic bulge is a precious witness of the past history of these regions and of their star formation (figures 5.3 and 6.1). Sitting in the very center of our Galaxy is the most fascinating object, a super-massive black hole of a few million Solar masses (§ 11.7).

6.3

Current Organization of Stars Resulting from the Milky Way History

Determining distances, positions and velocities of galactic stars vastly improved in the course of the last century. Understanding the nature and the luminosity of the stars enabled very early on to establish a correct distance scale (§ 5.1) and measuring the Doppler shift of the stellar lines allowed astronomers to estimate their velocities along the line of sight. A revolutionary advance came from space observations that eliminate the blurring effects of the turbulence of the Earth’s atmosphere. For example, the European satellite Hipparcos* (figure 2.11), launched in 1989, measured the distance and the proper motion* of tens of thousands of stars, located in the neighborhood of the Sun. It paved the way for another European astronomical satellite, GAIA55, launched in 2013, which has revolutionized Galactic structure studies by measuring more than a billion stars far out with an accuracy 100 times better than Hipparcos. Its main objective is to decipher the different components of the Galaxy, and unveil their structure, kinematics and history. To take full advantage of these observational achievements and of the enormous amount of data they produce, detailed theoretical models are needed to refine our 3D vision of the different components of our Galaxy. The goal is to reconstruct the past history of the Galaxy and its stars from the current instantaneous state of the position and velocity of the billions of stars that can be measured, replaying backward the film of their movement. With GAIA we are deciphering the succession of events that have marked the history of our Galaxy, such as the absorption of satellite dwarf galaxies and globular clusters*.

6.4

The Interstellar Gas53, a Key Player in the Evolution of Galaxies

An extremely diluted gas responsible for galactic discs Galaxies are not, however, uniquely composed of stars. Besides the mysterious dark matter (§ 9.5), they contain the “interstellar medium”, extremely diluted gas spread throughout the vast space, in between the stars. It is difficult to imagine how diluted

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the interstellar gas and dust is, as it contains on average only about 1 atom per cm3. This is more than one billion billion (1018) times less than the density of the air we breathe; we can consider that it is an “emptiness” more perfect than the best we are able to create in the laboratory. Although the total mass of this gas is low compared to that of the stars (typically less than 10% of the mass in stars), it still corresponds to huge values – exceeding 10 billion solar masses for the Milky Way. The interstellar gas is a crucial component of galaxies, at least as essential as the stars, because new stars are formed from it. Initially the proto-galaxies were only gas, and then over billions of years, galaxies like the Milky Way have grown mainly by the accretion of fresh gas from “intergalactic” space (that is, outside the galaxies). It is the interstellar gas that has given their disc form to all spiral galaxies. As we have seen for the formation of stars (§ 4.3), a gas ball can be compressed under the action of its own gravity, but only along its axis of rotation, resulting in the formation of a disc perpendicular to this axis.

TAB. 6.1 – Main components of the interstellar gas and their relative importance in a spiral galaxy such as the Milky Way. Component Very hot ionized Hot ionized Atomic warm Atomic cold Molecular

Temperature Kelvin (K) ≥300 000 K 10 000 K 5000 K 100 K 10–50 K

Number of atoms per cm3 0.005 0.3 0.6 10–100 102–106

Fraction of mass
0.003
0.03
0.25
0.4
0.3

Fraction of Galactic-disc volume
0.5
0.1
0.4
0.01
0.001

The interstellar medium exhibits extremely contrasting properties with very different temperatures and densities, corresponding mainly to five broad classes of regions: atomic cold, atomic warm, molecular, ionized hot and ionized very hot (table 6.1). However, the proportion of atomic nuclei of the different elements always has the same general characteristics and remains very close to the average “cosmic” abundances (figure 3.7). In addition to organized large-scale motions of the gas (rotation, contraction, expansion, etc.) and the random microscopic motions of its atoms and molecules corresponding to the gas temperature, a significant part of the kinetic energy of interstellar atoms and molecules is in the form of random macroscopic motions of the gas, often supersonic, at various scales, known as turbulence. Turbulence plays an important role in the dynamics of the interstellar gas at all scales, either by slowing down its contraction or through supersonic shock waves.

The atomic gas The cold atomic gas represents overall about 50% of the total mass of the interstellar gas. Condensed in clouds where the density is well above average, it is an essential

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reservoir that supplies the molecular clouds where new stars form. A little more than a century ago, the detection of spectral absorption lines of atoms, such as sodium and calcium, in the light of distant stars seen across the interstellar space, provided the first evidence of the presence of interstellar gas. However, for most atoms, such as hydrogen, carbon and oxygen, these lines fall in the far ultraviolet. It was not until the 1970s that this spectral domain became accessible by sending telescopes into space. Observing the wealth of different elements that have absorption lines in that part of the spectrum lead to a systematic measurement of the abundance of many atoms in the interstellar gas. However, the method of choice for studying the interstellar atomic hydrogen is the observation of its radio line at wavelength λ = 21 cm. Since the mid-twentieth century, astronomers have developed powerful radio telescopes to detect atomic hydrogen and probe the interstellar gas throughout the whole Milky Way and nearby galaxies. Radio frequencies are not absorbed by interstellar dust and can therefore probe the very centers of these galaxies. Since its first detection in 1951, the observation of the 21 cm line has remained one of the principal means for studying interstellar gas in external galaxies. The techniques used enable to measure the spectral line profiles and to determine the velocity of the atomic gas, providing key information about the dynamics of the gas in galaxies. These observations have benefited from the increase in the size of radio telescopes, often operated in interferometric networks (§ 2.4) such as the VLA* (figure 2.9a). The global SKA* project (Square Kilometer Array, figure 2.14) will mark a culmination in this search.

The molecular gas The molecular form is the natural state of most gases which are neither too hot nor excessively diluted, because the formation of the molecular bonds between the atoms gives off energy and leads to a more stable form of the gas than the atomic form. The interstellar molecular gas is a significant fraction of the gas mass in spiral galaxies – more than 30% in the Milky Way. The study of molecular gas at radio wavelengths began only a decade after that of the 21 cm line of the atomic gas because of the late development of the millimeter wave techniques where most molecular lines are found. However, since the 1970s, the study of interstellar molecules has emerged as a key sector of radio astronomy, mostly because all the stars of the Universe form from molecular gas. As the overwhelming dominant molecule H2 is difficult to detect directly, the next more abundant molecule CO (carbon monoxide) has become the “universal” tracer of molecular gas. Some other molecules, such as HCN and H2O, also provide complementary diagnostics of the conditions in the molecular gas and its chemistry. In ordinary spiral galaxies such as the Milky Way, the bulk of the molecular gas is located in the Galactic disc (figure 6.3) with a clear concentration in the spiral arms. It is very inhomogeneous and is dotted with clumps much denser than average. These small “dense clouds” are the sites where the new stars are individually formed (§ 4.2).

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FIG. 6.3 – Radio galactic emission of the CO molecule. Top, emission distribution in the millimeter lines of carbon monoxide, CO, in the Milky Way seen in 2012 by the Planck satellite (figure 9.4). It is thought to accurately reflect the mass distribution of the molecular gas (mainly H2). The bottom figure allows to compare it with the CO emission map observed from ground-based telescopes about thirty five years ago; the two images are remarkably identical. Credit: (top) Planck/ESA; (bottom) CFA Harvard, T. Dame.

A chemistry in extreme conditions without any equivalent on Earth The extreme conditions of density and temperature of the interstellar gas explain the presence of quite unusual molecules that reflect a unique chemistry unattainable in our laboratories. More than 200 molecules have been identified to date in space that are divided into two large groups. Unsurprisingly, we first encounter simple species such as water (H2O), carbon monoxide (CO) and ammonia (NH3), and species that are the basis of our organic chemistry, such as H2CO, HCN, CH3OH and other more complicated molecules containing up to twenty atoms. Considerable efforts have been made to detect amino acids in the interstellar medium. The detection of these elementary components of the proteins might shed some light on the origins of life, but they do not yet seem to have been confirmed in interstellar gas (unlike cometary nuclei and meteorites, § 12.7). Unusual, very unstable forms of “radicals” have also been detected, which are characterized by the presence of an unpaired electron such as OH, CN, and C2H, and molecular ions such as H3þ, and HCO+. Such species are highly reactive and therefore very unstable in our laboratories (HCO+ was unknown in the laboratory before its discovery in interstellar space). They play a key role in interstellar chemistry where they are created in reaction chains initiated by exposure to UV radiation in the atomic gas or cosmic rays in molecular clouds. Because of the extreme dilution of the gas, the interstellar chemistry is completely different from

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usual chemistry. The rate of individual chemical reactions may vary from years to million years instead of tiny fractions of a second in the laboratory, so that typical time constants for significant variations of interstellar molecular abundances range from thousands to billions of years.

The interstellar hot and very hot ionized gas Most of the volume of a galaxy like the Milky Way is occupied by gas where all the hydrogen atoms are ionized and the density is even lower than in the neutral atomic gas (table 6.1). In addition, each massive hot star is surrounded by a gas bubble of much higher density ionized by UV photons emitted by the star. The main tools used to study moderately hot ionized gas are observations of the red line (Hα) of hydrogen (which gives the pink color to images of interstellar nebulae) and continuous radio radiation. The extremely dilute, very hot gas, with temperature approaching a million Kelvin or more, occupies not only the majority of the disc of the Milky Way but also the totality of its halo. It is often called “coronal” because its temperature is close to that of the solar corona*, and is mainly observed by its X-ray emission.

6.5

Other Players in the Interstellar Medium

Dust and interstellar nanoparticles Solid micro- and nano-particles of “dust” (of sizes between a few micrometers to a few nanometers) appear ubiquitous in most components of the interstellar medium. They very effectively absorb ultraviolet and visible radiation, preventing us from seeing the inner regions of the Milky Way and shielding molecules inside the clouds from interstellar ultraviolet radiation. These grains consist mainly of silicates or graphitic carbon. The most stable central part of the grains is formed in the envelopes of red giant stars (figure 3.9a) and in the shock waves of supernova ejecta (figures 10.5 and 10.6a), but these grains constantly exchange atoms and molecules with the interstellar gas. As these grains are very cold, down to 10 K, many molecules, except H2, can stick to their surface and react with other accumulated species. Conversely, a variety of processes can eject the molecules from the dust grains: UV or X rays, shock waves, etc. This injection of new molecules, including H2, into the gas phase is a key contribution to interstellar chemistry. It has been known for only about thirty years that there is a large population of interstellar nanoparticles, smaller than conventional dust grains with sizes of the order of one nanometer. Their structure is based on fused hexagonal rings of aromatic carbon (as in graphene and graphite) surrounded at the periphery by hydrogen atoms. Called polycyclic aromatic hydrocarbons (PAH), they are revealed by their strong spectral bands emitted in the mid-infrared that dominate the mid-infrared spectra of galaxies (figure 6.4).

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FIG. 6.4 – PAH infrared emission of the Antennae Galaxies. Left, infrared image of these merging galaxies obtained from the Spitzer Space Telescope (figure 2.10) at 7–9 µm (red) and 3–4 µm (blue). The image is dominated by the 7–9 µm emission of polycyclic aromatic hydrocarbons (PAHs) distributed in the regions of interaction between the two galaxies, while their nuclei are visible at 3–4 µm. Right, infrared spectrum obtained with the ISO* satellite. The spectrum is dominated by the strong PAH bands at 6.2, 7.7, 8.5, 11.3 and 12.7 µm. Other lines of atomic ions and the H2 molecule are also visible. Credit: (left) Wikipedia; (right) A. Omont, ned.ipac.caltech.edu.

Supernovae: interstellar tsunamis The energy suddenly released under different forms by the explosion – or rather the implosion – of a supernova is enormous (§ 10.1). The mechanical energy injected into the ambient interstellar medium is much higher than the light energy (but smaller than the energy of emitted neutrinos). The expulsion of several solar masses of gas at speeds up to an appreciable fraction of light velocity maintains a cascade of energetic processes and causes a gigantic disturbance of the surrounding interstellar medium. The disturbance from multiple supernovae can spread to fantastic distances, up to several thousand light-years, sweeping the interstellar gas by powerful shock waves, making them real cosmic tsunamis. The explosions of supernovae deeply imprint the state of the interstellar medium at different scales. They leave behind a hot highly ionized gas that emits a very characteristic radiation at all wavelengths. Such “supernova remnants” are thus highly visible in the galactic sky (figures 10.5 and 10.6a), especially around historical supernovae such as the Crab (figure 6.5), whose expansion can be traced back from the initial explosion observed in 1054. Supernova remnants are thought to be the main originating site of cosmic rays (§ 10.4). Moreover, as the most massive stars live only a very short time, their supernova explosion is likely to occur within or adjacent to the giant molecular cloud that gave birth to them. The compression that the supernova induces into the interstellar gas can trigger the formation of a new generation of stars and, thus, of supernovae.

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FIG. 6.5 – Crab Nebula, remnant of the supernova of 1054. This highly luminous interstellar nebula in visible light (this image), in radio and X-rays (figure 10.7), corresponds to a well-documented supernova, whose explosion was observed in 1054 (§ 10.1). The presence of a neutron star at its center is attested by a powerful pulsar (figure 10.7). The nebula has continued its expansion for nearly 1000 years at a speed of several thousand km/s (still
1000 km/s today), so that its diameter is about six light-years today. Note the fragmented and filamentary structure, suggestive of the intensity of the explosion that has violently disrupted the surrounding interstellar matter in a large area. Credit: NASA/ESA/ASU/J. Hester & A. Loll.

FIG. 6.6 – Planck view of the Milky Way Galactic fountain. View of the submillimeter wavelength emission of interstellar dust by the Planck satellite* (figure 9.4), tracing the interstellar gas expelled from the disc of the Milky Way up to heights of thousands of light-years before it falls back onto the Galactic disc. Credit: Planck/ESA.

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The cumulative action of supernovae may form huge super-bubbles throughout the interstellar medium. Such super-shells may eventually expel significant amounts of interstellar gas outside the galactic disc. This mechanism is probably at the origin of the “galactic fountains”, which are huge rising currents of hot gas carried to great heights, thousands of light-years, above the Galactic disc, that ultimately fall back on the disk under the action of the gravitational attraction of the Milky Way (figure 6.6). The cumulative effect of supernovae is even more devastating in starburst galaxies where their number can be multiplied by several hundred compared to the Milky Way (§ 7.2).

Cosmic rays and the interstellar magnetic field The violent effects of supernova remnants on the interstellar medium also explains the presence of its most energetic particles, the “cosmic rays”. We will see in chapter 10, describing the violent Universe, how cosmic rays are important actors and the questions still raised by the most energetic of them (§ 10.4). These relativistic particles – mainly protons or He++ nuclei – play a key role in the physics and chemistry of the interstellar medium. Their energy allows them to penetrate into the depths of molecular clouds, for which they are the main source of gas heating and the drivers of all the interstellar chemistry. They can also react with ambient hydrogen atoms and protons to emit gamma rays that provide us with one of the best measurements of the mass of molecular clouds (§ 6.4). Strangely enough, the interstellar magnetic field remains relatively strong. Its strength is about 10 000–100 000 times lower on average in the disc of the Milky Way than the magnetic field of the Earth or the Sun’s heliosphere*. This is incomparably greater than the 10−18 factor between the mean gas density of the interstellar medium and the Earth’s or Sun’s atmosphere. As for the magnetic field of the Sun, stars or planets such as the Earth, the interstellar magnetic field must be generated by interstellar currents of electric charges producing a sort of natural dynamo. Its origin is today understood and known to be maintained by the galactic rotation. The interstellar medium is deeply affected by the universal action of its magnetic field on all moving charges. Although this force is not intrinsically very strong, if applied over astronomical scales and lengths it inevitably curves the paths of charged particles into spirals around the field lines. Charged particles of the interstellar gas are thus bound to the magnetic field and can only travel appreciable distances at the galactic scale along, but not in directions perpendicular to the magnetic field (this applies even to cosmic rays, § 10.4). Since charged particles strongly interact by collisions with neutral atoms and molecules in the gas, the entire interstellar medium is closely attached to the magnetic field. This is a key factor for the dynamics of the interstellar gas. Its movements perpendicular to the magnetic field are very difficult, but it can “flow” freely along field lines. This explains the universal filament structure of the interstellar medium (figures 6.6 and 6.7). If the interstellar gas is compressed, the magnetic field must “compress” with it so that its intensity increases. It can be shown that beside the pressure of the gas there is a kind of magnetic pressure, related to the magnetic field, which resists compression.

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This magnetic pressure is an obstacle to the gravitational contraction of dense cloud cores before they actually form stars. There is a remarkable approximate equality of energy per unit volume between its different forms, magnetic energy, kinetic energy of cosmic rays, thermal kinetic energy of gas atoms and kinetic energy of the turbulent motions (§ 6.4).

FIG. 6.7 – Filamentary structure of the interstellar medium. Views of the submillimeter wavelength emission of interstellar dust: on the right, by the Planck satellite (figures 6.6 and 9.4) on a very large scale of about one-tenth of the Milky Way disc; on the left by the Herschel satellite* (figure 2.10) a side region about ten times smaller. In both cases a filamentary structure of the dust distribution is clearly visible. Credit: ESA.

6.6

Exotic Components of the Milky Way

Even if the stars and the interstellar gas and dust are the major and most visible agents which condition the properties of the Milky Way, they are far from exhausting all its contents. The Milky Way, like other galaxies, shelters also many more discreet and mysterious components whose inventory can still hold many surprises. We refer to § 9.5 for the discussion of the dark matter that seems to provide the halo with a total mass about ten times that of the stars of the whole Milky Way, and § 11.7 concerning the super-massive black hole in the center of the Milky Way. In addition, our Galaxy, like the others, include a multitude of other discreet components which remain mysterious because it is nearly impossible to observe them directly. We may group them in two broad classes:

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– The inert stellar residues. At their “death”, when the stars have dispersed part of their gas into the interstellar medium, most of them do not disappear completely, but leave a compact (white dwarf) or hyper-compact (neutron star or black hole) inert residue (see § 3.3). Except if they are part of a binary system, such stellar corpses are condemned to a slow, quasi-mineral evolution. By inexorably cooling, white dwarfs grow indefinitely pale until becoming as invisible as ghost stars at galactic distances. In the same way, the pulsar phase of isolated neutron stars lasts only a moment and fades with the slowing down of their rotation (§ 10.2). The Milky Way is littered with billions of star corpses of both types. Similarly, brown dwarfs (§ 4.5) quickly reach a state of unlimited cooling that makes them invisible if they are some distance away. The estimation of the number of these dead bodies is based on models of evolution of the dead stars. There are thus billions of neutron stars produced from almost all isolated massive stars; at least a dozen billion white dwarfs, the outcome of all the isolated intermediate mass stars that have already ended their active life of a few billion years; several tens of billions of invisible brown dwarfs, the vast majority of all those formed since the dawn of the Galaxy. – Planets and asteroids floating freely in interstellar space. We do not know much about objects smaller than brown dwarfs, floating freely in the interstellar space. They may have a dual origin, either similar to that of brown dwarfs by contraction of interstellar nebulae, or from planets originally orbiting around a star that could have been detached by the action of another star or planet. Such ejections should be frequent in multiple star systems or star clusters. Due to the difficulty of detecting them, their number in interstellar space remains uncertain, but it could be compared to that of stars. If we still go down in mass range, there must be a larger number of interstellar objects of the size of asteroids, trans-Neptunian objects* and more generally planetesimals* (box 12.1), which have been ejected from the hundred billion planetary systems of the Milky Way. Their number remains highly uncertain. Before closing this chapter on our galaxy, one should remember that our galactic environment extends to the near suburbs of the Milky Way. These suburban areas include: (1) the Sagittarius Dwarf Galaxy that the Milky Way is currently engulfing; (2) the Magellanic Clouds (figures 5.6, 6.1 and 6.8a); (3) dwarf galaxies, satellites of the Milky Way (figure 6.8a); and (4) even all the galaxies of the group bound by gravitation, dominated by the Milky Way and Andromeda, which is called the Local Group (§ 7.2 and figure 6.8b). Despite the large distances, Galactic studies of stars and the interstellar medium are progressively extending to these objects which present more varied conditions than the Milky Way. Our cosmic environment at larger distance, up to nearly 100 million light-years, contains other groups of galaxies as well as a few clusters of galaxies (figure 6.8c). It is included within the Laniakea supercluster which encompasses about 100 000 galaxies stretched out over 500 million light-years.

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FIG. 6.8 – The Milky Way environment. (a) Satellites of the Milky Way. Diagram of the position of the dozen small dwarf galaxies which are satellites of the Milky Way (a few hundred thousand light-years away). (NB: These are the main ones, but there are other smaller ones). (b) The Local Group. Diagram of the galaxies of the group of galaxies (§ 7.2) to which belong the Milky Way, the Andromeda galaxy (figure 5.2), another massive galaxy (Triangulum Galaxy, M33) and more than twenty dwarf galaxies (up to a few million light years away). (c) Our farther neighbors. Our cosmic environment up to nearly 100 million light-years beyond the boundaries of the Local Group. It contains twenty or so other groups of galaxies as well as two clusters of galaxies: Virgo (the closest, about 50–70 million light-years away) and Fornax. Credit: Wikipedia Commons.

Chapter 7 Hundreds Billions of Galaxies 7.1

Galaxies at All Stages of Their Life

If the first half of the twentieth century was the era of discovering, understanding and exploring the galaxies through the Milky Way and its neighbors, the last half-century was the era of exploring galaxies in the distant Universe, up to the visible cosmic horizon. When we observe extremely distant galaxies, we see them in a state significantly younger than the Milky Way today, since their light had to travel for billions of years before reaching us. Thus, we observe the epoch of their most intense star formation, more than ten billion years ago. Comparison with nearby galaxies provides an increasingly refined picture of the global evolution of galaxies and, therefore, of the Universe. As we move closer and closer to the time of their formation, about 13 billion years ago, we can make decisive progress in understanding the complexity of the processes associated with formation of galaxies and the first generations of stars (cf. § 7.3). As for the study of large human populations, our approach to galaxies can only be statistical. Like demographers, astronomers conduct surveys and, more rarely, censuses. We are greatly assisted in this task by the assumption that, on large scales, the Universe is homogeneous and isotropic (§ 8). In any direction that we look at, a sample of galaxies at a given distance will have the same statistical properties (provided, however, that it is large enough to go beyond the limits of spatial structuring and clustering of galaxy distribution, see § 7.2). Our knowledge about distant galaxies often proceeds through deep probes. Very deep observations, using the best available telescopes, are done towards a very small portion of the sky, reaching the maximum sensitivity in order to detect the faintest galaxies, that is, the most distant or smallest ones. Currently, the deepest of these fields are those observed by the Hubble Space Telescope (HST) (figure 2.7a), which has devoted an impressive total amount of time to achieve the required sensitivity to detect galaxies at the edge of the Universe (figure 7.1). The incomparable quality of the images, taken outside the Earth’s atmosphere, makes it possible to achieve a higher sensitivity than with any of the much larger currently available ground-based optical telescopes. The detection of several thousand distant galaxies in each of these DOI: 10.1051/978-2-7598-2706-0.c007 © Science Press, EDP Sciences, 2022

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images made it possible to count them and measure their spatial density up to ages less than a billion years after the Big Bang. Extrapolating this density to the entire sky, we can deduce that the number of such galaxies in the accessible Universe exceeds one hundred billion! During the fifteen years since these images were taken, these few thousand deep field galaxies have been the subject to a large number of additional follow-up observations in order to determine their properties. The ever-increasing sensitivity of deep images allows us to study all the phases of the galaxy evolution, bringing us increasingly closer to the time of their formation.

FIG. 7.1 – A deep image of the distant Universe. Image of an extragalactic ultra-deep field observed by the Hubble Space Telescope (figure 2.7a) obtained after a ten-day exposure. It is the deepest image ever obtained of the distant Universe. In addition to a beautiful sample of various galaxies, a few billion light years away, the multitude of small spots in the image are galaxies with very large redshift observed at more than five billion years in the past. The most distant galaxies in this image, which can be seen up to more than a dozen billion years ago, are not far from the limits of the observable Universe. Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), Z. Levay (STScI).

Other major astronomical efforts of the last half-century were aimed at a complete census of galaxies brighter than a certain limit in the entire sky or a large part. Until the end of the twentieth century, global images of the extragalactic sky were provided to astronomers by photographic plates taken by dedicated telescopes with very large fields of view. The coordinated combination of decades of observation with telescopes from Mount Palomar for the northern sky, ESO* in Chile and AAO* in Australia for the southern sky, produced total sky coverage. Since the 1990s, all large area sky surveys have taken advantage of the progress of CCD20 detectors and

20

Charge Couple Devices (CCD) similar to those of a smartphone.

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their huge pixel number (figure 2.13). The most important of these surveys in the last two decades is undoubtedly the Sloan Digital Sky Survey (SDSS). In its main phase (2000–2008), this large and expensive project observed, from New Mexico, about a quarter of the sky – half of the northern sky, avoiding the vicinity of the Milky Way. Its sensitivity remains thousand times lower than that of the deep field of the Hubble Space Telescope (UHDF, figure 7.1), which covers an area that is ten million times smaller. Yet, it reproduces the sensitivity of Edwin Hubble’s own tiny deep fields done in the 1930s in an entire quarter of the sky. The SDSS survey also measured spectra, providing redshifts and thus precise distances of more than 700 000 galaxies, resulting in the best three-dimensional view of the local universe, namely: a quarter of the sky up to distances corresponding to a lookback time close to two billion years in the past (figure 9.2a). Global astronomy is now preparing for a new generation of much more ambitious sky surveys, that will be available in the next decade, both from the ground (LSST*, etc.) and from space (Euclid*, figure 9.4). The main objectives of these surveys are cosmological. Galaxies are the best tracers of the structure of the Universe and therefore of its history and of its major components. They inform us about dark matter (§ 9.5) and dark energy (§ 9.6), which were decisive in the genesis of galaxy structures. Such deep surveys also provide statistical information about galaxies and their large-scale aggregation. They also allow the identification of rare galaxies including, for instance, galaxies with very extreme properties or others that are seen during very brief phases of their evolution, such as collisions with neighboring galaxies, as well as line-of-sight coincidences that give rise to very strong gravitational lenses (figures 7.2 and 7.3).

7.2

The Turbulent Family Life of Galaxies

Aggregation of galaxies by gravity Although some of them appear isolated, galaxies have a strong tendency to form groups. This is not surprising given the spectacular architecture of filaments and density peaks (halos) that dark matter takes under the action of gravity (§ 9.1 and figure 9.2). Atoms of normal matter concentrate in these high-density regions to form primordial galaxies, which then grow by accreting dense surrounding gas. This gas is itself constantly renewed by fresh gas that condenses and flows along the dark matter filaments. Galaxies therefore tend to stretch along those filaments, particularly accumulating in dark matter density peaks at the confluence of filaments and avoiding regions of lower dark matter density that remain huge empty spaces, void of galaxies. For more than thirty years, we have had evidence that these large structures traced by galaxy accumulations and alignments are present not only in the numerical simulations (figure 9.2b) but also in the real Universe (figure 9.2a). The largest observation programs designed to map the sky first highlighted them in the relatively close Universe (  2 billion light years). New surveys are now revealing them earlier in the history of the Universe, up to about ten billion years in the past.

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Galaxy clusters typically contain hundreds to thousands of galaxies in a small space of several million light years, which corresponds to the largest concentrations (“halos”) of dark matter today. The attraction of this dark matter creates a potential well that confines galaxies and ensures the stability of the cluster. These structures formed early, more than ten billion years ago, and loosely. They then grew more recently by accretion of dark matter, gas and galaxies. Clusters contain not only galaxies in motion within the cluster but also very hot ionized gas, bound to the cluster. As discussed below, this hot gas (of order million degrees Kelvin) is evinced by its X-ray emission and by its interaction with the cosmological radiation that it diffuses with a small modification. Clusters have played a key role in the detection of dark matter (§ 9.5). Very soon after the discovery of galaxies, Fritz Zwicky52 studied the movement of galaxies within clusters. As early as the 1930s, he realized that their high velocities imply surprisingly high values for the total mass of the clusters. The problem was significant since the mass deduced from the dynamics of the galaxies reaches about ten times the sum of the masses of all the galaxies in the cluster, whereas one could think that they constituted the essential part of the cluster mass. This was the first time that astronomers encountered the problem of dark matter that is now thought to constitute about 80% of the mass of the clusters, and of the matter of the Universe (box 9.1). The model currently accepted for clusters is based on this idea. The large mass of dark matter dominates the gravitational force that structures the cluster and ensures its cohesion. Under the action of this force, dark matter organizes itself into a large spherical structure, which will ultimately govern the distribution of galaxies, their dynamics and physical properties. The persistent ignorance of the nature of dark matter may question its reality (§ 9.5). However, the concordant effects, observed in completely different fields, have reinforced the reality of dark matter. The spectacular gravitational lens effects, produced by massive clusters (figure 7.2), allow a precise characterization of their dark matter fraction and distribution. As shown in figure 7.3, the bending of light rays by gravitational attraction and the subsequent image distortion provide detailed information on the total mass of dark matter and its distribution in the halos of the clusters. This information is in perfect agreement with that obtained from the dynamics of the constituent galaxies and the studies of cluster gas. Clusters thus have the strongest accumulations of dark matter known in the Universe. They also have the specificity of existing in all their full development only in the present Universe, because the most massive of them, which reach one million billion (1015) solar masses, needed the entire lifetime of the Universe to accumulate this quantity of dark matter. At the crossroads of two filaments, they result from the combined over-densities of both (figure 9.2b). This makes clusters, their number and mass very constraining indicators for cosmological models. Clusters are also important for the distribution of their ordinary matter in galaxies and intergalactic gas. Their galaxies retain traces of their promiscuity from the time they were accreted into the cluster. Interactions between galaxies are frequent. Some can accumulate gas by tides, which strip others of their gaseous component. These frequent interactions eventually lead to many galaxy mergers, especially towards the center of the cluster. It is therefore not surprising that at the

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FIG. 7.2 – Gravitational arcs. In this deep image of a massive cluster of galaxies, MACS J0138.0-2155, next to the galaxies of the cluster, we see spectacular pieces of light arcs. These are images of a galaxy far behind the cluster that are distorted by space curvature due to the enormous mass of the cluster (see diagrams in figure 7.3). This mass produces a strong lens effect that stretches these images into arcs and magnifies them strongly. In addition, this image, made in 2016 has caught the explosion of a supernova in the lensed galaxy. This supernova is seen as red dots in three images of the galaxy (SN1, SN2, and SN3). As seen in the right zoom insets, the supernova disappeared in the new image made three years later in 2019. The supernova is not seen in the fourth 2016 arc image of the galaxy in the left top corner of the figure because of the longer lensed light path of this image; it is expected that the supernova image will only be observable in 2037. Its observation should allow astronomers to precisely measure the parameters of the expansion of the Universe, such as H0 and dark energy (§ 9.6). Credit: HST, NASA/ESA. cluster’s center there is an extremely massive galaxy (figure 5.4a), by far amongst the most massive galaxies in the Universe. They provide us with the opportunity to understand the various galactic properties under the most extreme conditions. In particular, these central galaxies contain the most massive black holes known in the Universe (§ 11). Since they are frequently active radio galaxies, emitting powerful relativistic jets, the central galaxies of the nearest clusters provide the most spectacular examples of the violent activity of a radio galaxy (figures 11.6 and 11.7). Despite the dominance of dark matter and the presence of these exceptional galaxies, the amount of cluster gas distributed between the galaxies is far from negligible. First of all, it is comparable in mass to the total of the galaxies. We now know that this gas contains a significant fraction of heavy elements, instead of being

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FIG. 7.3 – This diagram illustrates the principle of gravitational lensing: (1) the left panel shows the simplest case, where a massive spherical body (e.g., a massive elliptical galaxy), halfway in the direction of a distant galaxy (light source), distorts its image by bending the light rays by gravitation. If the two galaxies are perfectly aligned with the Earth, the image formed by the lens is a perfect ring (Einstein’s ring, as in the inset). In the more general case of a less perfect alignment, the ring breaks into a few arcs; (2) the right panel shows a more complex case of a galaxy cluster, such as in figure 7.2. Credit: W. Schaap (Kapteyn Institute, §U. Groningen) et al. 2dF Galaxy Redshift Survey; (inset) ESA/Hubble & NASA, T. Treu | Acknowledgment: J. Schmidt.

close to that of the primordial gas composed of only hydrogen and helium. These elements cannot have been formed anywhere else than in the galaxies of the cluster, from which they were expelled by powerful galactic winds caused by the blast of giant starbursts or active galaxy nuclei*. Subjected to the violence of supernovae and stellar winds, cluster gas particles heat up to extremely high temperatures, easily exceeding one million Kelvin. This very hot gas provides the basis for two of the most successful detection methods for distant clusters. Its temperature and mass make clusters among the most powerful X-ray sources in the Universe, so that the inventory of the most distant clusters still mainly relies on observations done with X-ray telescopes. However, the discovery of distant clusters by X-rays is now being challenged by the tenuous signature that clusters imprinted on cosmic background radiation. Known as the Sunyaev–Zel’dovich effect, named after its discoverers, this effect is now detectable in increasingly distant and less massive clusters as the accuracy of cosmological radiation maps improves (figure 8.4). While only a minority of galaxies do belong to a galaxy cluster, most are part of smaller associations of less than hundred galaxies, in majority dwarf galaxies, that are linked together by gravitation, and are known as galaxy groups. The Milky Way is a member of such a group of about fifty galaxies, called the “Local Group”, of a diameter of about ten million light years (figure 6.8b). It is dominated by two large spiral galaxies, the Milky Way and the Andromeda Galaxy (M31) that are 2.5 million light years apart. Most of the other galaxies of the Local Group are

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concentrated around them. The Large and Small Magellanic Cloud (figure 5.6) are the main satellites of the Milky Way, close neighbors that are, respectively, about 180 000 and 210 000 light years away. A third spiral, (the Triangle, M33) is located near Andromeda, 2.6 million light years from the Milky Way. The other forty or so less massive galaxies of the Local Group include mainly dwarf elliptical galaxies and some irregular galaxies.

Banality of collisions between galaxies We recall that galaxies are relatively close to each other than stars. Since they attract each other strongly and live in more or less large groups, confined by their mutual attraction, it is, thus, not surprising that collisions between galaxies are relatively frequent. Such collisions, which very generally lead to the fusion of the two galaxies, are rare although normal events in the long life of galaxies. At a minimum, any massive galaxy ends up engulfing the dwarf satellite galaxies that were formed at the same time and can orbit around it for billions of years. We could say that both “harassment” and “cannibalism” are frequent in the group life of galaxies! We have evidence of this in our Milky Way, where it has been known for about 20 years that such a collision is ongoing. The Dwarf Sagittarius Galaxy, hidden in a difficult to observe region near the Galactic Center, is already partially dismembered; the stars in its outer regions are stretched into a long filament of stars barely distinguishable from those of the Milky Way. In a few hundred million years, it will be incorporated into the Milky Way and its memory will only remain in dynamically distinct streams of stars in the outer regions of the Galaxy. We have identified other similar star streams, which are the witnesses to the similar tragic disappearance of at least three or four others of our little companions in the not so distant past. We know that the same fate will inevitably fall, in a few billion years, upon our nearest neighbors, the Large and Small Magellanic Clouds. Bearing in mind that the Large Magellanic Cloud (LMC) will be a much larger piece to swallow, this will certainly result in a major perturbation of the entire Milky Way, but without radically altering its structure. In general, it is thought that absorptions of small galaxies contributed to the thickening of the disc and bulge of the Milky Way. Based on data collected by GAIA*, evidence has been recently found that such a collision with another satellite galaxy comparable in mass to the LMC (about a tenth of that of the Milky Way) could have occurred during the first billions years of our Galaxy’s lifetime and could be responsible for most of the thicker part of its disc. The situation is quite different when the masses of the two galaxies that collide and merge are comparable. The well-organized fields to which stars were subjected in both galaxies before the collision are radically altered, as is the motion of the stars. This is perfectly confirmed by the increasingly detailed simulations of such collisions that have become common with the progress of computers. It is clear that the beautiful disc structures of the spirals must be violently disrupted. A fraction of the stars and gas is ejected into the intergalactic space in the form of spectacular filaments; however, a structure similar to elliptical galaxies is the most likely outcome of the reorganization of stars in a collision and merging of two galaxies of comparable masses without too much gas. This is probably the fate that awaits our

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FIG. 7.4 – Examples of galaxies in the process of merging. Images showing spectacular examples of galaxies, all relatively close to the Milky Way (less than about a billion light years away), at different stages of their collision. These collisions result in the fusion of most of the stars and gas into a single galaxy, while the rest is dispersed in space and seen in the form of filaments, similar to antennas, which are visible on many of the images. The strong disturbance of their gas components causes a violent outbreak of star formation and the appearance of large dust clouds that completely absorb UV and visible radiation from young, bright stars, re-emitting most of the energy in the far infrared. Credit: HST/NASA. Milky Way in a few billion years when it will merge with our neighbor Andromeda (figure 5.2) at the end of their long and ever closer round. Collisions between massive galaxies are relatively rare nowadays because the expansion of the Universe has spread the un-clustered galaxies apart and many of those that remain linked in clusters or groups have already merged. Nevertheless, there are many spectacular collisions in progress in systems close enough to us to reveal details of the collision process in its different phases (figures 7.4 and 7.6). In addition to the chaotic effects on the arrangement of stars, the consequences for the interstellar gas are even more dramatic. Unlike stellar populations, gas clouds cannot simply interpenetrate; their collision generates violent compressional shock waves (figure 7.5). Above all, however, the tidal force exerted by one galaxy on the gas of the other, even at great distances, very quickly disrupts the fragile balance that prevents gas clouds from collapsing into new stars (§ 4.2). This results in a violent outbreak of stellar formation in colliding galaxies, where the rate of

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FIG. 7.5 – Stephan’s galaxy quintet. This group of five galaxies is one of the most spectacular examples of multiple galaxy collisions. Galaxy A violently collides with the group of three other galaxies (B, C, D) that it crosses at a supersonic speed of nearly 1000 km/s (more than 3 million km/h). The fifth large galaxy (E) is a foreground galaxy that is not a member of the group. The blue vertical streak in the center of the figure is the superimposed X-ray image of the gigantic shock wave front that was produced by the collision in the gas between the galaxies of the group. Credit: Chandra/NASA, Harvard CFA.

stellar formation can increase by a factor of ten or even a hundred. In addition, the motion of all the interstellar gas is perturbed and it loses its original regular rotation pattern. A substantial part of this gas can move to the central regions where the local conditions can maintain the starburst even after the galaxy merging is completed. It should be noted that the super-massive central black holes of the two galaxies also participate in the fusion process. Buried in the heart of the newly fused galaxy, the two black holes undergo chaotic interactions with gas and stars that quickly bring them closer to the center and therefore to each other. They thus end up into a final merging process (§ 11). Such cataclysms completely reshape the structure of galaxies. They suddenly form a significant fraction of their stars in a single starburst. They are rare today, but we know that they were hundreds of times more frequent in the early Universe, about ten billion years ago. At these large redshifts, when the galaxies were still dominated by gas, such mergers were the origin of the gigantic stellar formation outbreaks that were commonplace then, reaching stellar formation rates that are unparalleled today. From an observational point of view, such giant star outbreaks appear as ultra-luminous infrared galaxies, because the dust associated with the interstellar gas absorbs almost all the energy radiated by young stars, re-emitting it in the far-infrared. However, it is thought that the main result of late collisions between spiral galaxies, where there is little gas left, is the redistribution of their stars into a system that is no longer flattened but more or less spherical. This is probably the origin of the shape of elliptical galaxies and should occur in the future collision between the Andromeda Galaxy and the Milky Way in about four billion years.

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FIG. 7.6 – Jet and magnetic field of radio galaxy Centaurus A. Cen A is a spectacular nearby radio galaxy only at 10 million light-years from the Milky Way. The remnant of a merger between an elliptical and a spiral galaxy, it displays giant jets extending up to one million light-years into the intergalactic medium and a central thick dusty disk that is the remnant of the spiral galaxy. The image is a composite of optical, submillimeter, X-ray and far-infrared observations. The huge disk is thread by magnetic field lines that appear as bright polarized infrared emission of gas and dust filaments. APOD, NASA. Credit: ESO/WFI (visible); MPIfR/ESO/APEX/A. Weiss et al. (microwave); NASA/CXC/CfA/R. Kraft et al. (X-ray).

7.3

Understanding the Formation and Evolution of Galaxies

Understanding the formation of galaxies remains very difficult. We are just beginning to directly observe the first steps of this formation for only a minority of galaxies. Such galaxies, observed in the Universe when it was only about a billion years old, were then too small and distant to be easily detectable by our current telescopes. Yet, the most important observational progress in recent years in this field is the detection of a significant sample of hundreds of such luminous galaxies. This gives us a good idea of what the brightest galaxies were like in the very early Universe. However, until the JWST (figure 2.7b) is in full operation, we still know very little about the whole range of fainter galaxies at that time and especially about the very early stages of formation of all galaxies. Our understanding of this stage of galaxy evolution is therefore mainly based, not on its direct observation, but first on what can be deduced from the properties of older galaxies and their stars up to the present time, and second from theoretical

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modelling. It is now agreed (see figure 9.1) that the first generations of stars may have formed 100 or 200 million years after the Big Bang, in clusters of stars of a size perhaps similar to that of globular clusters* (between a few thousand and a few hundred thousand stars), but smaller than that of even small galaxies. These first stars obviously formed in the first dark matter density peaks at a time when these over-densities had already grown large enough (§ 9.1). It was in the depths of these condensations that the first gas nebulae of sufficient mass and density had to gather to start the first collapses leading to the formation of stars, but compared to the later “normal” stellar formation, we see now throughout the Universe (§ 4), this initial stellar formation process had to overcome a serious handicap, the absence of heavy atoms other than hydrogen and helium. The spectral lines of molecules such as CO and H2O and atoms or ions such as O and C+, not to mention dust formed by heavy elements, play an essential role in the removal of energy from interstellar nebulae that is necessary to allow them to condense and eventually collapse to form stars (§ 4). Since all heavy atoms were formed later in the stars, there were none in the “primordial” gas that formed the first stars. The final cooling of the primordial nebulae, allowing them to contract and collapse, could only have been accomplished by the very rare H2 molecules. The low efficiency of this cooling must have resulted

Box 7.1 – Evolution scheme of spiral and elliptical galaxies Spiral galaxies The Milky Way is a good representation of this dominant type of galaxy. In their youth, up to 10 and even 5 billion years ago, they had much more gas than today. This gave them their flattened disc shape, resulting in a more intense stellar formation that produced the majority of today’s stars. Given the rate of gas consumption by star formation, this gas had to be constantly renewed by gravitational accretion of extragalactic gas, faster than at present. Successive mergers of small satellite galaxies took place in parallel (§ 7.2). Major mergers with another galaxy of comparable size have, however, remained relatively rare in the lifetime of most spiral galaxies (§ 7.2). The Milky Way seems to have never undergone such a major collision before, but it should have engulfed a satellite galaxy about a tenth of its mass during the first billions years of its lifetime (§ 7.2). It will have one major collision with Andromeda in a few billion years. Elliptical galaxies The antiquity of their stars and the current absence of gas in elliptical galaxies imply an early interruption of the star formation due to the exhaustion or expulsion of the interstellar gas and its non-renewal. The absence of a disk and the random distribution of star orbits can result mainly from fusion between two galaxies already rich in stars and without much gas. The tendency of ellipticals to be found more frequently in galaxy clusters makes mergers more likely at all times. However, the existence of ellipticals with very large redshifts shows that some of them may have formed very early in the history of the Universe.

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in stars quite different from those we know today. They were certainly much more massive, typically exceeding perhaps 100 M⨀, and therefore very unstable, exploding quickly and forming the first heavy atoms. During the rest of the first billion years after the Big Bang, as the size of dark matter condensations increased, stellar clusters could also grow by merging or accreting gas. They could then reach the mass of small galaxies which continued growing by accreting further gas or merging. Towards the end of this period, which is also the period of reionization of the intergalactic gas (§ 9, figure 9.1), we already observe some galaxies as massive as the Magellanic Clouds, especially around the first quasars* (§ 11). The evolution of galaxies is then increasingly documented for those that are less distant in space and time, and as observational data become more detailed and accurate. We can consider that the period of the second half of the Universe’s history, roughly the last 7 or 8 billion years, is beginning to be well covered by observations of massive galaxies like the Milky Way, providing a fairly clear idea of the main features of the evolution of spirals and ellipticals over this period (see box 7.1). As more important questions remain for earlier times, observational information continues to accumulate and our understanding is progressing rapidly for the galaxies of the young Universe.

Part IV

Cosmology, the Science of the Universe as a Whole81,82,85,86,88

Chapter 8 Birth of Cosmology 8.1

The Universe of Galaxies is Expanding84

The discovery, around 1930, of the apparent receding motion of nearby galaxies (figure 8.1) immediately followed the discovery of their actual nature as vast, distant, independent stellar systems. The expansion of the Universe revealed by this motion provided the observational basis for purely mathematical models of the Universe derived from the new theory of general relativity. This may be considered as the true birth date of cosmology, the science of the structure and evolution of the Universe as a whole. After discovering that the Universe was not limited to the stars of the Milky Way, humanity had an immense surprise when entering the world of galaxies. This is not just the next layer of an indefinite stack of nesting doll structures. Far from being immutable, the Cosmos is universally in an indefinite expansion revealed by the galaxies moving away as evidenced by the redshift* of their spectral lines. This galaxy recession is accelerating as we probe the depths of the Universe. For galaxies not too far away, the rate V of this motion is directly proportional to the distance D of the galaxy, i.e. V = H0 D, where H0 is the Hubble constant. We deduce from this that the relative rate of separation Vij between any two galaxies i and j increases similarly proportional to their distance Dij (figure 8.1), Vij = H0 × Dij, and therefore that the whole Universe is expanding. This fundamental result signaled the birth of observational cosmology, furnishing unexpected substance to mathematical models derived from Einstein’s founding ideas on general relativity. Thus, a new science of the global nature of the Universe and its history, known as cosmology, was born, which became more and more precise throughout the century. As early as 1917, Einstein had proposed a purely mathematical model of the Universe, supposed to be isotropic and homogeneous, but he had artificially forced the solution to obtain a static universe. In the same year, Willem de Sitter had favored another model that naturally involved the expansion of the Universe, but ignored the existence of matter by assuming zero density. It was only later that two young researchers, the Russian Alexander Friedmann in 1922 and the Belgian Georges Lemaitre in 1927, derived independently the complete solution of cosmological equations, although their works took time to be recognized because of the languages DOI: 10.1051/978-2-7598-2706-0.c008 © Science Press, EDP Sciences, 2022

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FIG. 8.1 – Model illustrating the expansion of the Universe (a, b). The figure shows the analogy of the expansion of the Universe with raisins in a swelling cake, illustrating the fact that, with Hubble’s law, V = H0 × D, the Milky Way is not at all a singular point, nor the privileged center of expansion: in the swelling cake, each raisin/galaxy moves away from all the others with a relative speed following the same law. The whole Universe swells, without there being a center of this expansion. Cosmic redshift (c). When we observe the light of a distant galaxy, there is a shift toward the red between the wavelength λ that we observe and the wavelength λ0 that was emitted by the galaxy. This redshift is defined as z = (λ − λ0)/λ0. The figure displays the relationship in the standard cosmological model (box 9.1) between the redshift z and the lookback time t that has elapsed in the past since the galaxy emitted the light that we observe now. Note that for nearby galaxies t = D/c and the redshift is proportional to D and t: z = V/c = (H0/c) × D = H0 × t. Credit: NASA/GSFC; (bottom right) D. Elbaz, CEA. in which these works were published (Russian and French) and the low visibility of the journals in which they appeared. Nevertheless, Lemaitre was the first to grasp the profound significance of his model in terms of the expansion of the Universe, which implied a linear relationship between V and D. He had even obtained, as early as 1927, the first approximate observational verification from existing data about redshifts and distances to nearby galaxies. New observations allowed Hubble in 1929 to give a slightly more solid basis to the linear relationship between the velocity of galaxies and their distance, V = H0 D, now known as the Hubble–Lemaitre law. However, Hubble himself did not interpret this relationship in terms of the expansion of the Universe. It was only after 1930 that this interpretation began to spread through the efforts of Eddington and de Sitter, popularizing Lemaitre’s work. By the 1930s, the connection between galaxy recession and relativistic models was effective, and the framework of modern cosmology was definitively provided with its essential ingredients, namely:

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– We see galaxies moving away from us uniformly in all directions. This can give us the illusion that we are at the center of the world. However, this is not the case, because an observer placed on any other galaxy would have an identical impression (figure 8.1). This justifies the basic cosmological assumption that the Universe is perfectly uniform on a large scale, and that the Universe as a whole and its space–time are expanding. – Only a relativistic theory can account for the forces that govern the whole Universe and its evolution. Nevertheless, we can define for each observer (or each galaxy) a privileged time, the cosmic time, counted from a common origin, in the initial moment when all matter was merged. We say today that it is the time elapsed since the Big Bang. When we observe a distant galaxy, there is a unique correspondence between the redshift z = (λ − λ0)/λ0 of its spectral lines and the cosmic time when it emitted the light we now observe (figure 8.1c). In fact, the redshift z is an observational parameter that can be directly measured on galaxy spectra, unlike cosmic time that depends on the cosmological model, and z is therefore systematically used to specify the age and distance of distant galaxies. – The rate of expansion of the Universe is not constant over time. It is possible to conceive of an acceleration or deceleration of this expansion. Among the factors that can modify the expansion, the most obvious is the gravitational attraction of other galaxies, or rather of all the matter in the Universe, which tends to slow down the expansion. As early as the 1930s, attempts were made to estimate the average density of galaxies per unit volume in the nearby Universe, but the errors were still considerable. In particular it could be imagined that, if the density of the Universe (mass per unit volume) was high enough, the expansion could be slowed down enough to stop and reverse into contraction at some time in the future. – It was also known that the addition of a cosmological constant Λ, allowed by cosmological equations, can modify the expansion. It was possible to consider that a cosmological constant could profoundly alter the history of the Universe, as Einstein had initially proposed before retracting his suggestion. However, it was not until the end of the 20th century that the rate of recession of distant galaxies was measured with sufficient accuracy to reveal an acceleration of the Universe (figure 9.3 right). The physical origin of this re-acceleration and its energy (“dark energy”) remains unclear, but it could be linked to the cosmological constant (§ 9.6). The implications of the mathematical framework of the equations of cosmology were clearly appreciated as well as their observational issues about the current density of Universe ρ0 and the future of the Universe, which is predicted from the current epoch values of the constants H0 and ρ0. On this basis, the main lines began to emerge in the late 1930s for the program of the next three quarters of a century that would seek to answer the fundamental questions of cosmology, namely the determination of values of H0, ρ0 and Λ. This proved to be much more difficult than expected and this objective was not achieved until the very end of the 20th century (box 9.1). Moreover, since the current expansion of the Universe naturally leads to the assumption of a hyper-dense and hot initial state, it was impossible to escape the

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question of the reality of this state, its physical properties and the limit to which these properties could be extrapolated by going back in time. This was the subject of the major debate on the Big Bang, which extended over the next three decades until the mid-1960s (§ 8.2), and which remains open even now for the very first phases.

8.2

The Saga of the Big Bang Confirmation

The universal flight of galaxies, which are all moving away from each other, irresistibly provokes the imaginary exercise of time-reversing the time course of their movement. By mentally running the film backwards, we inevitably bring the galaxies closer together, to achieve their fictitious dissolution in an initial dense state. We are now convinced that nothing in the laws of physics allows us to escape the conclusion that our Universe of galaxies, stars and their planets comes from a singular state with unimaginable values of density and temperature of a rapidly expanding gas of particles. This vision, described in the following chapter, is generally referred to as the Big Bang. There is now a broad consensus that this description should be considered as definitively confirmed, at least for the later phases of the Big Bang (§ 8.4) This was established in the 1960s by the discovery of microwave radiation from the cosmological background (figure 8.4) and the agreement of the abundances of helium and deuterium in the present universe with the nucleosynthesis models of the Big Bang (§ 8.4, figure 8.3a). It has been subsequently confirmed by the consistency of many other observations, including: detailed properties of the anisotropies of the cosmic radiation, their consistency with the observed large-scale structures of galaxies, the flatness of the Universe, the direct measurement of the temperature of the cosmic radiation in the past, the observation of the baryon acoustic oscillations*, the evolution properties of galaxies, the consistency of the measurement of the Hubble constant H0 by various methods. The period 1945–1970 provided the validation of the Big Bang model. It remains surprising that its reality was not fully established earlier. In fact, the theoretical foundations of particle physics were available from the late 1940s and had been used at that time by Gamow, Alpher and Hermann to publish in 1948 a paper, “The Origin of Chemical Elements”, which presented a coherent physical model of the phase of the Big Bang that included helium synthesis and the production of the microwave background radiation. It is strange that the impact of this work remained very limited during the next fifteen years, when the controversy was in full swing between astrophysicists who supported the mathematical model of the Big Bang and their opponents who advocated a stationary universe model. In particular, it seems that Gamow’s 1948 article predicting the existence of microwave relic radiation was long ignored by the radio astronomy community. This cosmological radiation*, which proved to be a decisive argument in favor of the Big Bang, could probably have been identified long before its accidental discovery in 1964. After that date, the general model of the Big Bang became definitively the standard model of cosmology.

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FIG. 8.2 – Schematic diagram showing the evolution of the Universe from the origin of the Big Bang to the present day. This diagram illustrates the different evolutionary phases of the current Big Bang model87,89, on an approximate logarithmic scale for time, temperature and mean kinetic energy of the particles of the “cosmic soup”. We note the key phases: first, the probable, still poorly understood, phase of highly accelerated expansion, generally known as “inflation” (  10−35 s), then, that of the disappearance of antimatter (the residual existence of a small amount of ordinary matter is still not understood), and, finally, the better understood later phases: the merging of quarks* into hadrons* (before 10−4 s), the nucleosynthesis of light elements (before 100 s), the recombination of electrons and hydrogen and helium nuclei and the decoupling between radiation and matter (  380 000 years). The diagram then continues by illustrating the formation of the structures of the current Universe, first by a long period of invisible growth of density fluctuations over a few hundred million years (figure 9.1), then by the formation of the first stars and galaxies and their evolution into large structures (figures 9.1 and 9.2) over more than 13 billion years to the present day. Credit: Particle Data Group, Lawrence Berkeley National Laboratory.

The modeling of this final part of the Big Bang (§ 8.4) progressively became robust because its physics is well understood and the parameters of the Universe are now precisely known (box 9.1). In retrospect, it is easy to appreciate the immense impact of the discovery of the expansion of the Universe by Lemaitre, Hubble, Slipher and

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their colleagues revealing the nature of our Universe and its history. Yet, it has taken nearly half a century of hard work to achieve the current consensus regarding the robustness of the Big Bang cosmological model.

8.3

The Very First Phase in the History of the Universe: Uncertain Physics (figure 8.2)

Not all phases of the history of the Universe are understood with the same precision because of the difficult physics of the extreme conditions at play. Today, we have a coherent view of its main features throughout its 13.8 billion years, except for the very first fraction of a second. We will quickly review the synthetic picture of the history of our Cosmos (figures 8.2 and 9.1), from this origin to our current world of galaxies, which represents one of the most prodigious scientific achievements of the last half-century. We can divide this story into two major phases: (1) first, the Universe, a gas of various particles, remained almost homogeneous at all scales; we will simply call this phase “Big Bang” for convenience; (2) it is followed by the formation and evolution of the structures of the current Universe, which is hierarchically structured at various scales into stars, galaxies, clusters and filaments of dark matter and galaxies (figure 9.2). It seems more appropriate to distinguish two periods in the development of the Big Bang itself. The details of the first period remain uncertain because the energies of its particles exceed those that can be studied on Earth (figure 8.2). We believe, however, that we have firmly reconstructed the second period, which is often called the “Big Bang standard model”, because we understand well both its physics and the constraints imposed by the parameters of the Universe today (box 9.1). As we said, if we go back in time over the course of the expansion of the Universe, we inevitably encounter increasingly dense hot states. The density and temperature, and, therefore, the energy of the particles, appear to tend towards infinity. For the era before about 10−12 s, physics becomes more and more uncertain as we go back in time because the energies eventually exceed those that can be produced in the most powerful particle accelerators. Finally, we enter into an unknown territory where physics remains speculative. In short, it is agreed that it probably makes little sense to imagine more remote times than when the properties of the Universe approach the fantastic Planck values. These are obtained by combining the three fundamental constants of gravitation and quantum physics (c, G and h): 5 10−44 s for the Planck time, 2 10−36 m for the Planck length, 1032 K (or 1028 eV) for the temperature (or energy). When approaching such values, any applicable physical theory must combine general relativity and quantum physics, but we still do not have such a theory despite enormous efforts towards achieving this goal in the last decades. As a result, we are neither able to describe under what conditions the history of the Universe began, nor to say whether this question makes any sense. The same is true of the question of whether it makes sense to talk about previous eras, despite our understandable curiosity about such inquiries.

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If we, nevertheless, let our imagination follow the Big Bang from the Planck time, 5 10−44 s (figure 8.2), the situation remains unclear before about 10−36 s. What we think may have happened in the period in between 10−36 and 10−32 s can be very constrained by the properties of the Universe in the later phases until the present time. It is indeed almost certain that the remarkable homogeneity and isotropy properties of the current Universe, as well as its “platitude*” (§ 9.4), testify that the Universe had to go through a fantastic acceleration phase that is most often referred to as inflation. In this picture, the Universe saw its characteristic dimensions multiplied by a gigantic factor of at least 1030, perhaps 1080, in a tiny fraction of a second! Although we still do not know the real behavior and physics of the Universe under these exceptional conditions, scenarios have been proposed for about thirty years on the constraints and the type of physics that could apply then, using the energy of the vacuum. A period of the inflationary type appears necessary for most current cosmological theories to explain the homogeneity and isotropy of the observable Universe. These properties indicate that all its parts could have been confined before this period in a space small enough for them to have been connectable at the speed of light and therefore in causal interaction. Inflation would also explain why the current Universe is so remarkably “flat” (§ 9.4). It also makes it possible to understand the existence of extremely small initial quantum fluctuations in energy density and how they could later sufficiently grow under gravitational attraction to be detectable as brightness fluctuations in the cosmic background radiation (figure 8.4) and ultimately generate the large structures of the Universe and galaxies (§ 9.1). The results of the Planck satellite (figures 8.4 and 9.4) have confirmed that the properties of these fluctuations are consistent with predictions of the most common inflation models, but the physical mechanisms remain to be clarified before these models are confirmed. At the end of this cataclysmic era, the Universe returned to the quieter state of a “normal” expansion that proceeded in accordance with the “standard Big Bang model”. As the temperature decreases, we can distinguish successive stages over time, depending on temperature and according to the type of dominant particles (figure 8.2). Between 10–32 and  10−10 s, the temperature dropped from about 1028 K to 1015 K and most particles gradually annihilated with their antiparticles. At the end of this period, with the temperature decrease accompanying the expansion, the Big Bang finally entered the range of energies that can be reached with current particle accelerators and to which elementary particle physics applies. For T  1015 K, the Universe’s particle soup was dominated by very massive exotic particles such as W and Z bosons and their antiparticles, Higgs bosons, electrons, neutrinos and photons, and quarks at the end of this period. Throughout this time, the Universe was almost perfectly symmetrical between matter and antimatter, the number of each type of particles being almost equal to the number of corresponding antiparticles. Some aspects of the microphysics of this period still remain unknown, in particular the reason why and the moment when this perfect symmetry was broken resulting in a very slight excess of each type of matter particle with respect to its antiparticle.

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A Well-Understood Second Phase: The Standard Big Bang Model

At the end of this period, around 10−10 s (  100 GeV*, T  1015 K), the Universe became dominated by quarks and leptons* (electrons, neutrinos, etc.) whose physics are mastered. It is this era of quarks that we will choose to date the beginning of the second phase of the Big Bang that we think if well understood. At the end of this quark era, around 10−6 s, the temperature had dropped sufficiently, to two trillion degrees Kelvin (  2 1012 K,  0.2 GeV), for the quarks to aggregate and become confined as “hadrons” (protons, neutrons, mesons and their antiparticles). The Universe then entered the hadronic era when hadrons dominated the cosmic soup with leptons* and photons. Until that time, the balance between matter and antimatter remained almost preserved. However, at some point in time, as the temperature continued to decrease, antiprotons and antineutrons annihilated with most protons and neutrons, leaving only the small excess of protons and neutrons in the present Universe. This tiny excess, about one part in a billion, was crucial for determining the Universe’s present composition of matter excluding antimatter. This process was certainly completed at t = 10−4 s (1012 K). As the temperature continuously decreased, all the other antimatter particles successively disappeared, ending with the disappearance of positrons (anti-electrons). The Universe was then a few seconds old. It was composed mainly of neutrinos and photons, plus a tiny percentage (about one billionth by number) of our conventional particles of matter: protons, neutrons and electrons. When the Universe was a few seconds to a few minutes old, the remaining free and unstable neutrons could spontaneously transform into protons, but then nucleosynthesis started and eventually incorporated almost all the remaining neutrons into helium nuclei (4He, with 2 protons and 2 neutrons), where they remained stable. The ratio of the total number of neutrons (now mainly in helium nuclei) to that of protons then froze close to its current value of about 1/6. After three or four minutes, the Big Bang nucleosynthesis, often referred to as primordial nucleosynthesis, was almost complete. In addition to 4He, it produced very small quantities of deuterium (2H), 3He, lithium, beryllium and boron (figure 8.3a), but it proved unable to achieve the very difficult synthesis of carbon (§ 3.4) and, consequently, did not form any of the heavier atoms. Helium nucleosynthesis was therefore proven remarkably effective using almost all the neutrons still present a few seconds after the Big Bang to fuse them with so many protons in 4He nuclei. These singularly stable nuclei contain nearly a quarter of the baryonic mass of the Universe and they are still nearly 100 times more numerous than all other nuclei combined, except hydrogen. It is absolutely impossible to form such a quantity of helium in stars, which have only synthesized a small fraction of the helium currently present in the Universe. This fundamental result that the Big Bang naturally formed the enormous quantity of helium present in the current Universe had been found as early as 1948 by Alpher, Gamow and Bethe. Later confirmed, it provided a strong argument in favor of the Big Bang, while advances in helium abundance measurement and stellar nucleosynthesis theory (§ 3.4) confirmed that stars were unable to produce the bulk of the observed helium.

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FIG. 8.3 – Big Bang confirmation. (left, a). Primary abundances of light elements90. The blue curves show the calculated variation in the mass fraction of light elements 4He, 2D, 3He and 7 Li to protons (note the change in scale by a factor  100 000 between the three panels of the figure) at the end of the Big Bang, in a standard Big Bang model, as a function of the amount of ordinary matter (ratio of the number of protons plus neutrons to the number of photons). The grey vertical bar shows the determination of this amount of ordinary matter from the properties of the cosmic microwave background measured by Planck and WMAP*. It makes it possible to deduce the theoretical primordial abundances of these four elements by the crossings of the grey bar with the blue curves. The horizontal green hatched areas represent the range of the values for these primordial abundances derived with some uncertainty from direct observations of stars and the interstellar medium in conditions not too much affected by stellar nucleosynthesis. These values are in good agreement with the theoretical estimates of these primordial abundances, except for 7Li which has to be also synthesized in stars and the interstellar medium (note, nevertheless, the slight tension for deuterium shown in the zoom insert). (right, b). CMB discovery. The discovery of microwave radiation from the cosmological background (CMB) in 1964 by A. Penzias and R. Wilson was the decisive confirmation of the Big Bang theory together with the abundances of light elements. This discovery was accidental, indeed, while they were looking for an explanation for the anomalous radio noise of their antenna. Their result was immediately interpreted by Dicke, Peebles, Roll & Wilkinson as the CMB. It is probable that, even without that finding, the CMB would have been discovered very soon since at the time several groups were working at experiments for this purpose. R. Dicke’s group confirmed the CMB detection within a few months. Penzias and Wilson earned a Nobel prize in 1978 for the CMB discovery, while J. Peebles was awarded a Nobel prize in 2020 for his theoretical work in cosmology. Credit: (left) Pitrou, Coc, Uzan, Vangioni, arXiv:1801.08023v2; (right) Princeton University.

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All other light elements, including deuterium (D or 2H), 3He and 7Li, have an observed abundance substantially lower than 4He, by factors greater than 1000 for D and 3He, and nearly one billion or more for all others (figure 8.3a). This is a direct result of the much greater fragility of these nuclei compared to 4He. These primordial abundances depend very strongly on the baryonic density of the Universe (figure 8.3a). However, their direct observational determination, when nucleosynthesis stopped a few minutes after the beginning of the Big Bang, is very difficult. Indeed, being very fragile, these elements are systematically destroyed later in the interior of stars. It is therefore necessary to measure their abundances in the interstellar medium. However, these measurements are made difficult as the gas released by the stars that no longer contain D or 3He are polluting the primordial abundances. Nevertheless, the success of Big Bang models in predicting the value of these primordial abundances is impressive (figure 8.3a). The estimates of the primordial abundance of deuterium and the Big Bang nucleosynthesis models provide one of the most important constraints for determining the density of baryons in the Universe (figure 8.3a and box 9.1). After the completion of primordial nucleosynthesis within a few minutes after the Big Bang, the gaseous Universe was a plasma of hydrogen (protons) and helium ions and electrons embedded in a background of photons and neutrinos about a billion times more numerous. Its expansion in this state continued for nearly 400 000 years. Two events occurred at that time, which were important for the rest of the history of the Universe and its current appearance. First, photons and neutrinos ceased to dominate the energy content of the Universe. This role was then played by dark matter (figure 9.3) with a small participation of atoms, waiting for dark energy to take over some ten billion years later (figure 9.3). At about the same time, electrons combined with ionized nuclei of hydrogen (protons) and helium to form a gas of neutral atoms (this process is referred to as “recombination”). Until then, the interaction of photons and electrons had maintained a close coupling between the two worlds of radiation and matter that had the same temperature and the same (very small) density fluctuations. However, this interaction ceased to operate as soon as the electrons became integrated into the hydrogen or helium atoms, which interact very little with the photons. At the same time, the total energy of photons had rapidly become much smaller than that of dark matter and atoms, so that from that time on, photons played only a marginal role in determining the history of the Universe. Since then, they have freely continued their own individual journey through the Universe, just carried along with the global expansion that, like all lengths, indefinitely increases their wavelength. They are now seen at a wavelength multiplied by a factor of about 1000 since the time of recombination, shifting from the near-infrared spectral domain at their origin to the millimeter spectral domain today creating the background of cosmological radiation*. Despite its tiny contribution to the Universe’s current energy content and its almost negligible role in the formation of galaxies and stars, this component of the Universe is a key role messenger containing the archive of its past history. It perfectly preserves the conditions in the Big Bang 380 000 years after its origin, just when matter and radiation decoupled.

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FIG. 8.4 – (Bottom c). Sky map of the anisotropies of the cosmological microwave background (CMB) obtained with the Planck satellite. It should be noted that these spatial fluctuations remain very small (  0.00001 = 10−5 K), about one hundred thousandth at most of the average intensity of the cosmological radiation (2.7 K). They essentially represent the density fluctuations of the Universe at the time when radiation decoupled from matter about 380 000 years after Big Bang. This map is the result of an extremely elaborate processing of the data collected by Planck over four years of observation. It includes in particular: (1) The combination in 10ʹ (0.17 degree) pixels of the maps obtained in each of the nine frequency bands observed by Planck, ranging from the centimeter to the submillimeter radio domain. (2) The subtraction in each of these bands of all foreground emissions that overlap the cosmological background in different directions. The mathematical analysis of the spatial fluctuations of this map allows us to compare them with those calculated from Big Bang models (figure 9.4) and to derive rich information on the parameters of the Universe (box 9.1). (Top and center). Predicted foregrounds. Examples of major contributions, including foregrounds, to be subtracted from the raw sky map observed by the Planck satellite to accurately extract the contribution of spatial fluctuations from the cosmic diffuse background (figures 8.4c and 9.4): (1) top panel a: CMB dipolar anisotropy resulting from the Sun’s motion (400 km/s) relative to the reference frame of the cosmological radiation emission (  0.001 K); (2) center panel b: submillimeter emission of interstellar dust in the Milky Way; its high intensity (  0.003 K–10 K) with respect to spatial fluctuations in cosmological radiation is a major cause of uncertainty in the analysis of the very low anisotropy of the latter. Credit: ESA.

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In addition to primordial nucleosynthesis, the discovery of the cosmological microwave radiation in 1964 (figure 8.3b) was decisive in confirming the principle of the Big Bang (§ 8.2). The behavior of this photon gas is extraordinarily simple. Its properties are essentially fixed by its temperature T, which determines the number of photons per unit volume and their spectral wavelength distribution. This type of radiation is known in physics as thermal radiation at temperature T, called “blackbody*” radiation (figure 3.4a). After this decoupling, all photons remained, for about 13.8 billion years, a perfect thermal distribution of energy whose temperature decreased from a value close to 3000 K to reach today 2.73 K (λmax  1 mm). The detailed study of the spatial fluctuations of this radiation in recent decades (figures 8.4c and 9.4) has provided crucial elements for understanding the cosmological parameters (box 9.1). The parallel history of atoms led to the formation of galaxies and stars and, consequently, of our world. It also started quite simply from this moment of decoupling 380 000 years after the beginning of the Big Bang. The conditions that prevailed then are indeed very well defined and are part of a perfectly ordinary physics, which has allowed us to model in detail how and when almost all electrons associated with protons (and helium atoms). This neutral gas then evolved passively, carried by expansion, until the first stars and galaxies were formed a few hundred million years later (figure 9.1). This grandiose and surprisingly simple fresco of the history of the Big Bang is one of the great revelations of the last century. Initiated by the discovery of galaxies and their recession, it was firmly established half a century ago on the foundations of modern physics, thanks to our increased astrophysical knowledge of the Universe and its content of stars, atoms and photons. It has since been fully validated with the amazing accuracy of our physics to describe the Big Bang since at least the first thousandth of a second of the history of the Universe.

Chapter 9 Content of the Universe and Structure Formation 9.1

Formation of Galaxies and Structures of the Present Universe

In recent decades, huge progress has been made in understanding the history of the Universe and its current structure, notably the formation of galaxies. We know today that galaxy formation is a problem that is more difficult to solve in practice than decoding the many phases of the Big Bang (excluding the initial period and the inflation one). The genesis of galaxies is a fundamental question, the key of the formation of the stars and, therefore, of the planets and all the elements. Since their discovery, the origin of galaxies has been known as an extremely difficult issue, at least as much as the formation of stars. It is probably even more difficult, because the result, a galaxy, is a much more complex object than the wonderfully simple stars (in first approximation). Where are the galaxies coming from? How did they form? What determines their size? What drives their gathering into groups and clusters? These are issues that deeply concern us because they directly affect the distant origins of the Milky Way. These problems were briefly discussed in chapter 7 where we noted that the general pattern of galaxy formation seems now clear. The seed of these structures must be in the quantum density fluctuations appearing very early in the Big Bang, probably in a phase of inflation. The resulting small density peaks continuously grew under the action of the gravitational forces they exert on their surroundings. We find their tiny signature 380 000 years after the start of the Big Bang in the cosmic radiation (figure 8.4c). Dominated by dark matter and its gravitational self-attraction, they gained enough contrast much later when the Universe was a few hundred million years old to form the first large clusters of matter, followed by the first galaxies (figure 9.1). The number and size of galaxies and structures then continuously grew, mainly under the gravitational action of the dark matter,

DOI: 10.1051/978-2-7598-2706-0.c009 © Science Press, EDP Sciences, 2022

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FIG. 9.1 – Dark cosmic age. Illustration of the evolution of the Universe after the end of the ionized phase of the Big Bang, 380 000 years after its origin, when the electrons combined with H+ and He++ nuclei to give neutral atoms of hydrogen and helium (“recombination”). For several hundred million years after recombination, this cold gas did not emit any radiation, a phase known as the dark age of the Cosmos. Then the density fluctuations resulting from the Big Bang enlarged enough to form the first clusters of stars, then the first galaxies. Their UV radiation gradually re-ionized all the remaining intergalactic gas less than 1 billion years after the Big Bang. In the next 13 million years or so, galaxies evolved and enlarged, accreting more gas and forming more stars. Credit: Illustration Nick Spencer. Source: NASA/WMAP Science Team et R. Ellis (Caltech)/ESO.

resulting in the distribution of the galaxies and structures that we observe today with our telescopes (§ 7.2 and figure 9.2a) and in numerical simulations (figure 9.2b). But, as soon as galaxies form, the physics becomes much more complicated because the gas obeys the laws of magneto-hydrodynamics and the stars inject their energy in the form of radiation and supernovae, not to mention the disturbing role of active nuclei of the galaxies (§ 11). Where are we now, nearly a century after the birth of modern cosmology and half a century after the validation of the Big Bang model? The program that emerged as early as in the 1930s is accomplished. We have filled the blanks with the parameters of the Universe. Among all the possible mathematical models, we know what Universe we live in, deriving with astonishing precision its global properties, which define, for example, its structure, composition and history, including the rate of expansion, the age, the density, and the cosmological constant (see box 9.1).

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FIG. 9.2 – Large structures. (left, a). Observed large structures in galaxy clustering within two Universe slices (depth
1.7 billion light-years, angular aperture
75°, angular thickness 2°) detected by the combination of two large sky surveys (Sloan/SDSS* and 2dF). We note the structure in filaments, superclusters (four structures corresponding to superclusters are listed, the “Sloan Great Wall” is probably itself an accidental grouping of several superclusters) and the presence of large void spaces almost without galaxies. (right, b). Simulated large structures. Numerical simulation of the major structures of the Universe today. They gradually formed during the 13.8 billion years of the Universe’s history under the action of the gravitational forces that gathered their material from the very small initial over-densities. The results of the simulation with the filamentary structures, the filament nodes and large voids is strikingly similar to the structures observed in the actual distribution of galaxies (figure 9.2a). Credit: (left) W. Schaap (Kapteyn Institute, U. Groningen) et al., 2dF Galaxy Redshift Survey; (right) IAP/Groupe INC.

9.2

Fundamental Parameters of the Universe are Better Known than Its Physics

There is no doubt that the Universe is nearly spatially “flat”, that is to say that the Universe’s geometry is similar to our classical Euclidean geometry. Its total energy density is very close to the “critical” density (§ 9.4), just at the boundary of the expansion of the Universe that is always going on or will eventually stop and reverse: this finding is satisfactory for the mind. However, a very large part of essential ingredients that characterize the Universe and their properties (figure 9.3) are still beyond our current physical understanding. The role of gravitation that conditioned the expansion during the Big Bang, and after until the formation of galaxies, is explained by the “critical” density. However, ordinary matter (baryonic) has a density five times smaller than the mysterious dark matter that dominated the formation of the structures of the Universe. The physical origin of the cosmological constant, or, in other words, the physics that takes its

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FIG. 9.3 – Dark matter and dark energy. (left). Composition of the Universe at different

cosmological times (from the measurements of the Planck satellite cf. figure 9.4). It was successively dominated by various components: first, a few minutes after the beginning of the Big Bang, photons and neutrinos, which dominated the “cosmic soup” after the annihilation of antimatter particles including positrons (b); then the dark matter during the end of the Big Bang (c) and during the first billions of years that followed (d); finally the dark energy today (e) and in 10 billion years from now (f ). (right). Dark energy and re-acceleration of the expansion. Diagram showing the different phases of the expansion of the Universe. During the first billions of years after the Big Bang dark energy played a negligible role in the rate of the expansion of the Universe, which was mainly slowed down by the gravitational interaction with dark matter. Then the acceleration by dark energy ramped up, counterbalancing braking by dark matter throughout the central phase of the history of the Universe. Finally, in the last few billion years, dark energy has clearly prevailed causing a re-acceleration of the expansion rate. Credit: (left) Planck Collaboration; (right) NASA/A. Feild (STScI).

place to explain the re-acceleration of the expansion for more than five billion years, remains unknown. The same is true of essential elements of the theory that are crucial for shaping the Universe in which we live, such as inflation, the small over-densities from which the structures of the Universe derived, and the asymmetry between matter and antimatter that determined the current density of ordinary matter (baryonic).

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Box 9.1 – Fundamental parameters of the Universe Since the cosmological parameters are not independent of each other, it is important to establish a consistent set of values. We give here those that were recently determined, mainly based on the results of the Planck satellite (figure 9.4) that are the most accurate to date, and, where appropriate, some of the associated problems.  The Hubble constant H0 and the age of the Universe H0 ¼ 66:9  0:6 km=s=Mpc The conventional units come from the definition V = H0 × D (§ 8.1), expressing the distance D of galaxies in Mpc (1 Mpc = 3.26 million light-years). This value from the Planck satellite results corresponds to an age of the Universe of 13.8 billion years. It should be noted that the more direct determination, based mainly on distance measurement using the Cepheid method from observations with HST*, is in significant tension, since it suggests H0 = 73.2 +/− 1.7 km/s/Mpc, or 12.8 billion years for the age of the Universe (see discussion in section 9.3).  Total density of the Universe The total energy density per unit volume (essentially the mass energy mc2 of the matter plus the dark energy § 9.6) is almost exactly the critical density (§ 9.4). q0 ¼ ð8:62  0:12Þ  1027 kg=m3 To represent the extremely small order of magnitude of this density, we can say that if it consisted entirely of hydrogen, this value would correspond to about six hydrogen atoms per cubic meter (but in reality, ordinary matter [the baryons] is only about 1/20 of this total energy). This near equality of the total density with the critical density means that the Universe is flat* (Euclidean geometry) to a very good precision.  Density of baryonic (ordinary) matter (figure 9.3e) q0B ¼ ð4:9  0:1Þ%  q0  Dark matter density (figure 9.3e) q0DM ¼ ð25:9  0:6Þ%  q0  Dark energy density (figure 9.3e) q0^ ¼ ð69:2  0:6Þ%  q0 The interpretation of dark energy in terms of a simple cosmological constant seems plausible. Besides these main parameters, the results of the Planck satellite have provided key constraints about other important parameters of the Universe, including the physics of the inflationary phase using statistical properties of the cosmic background anisotropies, which preserve those of the quantum fluctuations during inflation.

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9.3

Age of the Universe and Variations of the Determinations of the Hubble Constant

The first cosmological parameter that comes to mind is the Hubble constant H0, which determines the current rate of expansion of the Universe. Its importance is also that its inverse value, 1/H0, gives an approximate estimate of the age of the Universe. According to the results of the Planck satellite, the best value for the age of the Universe is 13.8 billion years with an accuracy of a few percent and that of 1/H0 is 14.7 billion years (box 9.1). For nearly three quarters of a century after its introduction by Hubble and Humason in 1929, H0 was difficult to measure. Whereas the speed of recession of a galaxy was immediately determined by the redshift of its spectral lines, the difficulty of measuring H0 came from estimating the distance to the galaxies. The method used to assess the distance has long been the relation between the period and the luminosity of Cepheids*; yet it is necessary to be sure to use the “good” Cepheids, i.e. the ones that obey such a relation. It soon became clear that there were at least two classes of Cepheids with very different period-luminosity relation, and that Hubble’s original work had mixed the two. It was demonstrated in the late 1940s that the age of the Universe was at least four times older than Hubble’s original estimate of 2 billion years. This was an essential step for cosmology as it partially removed the contradiction of an age of the Universe that was shorter than the lifetime of some stars. However, even this huge correction was not enough to resolve the controversy over the value of H0. Even as recently as thirty years ago, it still provoked passionate debates between supporters of various H0 values that differed by a factor of two, but gradually, the different methods from those based on the measurement of the Cepheids (with the Hubble Space Telescope, figure 2.7a) to others converged to intermediate values close to those given in box 9.1, although serious problems may still persist as quoted in this box.

9.4

An Overall Density Very Close to the Critical Density

A basic recurring question since the beginning of the discussions in the 1930s concerns the long-term future of the expansion of the Universe. In the simplest models, excluding a cosmological constant, it is clear that the expansion is slowed down by the gravitational attraction that matter exerts on itself. If the gravitational attraction exceeded a certain value, the expansion should stop after a certain moment and the movement then reverses. These models predict that if the density of the Universe exceeds a certain value, the “critical density”, the braking by the gravitational attraction would be strong enough to stop the expansion and to initiate a general contraction of the Universe, which would accelerate until the final crushing of galaxies into an extremely dense and hot gas. If, on the other hand, the density were less than the critical density, the braking of the expansion would never

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be sufficient to stop it and it would continue indefinitely at a constant rate. The intermediate limit case, where the density is exactly equal to the critical density, also corresponds to an indefinite expansion but with a rate asymptotically approaching zero. It is of special interest because the space of such a universe has a “flat” structure where Euclidean geometry and the Pythagorean Theorem are valid. The curvature of this space is zero, unlike universes of density greater than the critical density which would possess a three-dimensional convex structure similar to that of the surface of a sphere, whereas the low-density universes would exhibit a similar concave structure on the surface of a hyperboloid. The measurement of the density of the Universe has therefore become a major goal of cosmology, since it was thought that it would determine both the nature of the cosmic space and the long-term future of the Universe. This question is now solved and we know that we live in a flat space, which is a natural consequence of inflation models. However, the solution is more complex and surprising than could have been imagined. On the one hand, it is confirmed that a process of an unknown nature, possibly connected to a cosmological constant, re-accelerates the expansion of the Universe by winning for about 5 billion years on the braking exerted by the gravitational attraction of the Universe matter on itself (figure 9.3 right). Therefore, this accelerated expansion will continue indefinitely (§ 9.6). On the other hand, the density of matter, deduced from the analysis of the anisotropies of the cosmological radiation, considerably exceeds the total density of the atoms of the ordinary matter (one gives the name of “baryonic” to this ordinary matter because its mass is dominated by the “baryons” - protons and neutrons in atomic nuclei). We speak of “dark matter” for the excess material whose abundance, based on the latest estimates from the Planck satellite, is about five times greater than the baryonic matter.

9.5

Need and Nature of Dark Matter

As schematized in figure 9.3, dark matter dominated for a very long time the history of the energy content of the Universe when it played a decisive role in the formation of the structures of the Universe. Even at the time (until around 100 000 years after the origin of the Big Bang), when the energy density of dark matter remained low compared to that of photons and neutrinos, the lack of interaction of dark matter with anything but gravity allowed for an efficient growth of over-densities from inflation. Later, when dark matter dominated the energy density of the Universe (figure 9.3), the growth of dark matter over-densities continued until the formation of the first galaxies, hundreds of millions of years later (§ 9.1). Even after the onset of dark energy, that progressively diluted the density of dark matter, the structures, galaxies and their clusters continued their local growth under the influence of dark matter gravitation. In fact, as we have seen, the proposition of the need for an additional “black” material was formulated independently of the density of matter of the Universe and well before its introduction in cosmology. It was required to explain multiple properties of the galaxies, their clusters and other structures, such as the speed of

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rotation of stars at the periphery of galaxies54, the mass of clusters deduced from the velocity of their galaxies (§ 7.2), the different cases of gravitational lenses (figure 7.3), as well as the traces left in the current distribution of galaxies by oscillations of the baryons before their decoupling from the radiation. What is the nature of this mysterious material that fills the Universe and the massive halos that surround galaxies and their clusters? It was quickly realized that ordinary baryonic matter cannot account for it by a substantial factor. Actually, estimating this baryon density has not been easy because the most obvious baryons, those that make up the stars, account for only about 10% of the total number of baryons in the Universe. Other baryons, those found in the gas within clusters of galaxies or extragalactic gas, include a larger fraction, but the best direct measurements still remain uncertain. It turns out that probably the extragalactic gas contains more than half of the baryons of the Universe and that, as a result, a significant fraction of the baryons seems to have escaped detection in the local Universe. The best estimates of the baryonic density are thus indirect, and are based on two main methods that are perfectly concordant: one uses the primordial nucleosynthesis of the light elements, mainly deuterium, in the Big Bang (figure 8.3a), and the second relies on a detailed analysis of the anisotropies of the cosmic radiation. The most accurate estimate based on the latter method comes from the Planck satellite (box 9.1 and figure 9.4), which found that the density of baryons represents 4.9% of the critical density and 16% of the total density of matter. The additional matter that must be postulated to reach the total observed density of matter is only revealed by its gravitational attraction. It does not otherwise interact with electromagnetic radiation, unlike atoms and electrons, and remains therefore invisible. Hence, it is called dark matter. Its nature remains mysterious. It cannot consist of any of the known or predicted particles of the “standard model” of elementary particle physics, which was brilliantly confirmed by the discovery of the Higgs boson at the Large Hadron Collider (LHC) at CERN. Various hypotheses have been put forward regarding the nature of dark matter particles in “supersymmetry” models or other extensions of the “standard model”. However, there is currently no evidence of the existence of such particles in the energy domain explored by the LHC and it is necessary to consider particles of higher mass, probably greater than 1000 GeV, with no hope of detecting them in the near future with terrestrial particle accelerators. A parallel method to identify the dark matter particles that surround us and pass through us, as everywhere in our Galaxy and throughout the Universe, is to try to detect them directly. It is plausible that they interact very weakly with ordinary matter, in the same way as neutrinos, and that we can detect these extremely rare interactions using very special detectors. These experiments must be massive enough to increase the chance of interaction. They also must be protected from all parasitic radiation by burying them deep enough in underground laboratories located in tunnels (like the Gran Sasso in Italy) or in old mines. Huge efforts have been invested in a dozen experiments of this type around the world, and the sensitivity of the detectors has impressively increased over the past twenty years. The difficulty is that we do not know exactly what we are looking for and what sensitivity is ultimately needed, because the theoretical models remain vague. Despite recent

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announcements, there is not yet any plausible detection. It is also possible that the dark matter particles might be indirectly detected by the particles or radiation they could emit if they prove to be unstable. Important constraints on the type of expected particles are provided by the cosmological models of formation of galaxies and other structures of the Universe. It appears that the mass of particles must exceed a minimum mass to allow them to remain non-relativistic at low enough temperatures to trap them in the gravitational attraction of even small galaxies. The majority of cosmologists therefore agree that most of the dark matter must be “cold” and formed of massive particles. Particles of zero or very low mass, such as neutrinos, constituting a “warm” dark matter, are probably not suitable, although a small fraction of “warm” dark matter may explain some properties of galaxy clusters. It may also be that dark matter is composed of bosons of extremely low mass (axions). Certain models even propose that dark matter is composed of primordial black holes, which were generated in early phases of the Big Bang, despite the difficulty to reconciling that with observational evidence. The nature and even the existence of dark matter can only be conclusively confirmed once the particles it is made of are discovered. One must not forget that the hypothesis of the existence of dark matter is based essentially on the intensity of the gravitational forces at a great distance, which greatly exceeds the gravitational attraction by the baryonic matter predicted by general relativity. However, the theory of gravitation, that is to say general relativity, cannot yet be tested over distances greater than the size of the Solar System. Other tentative theories have been proposed avoiding dark matter but postulating modifications or extensions of the general relativity. So far none of them seems consistent enough to replace the simplicity and coherence of dark matter in the standard cosmology model. As a final comment, it is important to keep in mind the solidity of the pillars on which one based the introduction of dark matter in cosmology and the physics of galaxies and their clusters (§ 7.2).

9.6

A Last-Minute Surprise, the Re-Acceleration of the Expansion Involving an Unknown Source of Cosmic Energy

Since Einstein’s first attempts, in 1917, to apply general relativity to the Universe as a whole, it has been known that an arbitrary cosmological constant can be added to the equations of general relativity for cosmology. It was also understood very quickly that a suitable choice of this cosmological constant could reconcile a flat universe with the observed matter density, which was well below the critical density. Yet, until 1998, models with a cosmological constant were not taken as seriously as the dominant models where it was assumed to be zero. The measurement of the rate of expansion of distant galaxies using their Type Ia supernovae* as standard candles brought a slight surprise, because it did not show that the expansion of the Universe

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slows down, but, on the contrary, that it had accelerated in the last billions of years. Such acceleration involves a huge energy, whose origin remains unknown, and is referred to as “dark energy”. A simple way to account for this acceleration is to introduce a cosmological constant, but it is not yet known whether the behavior of dark energy and its variation over time require more complex models. A connection with the quantum vacuum energy seems the most obvious track to explore, but none of the current explanations appear entirely convincing. Despite the difficulty of calibrating the brightness of Type Ia supernovae at such distances, the reality of the re-acceleration of the expansion and thus the phenomenon described as dark energy is now considered real and fundamental. It has also been confirmed by various other independent methods, including cosmic background radiation and acoustic oscillations of baryons*. The importance of the discovery of the accelerated expansion of the Universe by observing supernovae was such that it was awarded the Physics Nobel Prize in 2011. The origin of dark energy is now one of the major problems of astrophysics and physics, justifying considerable observational efforts to better understand its nature. This is the main goal of the Euclid project for the detailed study of gravitational lens effects, one of the major missions of the European Space Agency in the next ten years (figure 9.4 right). Note that the role of dark energy on the evolution of the Universe changed dramatically over time (figure 9.3). It was negligible during all the phases of the Big Bang when the dark energy density remained tiny compared to that of the dark matter and even that of the baryons. Its effect was still negligible or minor during the first billions of years of evolution when galaxies were formed. Only after that time did it become important for slowing down the braking of expansion and finally accelerating it in the last five billion years (figure 9.3 right). The present preponderance of dark energy is essential to dilute the density of matter of the Universe and to slow down the growth of its structures.

9.7

Summarizing: An Unexpected Universe Model Validated in Multiple Ways

If we summarize the state of our knowledge of the overall properties of the Universe and its history, we can say that the achievements in cosmology during the past century are prodigious. We have fully validated the general model of the Big Bang that describes the origin of the current Universe expanding from an extremely hot state that cools as it expands. In its final phases, its temperature is successively dominated by various mixtures of elementary particles, most of which are well known in current physics. It carries the germs for formation of galaxies and their associations from over-densities that gravity has increased little by little. Based on a vast effort of independent key observations, we are able to build a detailed and surprisingly precise physical model with the framework of the equations of general relativity. This current standard cosmological model, commonly referred to as ΛCDM (Lambda Cold Dark Matter, or “concordance”), is based on the presence of a cold dark matter made of massive, though unknown, particles and a

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FIG. 9.4 – (left). Anisotropies of the cosmic background radiation decomposed into different angular scales. This spectrum of measured intensity (red dots with uncertainty bars) is compared to that (blue continuous line) calculated from a Big Bang model that estimates the properties of the radiation and its anisotropies at the moment when it stops interacting with baryonic matter before freely expanding up to now. By optimally adjusting the model parameters (box 9.1), we see that the agreement is remarkable for all scales of less than ten degrees. The comparison with the model is less significant for larger scales (l < 20), since the measurement uncertainties are greater. (right). ESA Satellites Planck* and Euclid*. The Planck mission (2009–2013) measured the anisotropies of the fossil cosmic radiation (CMB, Cosmic Microwave Background – figures 8.4c and 9.4) with unparalleled accuracy (better than one millionth) by observing the entire sky at millimeter wavelengths. This has made it possible to derive the best parameters of the current standard cosmological model (box 9.1). Here we see the two main mirrors of the Planck telescope, protected from solar radiation by a large screen, and the compartment containing the various service systems that occupies the bottom of the satellite. The most sensitive receivers (inside the orange block) were cooled to an extremely low temperature (0.1 K, −273.05 °C), which represents the greatest technological achievement of this satellite. The Euclid satellite (model) for an ESA mission (launch scheduled in 2023 for a mission of at least 6 years). It is designed to produce three-dimensional maps of several hundred million distant galaxies to try to understand the nature of dark energy (§ 9.6) that accelerates the expansion of the Universe (figure 9.3 right). Credit: (left) ESA and the Planck Collaboration; (right) ESA. cosmological constant accelerating the expansion. It leads to the conclusion that the Universe has a Euclidean geometry. The properties of the initial phases of the Universe are still debated, but the fact that it is flat and the properties of the tiny over-densities that will give birth to galaxies and all the structures of the current Universe are well explained by an initial phase of phenomenal swelling, generally referred to as inflation.

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This model describes our Universe and its history based on half a dozen main parameters (box 9.1, plus some additional parameters) including: the Hubble constant H0, which gives the current expansion rate, the ordinary matter (baryonic) and dark matter densities, the dark energy density possibly corresponding to a cosmological constant (the sum of these three energy densities shows with excellent accuracy that the Universe must be flat). All these parameters are now determined with remarkable precision. Their values have generally been confirmed by several completely independent, concordant methods, namely: observations of Cepheids, observations of supernovae Ia, nucleosynthesis of helium and deuterium, dynamics of galaxy clusters, gravitational lenses, acoustic oscillations of baryons, modeling the formation of structures and, especially, properties of the cosmic background radiation. The precision achieved in reproducing the properties of the anisotropies of the cosmological radiation is also quite striking (figure 9.4). The wealth of information collected on the parameters of the ΛCDM model has enabled the last space observation missions dedicated to cosmic radiation, WMAP* and Planck, to successively provide the best standard reference ΛCDM models (box 9.1). Despite this impressive overall consistency of the ΛCDM model, there are still some difficulties in explaining some observational facts that appear to challenge some of its predictions. The most important problems probably concern the structure of dark matter halos in galaxies, which should contain more condensation and satellite galaxies than observed, and have a higher density in the center than measured. It is not impossible that this can be solved by a better modeling of the formation of the halos, involving slight modifications of the model ΛCDM. The small divergence in determinations of the Hubble constant H0 from the cosmic background anisotropies and from the Cepheid/HST method (box 9.1) might be another problem. It is, however, certain that this model will develop. For example, the behavior of dark energy will be refined with new, targeted observations, in particular with the Euclid satellite, which will probe how the properties of dark energy might differ from a simple cosmological constant. While there are alternative theories, mainly based on changes in general relativity and, in particular, its behavior at great distances83, their foundations do not appear superior and their explanatory power of the various cosmological observations do not reach nearly as far as the ΛCDM model. We therefore have convincing reasons to consider that this standard model represents the properties of our Universe. However, it is clear that it cannot be definitively validated until we have identified the physical nature of dark matter and dark energy.

Part V

Singular Stars and Cataclysms in Extreme Physical Conditions A New World Under the Sign of Violence and Singularities Our cosmos is not only radically different from that of the 19th century because we have discovered the universe of galaxies and cosmology. It has also changed due to the discovery of the fascinating and terribly violent world of cataclysmic and hyperdense stars. We now know that the Universe is vastly richer than the multitude of suns and planets in the Milky Way and each of the hundreds of billions of galaxies. The last half-century has been dominated by the revelation of this extreme component of the Universe with an accumulation of extraordinary discoveries. While our understanding of supernovae had begun in 1930s, the discovery of pulsars beacons from neutron stars in 1967 dramatically revealed the existence of such fantastic stars and proved their genesis in supernova explosions. The supernova of 1987 in the Large Magellanic Cloud provided the detection of its neutrinos which dominate the energy released in these events (figure 10.2a). Flashes (“bursts”) of gamma rays (γ-rays) observed since the late 1960s must largely be produced by very energetic supernovae (§ 10.3). There is good reason to believe that most stellar black holes, identified as early as the 1970s, come from supernovae. Alone or in pairs, hyper-compact black holes or neutron stars, give rise to various violent manifestations studied in the X-rays and γ-rays since the 1960s. The fusion of such hyper-compact black holes causes a spectacular emission of gravitational waves that has been detected in 2015. The discovery of black holes, singular objects predicted by the general theory of relativity, and the progressive awareness of their cosmic existence, rank certainly as one of the most important discoveries of the century. This awareness was actually initiated by the surprise caused by the gigantic energy emitted by radio galaxies and quasars, respectively, in the 1950s and in 1963. This energy originates in the vicinity of central super-massive black holes whose mass can exceed one billion solar masses. Today, it is known that all large galaxies, including the Milky Way, contain a super-massive black hole in their center, but it generally remains inactive (see chapter 11 and § 11.7 for the Milky Way).

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Box V.1 – High energy astrophysics103, a new window on the Universe With the opening of the X and gamma spectral windows, and the direct detection of cosmic rays and neutrinos, high energy astrophysics has emerged as one of the major fields of astrophysics at the end of the 20th century. It has benefited from increasingly efficient techniques developed by particle physicists throughout the century, but despite the relative ease of individually detecting these particles and high-energy photons, this spectral range remains marked by the difficulty of achieving spectral devices and directional tracking. Except for cosmic rays and very high energy γ-rays, space observation is imperative. The X-ray and γ-ray space observatories are therefore among the most important missions of all space agencies in the recent years (figure 2.11). These telescopes brought us a vision of the sky in X-rays and γ-rays to which we are not accustomed with visible light. These images, easier to obtain in X-rays than in γ-rays, are dominated by hot media where the temperature of astrophysical plasmas reaches several million degrees, or by other regions traversed by very energetic particles. This emission often comes from unusual sources, such as hot gas in clusters of galaxies, supernova remnants, accretion disks and jets around neutron stars and stellar black holes in binary systems, or around super-massive black holes in the center of galaxies. However, it also includes eruptions in all kinds of stars, including the young ones (and the Sun), and the interstellar medium of galaxies. Some sources are more specific to γ-rays, such as gamma-ray bursts, the electron– positron annihilation spectral line, especially in the center of our Galaxy, and the emission from interstellar gas generated by cosmic rays. The Earth’s atmosphere is far from a perfect shield against high-energy cosmic particles. The most energetic ones easily cross at least the upper layers of the atmosphere and can be observed from the ground much more conveniently than in space. The detection of atmospheric showers of ultra-relativistic particles produced by cosmic rays and high energy γ-rays is the target of some of the most important instruments of current astronomy (§ 2.4 and figure 10.9b). The same is true for detectors of gravitational waves using laser interferometry (§ 11.3 and figure 11.1), underground detectors of cosmic neutrinos (figure 10.2b) and underground facilities seeking the detection of the mysterious dark matter particles. After the extraordinary harvest of the last fifty years in extreme physics and exotic astronomical objects, high energy astrophysics is one of the most active fronts of astronomy at the frontiers of fundamental physics. The very recent breakthroughs in the fields of gravitational waves and neutrinos appear particularly promising for rapid extensions of these techniques as major tools of astronomy of the 21st century.

Chapter 10 Explosions of Stars and Their Singular Residues 10.1

Extreme Physics of Supernova Implosion/Explosion

A monstrous star lights up in the Milky Way At first glance, the world of stars seems immutable. This immutability fascinated all civilizations as it stayed in contrast both to the ephemeral nature of our lives here below and the erratic movement of the planets. Night after night, the network of constellations is identically represented and the brightness of their stars remains the same. Yet the careful, precise and regular observation of the sky reveals that some rare stars violate this rule. Whole classes of stars are variable and have their brightness strongly changing in a regular and cyclic manner over periods ranging from a few days to a few years. More strikingly, from time to time, new stars can appear and disappear. The rarity and unpredictability of such phenomena had always struck awe in people until astronomical observations were sufficiently developed to allow them to be understood. Among these “new stars”, traditionally called “novae” by early astronomers, some of them have been noted in historical annals. Their brightness surpassed that of the brightest stars and planets so that they were visible in full daylight, but their brightness decreased over periods of a few months, before they finally completely disappeared. Such phenomena remain extremely rare. Only two events of this type, now called “supernova”, have been observed in the Milky Way since the beginnings of modern astronomy, in 1572 (by Tycho Brahe, figure 10.4) and in 1604 (by Kepler); but we know that a few others had to be hidden from the eyes of astronomers by interstellar dust screens in distant regions of the Milky Way (figure 10.6a). The mention of some other older appearances of such new extraordinary stars, with a good probability of being supernovae, has been found in various astronomical records (figure 10.5). The richest sample comes from the meticulous surveillance of celestial phenomena carried out over several millennia by order of the Chinese emperors. The most remarkable of these appearances, one observed in 1006 (SN 1006, probably the DOI: 10.1051/978-2-7598-2706-0.c010 © Science Press, EDP Sciences, 2022

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FIG. 10.1 – The supernova SN 1987A. It was observed on 24 February 1987 in the Large Magellanic Cloud (figure 5.6). It is the only supernova observed in the Milky Way and its suburbs since the invention of the telescope in the 17th century and it is notable for the detection of its neutrinos (figure 10.2a). It has therefore been intensively studied and the development of its impact on the surrounding circumstellar and interstellar medium has been followed ever since. It is not really a coincidence that it occurred in the Tarantula Nebula (also known as 30 Doradus, figure on the right (b), the supernova is the light source on the bottom left of this figure), because it is the richest region of massive young stars anywhere in the Milky Way and its surrounding companion galaxies (figure 4.1b). The supernova SN 1987A comes from the core collapse and explosion of a blue supergiant that was previously known among the brightest stars of this nebula. The high luminosity (  108 L⨀) of the supernova was maintained for several months, mainly fueled by the radioactivity of elements created in the explosion. By sweeping up the environment, the light flash revealed its structure produced by mass ejections by the star in the past. The current images (left, a) are thus marked by the presence of three spectacular luminous rings. Credit: Australian Astronomical Observatory/NASA, ESO.

brightest observed stellar event in recorded history), and the other in 1054 (the Crab nebula), have proven crucial for our recent understanding of the supernova phenomenon from the remnants (nebula, pulsar) still observed at their location (figures 6.5, 10.5, and 10.7). However, modern astronomy had the chance to observe the explosion of a supernova in the Large Magellanic Cloud in 1987 (figure 10.1). We know today that supernovae trace the collapse of the central part and explosion of a massive star, producing ephemeral “stars” nearly a billion times more luminous than the Sun. It is an event of incredible violence where the collapse of the star core releases, in one second, a thousand times more energy than the Sun does in ten billion years; yet, the vast luminous energy from the supernova represents only a tiny fraction of its total released energy. It was not until the 1920s, when the nature

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FIG. 10.2 – Detection of cosmic neutrinos. (Left, a). First detection of high energy cosmic neutrinos emitted by the supernova SN 1987A. The detection of two dozen neutrinos emitted in the nuclear reactions of the explosion agrees with the models and is the most important observational result of SN 1987A. They were mostly detected by Kamiokande, a water neutrino detector in Japan (other detectors, IMB in USA and Baksan in Russia, also detected each a few neutrinos). The temporal grouping of the arrival of these neutrinos coincided within a few seconds with that of the first photons of the supernova. Note that despite the unmistakable nature of the implosion of the massive star, no pulsar signal is observed, perhaps because the Earth is outside the space swept by its radio emission beam (figure 10.7). (Right, b). IceCube neutrino observatory is located at the South Pole. Its thousands of sensors are located deep underneath the Antarctic ice, distributed over about a kilometer. The very rare reactions of neutrinos with the molecules of water in the ice can create energetic electrons and other particles emitting Cherenkov* radiation which can be detected by the photomultiplier tubes of IceCube. Credit: (left) Super-Kamiokande, University of Tokyo; (right) Credit: Wikipedia. of galaxies was discovered, that astronomers were aware of the magnitude of the physical cataclysm that is a supernova, because it was impossible before to have an idea of the distance of the observed supernovae and therefore of their actual luminous power (box 10.1). It is now known that these few historical supernovae in the Milky Way were at considerable distances from the Sun, thousands of times farther away than the nearest stars. However, exceptional as the brightness of these supernovae is, it could also come from ordinary but abnormally close novae. In fact, the observation in 1885 of the only supernova ever seen in our sister galaxy, the Andromeda galaxy (figure 5.2), allowed for the first time to make a precise determination of the colossal luminous power emitted by a supernova. However, to confirm it, it was necessary to wait a few

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FIG. 10.3 – SN 1994D in the galaxy NGC 4526. The brightness of the supernova, seen at the edge of the galactic disk in the bottom left corner of the figure, is a significant fraction of that of the large spiral galaxy. Credit: HST/NASA/Lawrence Berkeley National Laboratory. decades until the controversy regarding the distance of Andromeda was resolved (§ 5.1). Nowadays, hundreds of supernovae are observed every year in nearby galaxies (figures 10.3 and 10.4).

Implosion of a supergiant star: the mass of the Sun volatilized in energy in one second A star is an incredible machine when we measure it on human scale, with its energy production equivalent to one billion hydrogen bombs per second in the case of the Sun (10 000 times more for a red supergiant). What can be said about a Type II supernova (box 10.1) that radiates like nearly a billion Suns and emits a thousand times more energy than that in the form of neutrinos! We know today that the thermonuclear reactions at work in the Sun and normal stars are unable to power such a monster. There is evidence that the fabulous energy of these supernovae actually comes from the gravitational energy released in the collapse of the core of a massive star. We have seen in the discussion of the structure and energy of the stars (§ 3.2), that when the core of the most massive stars (supergiants) exceeds a little more than the mass of the Sun, nothing can stop its contraction. This accelerates into a titanic collapse until the radius of the star is reduced to a few kilometers. Because of the enormous acceleration by the forces of gravity, the total duration of this collapse is less than one second. The violence of the event almost instantly

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FIG. 10.4 – Supernovae. (left, a). Tycho Brahe. Picture of the Danish astronomer Tycho Brahe, in the presence of the supernova which he discovered and observed in 1572. Its location is now marked by a spectacular interstellar nebula that continues to rapidly expand since then (figure 10.5) (right). The brightness of a supernova can be comparable to that of its host galaxy. In the right images (b,c) showing the galaxy CGCG 089-013 before (b) and during (c) the appearance of the supernova SN 1999BE, we see that the brightness of the supernova is almost equal to that of its low-brightness host galaxy. Credit: (left) “Astronomie Populaire” Camille Flammarion (Paris, 1884) Domaine public; (right) HST/Lawrence Berkeley National Laboratory.

compresses an enormous mass comparable to that of the Sun into a tiny volume of about ten km radius. The gigantic released energy equals about one-tenth of the mass energy mc2 of this collapsed core! The outer layers of the star violently explode and are expelled into the interstellar medium at a speed close to one-tenth of that of light. Their compression and heating drive all kinds of nuclear reactions that have various major consequences: synthesis of some of the heaviest chemical elements found throughout the Universe (figure 3.7); production of large quantities of radioactive nuclei, mainly Ni and Co, whose radioactivity powers the brightness of the supernova over a period of a few months; emission of ultra-energetic neutrinos which, interacting very little with matter, can escape from the star by carrying 99% of the energy released in the implosion of its core. Only a small part of this total energy, typically  1%, appears as kinetic energy of the expelled gas that is travelling at enormous speed through the surrounding interstellar medium while deeply disrupting it (§ 6.5).

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FIG. 10.5 – Remains of historical supernovae. Composite images (X-rays plus visible light, with supernova dates) of today remnants of supernovae that were observed in the Milky Way and its satellites, by Chinese astronomers and other observers in 185, 1006 and 1054, by Tycho Brahe in 1572, by Kepler in 1604 and, in the case of SN 1987A, discovered at the Las Campanas Observatory in Chile on February 24, 1987 (see figures 6.5 and 10.7 for the Crab nebula and figure 10.1 for SN 1987A). Their remnant nebulae are expanding rapidly since the time of the explosion (at 1500 km/s for the Crab); the hot ionized gas produces a bright emission of visible light and X-rays, which is structured by the enormous mechanical energy injected by the explosion. All these were Type Ia* supernovae (see box 10.1), except SN 1987A and that of the Crab nebula which were of Type II*. The Crab nebula is the only observed supernova (in 1054) whose nebula encloses a known pulsar (figure 10.7). Credit: NASA/ESO. The previous scenario only describes the most energetic class of supernovae, those denoted as “Type II”. This discussion does not apply to “Type Ia” supernovae that correspond to another less energetic process, namely the thermonuclear explosion of a whole white dwarf after the accretion of matter from another associated star in a tight binary system, as discussed in § 4.4. In the induced heating, carbon and oxygen, the main constituents of the white dwarf, turn into iron, but the slow loss of the heat produced and the limited overall mass of the system lead not to the formation of a neutron star, but to the complete volatilization of the star’s initial material. Besides the ability to form most of the iron in galaxies, these special supernovae, “Type Ia”, have the advantage of being remarkably regular. They can thus be used as standard astronomical candles to measure distances (box 3.1). This is all the more important because they are extremely luminous objects that can be seen at distances of billions of light-years. Type Ia supernovae were used for determining the distance of galaxies in this range. They have thus proven to be the crucial element in one of the major discoveries of astronomy in recent years: the re-acceleration of the Universe involving a “dark energy” that remains still mysterious (§ 9.6).

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FIG. 10.6 – Supernovae and hypernovae. (Top, a) Hidden supernova remnants. Interstellar nebulae whose radio and X-ray emission possesses all the characteristics of a Type II* supernova remnant (box 10.1), but which cannot be associated with a listed supernova observation, either because the explosion was not recorded, or because they are in star formation regions that are very obscured by interstellar dust. However, the characteristics of these nebulae and, in particular, their speed of expansion makes it possible to date with more or less precision the supernova that must be associated with them. Among these sources, Cas A has been dated to around 1670; it is located fairly close (  10 000 light years away) and has been particularly well studied for decades. The recently discovered G1.9 + 0.3 seems to date from not much more than a century ago. (Bottom left, b). Supergiant star on the brink of implosion. Diagram of the compact core of a red supergiant just before it collapses (§ 3.3) causing the explosion of a Type II supernova and the creation of a neutron star. The increase in temperature in the different internal layers triggers the fusion reactions of the various elements, in turn up to an iron core of maximum stability, which accumulates in the center. Note the fantastic difference in scale (of five orders of magnitude) between the diameter of the supergiant (over a billion km) and that of its core (  10 000 km). (Bottom right, c). Gamma-ray burst: diagram of a possible structure for the explosion of a hyper-massive star emitting a huge burst of gamma rays that is detected in the form of a “gamma-ray burst”. The implosion of the star’s core, which probably forms a black hole, can in specific conditions lead to a massive jet of ultra-relativistic particles, in which a powerful internal shock wave emits gamma rays with the brightness of “long” gamma-ray bursts (§ 10.3). The subsequent propagation of external shock waves in the relativistic jet moving away from the star could explain the residual emission, called “afterglow”, that is observed in X-rays and visible light for a few hours or a few days. Credit: (top line) NASA; (bottom line left) New Mexico State University; (bottom line right) NASA.

In fact, we can consider the supernova phenomenon from different angles, each of which corresponding to a major astrophysical role. These multiple astrophysical facets of supernovae are summarized in box 10.1.

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Box 10.1 – Supernovae at the crossroads of high energy astrophysics Violent death of stars (§ 10.2, § 10.3) Type II supernovae are the result of the collapse of the core of a massive star, releasing, mainly in the form of neutrinos, up to about 0.1 × mc2 of energy in a few seconds (where m is the mass of this core), amounting to almost thousand times the energy radiated by the Sun during its entire life. A small portion (<  1%) is transferred into the kinetic energy of the outer part of the star that is ejected at high speed by the explosion into the interstellar medium. Only a tiny part of this energy is emitted as light to feed the large luminosity of the supernova. This is the fate of all stars with masses greater than  6–8 M⨀ that explode as Type II supernovae or rarer types for the more massive stars. Type Ia supernovae are less energetic in total because they correspond to the implosion of a white dwarf mass only slightly above 1.4 M⨀. But their optical power (luminosity) is typically 10 times greater than those of Type II. Lighthouses of the galactic sky Typical luminosity: several billion times the luminosity of the Sun for Type Ia (hundreds million for Type II) for a few days with prolonged attenuation for a few months, powered by the radioactivity of some elements (Ni, Co…). Origin of chemical elements (§ 3.4) Supernovae are believed to be at the origin of a part of all the chemical elements of the Universe heavier than carbon: either by nucleosynthesis in their explosion (elements heavier than iron); or by rejection into the interstellar medium of elements that were previously synthesized in the massive star by nuclear reactions made possible by the very high temperatures inside the star (figure 10.6b; most elements between Na and Fe, and part of key, lighter elements such as C, N, O, etc.). The explosion injects these atoms into the interstellar medium where they survive for billions of years, and will later be included in the formation of new stars (and their planets) or, in some cases, be ejected into intergalactic space. Violent processing of the interstellar medium (§ 6.5) The gas projected by the explosion, mixed with the surrounding interstellar gas in “supernova remnants”, forms spectacular nebulae, seats of violent phenomena, including the acceleration of cosmic rays (§ 10.4). Supernova shock waves shape the interstellar medium and its evolution throughout the galaxy, including hot gas. They can also compress the interstellar clouds close to the explosion, causing new star formation, and eject some of the interstellar gas from the galactic disk. Matrix of neutron stars and black holes (§ 10.2, § 11.2) Implosions/explosions of massive stars resulting in supernovae normally leave a residue as a compact object of only a few kilometers radius, generally a neutron star of a few solar masses, or, in rarer cases, a black hole of mass up to a few tens of solar mass from the supernova of a very massive star. Pairs of such compact objects, linked by gravitational attraction, deriving from a binary star with very massive components, can eventually merge and emit powerful bursts of gravitational waves (figure 11.2).

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10.2

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Neutron Stars, Hyper-Dense Supernova Residues

A neutron star is an object almost beyond our imagination. Let us try, however, to consider a density of a few billion tons per cubic centimeter, that is to say one hundred thousand billion times greater than that of the matter of all the objects of our world! Nineteenth-century physicists could not conceive of the existence of such extravagant bodies. Yet, after the quantum revolution of microphysics, in the late 1920s, the notion of a neutron star emerged almost naturally from the most powerful minds of astrophysics, such as Landau, Chandrasekhar and others directly connected to the discoveries of quantum physics. As we progressed in the physics of stars, we realized that the tendency of the star’s core to contract under gravitation could be counterbalanced at the white dwarf stage for low stellar masses (§ 3.3). However, if this mass exceeds the limit of 1.4 M⨀, physics shows that nothing can stop the collapse of the white dwarf (of diameter around 10 000 km) before the stage of neutron star. Such a fantastic object is contained in the diameter of a few tens of kilometers and its hyper-dense material is composed of neutrons that are virtually in contact. Indeed, the physics of electrons and atomic nuclei shows that there is advantage, if the pressure is strong enough, to combine electrons and protons into neutrons. Even if energy has to be spent for each neutron formed, the volume is much reduced and gravitational energy much increased, forming a hyper-compact pile of neutrons with an extreme material density that is comparable to atomic nuclei. A neutron star of a few solar masses is thus the normal residue of the implosion of the most massive stars (§ 3.3), but a neutron star cannot exceed about 2 or 3 solar masses or it will collapse into a black hole. The connection between supernovae and such theoretical objects was proposed as early as 1934 by Baade and Zwicky, but in the absence of any means of testing it, this suggestion was virtually unexplored until the accidental discovery of pulsars in 1967 by Bell and Hewish. These radio astronomers observed mysterious radio signal pulses, periodically reappearing with a clock-like regularity and a frequency close to one per second (figure 10.7). After a few speculations, the interpretation of these signals quickly led to identify the source of these signals to a rapidly rotating neutron star, with a period of between a few milliseconds and a few seconds, because no other known type of star, being much larger, could have resisted the corresponding centrifugal force without disintegrating. This hypothesis was quickly confirmed by the detection of a pulsar in the center of the Crab Nebula, the site of the historical 1054 supernova (see figures 10.7 and 6.5, § 10.1). Since then, many other pulsars have been found in association with supernova remnants. But these associations are not universal and there are cases where no pulsar has been found in some supernova remnants. This is natural for Type Ia supernovae (box 10.1) that do not produce a neutron star. For Type II supernovae (box 10.1), the fact that, in some case, no pulsar is observed, can be explained if the axis of rotation of the neutron star is directed in such a way that the Earth is not in the cone that is swept by the emission beam of the pulsar. Additionally, the neighborhood of many pulsars does not show any sign of visible supernova remnant, because the pulsar emission lasts longer than the visible remnant.

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FIG. 10.7 – Pulsar and neutron star. The top left panel shows, a diagram of the classical model of the radio emission of pulsars by a neutron star whose magnetic axis is tilted relative to its axis of rotation. A relativistic plasma is trapped in the magnetic field and follows the extremely fast rotation of the neutron star. In the pole region, however, the magnetic field lines are open and a strong radio emission can occur in a small cone around the magnetic axis. This beam rotates around the axis of rotation of the star, like the beam of a lighthouse, with a typical period. Each time it crosses the direction of the Earth, a brief pulse of radio signal is detected by radio telescopes, a signal that repeats with an extremely regular period that ranges from milliseconds to seconds (bottom left figure). The right figure shows a composite X-ray and visible image of the central part of the Crab Nebula (figures 6.5 and 10.5), highlighting the hot gas pattern of the nebula shaped by the rapid rotation of the pulsar and its magnetic field. The central bright ring, perpendicular to the axis of rotation, around the heart of the nebula, traces the emission of hard X-rays by high-energy particles. Credit: (left) NRAO; (right) Gamma-ray-pulsar: NASA Fermi Cruz de Wilde. Optical: NASA/HST/ASU/J. Hester et al., X-Ray: NASA/CXC/ASU/J. Hester et al.

The physics of neutron stars is exciting in many ways and can be highlighted by the study of specific topics (see box 10.2): properties of their ultra-compact matter, organization of their internal structure in various layers, possibility that neutrons are unstable in the center; the highest known extreme magnetic fields; the mechanism for transmitting radio and X pulsar signals; extreme rotations accelerated by accretion; binary systems whose orbital properties are affected by the slow emission of gravitational waves; detection of strong bursts of gravitational waves emitted by the fusion of a neutron star with another neutron star or a black hole (§ 11.3).

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Box 10.2 – Objects related to neutron stars Broadly speaking, a neutron star is a compact star with a radius of about 20 km and a mass in between 1.4 and 2–3 M⨀, composed mainly of neutrons. Its internal layered structure is complex and still debated for the central regions where the neutrons, which are constituted of a three-quark assembly, might not be stable. Known isolated neutron stars were identified from their pulsar emission. They are characterized by rapid rotation and a very intense magnetic field resulting from the conservation of magnetic flux in the core collapse of the progenitor star. This rapid rotation of neutron stars gives rise to the different types of pulsars:  Normal pulsars. Rotating neutron stars (with periods around 0.2 to 2 s) emitting a strong periodic radio signal, due to the radiation emitted by accelerating electrons in an intense co-rotating magnetic field (figure 10.7).  Millisecond pulsars. They show pulses with rotation periods of a few milliseconds. Their very fast rotation must result from the accretion of matter on the neutron star from the other star of a close binary system.  Double pulsars. Two neutron stars in an orbit bound by gravitation. We receive the pulsar signal of one or both of them (figure 10.8). The ultra-precise measurement of the very slow variation of the rotation period of the binary system has confirmed, in the 1970s, that it is due to the emission of gravitational waves, in agreement with the theory of general relativity (Nobel Prize 1993). This was decades before the direct detection of gravitational waves from merging black holes in 2015 (chapter 11).  X and γ-pulsars. Pulsars where emission is detected in X or γ-rays: X-pulsars generally result from accretion of matter onto a neutron star coming from the companion star of a binary system. However, the isolated pulsar of the Crab Nebula (figure 10.7) emits pulses at all wavelengths, from radio waves to high energy γ-rays. A particular class of X and γ-pulsars, called magnetars, have an extremely high magnetic field, up to quadrillions of times –1015 times – more powerful than the field surrounding Earth.  Quark stars (or “strange” stars). Hypothetical objects similar to neutron stars but formed mainly of non-bound quarks.

10.3

Gamma-Ray Bursts, Even More Powerful Bursts

For a tiny fraction of the galactic population of massive stars, the final collapse can cause the emission of a gamma-ray (γ-ray) flash much brighter than the conventional visible flash of supernovae. The exceptional bursts of γ-rays received on Earth from these events are intense enough to have been accidentally detected in 1967 by US satellites monitoring Soviet thermonuclear tests, but they only began to be understood in the 1990s, and have since been intensively studied by space-based γ-ray observatories. They are traditionally “called gamma-ray bursts” (GRB*).

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FIG. 10.8 – Binary pulsar. (left, a). Schematic of the “binary pulsar” discovered by Hulse and Taylor in 1974 (Nobel Prize of 1993). The pulse frequency of this fast pulsar (about 17 pulses per second) has a very slow modulation with 7.75 h period that is interpreted as follows: the neutron star of the pulsar P is bound to another neutron star (non-pulsar) E’ by gravitational attraction and they follow elliptical trajectories around their center of mass G with this orbital period of 7.75 h. The Doppler effect* due to the fast motion of the pulsar star on its trajectory at a few hundred km/s causes a small variation (less than 0.1 percent) of the pulse frequency of the pulsar. As this variation depends on the projection of the star’s speed on the line of sight, it changes magnitude and sign with its position on the trajectory and thus has a modulation with the orbital period. (right, b). This modulation makes it possible to follow the movement of the pulsar on its trajectory with an extraordinary precision. It presents a slight advance on the movement that the star would have if its trajectory and its period were constant. This accumulated shift (“periastron advance”) increases with time and reaches about 30 s after 30 years. This effect is consistent with the prediction of general relativity of the emission of gravitational waves, which slightly decreases the energy of neutron stars on their trajectory. As shown in the figure, the comparison of the measurement of this shift with the predictions of general relativity gives a perfect agreement within the errors of measurement. This provided a strong demonstration of the existence of gravitational waves well before their direct detection (figure 11.2). Note, however, the enormous difference (factor around 10−15) in instantaneous power radiated by gravitational waves between this system and the coalescence of two massive black holes whose gravitational wave is detected by LIGO (figure 11.2). This difference comes mainly from the difference between the distances between the two members of the couple (about a million km for the binary pulsar against a few hundred km for the pair of black holes). Note also the difference in mass (  1.4 M⨀ against  30 M⨀). Credit: NRAO (Figure provided by J. Weisberg). The “long” bursts have a duration between about 2 s and 100 s (there are also shorter bursts of another kind discussed below) and come from very distant sources at very large redshift, z  2.5 on average (which corresponds to bursts emitted about 10 billion years ago!). It is agreed that their enormous γ-ray emission is produced by a particle jet of ultra-relativistic material (figure 10.6c). The complex physics of such

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jets probably implies that they come from initially very massive stars in rapid rotation, having lost their layers of hydrogen and helium, and therefore a large part of their mass. Although such events are rare (perhaps about one per million years in the Milky Way, so any possible effects on terrestrial life are weak, although it could deserve consideration), their fantastic gamma intensity makes them easy to detect throughout the observable universe. Today, the farthest detection is at z = 9.4, when the Universe was only about 500 million years old. The gamma flash is followed a few minutes later by an emission afterglow in the X-rays and visible, which can last a few days, and that results from the braking of the jet by the surrounding gas (figure 10.6c). The extreme intensity of this emission makes such objects intrinsically the brightest in the sky for some time, enabling to locate and identify their host galaxy and redshift. The distance and nature of these galaxies have confirmed the connection of long gamma-ray bursts to the death of massive stars. Moreover, for the very distant Universe, the (momentary) brightness of gamma-ray bursts, much higher than that of the brightest quasars, makes them unique tools for probing the extragalactic gas by its absorption on the line of sight (figure 11.5). Nonetheless, gamma-ray bursts are extremely rare (although on average about one per day is detected over the whole sky) as compared to the millions of identified quasars. The short gamma-ray bursts, of average duration around 0.3 s, are much less powerful and therefore of a different origin than the long gamma-ray bursts. Their redshift is difficult to measure because of the weakness of their visible emission, but we now know that their average redshift is probably only around z  0.3 (or 4 billion light–years). Although still uncertain, it is likely that these short gamma-ray bursts come from the fusion of a neutron star with another neutron star or a stellar black hole. The gravitational waves emitted in one of such fusion was detected for the first time in August 2017 by the LIGO and Virgo detectors (§ 11.3). Its localization by these detectors in a small area of the sky allowed the detection of associated signals in X-ray and visible light and as a weak gamma-ray burst. The optical signal provided a very accurate measurement of its position and redshift, and therefore distance (120 million light-years). Its optical power was intermediate between that of a nova and a supernova, so that such events are named “kilonova”. The very sensitive spectroscopy of this kilonova showed evidence of numerous lines of very heavy atoms, revealing for the first time that such events are a major source of cosmic nucleosynthesis for most elements heavier than iron (for example, the rare earth elements).

10.4

Cosmic Rays, Messenger Particles of the High Energy Universe

The Earth is surrounded by cosmic violence but we are protected Cosmic rays are charged particles of high energy – mainly H+ protons, see figure 10.11 for the other elements - similar to the particles that physicists study in their accelerators, but with an infinitely wider range of energy. The somewhat

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archaic term “rays” should not be confused with the electromagnetic radiation carried by photons; but it is quite similar to the common sense of the term “rays” used to designate the particles emitted by radioactivity, generally dangerous for our organism. Cosmic rays come mainly from the Sun, the remnants of supernovae (§ 6.5) and, incidentally, from other more uncertain sources. Channeled by the interstellar and interplanetary magnetic field, a good part of the cosmic rays reaches the vicinity of the Earth after a significant attenuation due to the interplanetary magnetic field of the Sun. Only a tiny fraction, however, reaches us on the surface of the Earth. Cosmic rays are thus responsible for only a small fraction, typically 10%–15%, of the low ionizing radiation to which we are subjected on average. Their residual harmful effect is practically negligible. However, it would be quite different, if we did

FIG. 10.9 – Cosmic rays. (Left, a) First evidence of cosmic rays. Photograph of the Austrian physicist Victor Hess embarking on the balloon from where he first discovered the existence of cosmic rays at an altitude of 5000 m in 1912. (Right, b) Energy distribution of cosmic rays (of non-solar origin in the vicinity of the Earth). The huge range of energy and the very fast and extraordinarily regular decay of their number with energy are remarkable. The intensity of these cosmic rays with energy below about 1 GeV (109 eV) is greatly attenuated in the Solar system by the solar interplanetary magnetic field. Most cosmic rays come from the interstellar medium of the Milky Way, but it is believed that those with energy greater than 1016 eV come from other close-by galaxies. The inset is a diagram of the Pierre Auger International Observatory in Argentina, which detect cosmic rays of extreme energy, either by observing their trajectory and the showers of particles they create in the atmosphere, or by direct detection of these particles on the ground. Credit: (left) Wikipedia (American Physical Society); (right): spectre CR: Observatoire Pierre Auger. Auger-design Auger LAL.

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not benefit from a very effective double protection against their bombardment: first, the terrestrial magnetic field of the magnetosphere which channels them, and, second, the absorption of the upper layers of the atmosphere. The best proof of the latter effect is the rapid intensity increase in ionizing cosmic rays as we rise in altitude. Relative to the sea level, this intensity is multiplied by nearly 10 at an altitude of 4 km, and by more than 100 at 20 km. It increases much further in the magnetosphere and beyond in the interplanetary regions that do not benefit from the trapping of relatively low energy cosmic rays by the Earth’s magnetic field. As for all charged particles, the magnetic field curves their trajectory with a radius of curvature increasing with the energy of the particles. For low energies, this is much smaller than the Earth’s radius and the trajectories become helices wrapping around the magnetic field lines without reaching the Earth. Another effect of cosmic rays resulting from their interaction with the nitrogen nuclei of the upper atmosphere is the continuous creation of radioactive carbon 14 (14C) nuclei whose importance for dating prehistory is well known. While the residual effect of attenuated cosmic rays at ground level is minor, their intensity in space, unscreened by the terrestrial environment, can present a serious problem for satellite equipment and precision measurements, especially when they orbit beyond the magnetosphere. Cosmic rays produce parasitic signals and can even damage nano-elements of cameras, computers, control systems, etc. The harmful effects of high-dose cosmic rays on the human body represent a major obstacle to potential future interplanetary (and interstellar) flights with humans on board, as it is almost impossible in practice to fully protect cosmonauts for a long duration.

Half a century of particle physics using cosmic rays The study of cosmic rays was intimately linked to developments in particle physics throughout the twentieth century, especially during its first half when their study formed a major sector of this area of physics. We can say that particle physics started at the turn of the twentieth century with the discovery of radioactivity in 1896. Very quickly, physicists realized that there were ionizing particles similar to those of radioactive sources but more penetrating (more energetic). Some physicists proposed that they could be of cosmic origin. This hypothesis was confirmed in 1912 by Hess by measuring the ionization with a balloon electroscope at 5000 m altitude (figure 10.9a). This discovery initiated a period of nearly half a century that can be considered the golden age of cosmic ray studies, when their basic properties were established and when they formed an essential tool for physics as a key source of high energy particles. A good part of these studies continued to be made from balloons and observatories located on high mountains. The basic composition of cosmic rays was thus established: approximately 84% protons (H+), 14% helium nuclei (He++): 1% electrons and less than 1% for all the nuclei of all other atoms (figure 10.11). At the same time, laboratory studies of cosmic rays and particles produced in their interactions with the atomic nuclei played a major role in the discovery of new elementary particles. This lasted until the 1950s, when high-performance particle accelerators took the lead. Cosmic ray studies produced many fundamental discoveries including

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that of the positron (antielectron, 1932), π meson (1936), muon (1947) and some other relatively light particles. From this time on, the energy distribution of cosmic rays was progressively established, constantly extending to higher energies (figure 10.9b). Whereas the average kinetic energy of the majority of cosmic rays is about 1 GeV (109 eV, which is also about the mass energy mc2 of a proton or a neutron), this distribution is quite striking by the enormous range of energies it now covers (a factor of more than 1012 or a trillion!). Their number decreases extremely rapidly with their energy. The ratio is about 1019 between the total number of cosmic rays bombarding the magnetosphere (about 10 000 particles per square meter per second) and the tiny fraction of those more energetic than 1020 eV (less than one particle per 10 km2 per year). This decrease also takes place with remarkable regularity, practically perfectly following a power law (figure 10.9b). As we will discuss in detail, most of the cosmic rays come from our galaxy (except the tiny fraction of ultra-high energy). It is believed that their arrival direction in the Solar system is uniform because of their strong interaction with the interstellar magnetic field. However, the flux of relatively low energy cosmic rays (of the order of 1 GeV and below) arriving at the Earth depends on the Sun and its interplanetary environment for two reasons. With its most violent eruptions, the activity of the Sun can sporadically increase in dramatic ways the intensity of cosmic rays, especially those of very low energy.

FIG. 10.10 – Aurora borealis. Photograph of a spectacular aurora borealis (right) and the triggering of such phenomena by a burst of cosmic rays emitted by a solar flare (left). Arriving near the Earth, these energetic particles strongly disturb the magnetic field of the magnetosphere, which allows them to penetrate deeply into the atmosphere following the field lines close to the poles. They ionize the air molecules and cause the illumination of the sky observed during the aurorae in the polar regions. Credit: Auroradiagram JPL/NASA. Aurora NASA Suomi NPP.

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The most powerful of these events propel huge bursts of energetic particles (especially protons) first in the solar corona*, and then in the interplanetary medium throughout the Solar system. They are the solar cosmic rays. Their abrupt arrival in the terrestrial magnetosphere and ionosphere produces giant auroral displays (figure 10.10), perturbing radio and satellite communications. In contrast, the Galactic particles of low energy are deviated by the interplanetary magnetic field created by the Sun. As this field depends on the Sun’s activity cycle (figure 4.6), the average cosmic ray intensity received at the Earth’s magnetosphere is substantially modulated with the 11-year period of the solar cycle period. The average intensity of the Galactic cosmic rays is minimum when the Sun is the most active, because its magnetic field is then maximum, which deviates more the cosmic rays.

FIG. 10.11 – Abundances of Galactic cosmic rays. Relative abundances of the number of atomic nuclei of different elements observed in cosmic rays compared to the chemical abundances measured in the Solar system that reflect cosmic abundances (figure 3.7). We note a good correlation of the two abundances for a number of elements, but a huge relative increase in the abundance of the very light elements (Li, Be and B) in the cosmic rays due to the “spallation” reactions (see text). Credit: ACE Caltech (NASA).

Witnesses and ubiquitous carriers of interstellar violence We have already encountered the cosmic rays of the Milky Way in § 6.5, with their key role in the physics and chemistry of interstellar molecular clouds. Their detection near the Earth brings us original information on the conditions and composition of the interstellar medium. Their coupling to the interstellar magnetic field ensures that these particles remain confined within the Milky Way and that they come from sources inside our Galaxy. The radius of gyration of charged particles around the magnetic field is directly proportional to their energy and mass and inversely proportional to the magnetic field strength. In the mean interstellar magnetic field (of which we have noted the relatively high value in § 6.5), the proton gyration radius remains smaller than the thickness of the Galactic disk up to about 1015 eV.

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More generally, cosmic rays have a close symbiosis with the interstellar magnetic field. The intensity and energy distribution of these cosmic rays reflect the conditions of their initial acceleration and their propagation in the interstellar medium. It now seems well established that cosmic rays, except those of the highest energy, are mainly accelerated in supernova remnants (§ 6.5, figures 6.5, 10.5, and 10.6a). It is known that these interstellar regions receive a colossal energy on a large volume from the explosion of a supernova. From the radio synchrotron radiation* of the electrons that spiral in the magnetic field and the γ and X emissions, there is a great deal of evidence that supernova remnants contain particles of energy comparable to cosmic rays, and that they are associated with violent shock waves. These conditions certainly explain how the acceleration of charged particles can occur in processes initially proposed by Fermi and developed later. It is enough that these particles remain trapped long enough in a supernova remnant, crossing multiple times the shock waves. The universal form of the energy distribution of cosmic rays as a power law is thus simply explained (figure 10.9b). So, we think we understand these phenomena quite well. However, the source of cosmic rays of extremely high energy, beyond about 1016 eV, remains uncertain. Their extragalactic origin is hardly in doubt because their radius of gyration in the Galactic magnetic field is too large for them to remain confined in the disk of the Milky Way or other galaxies. The acceleration mechanisms must be extraordinary to explain such energies that are several million times higher than that of the largest particle accelerators such as LHC*. Two environments with extreme conditions have been proposed: the vicinity of the super-massive black holes of the radio-galaxies from which gigantic relativistic jets are propelled (§ 11.5, figures 11.6 and 11.7), or the colossal magnetic fields of certain pulsars (§ 10.2). As can be seen in figure 10.11, the composition of the cosmic rays in nuclei of the different chemical elements reflects fairly well the standard composition of cosmic media. Yet, both distributions have important differences that provide information about the processes of acceleration and propagation of cosmic rays. In particular, the distribution of cosmic rays does not have the usual low abundance of the Li–Be–B group (Z = 3 − 5). This shows the importance of “spallation” nuclear reactions by which the cosmic rays of the abundant C–N–O group (Z = 6–8) lose some of their protons (and neutrons) to create the lighter nuclei of these elements by interacting with interstellar matter.

Chapter 11 Black Holes and Their Power 11.1

Black Holes, General Relativity and the Cosmos101,102,104,105

The discovery of the presence of black holes in the Cosmos, now variously confirmed, represents one of the most important astrophysical discoveries of the last half-century. Although the name “black hole” was coined only in 1967 by J.A. Wheeler, it can be said that the notion of black hole immediately emerged after the formulation of the theory of general relativity by Einstein in 1915. It stems from the work of K. Schwarzschild in 1916, although Einstein himself took a long time to accept the existence of such extreme objects! If the theory of general relativity is obviously necessary to describe the extreme curvature of space in and near a black hole, the problematic of such singular objects from which even light cannot escape can arise in classical physics. This problem was considered by J. Michell as early as 1783, before being independently developed by Laplace some time later. Indeed, it is well known that it is necessary to give a minimum speed (11.2 km/s) to a rocket so that it can escape the terrestrial attraction, otherwise it falls back onto the Earth. As this follows directly from Newton’s laws in mechanics, it is elementary to generalize to a body of mass M and of arbitrary radius R, for which the escape velocity ve is such that ve2 = 2GM/R (where G is the gravitational constant). Nothing prevents us from mentally considering the value given to ve by this formula when we increase M and/or reduce R indefinitely, and to ask the question of what happens when ve reaches the speed of light c = 300 000 km/s. We thus find that ve = c on the surface of a body of mass equal to that of the Sun if this mass is concentrated in a radius R = 3 km. It could therefore be considered that even light could not escape from such objects; but this situation appeared completely unimaginable in the eighteenth century. The question appeared less unconceivable when the theory of general relativity provided the adequate mathematical framework for dealing with gravity in all its generality even in such extreme cases. So, when Schwarzschild showed that spherical symmetry (as for a star or a black hole) allowed a simple solution of Einstein’s complex equations of general relativity, he naturally identified the fundamental property of what is now called a black hole with a mass M extraordinarily concentrated around a point. For the distances at this central point below the DOI: 10.1051/978-2-7598-2706-0.c011 © Science Press, EDP Sciences, 2022

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“Schwarzschild radius”, RS = 2GM/c2, the curvature impressed on the space by the mass M is such that nothing, not even light, can escape the black hole in crossing its “horizon” constituted by the surface of the sphere of radius RS. For decades, the properties of such curiosity have remained largely an object of mathematical discussion21. They began to interest physicists when the notion of neutron star slowly emerged before being spectacularly confirmed by the discovery of pulsars (1967, § 10.2). The fantastic density of neutron stars is indeed comparable to that of stellar black holes (1.6 billion tons per cm3 for a black hole of 3 M⨀!). Successive steps proceeded very quickly from the 1960s to seriously consider and then definitively confirm the presence of cosmic black holes. It was soon realized that the mass of a neutron star could not exceed 2 or 3 solar masses. Heavier objects must therefore collapse to the limit of the Schwarzschild radius RS and thus become black holes. We had very good reasons to think that it must be the fate of the core of the most massive stars. Simultaneously the young X-ray astronomy discovered X binaries (§ 4.4) with compact objects exceeding this mass limit. With the confirmation of the presence of such objects, the discovery of stellar black holes dates back to the 1970s, although some doubts were formulated. At the same time, the discovery of quasars (1963, § 11.4) quickly revealed the presence of super-massive black holes (up to billions of solar masses) as the most logical solution to explain their fantastic luminosity. This was to be confirmed by accumulation of data on quasars in the following decades. In fact, in addition to the extraordinary case of quasars and other active galactic nuclei, the presence of super-massive black holes in the center of all massive galaxies has gradually imposed itself. It is confirmed in the Milky Way by the precise measurement of the trajectory of the stars that revolve around its central black hole (§ 11.7, figure 11.9a). Finally, we can say that the recent detection of gravitational waves (2015, § 11.3, figure 11.2) is a redundant confirmation that the black holes are an essential class of the objects that populate the Cosmos. Before going into detail about these different types of extraordinary objects and their manifestations, it is useful to summarize their basic properties. Despite the complexity of some mathematical aspects, their fundamental properties are extremely uniform (see, for example, refs.101–105 for more details). It can be mathematically shown that every black hole is entirely defined by its mass and its angular momentum* (or spin*) S (plus its electric charge, but it is never important for cosmic black holes). In the absence of S, the mass M completely characterizes the black hole; all black holes of the same mass are then absolutely identical. Speaking of a particular structure or details inside a black hole defined by the radius RS of its horizon makes no sense. Likewise, a black hole has no distinctive extension, no “hair”, on the outside22. Communication between the inside and the outside of the RS horizon surface is forbidden. From outside we cannot see inside 21

Debates continue today about the properties of mathematical singularities and information theory associated with black holes and their possible physical consequences. 22 It was nevertheless shown by J. Bekenstein and S. Hawking that a black hole radiates very weakly like a black body of extremely low temperature thanks to quantum effects, but without transmitting any information about its inside.

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the black hole and anything that could happen inside has no impact on the outside. Nevertheless, as with all other masses, its mass M manifests itself far beyond RS, as a point mass, curving space in accordance with general relativity and simply exerting the classical gravitational attraction force, F = GM/R2, at great distance R when the Newtonian approximation is valid. Things are a little more complicated when the spin S is not zero. The black hole turns then on itself with a speed that can approach the speed of light. This rotation provides an additional contribution to the energy that is comparable to the rest mass energy Mc2 when the rotation speed approaches that of the light. In addition, the rotation causes some complex deformation of the surrounding space in addition to its overall curvature, but the other fundamental properties (no internal structure, no possibility of transmitting information to the outside, no “hair”) remain valid. The two great classes of cosmic black holes, the stellar-mass and super-massive ones, and their properties are evidenced by the manifestations of the enormous gravitational disturbances they exert on their environment. For stellar black holes in binary systems, it is first of all the X-rays emitted during the heating produced by the relativistic accretion of the companion’s gas, but also the gravitational waves emitted during the fusion of black-hole binaries. The masses currently measured for stellar black holes by November 2021 range from a few solar masses to more than 150 (figure 11.3). In a similar way, the emission of X or UV rays by the gas accreted by the super-massive black holes of the quasars is their most spectacular evidence. This violence can also manifest by triggering gigantic relativistic jets in radio galaxies and more rarely by the explosive tidal disruption of stars passing nearby. More peacefully, the action of the gravitational force of these black holes is also very often revealed in orbiting stars or gas that allow a direct measurement of their mass. We find that the mass of super-massive black holes can range from a few hundred thousand to a few billion solar masses. Their radius extends from about a hundredth to a hundred times the distance from the Earth to the Sun. Note that their average density is then very modest (1.6 kg/m3 for 3 billion solar masses, that is to say comparable to the density of the air).

11.2

Stellar Black Holes

If the initial mass of the super-massive star exceeds a certain limit that depends on its composition in heavy elements and is perhaps about 25 M⨀, the mass of the collapsed body produced by the implosion of the star’s core in a supernova exceeds the stability limit of a neutron star (  2–3 M⨀). Therefore, the collapse must continue up to the black hole stage (§ 11.1). However, given the rarity of so massive stars, such supernovae are extremely rare. There is no known example of a clear association of a black hole with a supernova. It is likely that most of the black holes identified in X binaries (§ 4.4) had rather to be formed by growing the mass of a neutron star by accretion until it exceeds the stability limit of neutron stars. The existence of ultra-dense objects too massive to be neutron stars has been confirmed by X-ray astronomy since the 1970s. Such objects can only be stellar black holes with a mass equal to a few solar masses.

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The objects that have been revealed by gravitational wave detection in 2015 (figure 11.2) are also stellar black holes, members of gravitationally bound black hole pairs with a higher mass (  10–150 M⨀) than black holes of X binaries. It is still unclear how precisely these pairs formed, perhaps in the successive explosion of two supernovae (or hypernovae*) in a pair of hyper-massive stars (perhaps similar to that of Eta Carinae, figure 4.3b, or more extreme).

11.3

Gravitational Waves, Propagation of Spacetime-Curvature Disturbances

The extreme gravitational effects that arise in the terminal phases of the fusion of two black holes are at the origin of the latest major astrophysical discovery, the direct detection of gravitational waves (figure 11.2). These waves are emitted by accelerated masses in the same way as electromagnetic waves (light, radio, etc.) are by accelerated charges. The two types of waves propagate in the same way at the speed of light with a decreasing amplitude as the inverse of the distance. Gravitational waves can be thought of as the propagation of space deformations, just as waves on water are deformations of its surface that propagate. The fundamental lesson of general relativity is that masses deform the space in their neighborhood and black holes produce the most extreme cases of these deformations. It is therefore conceivable that the relativistic rotation in the last phases of black-hole merging is

FIG. 11.1 – LIGO-Virgo gravitational wave detectors. Gravitational wave detectors using kilometer-based laser interferometry. The Laser Interferometer Gravitational Wave Observatory (LIGO) operates two detectors in the United States (Hanford, WA and Livingstone, LA), which have detected many events (mostly black-hole mergers) since September 2015 (figures 11.2 and 11.3). Since 2017 the Franco-Italian instrument Virgo (near Pisa) works in association with LIGO to improve the detection sensitivity and location of sources in the sky. In 2020 their joint operation was extended to the Japanese detector KAGRA. Credit: LIGO: Caltech. Virgo: The Virgo collaboration.

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FIG. 11.2 – Gravitational wave signals. Bottom, the first gravitational wave signals ever detected on Earth, on September 14, 2015 by the two LIGO detectors 3000 km away in the United States (figure 11.1). The final signal, oscillating with a period of less than one hundredth of a second, is produced by the gravitational wave emitted by a pair of black holes, located 1.2 billion light-years away, during their merging and just before it when they were spinning around one another at a speed close to that of light (see the artist’s view from the top of the figure). The signal represents the amplitude of the deformation of spacetime produced on the Earth by the passage of the gravitational wave. This modulation of the lengths results in a tiny variation of the 4 km arm length of the detectors. Note the perfect similarity (taking noise into account) of the two signals. They were received with an offset of 7 ms which corresponds to the difference between the propagation times of the waves at the speed of light up to the two detectors of LIGO 3000 km away. The detailed analysis of these signals made it possible to deduce the parameters discussed in the text for the two black holes that merged. The expected signal calculated with these data and the theory of general relativity is shown in gray in the figures above; we see that it coincides so perfectly with the two observed signals (orange and blue) that the gray is almost invisible in the figures. This confirms that the signal comes indeed from the coalescence of two black holes and justifies the precision of the values deduced for their masses and their distance. Credit: (bottom line) GW Signal IAU, Caltech/MIT/LIGO Lab; (top line) Credit: C. CARREAU/ESA.

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one of the most effective ways to produce a gigantic gravitational wave. We now know how to fully calculate such a phenomenon (gray curves in figure 11.2), by combining an elaborate theory and the power of computers. It is thus shown that, in the fusion of two black holes of a few tens of solar masses each, an appreciable part of their total mass energy, corresponding to a few solar masses, is carried away by the emitted gravitational wave. For example, the first gravitational wave detected in 2015 (figure 11.2) was produced in a distant galaxy 1.2 billion years ago by the fusion of two such black holes of masses 36 and 29 solar masses, converting 3 solar masses into gravitational wave energy and producing a black hole of 62 M⨀. However, as this enormous energy is diluted throughout the space through which the wave is traveling, it produces only tiny deformations of space when the wave reaches the Earth. Thus, this gravitational wave made our space vibrate with tiny relative variations in length ΔL/L  10−21 (figure 11.2, ΔL equals one millionth of the thickness of a hair for the distance L from the Earth to the Sun!). This value shows the extreme difficulty of the detection of gravitational waves and the extraordinary level of technological achievements obtained with the interferometric laser detectors (figure 11.1) of the LIGO* collaboration which realized the first detection. While the mass of these first merging black holes detected in 2015 proved to be a little higher than expected, a substantial increase in sensitivity was achieved in the following years with the improvement of the LIGO detectors and the addition of the European Virgo* and Japanese KAGRA detectors. This has multiplied black-hole merging detections (figure 11.3). One has even reached the neutron star detection limit of only a few solar masses producing gravitational waves an order of magnitude weaker than black-hole mergers such as that of figure 11.2. The detection of the merging of two neutron stars was achieved for the first time in 2017 by the combination of LIGO and Virgo. A key result was the detection of γ-ray, X-ray and visible emission simultaneous to that of the gravitational wave. This was made possible by the important mass of matter ejected in the neutron-star fusion and it allowed the precise localization of the host galaxy and a detailed study of the physics of the kilonova* explosion (§ 10.3). As the violent fusion of two black holes, especially super-massive ones, must have effects on their environment, it is hoped to also detect concomitant electromagnetic radiation (γ, X or visible), notably by a “short” γ-ray burst (§ 10.3). This made it possible to locate this event and characterize its environment and galaxy, allowing us to better understand these systems. With the prospect of detecting super-massive black-hole mergers (§ 11.5) by spatial interferometers (figure 11.4), a whole new field of astronomy opens up with unique ways to probe the properties of these exceptional objects and their merging processes governed by general relativity. It therefore seems that we are witnessing the opening of a spectacular new field of astrophysics. The detailed understanding of relativistic ballets of such singular objects in cataclysmic events is emblematic of the technological and theoretical achievements of contemporary astrophysics. It is also the most direct proof of the existence of black holes.

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FIG. 11.3 – Masses of detected black-holes and neutron stars (2021). Mergers detected through gravitational waves by the LIGO-Virgo collaboration are represented by grey arrows connecting the two initial masses to the final mass, in blue for black holes and orange for neutron stars. Masses of neutron stars and stellar black holes detected only from their electromagnetic radiation (mostly X-rays, § 11.2) are shown as yellow and violet dots, respectively. Credit: LIGO-Virgo, Aaron Geller, Northwestern.

FIG. 11.4 – LISA. Principle of the Laser Space Interferometer Array (LISA) project for gravitational wave space sensing, to be launched around 2034 by the European Space Agency. This major project is based on the same ultra-precise measurement of length variations by laser interferometry as the LIGO and Virgo terrestrial detectors (figure 11.1), but the much longer arms, 5 million km (about 10 times the Earth-Moon distance) between the three satellites of LISA against 3–4 km for Virgo/LIGO, will detect the coalescence of black holes much more massive in the center of distant galaxies. Credit: NASA.

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Quasars: New Stars a Thousand Times Brighter than Galaxies

The huge black holes that sit in the center of the galaxies have a completely different size than their little brothers formed from the stars. Instead of a few times the mass of the Sun, we are now dealing with masses that are millions, even billions of solar masses. As their Schwarzschild radius increases in proportion to the mass (§ 11.1), it approaches or greatly exceeds the distance from the Earth to the Sun (150 million km, instead of a few km for stellar black holes). The energies that these super-massive black holes imply by forming (  Mc2) and inject into their environment, increase in proportion. The light (or mechanical power) emitted can reach 10 000 times the total luminosity radiated by the hundreds billion stars of a large galaxy like ours. However, most galaxies are not the seat of such an outburst of violence, because this infernal activity is only sporadic. Yet, we know that virtually all massive galaxies contain such a super-massive black hole in their center, but this one is active to give birth to a quasar or other form of “active galaxy nucleus” only during certain periods of its existence which lasts about 13 billion years. These periods of activity were mainly in the past, more than 5 or 10 billion years ago for most quasars. Therefore, the black hole of most galaxies, and especially our neighbors, is now inactive. The same is true for the Milky Way (§ 11.7). It is not easy to reveal the presence of such inactive black holes and we only manage to do it for large, near galaxies. On the other hand, when such galactic “nuclei” are in an “active” state, the gigantic energy that the most powerful ones emit in visible light or radio makes it possible to easily detect them to the limits of the observable Universe. It is also this unique power that made possible their early identification. The closest radio galaxies, that emit such colossal energies only in the radio domain (figure 11.6), have been systematically identified and studied in the early days of radio astronomy in the 1950s using increasingly efficient radio telescopes (§ 2.4)23. Hence the name “radio galaxies” given to these radio sources which are among the brightest in the sky. Quasars emit power equivalent to radio galaxies but in the optical spectral range (from ultraviolet to infrared). At a large distance, they are much more numerous than radio galaxies. Yet, their discovery occurred more than ten years after that of radio galaxies because there are no bright quasars in the very near universe. Their appearance is not directly distinguishable from the stars which are infinitely closer and no one at the time was expecting the existence of such extraordinary objects. Although 90% of the quasars have only a modest radio emission, some are also radio galaxies. The surprise was great in 1963, when the first spectrum of such an optical source was analyzed, coinciding with a radio

The radio galaxy Cygnus A (figure 11.6) had even been seen as early as 1939 using a primitive radio telescope without any idea of its nature and structure.

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galaxy (listed under the name of 3C 273) that seemed very ordinary24. At first glance, the spectrum seemed incomprehensible; then it was realized that it was dominated by the spectral lines of the hydrogen atom with a redshift of z = Δλ/λ = 0.16. This seemed an incredible value at the time. The speed implied by such a Doppler shift is about 16% of that of light. According to Hubble’s law, it would place the object about 2 billion light-years away, far beyond all the known galaxies at the time. There was serious controversy about the validity of the general-relativity extension of Hubble’s law (§ 8.1) under such conditions, in order to try to escape the conclusion of the gigantic distances and luminosities it implied. However, the majority of astronomers quickly agreed on the reality of the enormous distance of this source and its corresponding huge luminosity (more than 1012 L⨀25). It was immediately recognized that the sources of stellar energy, which were already familiar then, were unable to explain such a power and that only the accretion energy onto an extremely massive and compact body could account for such a luminosity. As physics did not seem able to provide any other valid explanation for such an object, only a super-massive black hole of several million solar masses seemed to be suitable. This hypothesis finally prevailed despite the controversies and has been widely confirmed since. In the meantime, the quasars have emerged as an essential population of the extragalactic zoo. The extreme quasar brightness makes them practically the easiest objects to detect at very large distances up to the limits of the observable Universe. Quasars and radio galaxies remain attached to some of the most studied problems of contemporary astrophysics, as will be seen below.

Quasars, tools for studying intergalactic matter Regardless of their remarkable intrinsic properties, the exceptional quasar brightness is used to provide us with the best tool for probing intergalactic spaces to the limits of the observable Universe. Before reaching our telescopes, photons emitted by distant quasars can encounter thin clouds of intergalactic gas here and there in their path of billions of light-years. A small part of these photons can thus be absorbed if they have the right wavelength of the spectral atomic lines of the crossed gas. This results in a deficit in the number of photons arriving at the corresponding wavelengths and is seen as absorption lines in the observed spectrum of the quasar (figure 11.5). The absorbing intergalactic clouds are themselves carried away by the expansion of the Universe so that the absorption lines of their atoms undergo a large redshift but smaller than the quasar’s redshift. The quasar absorption spectra thus provide us with an unparalleled information about the 3D distribution of the atoms

24

It was first seen as a punctual object, then a jet was discovered. Because the quasar was so bright, it was not before the 1990s that it was possible to distinguish the galaxy itself thanks to the HST* power. 25 This is the power emitted in the visible spectral range. It was later realized that the total electromagnetic power emitted by including all wavelength domains, especially UV, X and γ, was at least ten times greater.

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in the intergalactic gas that contains more than 50% of all atomic nuclei (baryons*) of the Universe (§ 9.5). In general, the distribution of these atoms reflects that of dark matter which is itself organized in the large structures that we have described in § 9.1. Intergalactic gas concentrates (but not only) in and around these structures including shells, filaments, clusters of galaxies, halos of individual galaxies, at different stages of their formation. The quasar absorption systems thus provide information on these structures complementary to that provided by the distribution of galaxies. Nevertheless, most of the intergalactic gas is extremely hot and ionized because of its exposure to the background of UV and X radiation. Its detection is therefore more difficult than that of galaxies. The multiplication of the number of known quasars with their absorption spectrum makes possible more and more ambitious statistical studies on the distribution and properties of intergalactic gas clouds and their relationship with dark matter and galaxies. By their ability to probe various times distributed throughout the history of the Universe, absorption systems also contribute to the investigation of fundamental problems of cosmology and even of physics. For example, the ratios of absorption lines of certain molecules, such as the radical CN, make it possible to measure the temperature of the cosmological radiation at large redshift and to verify that its variation with this redshift agrees with the cosmological models. In the same way, the thin absorption lines offer the possibility of measuring the ratios of the line wavelengths of certain atoms with very great precision. This could be a sensitive way of demonstrating possible variations of these ratios with the redshift and thus with time, which might reflect very small variations in the fundamental “constants” of physics; but all attempts to confirm such variations have failed.

11.5

Manifestations of Super-Massive Black Holes and Their Interpretation

Spectra As with most stars, the richest information about the enormous energy emitted by quasars and other active galactic nuclei comes from their optical spectra. The main features of these spectra (figure 11.5) show the violence of the environment of these gigantic black holes. One finds there lines of highly ionized atoms like C+++ which attest the presence of extremely intense ultraviolet radiation. Moreover, the spectral lines are very broadened by Doppler effect involving speeds of several thousand km/s, that are interpreted either as the rapid rotation of a gas ring rotating around the enormous mass of the black hole, or as the ejection of large clumps of gas at a very high speed. The analysis of these spectral lines allows us to unravel and model the physical mechanisms that produce this emission as well as the growth of the black hole.

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FIG. 11.5 – Spectra of quasars. Left: Schemes of typical spectra of some of the first discovered quasars. It is possible to visualize the redshift of the hydrogen lines Hα, Hβ and Hγ and that of the doubly ionized oxygen ion O++ with respect to their wavelength at rest (6563, 4861, 4340 and 5007 Angström, left spectrum). On the right at the top: spectrum of a bright quasar with a large redshift, z = 2.32, showing the emission of redshifted UV spectral lines of highly ionized elements (C++, C+++, Si+++). They dominate its UV spectrum with the fundamental Lyα hydrogen line (wavelength at rest 1215 Angström). Right bottom: Absorption spectrum (like a line “forest”) of the Lyα line observed in the direction of a quasar at z = 3.09. These lines are produced by the absorption of the hydrogen atoms of a multitude of small clouds of intergalactic gas closer to us than the quasar (z < 3.09). Credit: (left) Minimum credit line: C. Pilachowski, M. Corbin/NOAO/AURA/NSF; (right) ATNF/CSIRO, 2dF quasar survey. Feeding the gas accretion onto a quasar black hole needs to transport large quantities of fresh interstellar gas to the central regions of the galaxy. Only a small fraction will be brought within reach of the gravitational attraction of the black hole. To do this, the galaxy faces the same difficulties as for supplying enhanced star formation in its center. The problem of angular momentum is crucial in all successive phases which result in the final accretion of a small portion of the gas onto the black hole. As in many other gravitational accretion processes, such as star formation (§ 4.3) and accretion by stellar black holes, an essential step is the flattening of the gas nebula captured by the gravitational attraction of the black hole into an accretion disk rotating around it. We now know that in such disks, the subtle play of turbulence and the magnetic field can convey a portion of the gas to the center. During the accretion process and especially in the final phases, the released gravitational energy produces a very hot ionized gas. The electrons of this plasma emit X-rays which evacuate the energy and constitute most of the original luminosity of the quasars, but we only see a part of it, because most X-rays are immediately reabsorbed by the colder ambient plasma, which eventually emits most of the quasar power in the form of UV, visible or infrared radiation.

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Radio emission The other most impressive manifestation of the colossal energy released in the black-hole accretion is the ejection of gigantic jets of matter that convey energy comparable to that of quasars at an initial velocity close to that of the light. These jets, which emit a strong radio intensity and sometimes γ-rays, can coexist with a quasar optical emission. About 10% of the quasars are thus active in radio. But most radio galaxies that catapult such huge jets are less extreme in optics. As with the quasar optical energy, the gigantic total mechanical energy produced during the hundreds of millions of years of radio source activity can represent a significant

FIG. 11.6 – Radio galaxies. Spectacular images of the most powerful near radio galaxies. The super-massive black hole (more than a billion solar masses) in the center of a giant elliptical galaxy (the galaxy is visualized by the gray visible image obtained with the HST* for Fornax A) launches into two opposite directions jets of ultra-relativistic particles. They propagate over millions of light-years, well beyond the limits of the galaxy, causing an intense radio emission of intergalactic gas in a huge volume. The bottom row gives some examples of the variety of these objects whose direction of the jet could vary over time (3C31 and Cen A) and whose composite images (radio, visible, X) better reveal the nature: X-rays highlight the jet of Pic A (in blue) and Cen A, while the combination of submillimeter and visible radiation shows the dusty structure of Cen A following the merging of two galaxies (figure 7.6). Credit: NRAO, NASA, MPIfR, CSIRO.

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fraction of the black-hole mass energy Mc2. Such energy is able to propel the jet well beyond the limits of the host galaxy of the black hole. We see that in the spectacular images of the most powerful radio galaxies of our neighborhood, which show the impact of the jet heating immense regions of interstellar and intergalactic gas (figures 11.6 and 11.7). The physical mechanisms that generate and violently accelerate such outflows remain highly uncertain despite forty years of theoretical efforts. These are obviously very complex relativistic, magneto-hydrodynamic effects, related to the rapid rotation of the accretion disk and the very strong acceleration that the gas undergoes when approaching the surface of the black hole. It seems agreed that a rapid rotation of the black hole on itself is an essential ingredient to the production of these jets. It is therefore necessary to make use of the difficult physics of rotating

FIG. 11.7 – Jets of radio galaxies (see also figure 11.6). To the left, multi-scale images of the Virgo A radio galaxy associated with the giant elliptical galaxy M 87 (figure 5.4a), in the center of the Virgo cluster of galaxies (closest to the Milky Way, figure 6.8c). The complex structure of the global radio image (A) reveals the evolution over hundreds thousand years of the relativistic jet launched by a black hole of a few billion solar masses (figure 11.9b). The detailed lumpy structure of the jet (BCD) shows that it results from successive ejections of relativistic particles with an ultimate period less than one year (1 pc = 3 light-years). To the right, powerful near radio galaxy, Hercules A, studied in detail at various wavelengths showing its various components: (1) central elliptical galaxy imaged by the HST*; (2) gigantic halo around this galaxy of very hot gas detected in X-rays (purple color); (3) different episodes of the jet ejection of ultra-relativistic particles by the central black hole. This jet is at first very thin and mono-directional in its radio image, but it produces diffuse images (in blue) when it interacts with the ambient intergalactic gas, revealing several shock waves several hundred thousand light-years away from the galaxy. They correspond to successive emissions of relativistic particles by the black hole. Credit: NRAO, NASA.

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black holes and their action on their environment (§ 11.1). It is furthermore suspected that the extreme and unique conditions of these relativistic infernal pots are excellent candidates for producing the cosmic rays of gigantic energy that we detect on Earth (§ 10.4). As we have seen (§ 11.1), general relativity teaches that all the properties of astrophysical black holes are entirely determined by their mass and their angular moment of rotation* (spin*). We can try to determine these two quantities by studying their manifestations outside the black hole, as well as the coalescence of two super-massive black holes.

Mass The mass of a black hole is by far the easiest parameter to evaluate. As usual in astronomy, it suffices to analyze the trajectory and speed of any body in orbit around the black hole under the action of its gravitational attraction. As we see below (§ 11.7), this method can be applied to individual stars gravitating around the central black hole of our Galaxy. It is more difficult to implement at the great distances of super-massive black holes in other galaxies. Contemporary astrophysics, however, routinely exploits such a method not only for nearby galaxies by observing the collective rotation of central stars or gas around their inactive black hole, but also in quasars up to the edge of the Universe, by measuring the speed of the ionized gas orbiting the black hole. During the last fifteen years, we have been able to measure the mass of the black hole of several tens of galaxies close to ours and more than 100 000 quasars, whatever their distance. It turns out that the energy radiated by a quasar always seems to be about a tenth of the mass energy mc2 of the matter accreted by its black hole. By measuring the luminosity of a quasar, one can therefore roughly estimate the increase rate of the black-hole mass. In the most extreme cases, the growth of the black-hole mass can be exponential with a doubling every 50 or 100 million years. One knows well the current mass distribution of super-massive galactic black holes and how it has varied over the history of the Universe. The majority of these super-massive black holes “weigh” a few million solar masses, such as ours in the center of the Milky Way (figure 11.9a). Their number decreases rapidly for higher masses. Those of more than one hundred million and even several billion solar masses exist today only in the center of massive elliptical galaxies where they often manifest themselves by feeding the most powerful radio galaxies (figure 11.6). They are also systematically found in the brightest quasars whatever their redshift. Recalling that the Schwartzchild radius RS of a black hole is proportional to its mass M (RS = 3 km × M/M⨀) (§ 11.1), we see that for super-massive black holes RS compares to the distance from the Earth to the Sun (150 million km), the most massive occupying a larger volume than the entire Solar System.

Tidal disruption events A spectacular manifestation of the influence of a black hole on its environment is the rare explosive tidal disruption of a closely passing star. Such events may be

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observed in visible light or in X-rays around otherwise dormant super-massive black holes. The number of detections is steadily increasing and already exceeds 50. This is expected to strongly increase with new all-sky survey facilities such as The Rubin Observatory LSST*.

Rotation The rotation properties of super-massive black holes are much more uncertain than their mass, because we have no way to directly measure their angular moment of rotation, nor even to determine the axis of this rotation. It is nevertheless agreed that this rotation must be important for a large fraction of the super-massive black holes. Such an assertion is solidly supported by the analysis of accretion processes. They can create, accelerate or brake this rotation of the black hole, by transferring angular momentum of rotation to the black hole from the accretion disks or from the pairs of black holes that merge. This ultra-relativistic rotation energy considerably modifies the influence of the black hole on its environment. It changes the conditions of the gas accretion and, coupled with the magnetic field, it is suspected to be at the origin of the complex mechanisms that provide the gigantic initial acceleration propelling the jets of radio galaxies (figures 11.6 and 11.7).

Coalescence Although no case has yet been observed, the coalescence of their two super-massive black holes is regarded as a probable phenomenon after the merging of two massive galaxies (§ 7.2). Indeed, in the post-merging complex reorganization of the new galaxy, the two black holes tend to spirally “fall” towards its center. Gravitational interactions with the surrounding stars and gas produce a friction effect which allows black holes to evacuate their kinetic energy and angular momentum, which is necessary for their sedimentation in the central region of the galaxy. After a few billion years they could end up orbiting around one another at the center of the galaxy, ever closer, until they merge. In the very last very brief phases where the black holes rotate very close to one another, at a speed approaching the velocity of light c, they emit a substantial fraction of their total mass energy in the form of gravitational waves (figure 11.2). Such coalescences between super-massive black holes had to occur at a similar rate as galaxy mergers which were frequent in the young Universe. As one expects the intensity of the gravitational waves is proportional to the masses of the black holes, the detection of such coalescences by giant laser interferometers seems relatively easy. However, it is necessary to locate these interferometers in space (figure 11.4) because of the very low frequency expected for the gravitational waves. This frequency is close to the Schwartzchild circumference of the resulting black hole divided by the speed of light. It becomes extremely low for super-massive black holes, which makes impossible its detection with ground detectors because of seismic noise. The realization of giant space interferometers, such as the LISA project (figure 11.4), is one of the major issues of astronomy in the coming decades.

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Co-evolution of Galaxies and Their Black Hole

The universal presence of a huge black hole in the center of large galaxies suggests that it is an essential constituent of galaxies, even when it is dormant as in most galaxies including ours (figure 11.9a). Moreover, we have discovered that there is a remarkable correlation between the mass of this black hole and the total mass of elliptical galaxies or that of their bulge for spiral galaxies: the mass of the black hole is always close to one to two thousandths of the star mass of the galaxy or bulge (figure 11.8). One may be surprised that there exists such a precise relation between two bodies of scales so different (the radius of their volume may differ by a factor one billion!). It can only be explained if the two objects have had a correlated evolution and growth, with periods of strong interaction. We have already seen that the intense phases of stellar formation and black hole activity feed from the same source, the transport of interstellar gas to the central regions of the galaxy. It is therefore normal that there is some correlation between their masses, but this does not explain the precise value of the observed mass ratio. One promising explanation is the devastating effects that the black hole may have on the interstellar gas at a distance throughout the galaxy or bulge, occurring from its radiation or jet. As these effects increase with the mass of the black hole, there comes a moment when the force exerted to the gas exceeds the gravitational attraction which maintains it inside the galaxy, so that it is expelled outside. The disappearance of the gas dries up

FIG. 11.8 – Bulge-black-hole correlation. Visualization of the correlation between the mass of super-massive black holes and that of the stellar system that hosts them: elliptical galaxy or spiral-galaxy bulge. The mass of the black hole is about 1/1000 of the mass of the star system. Credit: Courtesy A. Feild/STScI/NASA.

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the black-hole supply, which stops its growth. The mass of the black hole thus adjusts to that of the galaxy or bulge. The detail of these effects is obviously complex and uncertain, but models seem to be able to well reproduce the correlation observed between the masses of the black hole and the galaxy or the bulge.

11.7

The Super-Massive Black Hole of Our Galaxy and Others

Like other massive galaxies the Milky Way has a super-massive black hole in its center. It remains quite modest in comparison with that of giant elliptical galaxies that outclass it by a factor of 1000, but it still “weighs” 4.3 million solar masses. Yet, it remained totally unnoticed until 1974 when radio astronomers found an anomalous radio-emission source, SgrA*, just at the Galactic Center. They quickly suspected the presence of a black hole to explain it. We now know that all massive galaxies have a central black hole in the same way as active galaxies. Yet, as in the Milky Way, these black holes generally remain “dormant” for lack of accretion of a significant amount of gas, so that their surrounding emits only a modest energy similarly to SgrA*. Several decades have nevertheless been necessary to confirm this hypothesis for the Milky Way, by observing the effect of the black-hole gravitational attraction on the movement of stars or gas very close to it. It was a daunting challenge to achieve the extremely high precision needed for observing in a very difficult area in the heart of the Milky Way disk that is obscured by dust. Only the progress of telescope size, infrared astronomy and adaptive optics has allowed two teams, European and Californian working together, to confirm the presence of a fantastic mass concentration of 4.3 million of solar masses in a tiny space just at the position of SgrA* (figure 11.9a). It is inconceivable that such a mass concentration could stably exist there if not a black hole. The revelation of this black hole is clearly one of the major astronomical discoveries of our time. The 2020 physics Nobel Prize has rewarded the rigorous observational virtuosity and the enormous technological efforts of contemporary astronomy. Since we have discovered the presence of this sleeping monster not so far away (27 000 light-years away from the Earth), the question arises as to whether it is likely to wake up in a more or less distant future giving an active galaxy nucleus. The answer is not evident. Due to the current stability of the Milky Way, we are certainly safe from a real active-nucleus episode for hundreds of millions of years at least. But it is quite possible that more modest temporary increases of gas accretion, even of stars, regularly occur, singularly increasing the power of this radio source. The Galactic Center is therefore already being carefully monitored so as not to miss such possible events. It is also possible to show the black hole directly by the way it distorts the space at its periphery and can appear as a dark image on the radio background

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FIG. 11.9 – Local super-massive black holes. (left, a). Trajectory of a star around the central black hole of the Milky Way. Successive positions (in arc-seconds) of a star observed in infrared radiation between 2002 and 2011 around the SgrA* radio source at the center of the Milky Way. The knowledge of this elliptical trajectory allows a precise determination of the mass of the extremely compact object around which the star moves, bound by gravitational attraction. It is most likely a super-massive black hole of 4.3 million solar masses, which coincides with SgrA*. (right, b). Image of a black-hole event horizon. High angular resolution of the center of the giant elliptical galaxy M 87 (see figures 5.4a and 11.7) obtained by millimeter-wave inter-continental interferometry (VLBI*, figure 2.9b) in 2019 using ALMA* and seven other radio telescopes (Event Horizon Telescope collaboration). One sees a dark region – the black hole’s “shadow” – surrounded by a ring of emission produced by the distorted paths of the radiation emitted from the surrounding material – dust and gas that form an accreting disk at the base of the powerful jets of this active black hole of 6.5 billion solar masses. When the image is taken from polarized radio waves as above in 2021, it reveals the field lines of the strong magnetic field around the black hole. It is thought that knowing the properties of this magnetic field will help understand how the powerful jets of the radio galaxy are driven (figure 11.7). Credit: (left) MPG/MPE/R. Genzel; (right) ESO & Wikipedia. (the silhouette of the monster!), thanks to the extreme angular resolution of millimeter-wave inter-continental interferometry (VLBI*, § 2.4, figure 2.9b). This was recently achieved for the black hole of the giant elliptical galaxy M 87 which is 1000 times more massive than the SgrA* black hole (figure 11.9b). One soon expects similar images for SgrA* which is much closer but more difficult to image because of its variability and of perturbations of its radio images by the interstellar medium of the Milky Way disk.

Part VI

Planets, in the Solar System and Outside

It is difficult to give full account of this vibrant sector in the context of this book because of the lack of space and the rapidity of its development, especially for exoplanets. It is therefore strongly recommended that readers refer to the excellent specialized works existing in this field121–130.

Chapter 12 Direct Exploration of the Planets 12.1

Planets, Stars of Astronomy until the 19th Century

For millennia, the planets have been the objects of astronomy par excellence. Their brilliance especially Venus, Jupiter and Mars can surpass the brightest stars. Their erratic movement in the sky, compared to the unchangeable round of the stars, makes them fascinating objects. Understanding their nature appeared essential to deciphering the Universe. Since our first ancestors noticed that they were not stars like the others, the planets intrigued all civilizations and populated their pantheons with tutelary or evil gods. The brightest planets, clearly visible to the naked eye, Venus, Jupiter, Mars, Saturn, and Mercury have been known for ever and their motion was studied since the earliest Antiquity. Uranus was discovered in 1781 and Neptune in 1846. It has been well established for two centuries that the planets of the Solar System fall into two categories: (1) the four small planets, near the Sun, called “telluric”, mainly “rocky”, resemble the Earth by their high density and small radius; (2) the four outer planets, Jupiter, Saturn, Uranus and Neptune, “giant” compared to the Earth, are obviously mainly gaseous because of their low density. They are also called “Jovian” because of Jupiter, their leader. Almost all of the twenty or so major satellites (figures 12.7 and 12.8) of the Solar System planets and the two small Mars satellites had been discovered before 1900, mainly around the Jovian planets. There is no doubt that until the end of the nineteenth century, the study of the planets was the most important theme of astronomy, which most struck the minds. To be convinced of this, it is enough, for example, to leaf through Flammarion’s or Clerke’s popular astronomy books (figure 1.2). The main properties of the planets130 are summarized in table 12.1. Most of these were already well known at the end of the nineteenth century, except for the pressure and composition of the atmospheres. The parameters of the motion of planets on ellipses around the Sun, initially established by Kepler in 1609, were refined in the following centuries. These ellipses are close to a circle (except for Mercury). Since it was recognized by Copernicus that they revolve around the Sun, it was noticed that

DOI: 10.1051/978-2-7598-2706-0.c012 © Science Press, EDP Sciences, 2022

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their trajectories have the remarkable property of being all almost in the same plane. It is also that of the orbit of the Earth around the Sun, called the ecliptic plane26. Extraordinary precision of the measurements and mathematical analysis of these movements made it possible to account for the small effects of the mutual gravitational attraction between planets with remarkable fidelity. This mathematical mastery, also highly publicized, made possible the greatest triumphs of nineteenth-century astronomy: the prediction of the presence of Neptune, leading to its discovery in 1846; the discovery of an anomaly in the orbital motion of Mercury in 1859, which was explained in 1915 as one of the first proofs of Einstein’s general theory of relativity; the demonstration of the existence of chaotic mechanical systems by Poincaré in the 1890s126. The observation of the planets and especially of Mars aroused an incredible craze in the media and the general public in the last decades of the nineteenth century (§ 1.2 and figure 1.5). It arose from the widespread belief in the possible existence of extra-terrestrial inhabitants of these planets. This culminated with the announcement of the mysterious “canals” on Mars whose appearance changed with the seasons. From this time, a little before the turn of the century, dates the fantasy of the Martians, little green men! The Martian canals were in the news and kept controversies alive for a while, bringing new first-class observations and even the construction of one of the largest telescopes of the time in Arizona (figure 1.5f). The illusion gradually subsided and finally disappeared in the 1920s. Remarkable images of Mars remained, identifying its main reliefs and illustrating the profound changes in the aspect of this planet with its seasons. Some of the best images taken before 1900 already showed the appearance and disappearance of winter polar caps. However, the cause of the changes in appearance that had been mistaken for canals was recognized only in the 1920s as due to gigantic sandstorms. Since then, most astronomers agreed that the chances of existence of extraterrestrial civilizations and advanced life in the Solar System are almost nil because of the conditions prevailing in the different planets and their satellites. The evolution of knowledge about the small bodies of the Solar System followed a parallel course. All largest asteroids, “minor planets”, closer to the Sun than Jupiter, were known in the nineteenth century (e.g. most of those with a diameter greater than 200 km shown in figure 12.1). As for shooting stars, meteorites and zodiacal light, the understanding of comets was already quite advanced at the beginning of the twentieth century: their main components – solid core, bright head (coma), long, double diffuse tails – were not only identified, but quite correctly interpreted for the tail of dust. Here too, the public imagination was greatly aroused, as in the case of the passage of Halley’s Comet in 1910.

26

This fact is indeed directly related to the striking feature, known from ancient times, that the path of the planets in the sky is always very close to the “ecliptic”, the great circle of the vault of heaven that the apparent motion of the Sun follows over the course of one year.

Radius (REarth) 0.38

Mass (MEarth) 0.055

Density (g/cm3) 5.43

Orbit R (AU*) 0.39

Period (yr) 0.24

T. rot. (day) 59

Obliquity (degree) 0.01

p (ground) (bar) 10−15

Venus

0.95

0.82

5.24

0.72

0.62

243

177

90

Earth

1.00

1.00

5.52

1.00

1.00

1.00

23.4

1

Mars

0.53

0.11

3.93

1.88

1.88

1.03

25.2

0.006

Jupiter Saturn Uranus Neptune

11.21 9.43 4.01 3.88

318 95.2 14.5 17.2

1.33 0.69 1.27 1.64

5.20 9.56 19.2 30.1

11.9 29.5 84.0 165

0.41 0.44 0.72 0.67

3.1 26.7 98 28.3

Planet Mercury

Atmosphere

Direct Exploration of the Planets

TAB. 12.1 – Main properties of planets. (“Obliquity” is the angle of the axis of rotation of the planet with the perpendicular to the ecliptic plane*).

96% CO2 3.5% N2 78%N2 21% O2 95% CO2 3% N2

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FIG. 12.1 – Major asteroids*. Images of 35 of the largest objects in the asteroid belt (with diameter > 140 km), located between Mars and Jupiter, observed with ESO’s VLT* (2021). The observations reveal a wide range of peculiar shapes, from spherical to dog-bone, and are helping astronomers trace the origins of the asteroids in the Solar System. Credit: ESO. Some features of the history of the Solar System, in particular the hypothesis of a common origin and age, were addressed more and more clearly. At the same time, the ancient dating of terrestrial rocks provided a realistic order of magnitude of the time scale of Solar System history.

12.2

Half a Century Without Revolution for Planetology

Unlike many other areas of astronomy, the first half of the twentieth century saw no major revolutions in our knowledge of the Solar System. Certainly, considerable progress was made thanks to the improvement of different techniques of observation. The microphysics revolution played a crucial role in understanding the physics of the different planetary environments. As quoted, the advance of the perihelion of Mercury was immediately explained by the theory of general relativity in 1915. Planetary imaging was constantly improved by better quality telescopes and photographic techniques. The combination of the best sites and great observational skill produced remarkable images, but before the space age and adaptive optics, despite larger telescopes, the quality of these images remained unavoidably limited by the turbulence of the Earth’s atmosphere. Improved photometric techniques and understanding the thermodynamics of radiation made possible determination of the temperature of the superficial layers of planets and other bodies and to relate it to their physics. However, this analysis remained limited. For example, the true speed

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of rotation of the solid surface of Venus was only determined in 1962 using radar. The development of spectroscopy made considerable progress in analyzing the composition of the outer atmosphere of the different planets and gaseous compounds of comets. Yet we cannot say that all this had upended our understanding of the Solar System. Extending our knowledge of the Solar System beyond Neptune was more fundamental. It began in 1930 with the discovery of Pluto. For three quarters of a century, Pluto was considered the ninth planet, despite its peculiarities. Its orbit around the Sun, just beyond that of Neptune, is very elliptical and far from the plane of the ecliptic. While astronomers expected to find a massive giant planet comparable to Uranus and Neptune, Pluto is obviously rocky in nature and its mass is very small (just a few thousandths of Earth’s mass, 10 000 times less than that of Uranus and Neptune). Despite the absence of a known massive planet beyond Neptune, there is no physical reason for the mass distribution of the Solar System disk to change abruptly at Neptune’s distance. It was predicted in the mid-twentieth century that the mass of the outer disk of the Solar System had to be distributed between a large number of small objects arranged in a ring in the ecliptic plane beyond Neptune – the “Transneptunian” objects or “Kuiper Belt”. After a long hunt, these objects were detected in the 1990s; 2000 are currently known and the number is increasing rapidly. Pluto belongs to this family; it is one, but not the most, massive. The International Astronomical Union has rightly decided in 2006 to no longer consider it a major planet. Note that the early discovery of Pluto is due to the fact that it is the most massive object of a sub-family of Transneptunian objects whose orbits are strongly disturbed by and synchronized to that of Neptune by a phenomenon of resonance, which brings them closer to us. The Kuiper Belt is also considered the likely reservoir of comets whose orbit are practically in the plane of the ecliptic. Despite the acceleration of their discovery and study, much uncertainty remains about Transneptunian objects, and their distribution in the disk of the Solar System. They should not, however, be confused with the objects of the “Oort” cloud, which are thought to be the reservoir of the new comets, and are much more distant and much more numerous (probably more than a hundred billion). The Oort cloud is also not confined to the disk of the Solar System but is spread in a spherical distribution. To summarize, if the first half of the twentieth century saw a modest increase in our knowledge of the Solar System, it is far from the revolution produced by the discovery of galaxies, the expansion of the Universe and understanding the physics and evolution of the stars. Nor can this contribution be compared to the leaps that the next half-century brought with in situ exploration of the Solar System and the discovery of exoplanets. Paradoxically, a major contribution of the early twentieth century to our understanding of the Solar System is the definitive abandonment of the belief in the possibility that they are inhabited by aliens, a dream of centuries. Curiously, as we saw in § 1.2, the first decades of the twentieth century even saw a strong tendency to reject the model of flattened nebula proposed by Kant and Laplace from the eighteenth century, whose reality is now confirmed (box 12.1). Instead, essential contributions of the first decades of the twentieth century to better understanding planets concern the Earth: its age, plate tectonics and understanding

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the Earth’s internal structure, nucleus and magnetism. This knowledge strongly informs comparative planetology and our understanding of the Solar System.

12.3

Humans Went to the Moon!

Our generation has seen humanity’s relationship with the world of planets radically change since we were able to venture there, through multiple probes, cameras or robots, or infinitely better by the realization of the forever dream of leaving the Earth and walking on the Moon. Of course, before this achievement, our imagination, stimulated by the revelation of mountains and craters on the Moon by Galileo’s telescope, could build phantasmagorical visions of planetary landscapes. But, since 1959 (Luna 127) and especially 1969 (Apollo) we have entered the era of direct exploration first of the companion of our nights, then of all the bodies of the Solar System. Moreover, we have the feeling, somewhat deceptive to tell the truth, that we are eventually able to get out of the jar of our planet. This feat requires remarkable mastery of the necessary technologies, whether rockets, aeronautics and its materials, telecommunications or emerging informatics. These technologies are the result of the enormous efforts of the Second World War and the Cold War. Landing on the Moon was the culmination of the fierce strategic competition in all areas, especially in space, between the United States and the Soviet Union. In parallel with the NASA program – Ranger, Surveyor, Lunar Orbiter – the Soviet Luna missions were the first, between 1959 and 1966, successively flying over the Moon, crashing on its surface, orbiting and photographing its far side. The Soviets landed two automatic rovers that explored the surface of the Moon over several tens of kilometers for several months. They also managed to automatically return to Earth about 300 g of lunar rocks. The American program followed these steps a few years later. Then the Apollo program dispatched the first men around the Moon in 1968. There were six missions inaugurated by the historic Apollo-11 Moon landing on July 21, 1969 (figure 12.2) of human on-site exploration of the Moon. The crews of Apollo made different measurements on the lunar surface; they left various devices which continued these measurements and transmitted their results to the Earth during several years. Above all, they collected a large sample of lunar rocks taken from sites of different geology and history. The exorbitant cost of the operation explains why it has not been renewed fifty years later28. Nevertheless, the robotic lunar exploration has actively resumed during the last decade from orbit and even landers and rovers, by all the major space powers (United States, China, Europe, Japan and India). The impact on our collective imagination of Apollo landing on the Moon magnified by its live broadcast to a large part of humanity. Beyond the media blitz, these images convinced the whole world that we had entered the space age. In parallel, the 27

Soviet probe Luna 1 was the first to escape the Earth attraction and fly over the Moon in 1959. The huge technological investment of the Apollo program has been made highly profitable by spinoffs not only in space and military but in many high-technology fields.

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scientific contributions of exploration of the Moon are vast and crucial. Yet, the few days spent by men on the Moon proved inefficient compared to purely robotic exploration. The justification for the enormous difference in price is evidently in the symbolic power of the arrival of humans on the Moon. The automatic missions of recent years have significantly advanced the knowledge of our satellite. Yet, it is likely that crewed missions will resume in the coming years, despite their cost. Their motivation remains largely symbolic, triggered by competition between nations, but the Moon is also an important goal for the development of human space missions, as an ideal test bed for flight techniques and the installation of space stations. For astronomy, some craters on the far side or pole regions of the Moon may be exceptional observation sites when telescopes can be established. Although its cost may prove prohibitive, it is possible that such a project could be carried out during the course of the century.

FIG. 12.2 – Landing on the Moon. The arrival on the Moon in 1969 of astronauts from NASA’s Apollo mission remains one of the great events of the twentieth century and the history of humanity. The vast cost of such an operation has prevented its renewal so far. However, its technological rewards have matched the investment and its scientific return has been considerable because of the analysis of the composition of hundreds of kilograms of lunar rocks returned to the Earth. Credit: NASA.

12.4

We Broadly Understand the Origin of the Moon and Its Importance for the Earth

Since its phases punctuate our nights and were noted since the birth of humanity, the observation of the Moon goes back to our origins. A good part of the calendars is based on its cycle, the lunar month. Its influence on the tides must have been

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striking for our ancestors who lived on the shores of the oceans since most remote prehistory. The solar origin of its diffused radiation has been noted since antiquity by many civilizations. From the moment they were seen in Galileo’s refractive telescope, its mountains and craters became a reality. Since then, well before the space age, the surface of the Moon has been mapped and inventoried with much detail. These images were already remarkable around 1900 after the advent of large photographic plates. Spatial data, including the analysis of lunar rock samples, mainly from Apollo, provided a fundamental progress in our knowledge of the nature and age of these rocks, which gave remarkable information on the age, origin and history of the Moon. The Moon’s formation is radioactively dated about 4.5 billion years ago, some tens or hundreds of millions of years after the formation of the Solar System about 4.56 billion years ago. The analysis of the lunar samples shows a composition in different elements and their isotopes surprisingly similar to the Earth and significantly different from all other known samples of the Solar System. This led to revisiting the debate about the origin of the Moon. It was probably formed as a result of a huge impact on the Earth of a body about the size of Mars. Such an apocalyptic impact had to fuse the cores of both bodies and eject a good part of their superficial layers into the surrounding space. It is thought that these debris formed a disk around the Earth, where the particles gradually agglomerated by collisions until forming a single massive body, the Moon. The lunar internal structure is that of a differentiated body including a core, mantle and crust like the Earth. However, its iron-rich core is proportionally very small, which explains the low density of the Moon. Most of the volume is occupied by the mantle whose composition is dominated by magnesium and iron silicates. A thin layer, the crust, covers its surface. Plate tectonics is absent on the Moon in contrast with the Earth’s crust where it continuously acts to produce new landscapes. The surface of the Moon has thus kept a much longer memory of its history going back almost to its origin, so its appearance and shape are dominated by meteoritic impacts. Their craters are characteristic of the images of the Moon; there are more than 300 000 on the visible side. The majority date from the first billion years of the history of the Solar System where all members were subjected to meteorite bombardment much more intense than now. The most violent of these impacts caused intense volcanism evinced by large lava flows (“lunar seas”). Synchronization between the period of rotation of the Moon and that of its revolution around the Earth means we always see the same half-surface of the Moon while its other face is constantly “hidden”. It was a surprise, observing this far side on the first orbiting lunar probe photos, to see how much its appearance differs from the visible face with a virtual absence of lava seas and craters on all its surface. This probably reflects a difference in the thickness of the crust between the two faces, which itself might have been caused by a primitive giant impact. Our knowledge of the Moon has been enriched in many areas over space missions, especially in recent years, by gravimetric measurements of the deep structure, the demonstration of a very weak magnetic field (less than 1% of the Earth), the detection of the presence of significant amounts of ice below the surface, the study of the effect of cosmic rays on the chemical composition of the superficial layers, etc.

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Let us recall that the Moon is practically devoid of atmosphere, since the residual pressure on its surface is only a few millionths of a billionths (10−15) of the terrestrial atmosphere. Besides the possible influence of tides on the evolution of terrestrial life, the presence of the Moon has been essential to stabilize the axis of rotation of the Earth and thus limit the amplitude of climatic variations. It is possible that this influence was decisive for the success of life on Earth. It is also not impossible that the giant impact that had formed the Moon had other important consequences on the structure and evolution of the Earth. The uniqueness of the Earth-Moon pair in the Solar System suggests that it is a rare phenomenon in planetary systems. Such an unveiling of the Queen of our nights enriches the extreme importance that the Moon has always had for humanity since prehistoric times. In addition to its impact on the calendars and festivals of all civilizations, we would never stop wondering about the transmutation of the darkness of the night sky and the poetry it induces (to the chagrin of the astronomers!).

12.5

Very Rich Close-up Photos of All the Bodies of the Solar System

With the first successful flights to the Moon, it was obvious that the way was open to go further, to leave the Earth suburbs and launch our space probes, new caravels of planetary exploration, towards the depths of the Solar System. Whatever the cost, how could one resist the temptation to approach the ancient planets and unveil their mysteries. Very soon our cameras sent amazing close-up views of most bodies of the Solar System and constantly improved them for fifty years. We launched various measuring devices land on their ground, first brutally and then gently, and we sent robots capable of furrowing their surface. Very quickly, the various space agencies mastered the art of navigation in the Solar System. Trajectories and launch windows were optimized to reach the target body after a long trip that can stretch over many years, with clever detours to sling off encountered planets. Yet, we should not underestimate the difficulty of such expeditions, the severe limits imposed and the many risks and dangers to which they are exposed. The power of the rockets remains limited, as is the mass they can lift out of the Earth’s gravity to navigate the Solar System. We must therefore very carefully consider the allocation of this mass by serving first the vital functions of the probes. This ensures on the one hand navigation, solar power supply, protection against solar radiation and cosmic rays, etc. It is also crucial to ensure communications with the Earth to receive navigation and maneuvering instructions, even troubleshooting, and especially to transmit observation data. This is exacerbated in sophisticated missions. They are more and more resource-intensive for orbiting, landing, moving on the target or even returning samples to Earth. The scientific instruments carried by space missions en route to distant planetary expeditions are thus constrained by the imperative of miniaturization. Enormous ingenuity is applied to achieve this goal without compromising the reliability of the

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mission. The high proportion of failures that occurred in the first missions is a good illustration of the risks of such enterprises. Yet, the overall success of the Solar System exploration remains impressive. Throughout its 50-plus years of global exploration, missions have become increasingly sophisticated, deploying ever more powerful spacecrafts and executing complex and delicate sets of maneuvers, at distances so remote that communications management remains a central problem. The remote pilots of these probes are constantly demonstrating an amazing mastery to manage them from Earth, not only to successfully run the sequence of planned operations, but also to deal with the unexpected. As a result, more and more successful remote troubleshooting is taking advantage of the powerful computers embarked on the probes and the redundancy of the essential mechanisms to overcome their inevitable failures in the harsh space environment. A good example of such adventures is the ballet of the Rosetta probe around comet 67P/Churyumov-Gerasimenko and the thrilling landing of the Philae module on the comet core in November 2014 (figure 12.10).

12.6

Summary of Planetary Expeditions

Many excellent workse.g.124 and internet sites describe the many space missions launched towards the planets and their satellites in the last half century and provide syntheses of their results. We quickly review these missions’ contributions planet by planet. Mercury is the least studied of the four terrestrial planets because of its lower priority and the difficulty of approaching so close to the Sun. Its low mass and high temperature, due to its proximity to the Sun, explain the almost total absence of atmosphere. Because of its orbit-rotation locking, its surface temperature has a huge range of variation (−100 °C– + 90 °C between the night and the day sides). Yet, the presence of ice in regions close to the poles implies that they never see the Sun. Its cratered surface is reminiscent of the Moon and testifies to the absence of tectonics and the relative weakness of past volcanism. Two major characteristics of Mercury remain unexplained: the origin of the magnetic field, which is strong enough to create a magnetosphere, and the relative absence of fairly light elements such as silicon and aluminum. These are two major issues for future space exploration, such as the ESA-JAXA mission BepiColombo that is on the way to Mercury for an arrival in 2025. Venus (figure 12.3a–d), on the contrary, is of capital importance because of its relationship to the Earth. It is also the brightest planet in the sky, clearly visible at dusk or dawn, which made it an essential astronomical landmark of all civilizations. Its present properties, completely different from those of the Earth, show how slight differences such as the distance to the Sun, 0.7 times smaller for Venus than for Earth, can lead to radical divergences in planet evolution. Venus was visited by multiple Soviet and American probes, generally placed in orbit around it and sometimes trying to land modules dropped through the atmosphere. However, the study of the surface and the lower atmosphere is made very difficult by the extreme

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conditions of temperature and pressure prevailing there. The heating produced by the relative proximity of the Sun has caused the greenhouse effect to diverge, provoking the evaporation of the water and its almost total escape from the planet, and preventing CO2 trapping in carbonates unlike the Earth. Most of the information collected on our sister planet comes from telemetry from probes in orbit. The surface has been totally mapped by radar techniques. Unlike Mercury and the Moon, an essential characteristic of Venus, as of the Earth, is the low number of meteoritic

FIG. 12.3 – Images of Venus and Mars. (Top row) Space images of Venus. Venus is in many ways the most Earth-like planet (see table 12.1), but its atmosphere (95% CO2) is nearly 100 times more massive. It is made opaque by thick clouds of sulfuric acid, SO2, ice, etc., rotating at large speed as we see in figure (b), and even reflecting the UV radiation (c). Because of the reflection of the Sun’s light, its appearance has phases as for the Moon and it is often the brightest star in the sky after the Moon and the Sun. It is also known as the “Evening Star” or “Morning Star”. Studying the interior of the atmosphere is made difficult by the high temperature, of about 500 °C at the surface. The Soviet probe Venera landed in 1979 taking pictures (a), but could only work for 50 min. Most of our detailed information on the volcano-dominated Venus relief comes from radar images produced by orbiting NASA and ESA probes. Figure (d) shows the reconstructed radar image of the large Gula Mons volcano, 3000 m high. (2nd and 3rd rows). Pictures of Mars and his landscapes. I. Global images of Mars showing: (e) Polar ice caps formed mainly of ice, with a small layer of dry ice (CO2) in winter; (f ) the gigantic Valles Marineris canyon (3800 km), cf. also (i). II. Some of the countless remarkable sights: (h) image taken during the first flight over Mars (Mariner 4, 1965); (g) first color image taken from the ground (Viking 2, 1976); (i) the gigantic Valles Marineris canyon, cf. also (f); (j) Mount Olympus, the largest volcano in the Solar System (27 km high, 650 km in diameter) of recent formation; (k) panorama taken by the Curiosity rover (2014, figures 2.12a and 12.4). Credit: (top line) Venera 9/NASA; NASA; NASA/JPL; (bottom lines) NASA.

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craters. It is deduced that the surface of Venus must be young (less than a billion years), which proves the existence of a recent volcanism because Venus is devoid of plate tectonics. However, the origin of the current ceasing of volcanism is not really understood, nor is the absence of a magnetic field, the planet’s retrograde rotation, its internal state, or its initial history. On the other hand, the more recent probes that flew over Venus gave us a wealth of detail by infrared spectroscopy on the composition of the atmosphere and its movements. In addition to CO2 (95%) and a little nitrogen, many minor components, including sulphides, are present in traces. Water must have been abundant in the past, while only very small amounts remain. Compared to the Earth wind regime, the circulation in the upper atmosphere of Venus is radically different, with an extremely rapid rotation rate that occurs with a four-day period. Half-a-dozen future Venus space missions by various space agencies are under development for a launch in the next ten years, such as the selected missions VERITAS* and DAVINCI + * (NASA) and EnVision* (ESA) aimed at deeply exploring the surface, the atmosphere and the interior of the planet and understanding how this similar world to Earth ended up with such a different fate, and the projects VENERA-D (Russia) and Shukrayaan (India). Mars (figure 12.3e–k) has been the main destination for planetary expeditions, extending the passionate interest devoted in this planet for more than 150 years. It combines similarity with the Earth – albeit smaller, colder and with a much more rarefied atmosphere – and accessibility due to its relative proximity and its tenuous and transparent atmosphere when sand storms are absent. Its exploration has been the purpose of more than 40 space missions, mainly by NASA, since the Mariner 4 probe in 1965 (figure 12.4). They followed the same progression as for the Moon without involving human flights: overflight, orbiting observatories, landing (Viking 1 & 2, 1976, figure 12.4, etc.), then exploration by robotic vehicles (“rovers”, figures 12.4 and 12.5b) over very large distances.

FIG. 12.4 – More and more sophisticated Mars exploration instruments (NASA): Mariner 4, the first spacecraft to fly over and photograph Mars in 1965; Viking 1, first landing in 1976; rovers, mobile surface exploration robots of increasing size (note the relative size of their wheels in the inset) (see also figure 2.12). Credit: NASA.

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Since Voyager’s first close-up images, we have been overwhelmed with images of the surface of Mars, each more amazing. Almost the entire surface of Mars and its reliefs have been mapped from orbit with a precision approaching that of satellite imagery of the Earth a few years ago. These images are complemented by millions of close-up special views photographing varied Martian landscapes, most often close to familiar landscapes of terrestrial deserts. This detailed survey of Mars has already accumulated a wealth of data (figure 12.3): the composition of its rocks and its geology, volcanism, erosion and meteorite craters, the composition of the atmosphere, its winds, seasonal climate variations, magnetic field, internal structure, etc. We can thus reconstruct the evolution of many of these parameters since the formation of Mars122. It is necessary to underline the extraordinary degree of sophistication now reached by the experiments and the analyses carried out on the surface of Mars by the various robotic devices embarked on the landers and the rovers (figures 12.4, 12.5, and 2.12). The exploration of Mars is remarkably advanced. The current state of our knowledge can be found in several recent references121,124,125,129. Some highlights may be noted. Some have analogies with the Earth: the temperature is only a little lower; the inclination of the axis on the plane of the ecliptic* produces seasonal variations of climate with cyclic deposition of ice caps and solid CO2 and strong winds; the landscapes are reminiscent of the terrestrial deserts, with enormous canyons and volcanoes; liquid water has circulated in abundance in the past, marking certain reliefs (figure 12.9a–d); the initial evolution could have been similar to the Earth over nearly a billion years, with significant presence of liquid water, plate tectonics and an intrinsic magnetic field, but these three characteristics disappeared after a few hundred millions of years. The subsequent evolution of this planet has been very different from that of the Earth, leading to opposite properties. The current atmospheric pressure is more than a hundred times lower than the Earth, although it must have been higher in the past; it contains 95% CO2 and only trace amounts of oxygen and water. The prolonged absence of plate tectonics has left large areas of very old rocks up to 3.8 billion years old that are very cratered by asteroid impacts. The absence of a massive satellite like the Moon allowed strong variations of the inclination of the axis of the planet and thus important climatic variations in the past. All this could explain the absence of important evidence of present life (§ 12.7) and the failure so far of all trace-of-life searches, even primitive bacteria, now and in the past. Half-a-dozen future Mars space missions by various space agencies are in development for a launch before the mid-1920s. Twice more many proposals are considered for the next decade, including the major NASA-ESA space mission Mars Sample-Return aiming at returning samples of the Mars soil onto Earth by about 2030. With Jupiter123 we pass into another realm, that of massive and gaseous “Jovian” planets (figure 12.6). Its principal properties known for centuries have fully confirmed the supreme majesty conferred by the ancients to Jupiter on the other planets. Not only is Jupiter one of the two brightest planets, just after Venus, but it is definitely the most massive. Its mass exceeds that of terrestrial planets like the Earth by a factor of at least 300 and that of Saturn by a factor of 3. Its distance from

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FIG. 12.5 – Images of the Mars2020 rover and the Insight laboratory (NASA). The Insight seismographs, SEIS (a), are operating on the surface of Mars since 2018 until about 2023. Mars2020 was launched in 2020 and the Mars2020 rover, Perseverance (b), landed onto Mars in 2021. The Mars2020 camera, Supracam, and Insight seismographs were provided by France (CNES). The Insight mission is a robotic lander designed to study the deep interior of the planet. By 2021, its seismographs had detected hundreds of “marsquakes” and provided new information about the components of Mars structure: crust, mantle and core. Mars2020’s key mission with Perseverance is to prepare containers of Mars soil samples for eventual recovery by a future mission of NASA plus ESA to return them to Earth. Several containers have already been prepared by the end of 2021. Credit: NASA.

the Sun less than the other Jovian planets makes it shine brighter in our sky. Above all, it formed in a denser region of the protoplanetary disk. This explains its higher mass, but also its wealth of satellites (79) that is more imposing than all the other planets. It is also believed that it has had a decisive effect on the present peculiar structure of the Solar System, especially because of its limited migration. Since 1973, Jupiter has been visited by a half-dozen NASA spacecrafts. This includes Galileo, which remained in orbit around Jupiter from 1995 to 2003. These probes provided many close-up images of the planet and its satellites as those of figures 12.6 and 12.7. The latest, Juno, has been around the planet since mid-2016, with the main objectives being to study the composition and structure of the atmosphere, the magnetic field, the magnetosphere and the gravitational field to understand the internal structure. Juno has already produced a multitude of detailed images, including pole regions and their powerful auroras borealis (figure 12.6a), which are similar to the Earth (figure 10.10) but stronger. Jupiter’s satellites constitute a veritable miniature planetary system with four of the six largest satellites in the Solar System. These satellites all have interesting peculiarities that justify dedicated studies. Io is remarkable for its intense volcanism induced by tidal effect by nearby Jupiter. The low density of the three others, Europa, Ganymede and Callisto, attesting the presence of large quantities of solid or liquid water, is confirmed by their icy surface (figures 12.7 and 12.9), but Europa is

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FIG. 12.6 – Jovian planets. (a) Picture of Jupiter. In addition to the classical band structure showing the rotation of the upper layers of the atmosphere, as well as the large red spot, seat of a gigantic cyclone, we see a spectacular blue light (actually observed in the ultraviolet) above the North Pole. It emanates from a powerful aurora borealis, related to the trapping of charged particles of the solar wind in the intense magnetic field of the planet. (b) Saturn and rings. (c) Comparative schemes of internal structure. It can be seen that between the rocky core and the gaseous surface, dominated by H2 molecular hydrogen, different layers are intercalated: metallic hydrogen under high pressure in Jupiter and Saturn; ice in Uranus and Neptune. Credit: NASA; NASA/JPL. more subject to the tidal forces of Jupiter, so models predict that it could be made up largely of liquid water. Needless to say, this has captured the attention of astrobiologists (§ 12.7). Two major space missions, Europa Clipper* (NASA) and JUICE* (ESA) will be launched in the mid-2020s to visit Jupiter’s satellites by 2030. Saturn has been less visited because its great distance makes space missions difficult and expensive. Until 2004, this planet and its system of satellites (figure 12.8) and rings had received only three flights of a few hours by the probes Pioneer 11 and Voyager 1 and 2 of NASA between 1979 and 1981. They provided spectacular global images. However, the exploration of the Saturnian world was radically improved over the last decades by the Cassini-Huygens mission (NASA jointly with ESA, figures 2.11 and 12.8). Launched in 1997, this spacecraft was active in orbit around Saturn from 2004 to 2017. It achieved all its objectives by supplying spectacular amounts of data and images on Saturn, its rings, its magnetosphere, and its satellites, especially Enceladus and Titan. The latter is one of the most massive moons in the Solar System and it is the only one with an atmosphere. The European lander Huygens touched down successfully on its surface and transmitted amazing close-up images and data. The contributions made by the Cassini-Huygens mission to our knowledge of the fascinating world of Titan are

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FIG. 12.7 – Satellites of Jupiter (four main satellites with their relative sizes: each is comparable to that of the Moon). While the yellow color of the soil of Io is due to the sulfur ejected by its very active volcanism, the other three have an ice surface (figure 12.9). The bottom row of images shows details of this surface: cracks and ice packs on Europa; waves of pack ice on Ganymede; very old surface of Callisto, as inferred by the presence of many craters of meteorites, where tidal effects are weaker because of the greater distance of Jupiter. Credit: NASA.

FIG. 12.8 – Satellites of Saturn. We see on the left their relative sizes compared to those of the satellites of Jupiter. The figure also shows details of the two most interesting satellites observed by the Cassini mission. Above: (1) Huge fractures crisscrossing the ice floe that covers Enceladus, as well as its geysers of liquid water (in the box); both phenomena must come from the tidal effects of near Saturn. (2) Orange color given by the carbon compounds to the opaque gas atmosphere of Titan. Below, radar images of Titan’s surface showing lakes, probably liquid methane (CH4). Credit: NASA.

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impressive. While the aerosol opacity of the atmosphere radically limits visible light imaging, radar imagery revealed the astonishing features of its icy surface littered with carbon compounds. Although relatively smooth, its surface is very complex possessing small mountains, canyons, dune fields, and pseudo-volcanoes. The importance of liquid methane (CH4) is particularly striking. It can form many large lakes (figure 12.8) fed by rivers, whose canyons may have been dug by runoff following heavy rains of liquid methane! Their even greater distance from the Earth explains why Uranus and Neptune have not yet been the object of dedicated space missions. The two planets nevertheless benefited from a close visit by the Voyager 2 spacecraft, respectively, in 1986 and 1989. This considerably increased our knowledge about these two planets, for example, their rings, their satellites, their magnetic field, and their meteorology. The discovery of new satellites and rings has significantly increased their numbers and Triton, the large Neptune satellite, was specially studied.

12.7

Searching for Life in the Solar System: Where and When?

Since the beginning of the space age, the search for traces of extra-terrestrial life remains one of the major issues driving exploration of the Solar System. This is obviously part of the general problem of the quest for extra-terrestrial life in the Universe, but at a much more accessible level. The proximity of the planets and other bodies of the Solar System and the possibility of direct analysis of samples of their material could allow us to detect even miniscule traces of current or very old life. This begins to be made much more efficient by the possibility of significant sample return to the Earth for comprehensive analyses. Such a capability offered by the proximity of the bodies of the Solar System could much more easily achieve results than exobiological searches in the far more distant exoplanets. The latter has indeed a chance of succeeding only if life has had a major impact on the global properties of the planet, such as the radical modification by biota (oxygenation) of the composition of the Earth’s atmosphere. However, even if one finds traces of life on a Solar System planet or one of its satellites, the impact of such a sensational discovery could be limited if it turns out that it is a form of life based on the same genetic code as ours. This would be proof that such an emergence of life in the Solar System was not independent of that on Earth. The probability of a common origin for all life that could have developed in the Solar System seems rather high since the planets of the Solar System are far from being completely isolated bodies. This is attested by the dozens of meteorites recorded on Earth, cataloged as “Martian”. We are pretty sure that they come from Mars after probably being ejected in violent impacts of asteroids on that planet. Nevertheless, the conditions for evolution of the planets or satellites where one can still hope to find at least traces of past life can strongly differ from the Earth. The discovery of the same type of life under different conditions and the revelation of the characteristics of its evolution would be a fantastic contribution to our understanding of primitive life on Earth.

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Box 12.1 – Formation model of the Solar System Main steps (see box 4.1)  Accretion disk surrounding a protostar. Complex gas transport in the disk towards the star by tranferring the angular momentum through turbulence, magneto-hydrodynamic torques, wind from the disk, and loss of orbital energy.  Sedimentation of dust toward the mid-plane of the disk.  Coagulation of dust grains. Growth in size during grain collisions; their surface is covered with ice in the outer regions of the disk, but not inside (luminous young star).  Planetesimals. Growth of solid particles, by coagulation (facilitated by the presence of ice) up to the size of small asteroids. Then their mutual gravitational attraction makes them grow by collisional merging, and they clear the dust from the regions close to their orbits.  Protoplanets. The growth of large planetesimals reaches the size of large asteroids and even the Moon in the inner regions of the disk, and several Earth masses outside.  Rocky planets and rocky nuclei of the gaseous planets form in collisions between protoplanets (the Moon is formed in a final collision between the Earth and a protoplanet).  Solar wind and formation of gaseous planets. The wind from the T Tauri* phase and the radiation of the star drive the gas out of the inner disk; in the outer regions the gas accretion by the protoplanets forms the gaseous giant planets.  Migration of planets. The instability of the Jupiter-Saturn couple’s orbit causes them to migrate through part of the disk. This deeply affects the entire Solar System: absence of planet between Mars and Jupiter and low mass of Mars, and very distant trajectories for Uranus and Neptune. But this migration remains limited and the presence of massive Jupiter protects the whole Solar System from strong instability.  Intense meteorite bombardment of all Solar System bodies due to orbital perturbations. The Moon and Mercury among others have retained very visible traces (craters) as a memory of this epoch. Open questions The complexity and chaotic nature of the processes still leave many questions:  Initial structure of the disk. The complex processes that shape it are far from well understood. The models are quite diverse and uncertainty affects all of the following processes.  Formation of planetesimals. The physics of solid particles and their collisions (agglutination or rebound) is not well understood.  Migrations. Modeling these for the Solar System remains difficult and depends on the detailed properties of the disk.  The absence of super-Earths (1–4 terrestrial radius) in the Solar System is surprising in light of the census of such exoplanets, but it could be related to the peculiarity of the Solar System and Jupiter.  How exceptional is the Solar System? Systems similar to Sun + Jupiter and Saturn are rare, while the Jupiter-Saturn pair seems to have been essential in shaping the Solar System.

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FIG. 12.9 – Water in the Solar System. (Top row). Evidence of the presence of water on the surface of Mars in large quantities billions of years ago. (a) Dry beds of old rivers (Mariner 9, 1971, NASA). (b) Water ice in crater at Martian North Pole (Mars Express, 2014, ESA). (c) Hydrated silicates highlighted by infrared spectroscopy (blue areas in the image) (Mars Express). (d) Delta river structure in the ancient meteoritic Jezero crater lake (MAVEN, 2014, NASA; colors arbitrarily represent lands of different compositions). Note that the rover Perseverance (figure 12.5b) landed in Jezero crater in 2021 and has already confirmed lake features. (Lower row). Internal structure of the main satellites of Jupiter and Saturn. The diagrams show the likely generality of layers of liquid water (“oceans”) beneath the icy surface and over other solid layers, ice under pressure and rocky core. Liquid water directly in contact with rocks, as possible in Europa and Enceladus, is especially interesting for primitive life. Credit: (top line) NASA; NASA/JPL/Caltech; ESA/Mars Express; (bottom line) Wikipedia commons; NASA.

Nonetheless, after fifty years of negative results, it is clear that the probability of easily finding life throughout the Solar System is smaller than first thought. The rich lunar samples examined in great detail, showed that there is practically no chance of finding life on the Moon. Despite initial hopes (and some premature announcements), forty years of robotic research on the surface of Mars have not yet yielded tangible evidence of even primitive life now or far back in the past. However, the evidence has accumulated on the current presence of large amounts of water near the surface of Mars, especially in rocks, such as hydrated silicates that are outcropping everywhere (figure 12.9c), and perhaps in permafrost beneath the surface. There is also ample evidence that part of this water could have been liquid in a remote past, sculpting the relief by its runoff (figure 12.9 a and d). It even seems possible that for a long initial period, hundreds of millions of years, the evolution of the conditions on Mars could have been rather similar to those of the Earth121. In such a context, life far in the past on Mars remains a major question. We now better understand where we are more likely to find traces, by digging in very ancient land whose properties

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have been preserved over billions of years and whose composition attests to a prolonged exposure to liquid water. Research will actively continue in this direction in the next decade. It will focus on sophisticated on-site analyses (figure 12.5) and especially the return of samples for ultra-high analysis on Earth. Another environment where we are almost certain to find huge amounts of liquid water is the large satellites of Jupiter (Europe, Ganymede and Callisto) and Saturn (Enceladus and probably Titan) (figure 12.9). We know that their surface consists of a crust of nearly pure ice, a kind of thick pack ice (figure 12.7). However, the models of their internal structures, including considerable heat sources from tidal effects and the radioactivity of their solid cores, show that it is possible to find one or more layers of liquid water just under this crust (figure 12.9). There must be considerable quantities of liquid water, true “oceans”, covering the whole surface of these satellites, with masses comparable to terrestrial oceans. The search for the presence of life, probably primitive, analogous to bacteria, in these “oceans” is one of the major objectives of the exploration of the Solar System. However, the difficulties are considerable because it is necessary to find a way to reach this liquid water despite the obstacle of the thick shell of ice. One possibility could be to take advantage of recent or old exchanges that might have existed between the internal liquid “ocean” and the icy surface. From this perspective, methane-rich Enceladus geysers (figure 12.8) could be a priority objective. Titan is especially interesting because it is the only satellite to have an atmosphere. That is quite similar to that of the Earth for the total pressure. It also consists mainly of nitrogen, N2. It is thought that this could be close to the atmosphere of the primitive Earth, including prebiotic molecules, before the enrichment of the Earth’s oxygen atmosphere by living organisms. However, the chances of finding developed life on Titan appear small because of its extremely low surface temperature, 94 K, −179 °C. The absence of detection of conditions favorable to life by the Cassini-Huygens mission (§ 12.6) goes in this direction. The small bodies of the Solar System are important targets for the exploration of compounds likely to be connected to the appearance of life. It has been known for some time that certain carbon meteorites, the most primitive, may contain an impressive collection of amino acids, nitrogen bases of DNA and other molecules considered pre-biotic. Their precise provenance is unknown, but these meteorites probably originate from the disintegration of primitive carbon asteroids or cometary cores. It is therefore thought that these small bodies are important reservoirs of such pre-biotic molecules in the Solar System. Such molecules are likely partly survivors from the interstellar gas and dust from which the Solar System was formed. The first samples of cometary matter analyzed, in particular by the Rosetta probe for comet 67P/Churyumov-Gerasimenko and its abundant carbonaceous material (figure 12.10), point in this direction. We can therefore foresee a high priority in the future for space missions aimed at analyzing in detail the composition of the carbon component of the variety of cometary cores or carbonaceous asteroids, either in situ, or especially by the return to Earth of important samples of matter, as carried out by the JAXA* mission Hayabusa2 in 2020. It is thus hoped, through the discovery of this pre-biotic component of the least transformed bodies of the Solar System since

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FIG. 12.10 – Rosetta and Philae. Meeting a comet. After a long 10-year journey through the Solar System, the Rosetta probe (figure a), launched in 2004, had an appointment with comet 67P/Churyumov-Gerasimenko. Having joined the comet core (a and b) in 2014, Rosetta accompanied it until 2016, closely observing its core and the evolution of the chemical composition of its degassing during it passage near the Sun. The highlight of the observations was the dropping of the Philae module, packed with scientific instruments, which landed on the surface of the comet core. Although the operation of Philae’s instruments was interrupted after three days for lack of energy, the results of Rosetta and Philae were quite remarkable. We can cite, among other things, (i) close-up images of the comet’s core similar to those of figure b showing a global view of the comet’s core (length about 7 km) and figures c and d, close-up views of reliefs on its surface; and (ii) the demonstration of a more carbonaceous core composition than expected, with the presence of numerous pre-biotic molecules including glycine, a key amino acid. Credit: ESA/Rosetta; ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.

its formation, to understand the origin of these molecules and their connection with the primitive Earth and the interstellar medium. This is a unique way to unravel the development of molecular complexity at the time of formation of the Solar System and its role in the origin of life.

Chapter 13 Entering the Dream World of Exoplanets 13.1

Explosion of Discoveries of New Planets29

The actual detection of planets around stars other than the Sun, the exoplanets, was achieved in 1995 and is now one of the most active sectors of astronomy. This capital discovery had been awaited for centuries, in fact since Copernicus and his successors clarified the nature of planets and stars almost 500 years ago. However, it is no coincidence that the exoplanets were finally found at the very end of the 20th century after a stubborn quest, taking advantage of the extraordinary technological progress that was then made. It must also be emphasized that this success initially benefited from the unexpected existence of very specific planets that are much easier to detect than the planets we know in our Solar System. It is remarkable that, immediately after the discovery of the first exoplanet in 1995 around 51 Pegasus by Michel Mayor and Didier Queloz (who were awarded the Nobel Prize of Physics in 2019), the number of new planets increased significantly in last two decades, with, by the end of 2021, a total of about 5000 exoplanets confirmed. The great diversity of successful discovery methods should be stressed. It has been known for a long time that the direct detection of the dim light emitted by a planet orbiting a star lost in the sidereal immensities is a seemingly impossible task (but, see figure 13.3), as it requires overcoming the contamination of the stellar light that is typically at least a hundred million times brighter than that of the planet. Overcoming this major hurdle is at the basis of the three main current and subtle methods that have enabled the discovery of most exoplanets. The first detections of exoplanets took advantage of the remarkable precision with which one can measure very small variations of the speed of a star thanks to the Doppler* shift of its spectral lines. Planets do not orbit precisely around their star,

29 For this topic in full revolution, it is strongly recommended to refer to very recent dedicated referencese.g.127, exoplanets.org which give a detailed but accessible view of this key question of the current astronomy.

DOI: 10.1051/978-2-7598-2706-0.c013 © Science Press, EDP Sciences, 2022

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but it is the system of the star and its planets that do orbit around the center of mass. As the star is much more massive than the planets, the center of mass of the system almost coincides with the center of the star but not exactly. The star moves on a small orbit around this center. The velocity in this orbit is very small, typically one thousandth that of the planets. Its detection, therefore, requires an extraordinary precision on the measurement of the Doppler shift of its spectral lines. By the late 1980s, we were not far from being able to detect such tiny movements of the stars closest to the Sun, provided they have massive planets close enough to them. This justified undertaking long-term programs to accurately measure the movement of the most promising nearby stars. These efforts were crowned with success in 199530 by the discovery that one and then several stars displayed small periodic movements that could only be explained by the presence of at least one planet orbiting around them. In fact, this key discovery was greatly accelerated by the unexpected existence of some systems where planets, at least as massive as Jupiter, are surprisingly very close to their star (as close as Mercury is to the Sun). At such close distance, they orbit very rapidly (figure 13.1), and the detection of the perturbation of the star’s motion becomes easier to measure. After this fundamental discovery, the hunt for exoplanets was open. The importance of this area has justified dedicating certain large telescopes and building several space observatories for this purpose (figure 13.2), not to mention the more punctual use of all best telescopes.

FIG. 13.1 – Exo-planets and detection methods. Mass and orbital period of known exoplanets, with colors indicating the detection method; for clarity, the many detections by the Kepler* satellite are omitted in the figure on the left, while they are added to the right. The methods by variation of the radial velocity (for the big planets) and by transit (especially with Kepler) completely dominate the current detections. Credit: IPAC/Caltech/NASA. 30

First around the star 51 Pegasus by M. Mayor and D. Queloz (Physics Nobel Prize 2019) and then confirmed around other stars and by G. Marcy’s group.

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As shown in figure 13.1, the initial method of measuring radial stellar velocity variations first remained the most effective method for discovering new exoplanets and analyzing multiple systems around the same star. This method was later complemented by the detection of the slight dimming of the stellar light when the exoplanets pass exactly between us and their star. During such “transits”, the exoplanets block typically, at most, about 1% of the light of the star. Their presence can therefore be detected by monitoring and measuring accurately the brightness of the star and its variations. Although the probability to have the plane of the planet’s orbit aligned with the direction of the star seen from the Earth is pretty small, this technique proves to be extremely fruitful if one can simultaneously monitor a large number of stars. This technique is behind the remarkable success of the satellites that have been dedicated to this research, namely CoRot, TESS and, especially, Kepler (figures 13.1 right and 13.2). Reaching the needed hyper-accurate stellar photometry (  0.01%) is generally only feasible with space telescopes (but see figure 13.3), even small ones, as they are unhindered by the disturbances of the Earth’s atmosphere.

FIG. 13.2 – Exo-planetary space missions, including projects under construction. Half of them are missions mainly dedicated to the search for exoplanets (CoRoT (2007–2014), Kepler (2009–2019), Cheops (2017), TESS (2018–2022), Plato (2025), Ariel (2029). Most (except WFIRST-Nancy Grace Roman Space Telescope, ca2027) mainly use the transit method. Credit: NASA. The above two methods promise rich future developments, including that of transits with the GAIA space mission (§ 6.3) which monitors a billion stars.31 In the meantime, plans are already being made to use significant resources to develop other 31

GAIA will also detect tens of thousands of planets, mostly giant gaseous, by astrometry.

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techniques that offer special advantages. One of the most promising is the detection of very small exoplanets around distant stars by their gravitational lens effect* (figure 7.3). This effect produces a brief amplification of the brightness of one of the billions of distant stars of the Milky Way when an exoplanet of a closer star accidentally crosses its line of sight from the Earth. Such events are very rare, but they are relatively easy to detect by continuously observing a very large number of stars and they have the advantage of being able to find planets of small mass comparable to or less than the Earth. The number of exoplanets detected so far by this method is still relatively small (figure 13.1, although it is already much larger than that seen in this figure). However, major projects are under construction or planned (such as WFIRST), whose objective is the detection of such exoplanets throughout the Milky Way through microlensing using the infrared to reduce the absorption by interstellar dust. In addition, enormous resources are programmed with the goal to directly observe exoplanets, and some projects have already been successful (figure 13.3). This will pave the way for numerous in-depth studies, including spectroscopy of the exoplanet’s atmospheres. This extremely difficult objective depends on two main conditions. The first condition is to achieve a very great sensitivity in order to detect such pale bodies as planets at distances of ten light-years from the Sun, which requires the new generation of planned giant telescopes of 30–40 m diameter (figure 2.1b). The second imperative condition is to free oneself from the dazzling much brighter close-by parent star which can be achieved by combining a very high angular resolution, using sophisticated adaptive optics (§ 2.2), and implementing

FIG. 13.3 – Direct images of exoplanets. (left) First image of an exoplanet (with the VLT*/ ESO, 2004), around a brown dwarf* 230 light-years from the Sun. The low brightness of the brown dwarf and its great distance from the planet (55 times the distance from the Earth to the Sun, twice that of Neptune to the Sun) facilitated this detection by direct imaging. (right) First multi-planet system to be directly imaged around a Sun-like star (VLT/ESO, 2020). The two planets (TYC 8998-760-1 b and c) are massive, bright, young and very far from their star (about 30 and 60 times the distance from Jupiter to the Sun). Credit: ESO.

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various devices to occult the star. Such systems are already successfully used on very large existing telescopes such as the VLT (figure 13.3) and will be installed on the planned ground-based 30–40 m class telescopes, as well as in future space missions.

13.2

The Majority of Stars have a Planetary System

The analysis of the still recent information on exoplanets already provides us with a rich vision of this new major field of astronomy. The  5000 planets already identified (late 2021), often by several complementary methods, are sufficient to provide us with fairly reliable statistical data on the frequency of planets around stars in the Milky Way and, by extrapolation, in all galaxies. We can already say that at least half of the stars not too different from the Sun, and probably the majority of all stars in the Milky Way, have a system of planets orbiting around them in the same plane, like the ecliptic of the Solar System. They have a distribution of distance to their star, temperature, mass and composition that somewhat resembles the Solar

FIG. 13.4 – Proto-planetary disks. Stunning, ALMA* high-resolution images of 20 nearby protoplanetary disks (similar to that of figure 4.1a) observed within ALMA DSHARP collaboration. The images reveal a striking diversity and surprising structures, including prominent rings and gaps, which appear to be the hallmarks of planets. These observations allow astronomers to better understand how planets form, including earth-sized ones. Credit: ALMA DSHARP collaboration.

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System. However, it is necessary to emphasize the extreme variety of planetary systems that have been identified, as discussed below (§ 13.3). If the Sun is an ordinary intermediate-mass star, and if its planetary system also seemed relatively ordinary at first sight, it now appears more and more exceptional for the peculiarities of the Jupiter–Saturn history (box 12.1), the Earth–Moon system and the absence of a key class of exoplanets, namely the super-Earths. Even if it was anticipated, it is useful to stress the importance of the recent results on exoplanets which demonstrate the almost universal presence of planetary systems around the stars. As a result, the number of planets is probably several hundred billion in each of the one hundred billion galaxies or so in the accessible Universe! The planets are likely even more numerous than the stars, since the majority of the stars must have a system of several planets. It is truly fascinating to realise that we have just entered into the vast and diversified world of planets around stars other than the Sun, at about the same time when all the planets of our Solar System became more and more familiar as a result of their in-situ exploration by space probes. This ubiquity of planetary systems a posteriori validates the model for the formation of the Solar System (box 12.1) from the coagulation of dust in the pre-planetary disc to form increasingly large planetesimals. If this model holds for the planets of the Solar System, there is no reason why it does not work to form planets around stars that are more or less similar to the Sun (we have other proofs, such as the generality of pre-planetary discs, figures 4.1a and 13.4). On the contrary, the study of multiple planetary systems will help us understand the peculiarities of the Solar System and the formation and evolution of each of our planets. Above all, the broad interest of exoplanets is because they are the focus of the quest for extra-terrestrial life (box 13.1). With the confirmation that planets are common around stars, a very important step has been taken on the long road to understand the situation of life in the Universe. In particular, it opens the way to the next steps including: (i) the systematic search for “habitable” planets that could look like the Earth; this is already under way (§ 13.4); (ii) in the future, the study of their properties to detect any indication of life, such as an oxygen-rich atmosphere or seasonal changes in the planets color similar to that induced by vegetation on Earth.

13.3

Surprising Variety of Exoplanets

A major surprise of the known exoplanets is their extraordinary variety and especially the diversity of their distances from their star. These distances are often quite different compared to the Solar System, but can we consider the Solar System to be normal? This is also a problem for the simplest models of their formation in pre-planetary discs. Although this diversity is surely biased by the current limits of detection methods, it certainly represents a fundamental property of planetary systems and proves that the relative stability of our current Solar System126 is not general. This does not disfavor the general pattern of formation of planetary systems and the hierarchy of formation of small rocky planets and large gaseous discs as a function of the distance to the star in the pre-planetary disc. However, this process

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FIG. 13.5 – Variety of exoplanets. Artist views of exoplanets discovered by the method of transits by the Transiting Exoplanet Survey Satellite (TESS) launched in 2018 by NASA. During its four-year mission, it discovered more than 2000 new exoplanet candidates, with a great variety of sizes, masses, distances to their star and temperatures. Credit: TESS, MIT. must often result in the formation of unstable planetary systems because of interactions between the planets and the protoplanetary disc. Such instabilities are essential to explain the presence of massive planets very close to the star (“hot Jupiters”). Massive gaseous planets can only be formed under the conditions prevailing in the outer regions of the preplanetary disc. Their position near the star implies their subsequent migration through the entire planetary system that they must have strongly perturbed. Moreover, many factors must contribute to the variety of exoplanets and there is no reason that the eight examples of solar planets exhaust the diversity of planets that can be formed depending on the characteristics of the star and its protostellar accretion disc. In particular, the Solar System does not have “super-Earths” planets of about 5–10 Earth masses while they are very common elsewhere (see box 12.1). The luminosity of the star is obviously a key factor in determining the surface temperature of nearby planets and their ability to accrete and retain different atoms in their atmosphere. It has been proposed that massive “ocean” planets, completely covered with liquid water, exist under appropriate conditions and that they are numerous among the detected exoplanets (figures 13.1 and 13.5). Many other factors can enter in determining various properties of planets from their formation and evolution, such as stellar wind intensity, accretion disc conditions, and the proportion of heavy elements. The competition in the formation of the different planets plays a role in the distribution of those that finally emerge. This is seen in the Solar System where the great mass of Jupiter prevented the formation of an additional planet in between Mars and Jupiter.

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The Search for Earth-Like Planets

As we have mentioned on several occasions, the main focus of all research on exoplanets is to detect and study planets similar to the Earth, with the more distant goal of finding signs of the presence of life outside the Solar System (box 13.1). There are very strong arguments to think that life in the Universe should be based on carbon chemistry and liquid water. We know that it has succeeded on Earth. Therefore, the most obvious strategy to look for life outside of the Solar System is to focus on planets that are quite similar to the Earth where the water is mainly liquid form. A first rather crude criterion for this is to favor planets found at the right distance from their star, where the conditions are such that water can be liquid, and located in the so-called “habitable” zone. On the one hand, this distance is not enough to determine the surface temperature of a planet. As we know for the Earth, Venus and Mars, this temperature also depends on the radioactivity heat supply and therefore the mass of the planet, and on the very complex greenhouse and carbon cycle effects that determine the fraction of carbon trapped in carbonates and that in the atmosphere.32 On the other hand, other factors also seem to be decisive for the development of life, such as a magnetic field shielding against cosmic rays, the presence of a large satellite like the Moon, volcanism (which also ultimately depends on radioactivity), and plate tectonics. However, it is necessary to first make a full inventory of the exoplanets located within distances commensurable with the “habitable” area and, more specifically, those that are the most similar to Earth, i.e. rocky and with a comparable mass. This cataloguing has begun and the number of exoplanets already known in the “habitable” zone could exceed hundred (figures 13.1, 13.6 and 13.7). It is already confirmed that they are counted in the billions in the Milky Way and the number of identifications will increase very quickly with the various projects underway to reach this objective. As shown by the example of Mars and Venus, however, the range of conditions for liquid water is quite narrow, between the Venus greenhouse furnace and the Martian permafrost. It is, therefore, essential to obtain the basic information on the distance to the star, the mass and the radius of the exoplanet, and determine its surface temperature and the characteristics of its atmosphere, which makes it possible to predict the presence of oceans as on Earth. The discovery of signs of life on such a planet (or the demonstration of their absence on a sufficiently large number of these planets) is much more difficult. By far, the most promising route is the search for signatures of life on a planetary scale. We know for the Earth that the presence of a large quantity of oxygen was released by living organisms. Finding a spectral absorption signature of oxygen (or rather ozone which derives from it and is easier to detect) seems still difficult today, but it could be achieved in a few decades from now. Preliminary steps are already planned for spectroscopic analyzes of more massive

32

We know that for the Earth, more than 99.9% of the carbon is in the solid Earth (lithosphere), less than 0.1% in the oceans, less than 0.01% in the biomass and only about 2 × 10−5 (1/50 000) in the atmosphere. For Venus, the atmosphere and lithosphere each contain a significant fraction of carbon, so that the greenhouse effect has produced much higher temperatures.

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FIG. 13.6 – Habitable exoplanets. An artist view of some of the few dozens of potentially habitable exoplanets identified by 2020, with their distances from the Earth and their sizes compared to the planets of the Solar System. Credit: PHR, UPR Arecibo Observatory.

FIG. 13.7 – Trappist 1 planetary system. This system is remarkable for two reasons: it was discovered around an extremely dim red dwarf star (using the transit method and a small robotic telescope on the ground); and, above all, it has seven planets of size and temperature comparable to the Earth. The artist-view figure shows that all their orbits are much smaller than that of Mercury. This explains the probable similarity of temperature with the Earth since the star is much less luminous than the Sun. Credit: NASA/JPL/Caltech.

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gaseous exoplanets with space telescopes and ground-based 30–40 m telescopes, but without hope of finding life tracers in these purely gaseous environments.

Box 13.1 – Quest for extra-terrestrial life (exobiology)122,125,127,131 It is not possible in this book to present an in-depth discussion of a problem as complex as that of life in the Cosmos, especially as it is intimately related to the fundamental question of the conditions of the appearance of life on Earth, which, to this day, remains poorly understood. This question is more and more linked to the research on exoplanets, in fact representing its key motivation. The following box aims to give some benchmarks and provide the general context of the quest of extra-terrestrial life. Basic problem The definition of a “living thing” is not obvious, but it is more or less agreed that it consists of individualized entities in relation to, but isolated from the external environment, capable of self-maintenance and able “to reproduce”. The fundamental problem is that we only know one form of life, that which developed on Earth. Ignorant of the conditions of appearance of terrestrial life, which is probably contingent, or perhaps purely accidental, it is very difficult to imagine the characteristics and viability of other forms of life that could exist in the Cosmos, unless they are similar to ours. There is, however, a very broad consensus that life based on carbon chemistry and liquid water is by far the most likely. It combines the richness of the structures of organic chemistry with the incomparable exchanges allowed by the dissolution of all kinds of substances in water. It seems very difficult to avoid an elaborate system of transmission of information like our genetic code, in principle, but probably different in its realization. If we are interested in the development of complexity, or even the appearance of intelligence, various stages of complexification of organisms, more or less similar to that of the evolution of terrestrial life, seem mandatory, but they are uncertain and surely different from those we know on Earth. Relative stability of external conditions (climate, etc.) over very long periods may be necessary for the Darwinian evolution of the complexification. Number of planets where life and intelligence have developed An estimate of the number of planets in the Milky Way where life is present can be written as the product NT × fl where NT is the number of stars with at least one planet with favorable conditions for life – that is, similar to the Earth with the presence of liquid water – and fl is the fraction of these planets where life actually appeared and was maintained. With the current detections of exoplanets, NT is estimated at about 10 billion, with an uncertainty of about a factor of 10. This uncertainty seems negligible compared to our ignorance of the probability fl of life appearance, which in the current state of our knowledge, may as well approach unity (for microbial life) or zero.

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Regarding extraterrestrial intelligence, Drake’s formula has often been used in the last decades to estimate the necessarily smaller number of extraterrestrial civilizations (NC) in the Milky Way with which we might be able to communicate. The number of life-bearing planets is obviously an essential element of this formula. The current number of extraterrestrial civilizations NC can be written as NC = NT × fl × fi × (tC/3 × 109 years), where fi is the fraction of life-bearing planets that produced intelligent beings developing a technical civilization, and tC is the average lifespan of these civilizations, if one thought that the first technical civilizations could have appeared around 3 billion years ago. The values of fi and tC remain basically unknown. If the stages of evolution that led from the earliest forms of terrestrial life to our civilization seem retrospectively natural, it is possible that certain key passages are actually extremely difficult and therefore very unlikely over the course of a few billion year evolution of life on any planet. As regards the future of humanity and its descendants in a few millennia and, therefore, the value of tC, this is a completely open question. While an extremely advanced technical civilization should have acquired the means to colonize the whole Galaxy quickly enough, the reasons for the absence of evidence of extraterrestrial intelligence have been much debated, especially since the 1950s (“Fermi paradox”, which states the apparent contradiction between the lack of evidence for extraterrestrial civilizations and various high estimates for their probability.). One of the most logical answers would simply be that such highly advanced technical civilizations are nonexistent or very rare, perhaps because they are too brief. Another explanation could be that they reach a level of intelligence and behavior that so much exceeds ours that they have no interest to communicate! In a similar context, the “anthropic principle” has been proposed for half a century80. Based on various apparent coincidences in the values of the parameters and constants of the Universe, it argues that our very existence implies special values of these parameters. One of the scientific reasons put forward might be the existence of an infinity of “parallel universes” (“multiverse”) as foreseen by certain physical theories. Current research on the appearance of life and extra-terrestrial life Research in exobiology is in constant development, in link with appearance and development of life on Earth. This includes amongst other: characterization of the oldest forms of life from fossil traces in rocks to nearly four billion years ago; the reconstruction of the evolutionary characteristics of the primitive Earth, notably the transformation and oxygenation of its atmosphere by primitive micro-organisms; modeling chemical reactions that allowed the emergence of life and their study in the laboratory; the reconstruction of the evolution of the oldest micro-organisms; the study of life in the most extreme current conditions that may be related to those of the primitive Earth (extremophiles); etc. As we have seen, the major effort of exobiology is currently the search for evidence of life in the Solar System, especially on Mars (§ 12.7), and in the longer term on exoplanets (§ 13.4). This effort is accompanied by an extensive program to gather all the information on the pre-biotic conditions of the environments where life could have appeared, notably the pre-biotic molecules of the interstellar medium and the Solar System in formation. Particular attention is paid to samples of pre-biotic

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material that could have been preserved without major alteration, for example, cometary nuclei, asteroids, meteorites, interstellar molecules. The start of life on Earth by interstellar seeding from an exoplanet (“panspermia”) does not seem totally excluded. This might perhaps be supported by the confirmation we recently had that a few cometary nuclei come directly from the interstellar space and not from the Solar System, but it would only shift the question of the initial emergence of life further in the past on another planet, without much changing the fundamental questions related to it.

Conclusion

Chapter 14 A New Cosmos in the 21st Century? Mankind on the shores of another world At the end of this rapid survey of a century of prodigious exploration of the new Cosmos, let us try to summarize where we stand and to imagine a roadmap for astronomy of the 21st century.

14.1

A New Cosmos

The road traveled in the past century has been immense. The universe revealed to us in the 20th century is vastly richer than that of our great grandparents. First, not to mention the fact that the basic structures of the Universe are galaxies and not stars, the world of galaxies has multiplied their number in the visible Universe by a factor of about 100 billion (1011) and their volume by a factor of more than 1014. Of course, the 100 000 light-years of the diameter of the Milky Way, which was roughly the astronomical horizon a century ago, already gave an impression of infinity compared to the scale of our little planet Earth. Multiplying this scale by a million changes little. We remain absolutely minute on the scale of the Universe, as was already revealed to Galileo when he first pointed his telescope towards the sky. Yet, by contemplating and reflecting upon the new world of galaxies amplifies even more the vertigo that one feels when looking into the abyss in which we are immersed. We share the fate of hundreds of billions of galactic worlds, even if, with our current knowledge of physics, it is inconceivable that our descendants, whoever they are, will ever enter into communication with them. Nonetheless, the essential difference of our Cosmos lies elsewhere. We now know that our Universe has a history and we have been able to decipher it, revealing a surprising story for our common sense. It traces our ultimate origins back to the “infernal boiling of the cosmic soup” of the Big Bang universe from which galactic and star structures have sprung. This origin is revealed by the fossil traces it left in DOI: 10.1051/978-2-7598-2706-0.c014 © Science Press, EDP Sciences, 2022

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the physical state of our current universe. It unfolds before our eyes as we go back in time observing the most distant galaxies that are captured in their youthful state when they emitted the photons we detect today after a journey of billions of years. We have begun to appreciate the importance of understanding that the story of the life we came from on our tiny planet is entwined in cosmic history on almost the same time scales. These jumps in the spatial and temporal dimensions of the world in which we live are far from exhausting the extraordinary revelations of the richness of its content. We now very well understand the world of stars. We know the origin of their energy, how it conditions their structure and evolution according to fairly simple patterns, and how they synthesized the atoms we are made of. We have finally proven that the systems of planets which revolve around them are the rule, and that the Solar System is not an exception. This has fundamental consequences for our understanding of the origin of life. Yet, the variety of these planetary systems seems endless, like that of innumerable multiple stellar systems, but we also know that these “normal” stars are accompanied in almost comparable numbers by the pale brown dwarfs and by compact stars (white dwarfs) or hyper-compact (neutron stars), and even black holes. We know that black holes, from a few to billions of solar masses, are an important component of the Universe and we are witnessing the imposing spectacle of their mergers.

14.2

Auguries for 21st Century Astronomy?

After a century, and especially the last half-century, so glorious for astronomy that we can speak of a golden age, we naturally are led to ask the question of the future. For the cosmologist, the question of the ultimate future of the Universe seems to be settled for the most part. The standard model predicts that the expansion of the Universe will continue indefinitely, accelerating even under the pressure of dark energy. Galaxies, when they are not linked in groups or clusters, will become infinitely distant, ceasing to be fueled by gas and thus completely ceasing the formation of new stars. With the inevitable death of all stars, the Universe will proceed towards its ultimate fate of inert graveyard, populated with residue-cadavers of stars that are white or brown dwarfs, neutron stars and black holes. An esoteric alternative, nevertheless, arises within this nightmarish future in consequence of the possibility that the proton has a finite life of some 1031 years that would imply a final Universe consisting of only photons and neutrinos, but what is the point of worrying excessively about such Byzantine questions? It is so far, so infinitely beyond the horizon of humanity. If, instead, we pose the question of the future of this humanity to a horizon of a few millennia or even a few centuries and the eventual impact of the discovery or the absence of an extraterrestrial life, the answers appear so uncertain that such questions are definitely beyond the scope of this book. Let us therefore keep this sketch of the astronomical prospective on the same time scale as that of this volume, the century to come. Even here, we know, by

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comparing the predictions of the auguries of the Belle Epoque around 1900 to the reality of the explosion of astronomy in the twentieth century, that such an exercise is a nearly impossible task. However, let us try to list the questions that we can ask for the next decades, and then until the end of the century. The beginning of this exercise is quite easy. Research in astronomy is, characteristically, inertial. The biggest projects are decided or come out 20 or 30 years in advance. The roadmap of observational astronomy for the next decades is therefore well drawn. It aims mainly at key objectives that can bring decisive progress in the various fields, but more often than not, the objectives of the new large telescopes imply a broad front of astronomical knowledge. This is an essential foundation for breakthroughs in our understanding of the Universe. It also helps preserve the essential individual initiative of astronomers in the exploration of the complexity of the Universe, and the precious serendipity of accidental discoveries. Substantial progress is thus definitely planned (except for possible disaster in space projects or on Earth) in the next ten or thirty years in most of the major areas we have reviewed. The most promising area is exoplanets, which has a promise of spectacular progress, perhaps greater because of its youth. It is dominated by the capital stakes of exobiology. The number of known exoplanets will continue to increase and the discoveries of planets more or less comparable to the Earth will multiply. At the same time, the analysis of the fundamental properties of these exoplanets will rapidly progress, starting with the most massive and the closest to us. In particular, the developments in infrared spectroscopy will bring us closer to the grail of spectroscopic signatures of biological transformations of their atmospheres, such as oxygenation, similar to those undergone by the Earth’s atmosphere. However, I will not take the bet that this last objective can be definitely achieved before the middle or the end of the century. The other flagship area, cosmology, is assured of substantial progress in the constraints on the nature of dark energy (thanks, in particular, to the Euclid satellite, figure 9.4), on the distribution of dark matter and on the properties of the period called inflation through the polarization of cosmic radiation. Because of its complexity, the field of evolution and formation of galaxies holds a great potential for discovery and deepening. It will fully benefit from the panoply of advanced astronomy facilities in the first half of the 21st century, including the JWST (figure 2.7b), the giant 20–40 m ground telescopes, Euclid, ALMA (figure 2.5), and SKA (figure 2.14), as well as GAIA (§ 6.3) for the structure and history of the Milky Way. Variable phenomena will be constantly surveyed and might promise key discoveries. The world of high energies remains another particularly hot sector, with present and future facilities deployed for the detection of high-energy cosmic rays, gamma rays and neutrinos, and X-rays. The detection of gravitational waves generated by the merger of compact objects, black holes and neutron stars, has just left the realm of predictions to become almost routinely observational. It will be a key tool to study the physics of these phenomena and objects. This gravitational wave detection will be extended to the merger of super-massive black holes by space observatories, such as LISA (figure 11.4). We can also expect other important results in the

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exploration of the limits and extensions of the theory of general relativity, including space experiments. Even the stellar sector, including the Sun, can expect substantial progress in areas such as differential rotation, convection, magnetism and solar activity. Exploration of the complexity of stellar formation, terminal stages of stellar evolution and binary systems will also undoubtedly progress substantially. The continuation of this prediction exercise becomes much more uncertain and perilous beyond these few decades and current projects of astronomy. I will distinguish two levels of difficulty when one is trying to say something sensible about the evolution of astronomy toward the end of the century: the fundamental vanity of such distant predictions as we have learned from the experience of the past century, and the fundamental difficulty of the key questions that dominate the landscape of exobiology and that of physics and cosmology. First, the example of the twentieth century shows that the pace of evolution of technological development and discoveries is such that the situation a century ahead is virtually unpredictable. It is rather futile to try to imagine the state of astronomy in a century, in so far as it depends on progress that cannot be anticipated from technology and physics, without forgetting the enormous power of serendipity or accidental discoveries as it has happened in the past. Let us also keep in mind the uncertainties that hang over the future of humanity and its planet that could undermine the ambitions and magnitude of massive efforts for astronomical exploration. However, there is no doubt that all areas that we have overflown will have progressed decidedly on a broad front. It is safe to say that most of the current issues, which simply depend on technology without conceptual implications, will be overcome and more or less closed in a century from now. For example, I will place the following objectives in this category: the human landing on Mars (despite the limited scientific interest); advanced exploration of life forms in the whole Solar System; the existence or not of exoplanets with an oxygenated atmosphere like the Earth; the properties of sources of very high energy particles, γ-rays, neutrinos and cosmic rays; the properties of the various sources of gravitational waves; the detailed history of the Milky Way and its neighbors; the precise physics of accretion on super-massive black holes and the generation of powerful jets of radio galaxies; the initial formation of these super-massive black holes and the existence or not of a large population of such “bare” black holes in the intergalactic space. But we should not forget the key role of serendipity for discovering new problems and solving them or not. The second level of difficulty in predicting the evolution of our understanding of the Universe is that it depends essentially on a few key questions that we are today unable to really answer. As we have seen, this includes the nature of dark matter and dark energy, that of “inflation” and the physics of instants close to the origin of the Big Bang, and the prevalence of matter on antimatter. It is almost certain that the answers to these questions are beyond the limits of present-day physics. It is nearly impossible to predict what will be the type of answers and when they will come to us; maybe tomorrow for some, in ten years, in a hundred years or maybe never? We cannot dismiss the idea that they can lead us to question current physics by going

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beyond it or even by putting it in abyss, leading to strange concepts such as the multiverse (box 13.1) or the limits of the human brain to understand the Universe. At the end of this century of intense activity and extraordinary discoveries, the perspectives of astronomy remain immense, as they bring to forefront among the most fundamental questions for humanity. However, we should be careful to avoid any triumphalism. We remain fundamentally in the situation where we have been placed since Copernicus with the vision of our real position in an almost infinite universe. To reuse the beautiful image of Newton, we are still on the shores of the immensity and mysteries of this new Cosmos. We play with the shimmering discoveries of this new world and its dimensions always vastly exceed us. Experience has taught us that every time we have explored new corners of our universe, the result has surprised us. We are unable to imagine the questions on the Cosmos that could arise in a few centuries for our descendants. However, our thirst to immerse ourselves more and more in its mysteries remains unquenchable, because we feel that it contains an essential key to our future.

Glossary

Abundance. Ratio between the number of nuclei of a chemical element and the number of hydrogen nuclei. Accretion disk. Disk of gas (and possibly dust) accompanying and allowing the accretion onto a relatively compact object (star, black hole, etc.). Active galactic nucleus (AGN). Central region of a galaxy surrounding a supermassive black hole, the site of intense energy emission in the form of radiation or relativistic particle jets, related to the accretion of matter by the black hole. Adaptive optics. Compensation for the effects of atmospheric turbulence on astronomical images by deforming the mirrors of a telescope. Angular momentum. Vector product of the position R and momentum mV of a particle. Sum of these products for a set of particles. AU. Abbreviation for Astronomical Unit, equal to the average distance from the Earth to the Sun, about 150 million km. Anthropic principle. Statement that the parameters of the Universe, including those of physics, must necessarily allow for the development of life and intelligence, otherwise we would not be here to observe it. Asteroid. A solid body of a few hundred kilometers to a few meters in diameter orbiting the Sun. Baryon. Particles formed of three quarks: proton, neutron, etc. Baryon acoustic oscillations. Acoustic oscillations in the early Universe toward the end of the Big Bang which left their imprint on the distribution of galaxies. Blackbody. A body emitting “thermal” electromagnetic radiation whose spectrum is entirely defined by its temperature (figure 3.4a). Brown dwarf. A substellar astronomical object whose mass is too small to sustain hydrogen fusion (M