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The Evolution of Stars The Evolution of Stars The Evolution of Stars: From Birth to Death By Graham Hill The Evolution of Stars: From Birth to Death By Graham Hill This book first published 2020 Cambridge Scholars Publishing Lady Stephenson Library, Newcastle upon Tyne, NE6 2PA, UK British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Copyright © 2020 by Graham Hill All rights for this book reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owner. ISBN (10): 1-5275-5052-4 ISBN (13): 978-1-5275-5052-0 To Willem J Luyten who gave me the opportunity to become a professional astronomer and who, each day, demonstrated that age is no barrier to imagination and innovation. And to my wife Joanie, who has been my rock over many years, particularly over this last year. Frontispiece. See Centrefold. The Spirograph Nebula. Credit: NASA and The Hubble Heritage Team/(STScI/AURA) CONTENTS List of Tables ........................................................................................... xii Preface .................................................................................................... xiii Prologue ................................................................................................... xv 1 Background ............................................................................................ 1 1.1 What’s with astronomy? Who cares? 1.2 The nature of astronomy 1.3 The window to the Universe: The electromagnetic spectrum 1.4 Neutrinos 1.5 Gravity waves (GWs) 1.6 Cosmic Rays 1.7 A brief reflection 2 The sky and how we view it ................................................................ 27 2.1 Twinkles 2.2 Attenuation of starlight 2.3 Constellations: Not the only legacy from ancient times 2.4 Telescopes: What does a telescope do? 2.5 Refracting telescopes 2.6 Reflecting telescopes 2.7 The current crop of behemoths 2.8 Interferometers: Getting much more for your bucks 2.9 The astronomer's night-time world 2.10 Detectors: The astronomer's eyes 2.11 Advances in optics 2.12 Beating the atmosphere 2.13 Why are telescopes located in weird places? 2.14 Back to the “weird places” 2.15 Uses of computers in astronomy 2.16 Next steps viii Contents 3 Learning about the Sun ....................................................................... 67 3.1 Early days 3.2 Ancient book burning: The library at Alexandria 3.3 Getting started 3.4 Measuring the distance to the Sun 3.5 The size of the Earth 3.6 Aristotle’s views hold things up 3.7 The Copernicum revolution 3.8 Galileo and the telescope 3.9 Kepler 3.10 Newton 3.11 How far away is the Sun? The solar parallax 3.12 The velocity of light 3.13 The Sun’s size 3.14 Measuring the Sun's mass 3.15 The discovery of Uranus: The power of analysis 3.16 An instrumental interlude: The spectrograph 3.17 Spectral lines in the Sun 3.18 Spectra 3.19 The Doppler shift 4 Our first peek: The Sun as a star ....................................................... 94 4.1 Some solar details 4.2 At the Sun's surface 4.3 The solar atmosphere 4.4 What is the Sun made of? 4.5 The Sun's magnetic field 4.6 The solar cycle 4.7 Sunspots and the weather 4.8 Back to Sunspots 4.9 Solar seismology 5 The solar interior ............................................................................... 115 5.1 Origins 5.2 Hubble’s law and the fifth force of Nature? 5.3 Birth of the Sun 5.4 Sources of solar energy generation 5.5 The need for mathematical modelling: Modelling the Sun The Evolution of Stars: From Birth to Death ix 6 What’s out there? First glimmerings ............................................... 137 6.1 Double or binary stars 6.2 Measuring the distance to the stars: The elusive parallax 6.3 A sidebar: The development of mathematical analysis 6.4 Stellar photometry: Measuring brightness and variability 6.5 Stellar positions and motions 6.6 Brightness and colour 7 Stars are not just points of light ....................................................... 152 7.1 The growth of spectroscopy 7.2 The appearance of spectra: Absorption lines 7.3 Stellar classifications 7.4 The Hertzsprung-Russell diagram 7.5 The effects of gravity seen in spectra 7.6 More binary stars 7.7 An aside: The detection of extrasolar planets 8 Assembling our ingredients .............................................................. 178 8.1 Preamble 8.2 The mass-luminosity relation 8.3 Measuring stellar temperatures 8.4 Stellar size 8.5 Stellar densities 8.6 Variable stars 8.7 Abundances: What are stars made of? 8.8 A summary: Our bits and pieces 9 Snapshots of evolution ....................................................................... 199 9.1 A first look at evolution: Early days 9.2 Clusters of stars 9.3 The concept of zero age 9.4 Extending the distance scale 9.5 Cepheid variables: Stars that pulsate 9.6 RR Lyrae variables: Other stars that pulsate 9.7 A complication: Interstellar reddening or dust 9.8 The spacecraft Hipparcos 10 A pathway to diamonds ................................................................... 220 10.1 Boundaries 10.2 A cosmic jigsaw puzzle 10.3 Stellar nurseries x Contents 10.4 How do we evolve a model star? 10.5 From interstellar gas and dust, to the Sun 10.6 The main sequence Sun 10.7 Off the main sequence 10.8 The helium flash 10.9 Pulsating RR Lyrae stars 10.10 To the asymptotic giant branch 10.11 Smoke rings: The formation of a planetary nebula 10.12 From under the carpet 10.13 The end of the line: The formation of a white dwarf 10.14 Stars less than 0.075 solar masse 10.15 Stars less than 0.7 solar masses 10.16 A summary. What have we got so far 10.17 Complications: Surprise! Surprise! 11 On the fast track to oblivion ........................................................... 252 11.1 Helter skelter 11.2 Back to the story 11.3 The end of the line 11.4 More about supernovae 11.5 Summary 12 Resulting weirdoes, the products of evolution ............................... 266 12.1 Preamble 12.2 White dwarfs 12.3 Neutron stars 12.4 Black Holes 12.5 Brown dwarfs: Little stars that couldn’t 12.6 Missing mass: Yet more of it! 13 Prove it! ............................................................................................ 291 13.1 How do you know you’ve got it right? 13.2 The data 13.3 Obtaining cluster data 13.4 Generating the models 13.5 On the way to evolution: Generating a ZAMS 13.6 How do you match theory with observations? 13.7 The Cepheids 13.8 RR Lyrae stars and the Horizontal Branch 13.9 What can eclipsing binaries tell us? 13.10 White dwarfs The Evolution of Stars: From Birth to Death xi 13.11 Neutron stars and pulsars 13.12 Black holes 13.13 Summing up 14 And so they came, two by two ......................................................... 334 14.1 Binary star evolution: The Roche model 14.2 Modelling a binary system: Getting started 14.3 Detached systems 14.4 Back to detached systems 14.5 Semi-detached systems 14.6 Contact systems (W UMa binaries) 14.7 End-of-life variations 14.8 Detecting neutron stars in orbit 14.9 A brief wrap-up 15 Where is astronomy heading? ........................................................ 355 15.1 Big data 15.2 OGLE 15.3 Gaia 15.4 The behemoths: Ground-based 15.5 Satellite-borne telescopes 15.6 Telescopes: A census 15.7 A fantasy? The Overwhelmingly Large Telescope (OWL) 15.8 Where is astronomy heading? 15.9 My omissions 15.10 Last words Glossary ................................................................................................. 383 Journal Abbreviations .......................................................................... 417 List of Illustrations and Figures .......................................................... 418 LIST OF TABLES Table 3.1. A comparison of planet-Sun distances Table 5.1. The run of temperature, density and mass as a function of radius in the solar interior Table 7.1. A coarse summary of spectral classes Table 8.1. Some comparative mean densities Table 10.1. Mass limits and evolutionary destinations PREFACE “Twinkle, twinkle, little star How I wonder what you are Up above the world so high Like a diamond in the sky Twinkle, twinkle little star How I wonder what you are” This book had its genesis more than sixty years ago when I read W. M. Smart's book “Some Famous Stars”. It was, and still is, a wonderfully informative book on astronomy and was influential in setting me on my path to becoming an astronomer. Now, in my mature years, I've responded to the heeding of my wife who has long urged me to write about astronomy though not in a technical manner for I have authored and co-authored many papers on the subject. Her drive, and that of our children and close friends, was for me to write a popular book about the stars that might interest a public who have long been fascinated by astronomy. This is an interest only whetted further by the flow of results from the Hubble Space Telescope, the huge ten-metre-class ground-based telescopes such as the Keck twins on Mauna Kea, the quartet of eight-metre European telescopes in northern Chile and other more esoteric observatories currently in space. Unlike Smart who wrote about a few specific stars, I have chosen to write the story of the evolution of stars from birth to death, but not just of single stars. About 50% of the stars in the sky come in pairs that are gravitationally linked, just as the Earth and Moon are linked, as is the Earth-Moon system to the Sun. Because of this physical connection between stars, the possibilities involved in their evolution take some curious and very interesting paths, much as in our own lives which are more complex when lived with a partner. Most people know little about those little points of light out there we call the stars.
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