David Stevenson The Complex Lives of Star Clusters Astronomers’ Universe More information about this series at http://www.springer.com/series/6960 David Stevenson The Complex Lives of Star Clusters David Stevenson Sherwood , UK ISSN 1614-659X ISSN 2197-6651 (electronic) Astronomers’ Universe ISBN 978-3-319-14233-3 ISBN 978-3-319-14234-0 (eBook) DOI 10.1007/978-3-319-14234-0 Library of Congress Control Number: 2015936043 Springer Cham Heidelberg New York Dordrecht London © Springer International Publishing Switzerland 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifi cally the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfi lms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specifi c statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Cover photo of the Orion Nebula M42 courtesy of NASA Spitzer Printed on acid-free paper Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www.springer.com) For my late mother, Margaret Weir Smith Stevenson Pref ace Stars are fairly social beasts. They are mostly born in extended, open families and then stay together for most of their childhood. Low and intermediate mass stars tend to wander with increasing time, but many stars stay with their siblings right through to the moments they die. Of the types of relationships stars are born into, some are very tight and others much more relaxed. The densest associations are globular clusters, while the loosest are stellar associations. For some considerable time, it was thought that these stellar group- ings were distinct. However , as further evidence has come to light, it has become clear that the boundaries between these cluster groupings and the smallest dwarf galaxies have become blurred. Meanwhile, new types of clusters have emerged as technology has improved and more observations have been carried out. This book documents many of these discoveries, as well as new findings about the structure, origin and evolution of star clus- ters. For example, globular clusters were once thought to host a single population of stars. However, many appear to hold two or more generations of stars. How exactly is this possible, given the apparently small mass of these clusters? Are all globular clusters really star clusters, or are some the denuded remains of something much grander? As a result of containing dense assemblages of stars, clusters also allow relatively rare events to transpire that seemingly mock conventional rules of stellar evolution and engagement. Fairly massive stars can emerge from populations of low mass objects as a result of collisions or downright theft, while planetary systems might form and then degrade as stars harass one another. Some astronomers have come to question whether planetary systems could exist that might host life. Are conditions within clusters too severe to permit the acquisition of life, in particular intelligent life? vii viii Preface By considering the fates of stars, stellar partnerships and the star cities in which they reside as a whole, the outcome for life emerges—at least in outline. Many of these questions, and thus the story as a whole, will only be resolved with further observa- tion and a considerable amount of deliberation. Remember that as you are reading much of the current work is still contentious and clouded in considerable uncertainty. It will remain so until further observations are made. Sherwood, UK David Stevenson About the Author David Stevenson was born in Paisley, Scotland, in 1968, and stud- ied molecular biology at Glasgow University and a PhD in genet- ics from the University of Cambridge. After a stint in academia, he became a teacher, but continued to write science articles for vari- ous magazines including Sky & Telescope. His publications with Springer include Extreme Explosions (2014) and “Under a Crimson Sun” (2014), and he is at work on a new book about weather and exoplanets. Despite a background in Biology, David’s father inspired his interest in astronomy from an early age. Exploring the crossroads of different scientific disciplines is a key interest and motivation in both teaching and writing. David lives in Nottingham, UK, with his wife and family. ix Contents 1. Initial Observations ............................................................. 1 Introduction .......................................................................... 1 Historical Observations ....................................................... 1 Open, Globular or Simply Associated? ............................... 3 The Hertzsprung-Russell and Color- Magnitude Diagrams ............................................................................... 5 Luminosity Classes of Stars ................................................ 11 A Schism Between Open and Globular Clusters: Observational Bias? .............................................................. 14 How Star Clusters Revealed the Structure of the Milky Way .................................................................. 20 Conclusions .......................................................................... 23 2. Adventures in Stellar Evolution .......................................... 25 Introduction .......................................................................... 25 Star Formation: Standard Monolithic Models .................... 25 Discs and Jets ....................................................................... 27 Cracks Within the Monolith ............................................... 28 Realistic Models for the Formation of Star Clusters .......... 30 Starburst Formation of Clusters .......................................... 32 Cluster Formation and the Evolution of Galaxies ............. 32 The Lives of Stars ................................................................. 36 Brief Lives: An Overview of the Lives of Massive Stars ................................................................ 36 Intermediate Mass Stars ................................................... 43 Sun-Like Stars ................................................................... 48 The Fate of the Smallest Stars in the Universe ............... 51 Corpses ................................................................................. 53 Cosmic Recycling ................................................................ 54 Conclusions .......................................................................... 55 xi xii Contents 3. Variable Stars ........................................................................ 57 Introduction .......................................................................... 57 Low and Intermediate Mass Stars from Birth Through Middle Age and Death .......................................... 58 The Music of the Stars ......................................................... 61 A Varying Journey Through Time ....................................... 62 T Tauri Stars ...................................................................... 63 Delta Scuti Stars ............................................................... 63 Exit Stage Left ................................................................... 65 Semi-regular Variable Stars .............................................. 66 The Horizontal Branch ..................................................... 67 W Virginis Stars................................................................. 72 Mira Variables ................................................................... 73 OH/IR Variables ................................................................ 75 RV Tauri Stars ................................................................... 76 ZZ Ceti Stars ..................................................................... 77 Young, Fickle and Massive .................................................. 80 Herbig Ae/Be stars ............................................................ 80 Be Stars ................................................................................. 83 Supergiant Variable Stars ..................................................... 87 Luminous Blue Variables and Wolf-Rayet Stars ................. 90 Cataclysmic Variables .......................................................... 91 Conclusions .......................................................................... 98 4. Globular Cluster Formation ................................................ 101 Introduction .......................................................................... 101 The Stars of Globular Clusters ............................................ 101 Cluster Formation: A Reprise .............................................. 102 What Do Observations of