Physics SERIES IN ASTRONOMY AND ASTROPHYSICS LONGSTAFF SERIES IN ASTRONOMY AND ASTROPHYSICS SERIES EDITORS: M BIRKINSHAW, J SILK, AND G FULLER ASTROBIOLOGY ASTROBIOLOGY ASTROBIOLOGY: An Introduction Astrobiology is a multidisciplinary pursuit that in various guises encompasses astronomy, chemistry, planetary and Earth sciences, and biology. It relies on mathematical, statistical, and computer modeling for theory, and space science, engineering, and computing to implement observational and experimental work. Consequently, when studying astrobiology, a broad scientific canvas is needed. For example, it is now clear that the Earth operates as a system; it is no longer appropriate to think in terms of geology, oceans, atmosphere, and life as being separate. Reflecting this multiscience approach, Astrobiology: An Introduction • Covers topics such as stellar evolution, cosmic chemistry, planet formation, habitable zones, terrestrial biochemistry, and exoplanetary systems An Introduction • Discusses the origin, evolution, distribution, and future of life in the universe in an accessible manner, sparing calculus, curly arrow chemistry, and modeling details • Contains problems and worked examples and includes a solutions manual with qualifying course adoption Astrobiology: An Introduction provides a full introduction to astrobiology suitable for university students at all levels. ASTROBIOLOGY An Introduction ALAN LONGSTAFF K13515 ISBN: 978-1-4398-7576-6 90000 9 781439 875766 K13515_COVER_final.indd 1 10/10/14 2:15 PM ASTROBIOLOGY An Introduction Series in Astronomy and Astrophysics The Series in Astronomy and Astrophysics includes books on all aspects of theoretical and experimental astronomy and astrophysics. Books in the series range in level from textbooks and handbooks to more advanced expositions of current research. Series Editors: M Birkinshaw, University of Bristol, UK J Silk, University of Oxford, UK G Fuller, University of Manchester, UK Recent books in the series Nuclear Processing of Heavy Elements Hermann Beer An Introduction to the Physics of Interstellar Dust Endrik Krugel Numerical Methods in Astrophysics: An Introduction P Bodenheimer, G P Laughlin, M Rózyczka, H W Yorke Very High Energy Gamma-Ray Astronomy T C Weekes The Physics of Interstellar Dust E Krügel Dust in the Galactic Environment, 2nd Edition D C B Whittet Dark Sky, Dark Matter J M Overduin and P S Wesson An Introduction to the Science of Cosmology D J Raine and E G Thomas The Origin and Evolution of the Solar System M M Woolfson The Physics of the Interstellar Medium, 2nd Edition J E Dyson and D A Williams Optical Astronomical Spectroscopy C R Kitchin Dust and Chemistry in Astronomy T J Millar and D A Williams (eds) Series in Astronomy and Astrophysics ASTROBIOLOGY An Introduction Alan Longstaff Associate Lecturer in Astronomy and Earth Sciences Open University, UK Boca Raton London New York CRC Press is an imprint of the Taylor & Francis Group, an informa business Cover credit: Carter Roberts, Eastbay Astronomical Society, CA. 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Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents Constants ..................................................................................................................xxv Acknowledgments ..................................................................................................xxvii Author .....................................................................................................................xxix Introduction .............................................................................................................xxxi 1. Origin of the Elements .......................................................................................1 1.1 Elements for Life ......................................................................................1 1.2 The Universe Started from a Hot and Dense State ...................................1 1.2.1 Abundances of Primordial Elements Are Predicted by the Big Bang Hypothesis ...................................................................1 1.2.1.1 The Early Universe Was Dominated by Radiation ................................................................1 1.2.1.2 Primordial Hydrogen and Helium Were Synthesized in the First 20 min ..................................2 1.2.1.3 Lithium Abundance Is Problematic ............................4 1.3 The Message of Light ...............................................................................4 1.3.1 Atoms and Molecules Process Electromagnetic Radiation ........4 1.3.2 Electronic Transitions Are Quantized ........................................6 1.3.3 Energy Levels Govern Electronic Transitions in the Hydrogen Atom ...........................................................................8 1.3.4 Spectrographs Are Used to Capture Spectra ............................ 11 1.3.5 Stellar Spectra Encode Temperature and Elemental Abundances ............................................................................... 11 1.3.5.1 Harvard Spectral Classification Is Based on the Strength of Absorption Lines ...................................12 1.3.5.2 Luminosity Classes Are Based on the Sizes of Stars .......................................................................... 14 1.3.5.3 A Plot of Luminosity against Temperature Is a Hertzsprung–Russell Diagram .............................. 15 1.4 Stellar Evolution ...................................................................................... 16 1.4.1 The Properties of Main Sequence Stars Are Determined by Their Masses ........................................................................ 17 1.4.1.1 Brown Dwarfs Are Not Massive Enough to Become Stars ............................................................ 18 1.4.1.2 The Boundary between Brown Dwarf and Planet Is Hard to Define ........................................... 18 1.4.1.3 The Upper Limit for Stellar Masses Is Not Well Defined ..................................................................... 19 1.4.2 Stars Form by the Collapse of Giant Molecular Clouds ........... 19 1.4.2.1 Cloud Collapse Can Be Modeled by Simple Physics .....................................................20 v vi Contents 1.4.2.2 Any Plausible Model of Star Formation Must Be Able to Explain Several Key Facts...................... 21 1.4.2.3 Fragmentation and Accretion Are Key Processes in Star Formation ..................................... 21 1.4.3 Protostars Contract Down Onto the Main Sequence ................22 1.4.3.1 Initial Protostar Contraction Is Isothermal ..............22 1.4.3.2 Contraction onto the Main Sequence .......................22 1.4.4 Main Sequence Stars Fuse Hydrogen to Helium ......................22 1.4.4.1 Proton–Proton (PP) Chains Are the Main Energy Yielding Fusion Reactions in Main Sequence Stars ..........................................................23 1.4.4.2 Stars Process γ Radiation to Lower Energy Radiation ...................................................................24 1.4.5 Many Low Mass Stars Become Red Giants .............................24 1.4.5.1 Hydrogen Fusion Moves from Core to Shell ............24 1.4.5.2 Core Fusion Reactions Generate Carbon and Oxygen in Red Giant Stars .......................................25 1.4.5.3 Asymptotic Giant Branch (AGB) Stars Build Elements by Slow Neutron Capture .........................26 1.4.5.4 AGB Stars Distribute Metals into the Interstellar Medium ..................................................27 1.4.5.5 Very Low