Astronomy: A Self-Teaching Guide (7th ed.) PDF

ASTRONOMY ASTRONOMY A Self-Teaching Guide Seventh Edition Dinah L. Moché, Ph.D. John Wiley & Sons, Inc. This book is printed on acid-free paper. Copyright © 1978, 1981, 1987, 1993, 2000, 2004, 2009 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc...

ASTRONOMY ASTRONOMY A Self-Teaching Guide Seventh Edition Dinah L. Moché, Ph.D. John Wiley & Sons, Inc. This book is printed on acid-free paper. Copyright © 1978, 1981, 1987, 1993, 2000, 2004, 2009 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system or trans- mitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8600, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008. Limit of Liability/Disclaimer of Warranty: While the publisher and the author have used their best efforts in preparing this book, they make no representations or war- ranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a partic- ular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suit- able for your situation. You should consult with a professional where appropriate. Neither the publisher nor the author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information about our products and services, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data Moché, Dinah L., date. Astronomy : a self-teaching guide / Dinah L. Moché—7th ed. p. cm. — (Wiley self-teaching guides ; 190) Includes bibliographical references and index. ISBN 978-0-470-23083-1 (paper) 1. Astronomy. I. Title. QB45.2.M63 2009 520—dc22 2009025983 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 CREDITS We thank the American Astronomical Society for providing dozens of press releases, fact sheets, and celestial images. This service enables us to keep this text accurate and up to date. Figures are courtesy of the following organizations and individuals: David Aguilar/Harvard-Smithsonian Center for Astrophysics: 7.8 California Association for Research in Astronomy (adapted from): 2.15 C.S.I.R.O.: 2.17, 6.19c ESA/NASA/SOHO: 4.1, 4.8 ESO: 6.3, 6.16c Ann Feild/Space Science Telescope Institute: 6.23 Gemini Observatory/Neelon Crawford-Polar Fine Arts: 2.6 Hale Observatories: 6.4, 9.28, 11.5, 11.7 The Hubble Heritage Team (AURA/STScI/NASA: 6.21 Hubble Space Telescope WFPC Team, NASA, STScI: 6.16a Dr. Thomas Jarrett: 6.17 JAXA: 4.9 Lowell Observatory: 9.22 Dinah L. Moché/George Tremberger Jr.: I.2, I.3, 1.12, 2.4, 3.8, 3.9, 3.16, 3.17, 3.18, 3.20, 4.6, 5.3, 5.10 ([selected] data from Barbara J. Anthony-Twarog), 6.6, 6.7, 6.8, 6.12, 6.24a, 6.24b, 6.25, 7.1, 7.2, 7.4, 8.1 ([selected] from NASA), 8.2, 8.4, 8.11, 8.12, 9.12, 9.14, 9.15, 10.8 NASA: I.1, 5.1a, 5.14, 8.14 (adapted), 8.16, 9.2, 9.6, 9.8, 9.9, 9.10, 9.18, 9.19, 9.21, 9.23, 9.26, 9.27, 10.1, 10.5, 10.6, 10.7, 11.1, 11.4, 12.3, 12.4, 12.5 NASA, Reta Beebe, and Amy Simon (New Mexico State University): 9.20 NASA/CXC/ASU/J. Hester, et al.: 5.12b NASA/CXC/CfA/R. Kraft, et al.: 6.19a NASA/CXC/MIT/F. K. Baganoff, et al.: 6.11 NASA and ESA: 6.16d NASA/ESA/ASU/J. Hester & A. Loll: 5.12a NASA and STScI: 6.16f NASA, ESA, S. Beckwith (STScI) and the HUDF Team: I.4 NASA, ESA, R. Gendler, T. Lauer (NOAO/AURA/NSF) and A. Feild (STScI): 6.14 NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration: 6.16b, 6.16e NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University): 6.20 NASA and the Hubble Heritage Team (STScI/AURA): 5.9, 6.2 NASA, ESA and T. Lauer (NOAO/AURA/NSF): 6.9 NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington: 9.1a, 9.1b NASA/JPL-Caltech: 6.10, 9.17 NASA/JPL-Caltech/M. Meixner (STScI) & the SAGE Legacy Team: 6.13 NASA/JPL-Caltech/T. Pyle (SSC): 12.2 NASA/JPL-Caltech/Univ. Minn./R. Gehrz : 5.12c NASA/JPL/Space Science Institute: 9.24 NASA/JPL/STScI: 9.25 NASA, Steve Lee (University of Colorado), Jim Bell (Cornell University), Mike Wolff (Space Science Institute): 9.16 NASA/WMAP Science Team: 7.9 National Optical Astronomy Observatories: 1.3, 5.11, 6.1, 6.5, 6.22a National Radio Astronomy Observatory/AUI, J. O. Burns, E. J. Schrier, and E. D. Feigelson: 6.18 v vi  ASTRONOMY Image courtesy of NRAO/AUI and Earth image courtesy of the SeaWiFS Project NASA/GSFC and ORBIMAGE: 2.18 J. William Schopf, Elso S. Barghoorn, Morton D. Masser, and Robert O. Gordon: 12.1 Dr. Martin Schwartzchild, Princeton University: 4.10 Seth Shostak: 12.7 Aurore Simonnet, Sonoma State University, NASA E/PO: 2.19 SOHO (ESA & NASA): 4.7a, 4.12, 4.13 SOHO (ESA & NASA); assembled by Steele Hill (NASA GSFC): 4.11, 4.12, 4.13 SOHO/MDI magnetic map, white light, TRACE 1700A continuum, TRACE Lyman alpha, TRACE 171Å, TRACE 195Å, TRACE 284Å, YOHKOH/SXT X-ray image; composite prepared by Joe Covington (Lockheed-Martin Missiles and Space, Palo Alto): 4.7b (clockwise from top) STScI and NASA: 2.12 Tass/Sovfoto: 9.7 United States Air Force: 11.10 Courtesy John Walker: 4.5 Ryan Wyatt (adapted from): 8.3 Photo Insert Page 1 top: NASA/Lockheed Martin [images from the NASA Transition Region and Coronal Explorer (TRACE), the Extreme ultraviolet Imaging Telescope (EIT), the Large Angle and Spectrometric coronagraph (LASCO), and the Michelson Doppler Imager (MDI) tele- scopes on the ESA/NASA Solar and Heliospheric Observatory (SOHO)]; page 1 bottom: Courtesy NASA/JPL-Caltech; page 2 top: X-ray: NASA/CXC/CfA/R.Kraft et al; Radio: NSF/VLA/Univ.Hertfordshire/M.Hardcastle; Optical: ESO/VLT/ISAAC/M. Rejkuba, et al; page 2 bottom: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); page 3 top left and right: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); page 3 bot- tom: NASA/WMAP Science Team; page 4 top: X-ray: NASA/CXC/Wesleyan Univ./R. Kilgard, et al; UV: NASA/JPL-Caltech; Optical: NASA/ESA/S. Beckwith & Hubble Heritage Team (STScI/AURA); IR: NASA/JPL-Caltech/ Univ. of AZ/R. Kennicutt; page 4 bottom: X-ray: NASA/UMass/Q. D. Wang, et al.; Optical: NASA/STScI/AURA/Hubble Heritage; Infrared: NASA/JPL-Caltech/Univ. AZ/R. Kennicutt/SINGS Team Tables and illustrations are adapted, redrawn, or used by permission of the following authors and publishers: Table 1.1: Robert Garrison and Toomas Karmo, Observer’s Handbook 2008, with permis- sion of the Royal Astronomical Society of Canada. Table 2.1: Astronomy: Fundamentals and Frontiers, 3rd edition, by Robert Jastrow and Malcolm H. Thompson. Copyright © 1972, 1974, 1977 by Robert Jastrow (John Wiley & Sons, New York). Table 3.1 (adapted); 11.2 (selected): Allen’s Astrophysical Quantities, 4th edition. Copyright © 1999 by A. N. Cox, ed. (Springer-Verlag, New York). Table 6.2 (adapted): Realm of the Universe, by George O. Abell. Copyright © 1964, 1969, 1973, 1980, by Holt, Rinehart and Winston, Inc. Copyright © 1976 by George O. Abell. Used by permission of Holt, Rinehart and Winston, Inc. Tables 8.2 and 8.3: (selected) National Aeronautics and Space Administration public information. Updates by JPL’s Solar System Dynamics Group, URL: http://ssd.jpl.nasa.gov/sat_elem.html Tables 10.2 and 10.3: with permission from Solar Eclipses: 1996–2020 and Lunar Eclipses: 1996–2020, by Fred Espenak, NASA/Goddard Space Flight Center. Tables 8.3 and 11.1: (selected) Brian G. Marsden, Smithsonian Astrophysical Observatory. Appendix 5: Alan Batten, Observer’s Handbook 2009, with permission of the Royal Astronomical Society of Canada.  TO THE READER Astronomy is a user-friendly guide for beginners. Chapters make it easy for you to quickly learn the main topics of a college level course. Sections clarify basic principles and contemporary advances. The Index enables you to look up concepts, definitions, facts and famous astronomers, fast. You can use the book alone or with a conventional textbook, Internet- based or distance-learning course, computer software, telescope manual, or as a handy reference. PARTICULARLY USEFUL FEATURES Web site addresses throughout for the best astronomy online. Mathematics is not required. Line art makes technical ideas obvious. Star and Moon maps for fun stargazing. Up-to-date, accurate star, constellation, and astronomical data. Popular sky targets for hobby telescopes. Tips for hands-on, active learning. Objectives, reviews, and self-tests to monitor your progress. WHAT’S NEW IN THE SEVENTH EDITION? While keeping its successful self-teaching format, this seventh edition incorpo- rates Web site addresses for spectacular color images. The entire book was revised to include revolutionary discoveries and the best suggestions from many readers and educators who profitably used prior editions. Frontier twenty-first-century research into black holes, active galaxies and quasars, searches for life in space, origin and structure of our universe, and the newest ground and space telescopes are described. vii viii  ASTRONOMY Web sites with daily astro-news and space scenes never before viewed by humans are specified. Labeled drawings of the Keck Telescope, Fermi Gamma Ray Observatory, and Hubble Space Telescope data path clarify space technol- ogy. New art illustrates fundamental concepts, such as the electromagnetic spectrum, phases of the Moon, planet orbits, and H-R diagrams. STUDY AIDS A list of objectives for each chapter tells you instantly what information is contained there. The first time a new term is introduced, it appears in bold type and is defined. Topics in each chapter are presented in short, numbered sections. Each section contains new information and usually asks you to answer a question or asks you to suggest an explanation, analyze, or summa- rize as you go along. You will always see the answer to the question right after you have answered it. If your answer agrees with the book’s, you understand the material and are ready to proceed to the next section. If it does not, you should review some previous sections to make sure you understand the mater- ial before you proceed. A self-test at the end of each chapter lets you find out fast how well you understand the material in the chapter. You may test yourself right after com- pleting a chapter, or you might take a break and then take the self-test as a review before beginning a new chapter. Compare your answers with the book’s. If your answers do not agree with the printed ones, review the appro- priate sections (listed next to each answer). USEFUL RESOURCES AND WEB SITES Sources of excellent print and online astronomy materials, activities, and refer- ences are included in the Useful Resources and Web Sites section. Here you will also find a list of other books for stargazers of all ages by the author, Dinah L. Moché, Ph.D. The author and publisher have tried to make this book accurate, up-to- date, enjoyable, and useful for you. It has been read by astronomers and many students, hobbyists, and educators who have contributed helpful suggestions during the preparation of the final manuscript. If, after completing the book, you have suggestions to improve it for future readers or for an author’s visit, please let the author know: Dinah L. Moché, Ph.D., c/o Professional & Trade Group, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030. www.spacelady.com Check this book’s Web site for exciting new discoveries, and updates and corrections in press for the next printing. www.wiley.com/go/moche  ACKNOWLEDGMENTS I am especially grateful to my numerous students and lecture audiences and to readers of earlier editions of Astronomy whose questions and comments shaped the seventh edition. Special thanks for always enthusiastically sharing the wonder and excite- ment of space with me to: My home galaxy of stars Mollie and Bertram A. Levine; Elizabeth, Stephen, Lucy, Benjamin, Robert and Melanie Schwartz; and Rebecca, Richard, Cynthia, Jessica, Caroline, and Amanda Kahlenberg. My counselor Ernest Holzberg, Esq. and friend Bonnie Brown. The National Science Foundation Faculty Fellowship in Science awarded to me made possible advanced studies in astronomy. Stephen Kippur, Dean Karrel, Kitt Allan, Eric Nelson, Constance Santisteban, Ellen Wright, Camille Acker, Megan Burke, and Barbara Mele at John Wiley & Sons, Inc. I appreciate the continued encouragement and support of those who con- tributed to earlier editions. Thanks for the Seventh Edition go to: Stephen P. Maran, American Astronomical Society; T. H. Jarrett (IPAC) and J. Davy Kirkpatrick, California Institute of Technology; Peter B. Stetson, Dominion Astrophysical Observatory; John Pazmino, Federal Energy Regulatory Commission; Peter Michaud, Gemini Observatory; Laurence A. Marschall, Gettysburgh College; Kimberly Kowal Arcand, Harvard Smithsonian Center for Astrophysics; Megan Watzke, Harvard Smithsonian Center for Astrophysics; Antoinette Beiser, Lowell Observatory; Deidre Hunter, Lowell Observatory; Dawn Myers (GSFC), William Steigerwald (GSFC), Cheryl Gundy (STScI), Eric L. Winter (IPAC) of NASA; Dave Finley, National Radio Astronomy Observatory; Ruth A. Kneale, National Solar Observatory; Santi Cassisi, Osservatorio di Teramo; President Eduardo Marti, Tak D. Cheung, Thomas P. Como, Alex L. Flamholz, Francesca R. Gianferrara, Todd M. Holden, Alec Kisselev, Liza Larios, David H. Lieberman, Paul J.Marchese, Bruce Naples, Charles P. ix x  ASTRONOMY Neuman, Charles Prancl, Ralph Romanelli, Robert Taylor, George Tremberger Jr., Queensborough Community College of the City University of New York; Seth Shostak, SETI Institute; Aurore Simonnet, Sonoma State University; Lynn Cominsky, Sonoma State University; Patrick Kelly, The Royal Astronomical Society of Canada; Barbara J. Anthony-Twarog, University of Kansas; Don Vandenberg, University of Victoria; and Robert Benjamin, University of Wisconsin. Sixth Edition: Davy Kirkpatrick, California Institute of Technology; Marion Schmitz, California Institute of Technology; Peter Michaud, Gemini Observatory; Laurence A. Marschall, Gettysburg College; Robert Kirshner, Harvard University; David Aguilar, Harvard-Smithsonian Center for Astro- physics; Brian G. Marsden, Harvard-Smithsonian Center for Astrophysics; Gareth V. Williams, Harvard-Smithsonian Center for Astrophysics; Holland Ford, Johns Hopkins University; Jim Lochner, NASA Goddard Space Flight Center; Stephen Maran, NASA Goddard Space Flight Center; Rajiv Gupta, Royal Astronomical Society of Canada; Lynn Cominski, Sonoma State University; Cheryl Gundy, Space Telescope Science Institute; Zoltan Levay, Space Telescope Science Institute; Ray Villard, Space Telescope Science Institute; Alex Filippenko, University of California/Berkeley; Harold Epps, University of California/Santa Cruz. Fifth Edition: Joseph F. Veverka, Cornell University; Robert Garrison, David Dunlap Observatory; Alan Batten, Dominion Astrophysical Observatory; Peter Michaud, Gemini Observatory; Francois Spite, IAU; Gerard Helferich, John Wiley & Sons, Inc.; Christopher Jackson and Diana C. Madrigal, John Wiley & Sons; Michael Arida, Fred Espenak, Stephen P. Maran, Wayne Warren (GSFC), Alan Chamberlain, Mary Beth Murrill, Jane Platt (JPL), and Cheryl Gundy (STScI), NASA; David G. Finley, National Radio Astronomy Observatory; Roy L. Bishop, Royal Astronomical Society of Canada; Brian Marsden, Smithsonian Astrophysical Observatory; Geoff Chester, U.S. Naval Observatory; Harry Shipman, University of Delaware; and Helene Dickel, University of Illinois. Fourth Edition: Steve Maran, American Astronomical Society; Maria Pallante, Authors Guild; Bob Finn, California Institute of Technology; Richard Dannay, Esq.; Pat Peterson, de Grummond Collection, University of Southern Mississippi; Carol R. Leven, Freelance Administrator; Laurence A. Marschall, Gettysburg College; Nicholas L. Johnson, Kaman Sciences Corporation; Mary Beth Murrill, W. M. Keck Observatory; Keith Mordoff, Lockheed Missiles & Space Company, Inc.; Richard Jackson, Bill Santoro, Joe Schank, Mamaro- neck Post Office; Constance Moore, Althea Washington (Headquarters), Alan S. Wood, Kimberly Lievense, Sharon Miller, Mary Hardin, Ed McNevin, Jurrie van der Woude, Gil Yanow (JPL), Charles Borland, Billie A. Deason, Lisa Vazquez (JSC), Allen Kenitzer (MSFC), Ray Villard (STSI) of the National Aeronautics and Space Administration; Emma Hardesty, Karie Myers, National Optical Astronomy Observatories; Director Paul A. Vanden Bout,  ACKNOWLEDGMENTS  xi Patrick C. Crane (VLA), Pat Smiley, National Radio Astronomy Observatory; Array; Roy Bishop, Observer’s Handbook; Gloria Lubkin, Physics Today; Jacqueline Mitton, Royal Astronomical Society (U.K.); David Okerson, Science Applications International Corporation; George Lovi, Sky and Telescope columnist; Preston J. Campbell, TRW Federal Systems Division; John Percy, University of Toronto; Jay Pasachoff, Williams College. Third Edition: I. Robert, Victor and Esther Rozen; Jack Flynn, Andrew Fraknoi, Juliana Ver Steeg, Astronomical Society of the Pacific; Director Sidney Wolff, Carl A. Posey, and Jeff Stoner, Kitt Peak National Observatory; Elyse Murray, Bernard Oliver, and Charles Seeger (Ames), Donald K. Yeomans (JPL), NASA; Ronald Ekers, Arnold H. Rots, and Don L. Swann, NRAO/VLA; Tobias Owen, SUNY/Stony Brook; Larry Esposito, University of Colorado; and Paul W. Hodge, University of Washington. Second Edition: Lloyd Motz and Chien Shiung Wu, Columbia University; Harry L. Shipman, University of Delaware; Frank E. Bristow (JPL), Les Gaver, David W. Garrett, Curtis M. Graves, William D. Nixon (Headquarters), Peter W. Waller (Ames), and Terry White (JPL), NASA; Janet K. Wolfe, National Air and Space Museum; Richard W. West, NSF; Henry D. Berney, Thomas Como, Donald Cotten, Julius Feit, Sheldon E. Kaufman, Valdar Oinas, Robert Taylor, and Kurt R. Schmeller, Queensborough Community College of CUNY; and Arnold A. Sterassenburg, SUNY/Stony Brook.  CONTENTS List of Tables xv Introduction Cosmic View 1 Chapter 1 Understanding the Starry Sky 5 Chapter 2 Light and Telescopes 31 Chapter 3 The Stars 65 Chapter 4 The Sun 95 Chapter 5 Stellar Evolution 121 Chapter 6 Galaxies 146 Chapter 7 The Universe 181 Chapter 8 Exploring the Solar System 201 Chapter 9 The Planets 229 Chapter 10 The Moon 270 Chapter 11 Comets, Meteors, and Meteorites 293 Chapter 12 Life on Other Worlds? 314 Epilogue 334 Useful Resources and Web Sites Periodicals: Print and Online 335 Databases 335 xiii xiv  CONTENTS Books by Dinah L. Moché 336 Career Information 336 Almanacs, Observing Guides, and Star Atlases 337 Organizations 337 Stunning Color Images and News Online 338 Appendixes 1. The Constellations 341 2. Physical and Astronomical Constants 343 3. Measurements and Symbols 344 4. Periodic Table of the Elements 346 5. The Nearest Stars 348 6. The Messier Objects 349 Index 352 Star and Moon Maps Back of Book Spring Skies Summer Skies Autumn Skies Winter Skies Moon Map  LIST OF TABLES 1.1 The Brightest Stars 14 2.1 Four Hot and Cool Stars 39 2.2 Major Optical Telescopes in the World 52 3.1 Spectral Class Characteristics 74 3.2 Magnitude Differences and Brightness Ratios 80 3.3 Sample Magnitude Data 81 4.1 Properties of the Sun 103 6.1 Some Properties of Open and Globular Star Clusters 152 6.2 Rough Values of Galactic Data 165 8.1 Days of the Week 204 8.2 Properties of the Planets 218 8.3 Selected Moons of the Planets 222 10.1 Properties of the Moon 276 10.2 Total Solar Eclipses 286 10.3 Total Lunar Eclipses 287 11.1 Some Periodic Comets 301 11.2 Principal Annual Meteor Showers 306 11.3 Large Meteorites on Display in the U.S. 307 11.4 The Occurrence of Meteorite Types 308 xv  INTRODUCTION: COSMIC VIEW Strange is our situation here upon Earth. Each of us comes for a short visit, not knowing why, Yet sometimes seeming to divine a purpose. Albert Einstein (1879–1955) On a clear night in a place where the sky is really dark, you can see about 2000 stars with your unaided eye. You can look trillions of kilometers into space and peer thousands of years back into the distant past. As you gaze at the stars you may wonder: What is the pattern or meaning of the starry heavens? What is my place in the vast cosmos? You are not alone in asking these questions. The beauty and mystery of space have always fasci- nated people. Astronomy is the oldest science—and the newest. Exciting discoveries are being made today with the most sophisticated tools and techniques ever avail- able. Yet dedicated amateurs can still make important contributions. This book will teach you the basic concepts of astronomy and space exploration. You will more fully enjoy observing the stars as your knowledge and understanding grow. You will be better able to surf the Web and to read more on topics that intrigue you, from ancient astronomy to the latest astro- physical theories and spaceflights. As you teach yourself astronomy, refer to: The Star maps and Moon map at the back of this book. These special, easy-to-read maps will help you locate and identify particularly inter- esting objects in the sky. Simple activities you can do that demonstrate a basic idea. 1 2  ASTRONOMY  Internet link to spectacular images and new reports. Now, begin reading about the enormous tracts of space and time we call the universe, and stretch your mind! Our home is planet Earth, a rocky ball about 13,000 km (8000 miles) in diameter suspended in the vastness of space-time (Figure I.1). Figure I.1. Earth photographed from space. Sunshine dramatically spotlights Earth’s blue ocean, reddish-brown land masses, and white clouds from the Mediterranean Sea area to the Antarctica polar ice cap.  INTRODUCTION: COSMIC VIEW  3 Figure I.2. Planets orbiting the Sun in the solar system. (Drawing not to scale.) Earth belongs to the solar system (Figure I.2). The solar system consists of one star—our Sun—plus planets, moons, small solar system bodies, and dust particles, all of which revolve around the Sun. The solar system is more than 15 trillion km (9 trillion miles) across. The Sun and the solar system are located in one of the great spiral arms of the Milky Way Galaxy (Figure I.3). Our immense Milky Way Galaxy Figure I.3. The solar system in the Milky Way Galaxy. 4  ASTRONOMY includes over 200 billion stars plus interstellar gas and dust, all revolving around the center. The Milky Way Galaxy is about 100,000 light-years across. (One light-year is practically 10 trillion km, or 6 trillion miles.) Our Milky Way Galaxy is only one of billions of galaxies that exist to the edge of the observable universe, some 14 billion light-years away (Figure I.4). Figure I.4. Nearly 10,000 distant gallaxies in a patch of sky just one-tenth as big as the full Moon, in the constellation Fornax. Each galaxy includes billions of stars.  1 UNDERSTANDING THE STARRY SKY And that inverted bowl we call the Sky Where under crawling coop’t we live and die Lift not your hands to it for help—for It As impotently rolls as you and I. Rubáiyát of Omar Khayyám (1048–1131) Objectives  Locate sky objects by their right ascension and declination on the celestial sphere.  Identify some bright stars and constellations visible each season.  Explain why the stars appear to move along arcs in the sky during the night.  Explain why some different constellations appear in the sky each season.  Explain the apparent daily and annual motions of the Sun.  Define the zodiac.  Describe how the starry sky looks when viewed from different latitudes on Earth.  Define a sidereal day and a solar day, and explain why they differ.  Explain how astronomers classify objects according to their apparent bright- ness (magnitude).  Explain why the polestar and the location of the vernal equinox change over a period of thousands of years. 5 6  ASTRONOMY 1.1 STARGAZER’S VIEW On a clear, dark night the sky looks like a gigantic dome studded with stars. We can easily see why the ancients believed that the starry sky was a huge sphere turning around Earth. Today we know that stars are remote, blazing Suns racing through space at different distances from Earth. The Earth rotates, or turns, daily around its axis (the imaginary line running through its center between the North and South Poles). But the picture of the sky as a huge, hollow globe of stars that turns around Earth is still useful. Astronomers call this fictitious picture of the sky the celestial sphere. “Celestial” comes from the Latin word for heaven. Astronomers use the celestial sphere to locate stars and galaxies and to plot the courses of the Sun, Moon, and planets throughout the year. When you look at the stars, imagine yourself inside the celestial sphere looking out (Figure 1.1). Why do the stars on the celestial sphere appear to move during the night when you observe them from Earth? ________________________________________ ________________________________________________________________________ Answer: Because the Earth is rotating on its axis inside the celestial sphere. (a) (b) Figure 1.1. (a) To a stargazer on Earth, all stars appear equally remote. (b) We picture the stars as fixed on a celestial sphere that spins westward daily (opposite to Earth’s actual rotation).  UNDERSTANDING THE STARRY SKY  7 1.2 CONSTELLATIONS It is fun to go outside and see a young blue-white star or a dying red giant star in the sky right after you read about them. You may think you will never be able to tell one star from another when you begin stargazing, but you will. The removable star maps at the back of this book have been drawn espe- cially for beginning stargazers observing from around 40°N latitude. (They should be useful to new stargazers throughout the midlatitudes of the north- ern hemisphere.) Stars appear to belong to groups that form recognizable patterns in the sky. These star patterns are called constellations. Learning to identify the most prominent constellations will help you pick out individual stars. The 88 constellations officially recognized by the International Astro- nomical Union are listed in Appendix 1. Famous ones that shine in these lati- tudes are shown on your star maps. Their Latin names, and the names of asterisms, or popular unofficial star patterns, are printed in capital letters. Thousands of years ago people named the constellations after animals, such as Leo the Lion (Figure 1.2), or mythological characters, such as Orion the Hunter (Figure 5.1). More than 2000 years ago the ancient Greeks recog- nized 48 constellations. Modern astronomers use the historical names of the constellations to refer to 88 sections of the sky rather than to the mythical figures of long ago. They refer to constellations in order to locate sky objects. For instance, saying that Mars is in Leo helps locate that planet, just as saying that Houston is in Texas helps locate that city. Look over your star maps. Notice that the dashed line indicates the ecliptic, the apparent path of the Sun against the background stars. The 12 (a) (b) Figure 1.2. Constellation Leo is best seen in early spring when it is high in the sky. (a) Brightest star Regulus marks the lion’s heart, a sickle of stars his mane, and a triangle of stars his hindquarters and tail. (b) Leo the Lion. 8  ASTRONOMY constellations located around the ecliptic are the constellations of the zodiac whose names are familiar to horoscope readers. List the 12 constellations of the zodiac. __________________________________ ________________________________________________________________________ Answer: Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius. 1.3 CIRCUMPOLAR CONSTELLATIONS Study your star maps carefully. You will notice that several circumpolar con- stellations, near the north celestial pole (marked POLE +), appear on all four maps. These are north circumpolar constellations, visible above the northern horizon all year long at around 40°N latitude (Figure 1.3). At this latitude, the south celestial pole and nearby south circumpolar constellations do not rise above the horizon any night of the year. List the three circumpolar constellations closest to Polaris (the North Star) and sketch their outlines. ______________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: Three circumpolar constellations that you should be able to pick out on the star maps are Cassiopeia, Cepheus, and Ursa Minor. After you know their outlines, try to find them in the sky above the northern horizon. Note: At latitude 40°N or higher, Ursa Major and Draco are also circumpolar. 1.4 HOW TO USE THE STAR MAPS You can use the star maps outdoors to identify the constellations and stars you see in the night sky and to locate those you want to observe.  UNDERSTANDING THE STARRY SKY  9 Figure 1.3. A time exposure taken with a camera aimed at the north celestial pole over the U.S. Kitt Peak National Observatory shows star trails that mirror Earth’s actual rotation. Kitt Peak is a 2100-m- (6900-ft)-high site about 30 km (50 miles) outside of Tucson, Arizona. www.noao.edu/kpno 10  ASTRONOMY Choose the map that pictures the sky at the month and time you are stargazing. Turn the map so that the name of the compass direction you are facing appears across the bottom. Then, from bottom to center, your star map pictures the sky as you are viewing it from your horizon to the point directly over your head. For example, if you are facing north about 10:00 P.M. in early April, turn the map so that the word NORTH is at the bottom. From the horizon up, you may observe Cassiopeia, Cepheus, the Little Dipper in Ursa Minor, and the Big Dipper in Ursa Major. Name a prominent constellation that shines in the south at about 8:00 P.M. in early February. __________ Answer: Orion. 1.5 HOW TO IDENTIFY CONSTELLATIONS The constellations above the southern horizon parade by during the night and change with the seasons. Turn each map so that the word SOUTH is at the bot- tom. Use your star maps to identify the most prominent constellations that shine each season (such as Leo in the spring and Orion in the winter). Identify and sketch three constellations that you can see this season. ________________________________________________________________________ ________________________________________________________________________ Answer: Your answer will depend on the season. For example, if you are reading this book in the spring, you might choose Leo, Virgo, and Boötes. 1.6 STAR NAMES Long ago, more than 50 of the brightest stars were given proper names in Arabic, Greek, and Latin. The names of bright or famous stars to look for are printed on your star maps with the initial letters capitalized. Today astronomers use alphabets and numerals to identify hundreds of thousands of stars. They refer to each of the brightest stars in a constellation by a Greek letter plus the Latin genitive (possessive) form of the constellation  UNDERSTANDING THE STARRY SKY  11 name. Usually the brightest star in a constellation is α, the next brightest is β, and so on. (The Greek alphabet is listed in Appendix 3.) Thus, Regulus is called α Leonis, or the brightest star of Leo. Fainter stars, not shown on your maps, are identified by numbers in star catalogs. In a built-up metropolitan area you can see only the brightest stars. When you are far from city lights and buildings and the sky is very dark and clear, you can see about 2000 stars with your unaided eye. Name the three bright stars that mark the points of the famous Summer Triangle. Refer to your summer skies map. ___________________________________ ________________________________________________________________________ Answer: Vega, Deneb, and Altair. Look for the Summer Triangle overhead during the summer. 1.7 BRIGHTNESS Some stars in the sky look brighter than others. The apparent magnitude of a sky object is a measure of its observed brightness as seen from Earth. Stars may look bright because they send out a lot of light or because they are rela- tively close to Earth. In the second century B.C., the Greek astronomer Hipparchus divided the visible stars into six classes, or magnitudes, by their relative brightness. He numbered the magnitudes from 1 (the brightest) through 6 (the least bright). Modern astronomers use a more precise version of the ancient classifying system. Instead of judging brightness by the eye, they use an instrument called a photometer to measure brightness. Magnitudes for the brightest stars are negative—the brightest night star, Sirius, measures –1.44. Magnitudes range from –26.72 for the Sun to about +31 for the faintest objects observed in a space telescope. A difference of 1 magnitude means a brightness ratio of about 2.5. Magnitudes are shown on your star maps and in Table 1.1. For example, we receive about 2.5 times as much light from Vega, a star of magnitude 0, as we do from Deneb, a star of magnitude 1, and about 6.3 times as much light as from Polaris, of magnitude 2. (Magnitudes are discussed further in Section 3.14.) What do astronomers mean by “apparent magnitude”? ___________________ ________________________________________________________________________ Answer: How bright a sky object looks. 12  ASTRONOMY 1.8 LOCATION ON EARTH The more you understand about stars and their motions, the more you will enjoy stargazing. A celestial globe helps you locate sky objects as a terres- trial (Earth) globe helps you locate places on Earth. Remember how Earth maps work. We picture the Earth as a sphere and draw imaginary guidelines on it. All distances and locations are measured from two main reference lines, each marked 0°. One line, the equator, is the great circle halfway between the North and South Poles that divides the globe into halves. The other line, the prime meridian, runs from pole to pole through Greenwich, England. Imaginary lines parallel to the equator are called latitude lines. Those from pole to pole are called longitude lines, or meridians. You can locate any city on Earth if you know its coordinates of latitude and longitude. Distance on the terrestrial sphere can be measured by dividing the sphere into 360 sec- tions, called degrees (°). (Angular measure is defined in Appendix 3.) Refer to the globe in Figure 1.4. Identify the equator; prime meridian; 30°N latitude line; and 30°E longitude line. (a) __________ ; (b) __________ ; (c) __________ ; (d) __________ Answer: (a) 30°N; (b) 30°E; (c) equator; (d) prime meridian. Figure 1.4. Terrestrial globe.  UNDERSTANDING THE STARRY SKY  13 1.9 CELESTIAL COORDINATES Astronomers draw imaginary horizontal and vertical lines on the celestial sphere similar to the latitude and longitude lines on Earth. They use celestial coordinates to specify directions to sky objects. The celestial equator is the projection of the Earth’s equator out to the sky. Angular distance above or below the celestial equator is called declina- tion (dec). Distance measured eastward along the celestial equator from the zero point, the vernal equinox, is called right ascension (RA). Right ascen- sion is commonly measured in hours (h), with 1h = 15°. Just as any city on Earth can be located by its coordinates of longitude and latitude, any sky object can be located on the celestial sphere by its coor- dinates of right ascension and declination. Give the location of the star shown in Figure 1.5. _________________________ Answer: 20h RA, 30°N declination. North 60°N Celestial Pole 30°N 18 h Earth 20 h 30°S Celestial 22 h Equator 0° 0h 320h° 60°S South Zero-point Celestial Pole Vernal Equinox Figure 1.5. Celestial globe. 14  ASTRONOMY 1.10 LOCATION ON THE CELESTIAL SPHERE Every star has a location on the celestial sphere, where it appears to be when sighted from Earth. The right ascension and declination of stars for a stan- dard epoch, or point of time selected as a fixed reference, change little over a period of many years. They can be read from a celestial globe, star atlas, or computer software. (See Table 1.1, for example. You’ll be referring to this table when the information it contains is discussed in later chapters.) TABLE 1.1 The Brightest Stars Right Ascension Declination Apparent Spectral Distance Absolute Star Name h m º ′ Magnitude Class (ly) Magnitude Sun — — — — –26.75 G 8 lm 4.8 Sirius α Canis Majoris 06 46 –16 44 –1.44 A 9 1.5 Canopus α Carinae 06 24 –52 42 –0.62 A 310 –5.4 Arcturus α Bootis 14 16 +19 10 –0.05 K 37 –0.6 Rigil Kentaurus α Centauri 14 40 –60 52 –0.01 G 4 4.2 Vega α Lyrae 18 37 +38 47 0.03 A 25 0.6 Capella α Aurigae 05 17 +46 00 0.08 G 42 –0.8 Rigel ß Orionis 05 15 –08 12 0.18 B 800 –6.6 Procyon α Canis Minoris 07 40 +05 12 0.40 F 11 2.8 Achernar α Eridani 01 38 –57 12 0.45 B 144 –2.9 Betelgeuse α Orionis 05 56 +07 24 0.45 M 520 –5.0 Hadar ß Centauri 14 04 –60 25 0.58 B 500 –5.5 Altair α Aquilae 19 51 +08 53 0.76 A 17 2.1 Aldebaran α Tauri 04 36 +16 31 0.87 K 65 –0.8 Spica α Virginis 13 26 –11 12 0.98 B 260 –3.6 Antares α Scorpii 16 30 –26 27 1.06 M 600 –5.8 Pollux ß Geminorum 07 46 +28 01 1.16 K 34 1.1 Formalhaut α Piscis Austrini 22 58 -29 35 1.17 A 25 1.6 Deneb α Cygni 20 42 +45 18 1.25 A 1500 –7.5 Acrux α Crucis 12 27 –63 08 1.25 B 320 –4.0 Becrux ß Crucis 12 48 –59 43 1.25 B 352 –4.0 Note: Magnitudes are visual magnitudes, measured over visible wavelengths. Abbreviations Right Ascension: h = hours; m = minutes of time Declination: ° = degrees; ´ = minutes of arc Distance: ly = light-year and lm = light-minute  UNDERSTANDING THE STARRY SKY  15 The locations of the Sun, Moon, and planets on the celestial sphere change regularly. You can find their monthly positions, rise and set times, and other practical data in current astronomical publications, computer soft- ware (see Useful Resources and Web Sites) and at the U.S. Naval Observatory Web site. http://aa.usno.navy.mil Explain why in any given era the stars may be found at practically the same coordinates on the celestial sphere, while the Sun, Moon, and planets change their locations regularly. ___________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: The stars are too far from Earth for the unaided eye to see them move even though they are traveling many kilometers per second in various directions. The Sun, Moon, and planets are much closer to Earth. We see them move relative to the distant stars. 1.11 LOCAL REFERENCE LINES Lines of declination and right ascension are fixed in relation to the celestial sphere and move with it as it rotates around an observer. Other useful refer- ence lines relate to the local position of each observer and stay fixed with the observer while sky objects pass by. At your site, the zenith is the point on the celestial sphere directly over your head. The celestial horizon is the great circle on the celestial sphere 90° from your zenith. Although the celestial sphere is filled with stars, you can see only those that are above your horizon. The celestial meridian is the great circle passing through your zenith and the north and south points on your horizon. Only half of the celestial meridian is above the horizon. Refer to Figure 1.6. Identify the stargazer’s zenith; celestial horizon; and celes- tial meridian. (a) __________ ; (b) __________ ; (c) __________ Answer: (a) Zenith; (b) meridian; (c) horizon. 16  ASTRONOMY Figure 1.6. A stargazer’s local reference lines. 1.12 CELESTIAL MERIDIAN Go outside and trace out your zenith, celestial horizon, and celestial meridian by imagining yourself, like that stargazer, at the center of the huge celestial sphere. If possible, try this on a clear, dark, starry night. Face south. Observe the stars near your celestial meridian several times during the night. Describe what you observe. _________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: The stars move from east to west and transit, or cross, your celestial meridian. This is because of the Earth’s rotation from west to east. A star culminates, or reaches its highest altitude, when it is on the celestial meridian. 1.13 LATITUDE AND STARGAZING The stars that appear above your horizon and their paths across the sky depend on your latitude on Earth. The sky looks different from different lati- tudes (Figure 1.7).  UNDERSTANDING THE STARRY SKY  17 Figure 1.7. Local orientation of the celestial sphere at 40°N latitude. (a) View from a ficti- tious spot on the outside. (b) Stargazer’s view. If you could look at the sky from the North Pole and then from the South Pole you would see completely different stars. The Earth cuts your view of the celestial sphere in half. You can determine how the celestial sphere is oriented with respect to your horizon and zenith at any place on Earth. In the northern hemisphere, the north celestial pole is located above your northern horizon at an altitude equal to your latitude. Polaris, the polestar, or North Star, is less than one degree away from the north celestial pole and marks the position of the pole in the sky. The declina- tion circle that is numerically equal to your latitude passes through your zenith. In the southern hemisphere, the south celestial pole is located above your south- ern horizon at an altitude equal to your latitude. It is not marked by a polestar. Where would you look for the North Star if you were at each of the following locations: (a) the North Pole? __________ (b) the equator? __________ (c) 40°N latitude? __________ (d) your home? __________ Answer: (a) At your zenith; (b) on your horizon; (c) 40° above your northern horizon; (d) at an altitude above your northern horizon equal to your home latitude. 1.14 APPARENT DAILY MOTION OF THE STARS The stars appear to move in diurnal circles, or daily paths, around the celes- tial poles when you observe them from the spinning Earth. 18  ASTRONOMY Although the North Star, Polaris, is not a very bright star, it has long been important for navigation. Closest to the north celestial pole, it is the only star that seems to stay in the same spot in the sky. You can find Polaris by follow- ing the “pointer stars,” Dubhe and Merak, in the bowl of the Big Dipper in the constellation Ursa Major (Figure 1.8). Since the celestial poles are at distinct altitudes in the sky at distinct lati- tudes, the part of a star’s diurnal circle that is above the horizon is different at different latitudes on Earth (Figure 1.9). For example, if you stargaze at 40°N latitude, about the latitude of Denver, Colorado, U.S., you will see (Figure 1.9): (1) Stars within 40° (your lat- itude) of the north celestial pole (those stars between +50° and +90° declina- tion) are always above your horizon. These stars that never set—such as the stars in the Big Dipper—are north circumpolar stars. (2) Stars that are Figure 1.8. The “pointer” stars, Dubhe and Merak, in the bowl of the Big Dipper lead you to the North Star, Polaris. The angular distance between these pointer stars is about 5° on the celestial sphere. A fist at arm’s length marks about 10°. These examples will help you judge other angular distances in the sky.  UNDERSTANDING THE STARRY SKY  19 Figure 1.9. The sky from 40°N latitude. The north celestial pole is 40° above the north- ern horizon, and the celestial sphere rotates around it. Parallels of declination mark the stars’ diurnal circles. within 40° (your latitude) of the south celestial pole never appear above your horizon. These stars that never rise—such as the stars in the constellation Crux, the Southern Cross—are south circumpolar stars. (3) The other stars, in a band around the celestial equator, rise and set. Those stars that are located at 40°N declination (equal to your latitude) pass directly across your zenith when they cross your celestial meridian. Assume you are stargazing at 50°N latitude, about the latitude of Vancouver, Canada. Refer to Table 1.1 for the declinations of the bright stars Capella, Vega, and Canopus. Which of these stars will be above your horizon: (a) always? __________ (b) sometimes? __________ (c) never? __________ 20  ASTRONOMY Answer: (a) Capella (+46°00′ declination). Stars within 50° of the north celestial pole (between +40° and +90° declination) are always above the horizon. (b) Vega (+38°47′ dec- lination). This star rises and sets. (c) Canopus (–52°42′ declination) is within 50° of the south celestial pole (between –40° and –90° declination). 1.15 UNUSUAL VIEWS Describe how the diurnal circles of the stars would look if you were stargazing at (a) the North Pole and (b) the equator. Explain your answer. Tip: Remember that the celestial sphere rotates around the celestial poles. (a) ______________________ ________________________________________________________________________ ________________________________________________________________________ (b) ______________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: (a) All stars would seem to move along circles around the sky parallel to your horizon. The celestial sphere rotates around the north celestial pole, which is located at your zenith at the North Pole. (b) All stars would seem to rise at right angles to the horizon in the east and set at right angles to the horizon in the west. The celestial sphere rotates around the celestial poles, which are located on your horizon at the equator. 1.16 APPARENT ANNUAL MOTION OF THE STARS The appearance of the sky changes during the night because of Earth’s rota- tion. It also changes slowly from one night to the next. Every night the stars appear a little farther west than they did at the same time the night before. A star rises about 4 minutes earlier each evening. Four minutes a day for 30 days adds up to about 2 hours a month. If a star is above the horizon during the daytime, the bright Sun will obscure it from view. Thus the stars that shine in your sky at a particular time change noticeably from month to month and from season to season. In 12 months, that 4 minutes a day adds up to 24 hours. After a year, the starry sky looks the same again. The change in the appearance of the sky with the change in seasons is due to the motion of the Earth around the Sun. The Earth revolves, or travels around, the Sun every year.  UNDERSTANDING THE STARRY SKY  21 Picture yourself riding on Earth around the Sun, inside the celestial sphere, looking straight out. As Earth moves along in its orbit, your line of sight points toward different stars in the night sky. During a whole year you view a full circle of stars. (a) If a star is on your zenith at 9:00 P.M. on September 1, about what time will it be on your zenith on March 1? __________ (b) Will you be able to see it? __________ Explain your answer.____________________________________________ ________________________________________________________________________ Answer: (a) About 9:00 A.M. Stars rise about 2 hours earlier every month. (b) No. At that hour of the day the bright Sun obscures the distant stars from view. 1.17 THE ECLIPTIC If the stars were visible during the day, you would see the Sun apparently move eastward among them during the year. The ecliptic, the apparent path of the Sun against the background stars, is drawn on sky globes and star maps for reference. The band about 16° wide around the sky that is centered on the ecliptic is called the zodiac. Ancient astrologers divided the zodiac into 12 constella- tions, or signs, each taken to extend 30° of longitude (see Appendix 3). The zodiac has attracted special attention because the Moon and planets, when they appear in the sky, also follow paths near the ecliptic through these 12 constellations (Figure 1.10). What is the zodiac? ___________________________________________________ ________________________________________________________________________ Answer: A belt about 16° wide around the sky, centered on the ecliptic, containing 12 constellations. 1.18 APPARENT ANNUAL MOTION OF THE SUN The apparent easterly motion of the Sun among the stars is caused by the real revolution of Earth around the Sun. The Sun seems to move in a full circle around the celestial sphere every year. 22  ASTRONOMY Figure 1.10. The Sun’s apparent annual motion around the celestial sphere results from Earth’s real motion around the Sun. As Earth orbits the Sun, different constellations of the zodiac appear in the night sky. About how far does the Sun move on the ecliptic every day? Tip: Use the fact that the Sun moves 360° around the ecliptic in a year (about 365 days). __________ Answer: About 1°. Solution: 360° ≅ 1° per day 365 days 1.19 EARTH’S SEASONS The Sun’s path across the sky is highest in summer and lowest in winter. The altitude of the Sun above the horizon at noon varies during the year because Earth’s axis is tilted to the plane of its orbit around the Sun (Figure 1.11). Earth’s equator remains tilted at about 23.5° to its orbital plane all year long. So as Earth travels around the Sun, the slant of the Earth–Sun line  UNDERSTANDING THE STARRY SKY  23 Figure 1.11. Because Earth’s axis is tilted, each hemisphere gets varying amounts of sun- light during the year as our planet orbits the Sun. changes. Sunlight pours down to Earth from different angles during the year, causing the change of seasons as well as seasonal variations in the length of days and nights. Refer to Figure 1.11. Is the northern hemisphere tipped toward or away from the Sun (a) in December? __________ (b) in June? __________ Answer: (a) Away from; (b) toward. 1.20 EQUINOXES AND SOLSTICES You can determine what the Sun’s apparent position in the sky will be on any given day by checking the ecliptic on a celestial globe or a flat sky map like the one in Figure 1.12. The vernal equinox, which occurs about March 20, is the Sun’s position as it crosses the celestial equator going north. It is the point on the celestial sphere chosen to be the 0h of right ascension (see Section 1.9). The autumnal equinox, which occurs about September 23, is the Sun’s position as it crosses the celestial equator going south. At the equinoxes, day and night are equal in length. The summer solstice, which occurs about June 21, and the winter sol- stice, which occurs about December 21, are the most northern and most southern positions of the Sun during the year. At these times we have the longest and shortest days, respectively, in the northern hemisphere. 24  ASTRONOMY Figure 1.12. Flat sky map. Refer to Figure 1.12. Identify the vernal equinox __________ ; autumnal equinox __________ ; summer solstice __________ ; and winter solstice __________ Answer: vernal equinox (c); autumnal equinox (a); summer solstice (b); winter solstice (d) 1.21 SUN’S ALTITUDE The Sun is never directly overhead for stargazers in the midlatitudes. On a given day, the maximum altitude of the Sun in your sky depends on its decli- nation and your latitude. Where would you have to stand on Earth to have the Sun pass directly across your zenith at the time of the (a) vernal equinox? __________ (b) summer solstice? __________ (c) autumnal equinox? __________ (d) winter solstice? __________ Answer: (a) Equator; (b) 23.5°N latitude (Tropic of Cancer); (c) equator; (d) 23.5°S lati- tude (Tropic of Capricorn).  UNDERSTANDING THE STARRY SKY  25 1.22 OBSERVABLE EFFECTS OF EARTH’S MOTIONS How do the motions of Earth in space cause noticeable changes in the appear- ance of the sky for an observer on Earth? ____________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: Your summary should include the following concepts: The starry sky changes during the night because of Earth’s daily rotation. The visible stars change with the seasons because of Earth’s annual revolution around the Sun. The Sun’s apparent daily motion across the sky is due to Earth’s real rotation. The Sun’s apparent annual motion is due to Earth’s real revolution. 1.23 THE DAY Earth’s rotation provides a basis for keeping time using astronomical observa- tions. The solar day of everyday affairs measures the time interval of Earth’s rotation using the Sun for reference. The sidereal day measures the time interval of Earth’s rotation using the stars for reference. A sidereal day is 23 hours, 56 minutes, 4 seconds long. It is the time inter- val required for a star to cross your meridian two times successively, or the time for Earth to complete one whole turn in space. A solar day is 24 hours long, the length of time required for two successive meridian transits by the Sun. A solar day is about 4 minutes longer than a sidereal day because while Earth rotates on its axis it also moves along in its orbit around the Sun. Earth must complete slightly more than one whole turn in space before the Sun reappears on your meridian (Figure 1.13). A clock that keeps sidereal time is useful for stargazing. In sidereal time, all stars return to their identical positions in the sky every 24 hours. So a star rises, transits the meridian, and sets at the same sidereal time all year long. You can use celestial coordinates (see Table 1.1) to determine the sidereal time at any instant when you are stargazing. Local sidereal time is equal to the right ascension of stars on your meridian. For example, if you see brilliant Sirius transit, the sidereal time is 6 hours, 45 minutes. 26  ASTRONOMY Figure 1.13. A solar day is longer than a sidereal day because during the time Earth rotates it also moves along its orbit around the Sun. In the interval from one noon to the next, Earth completes slightly more than one whole turn in space. What motion of Earth causes the 4-minute difference between a sidereal and a solar day? ______________________________________________________________ Answer: Earth’s revolution around the Sun. 1.24 PRECESSION Your star maps will be useful to you for the rest of your life. You may be inter- ested to know, however, that they will finally go out of date hundreds of years from now. Earth’s axis of rotation shifts extremely slowly around a cone in space once about every 25,800 years. This slow motion of Earth’s axis, caused  UNDERSTANDING THE STARRY SKY  27 mainly by the tug of the gravity of the Sun and Moon on Earth’s equatorial bulge, is called precession. Earth’s axis always tilts 23.5° to its orbital plane, so precession causes the north celestial pole to circle among the stars. After thousands of years, the polestar changes (Figure 1.14). The vernal equinox, the zero point of right ascension, drifts westward around the ecliptic at a rate of about 50 seconds a year. It drifts 30°, a whole zodiac constellation, in 2150 years. Then all star charts are out of date. (Astronomers revise their precise star charts regularly.) In astrology today, each sign of the zodiac bears the name of the constel- lation for which it was originally named but with which it no longer coincides due to precession of the equinoxes. Refer to Figure 1.14. The present polestar is Polaris, and the vernal equinox is located in the constellation Pisces. (a) What was the polestar in the year 3000 B.C.? __________ (b) What will it be in the year A.D. 14,000? __________ Answer: (a) Thuban; (b) Vega. Figure 1.14. Precession. Earth’s axis very slowly traces out a cone in space, so eventually the polestar changes. 28  ASTRONOMY SELF-TEST This self-test is designed to show you whether or not you have mastered the material in Chapter 1. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test. 1. For each of the following references used on a terrestrial globe, list the corre- sponding name on the celestial sphere: (a) Equator. ___________________________________________ (b) North Pole. ___________________________________________ (c) South Pole. ___________________________________________ (d) Latitude. ___________________________________________ (e) Longitude. ___________________________________________ (f) Greenwich, England. ___________________________________________ 2. Refer to Table 1.1. Which of the five brightest stars in the sky are above the celestial equator, and which are below? _________________________________ ____________________________________________________________________ 3.. Refer to Table 1.1. Which of the five brightest stars never appear above the horizon at latitude 40° (about New York City)? ___________________________ ____________________________________________________________________ 4. Match where you might be on Earth with the correct description of the stars: _____ (a) The stars seem to move (1) Antarctica (below 61°S). along circles around sky (2) Equator. parallel to your horizon. (3) Jacksonville, Florida, U.S. _____ (b) The stars rise at right (30°22′N). angles to the horizon in (4) North Pole. the east and set at right angles to the horizon in (5) Sacramento, California, the west. U.S. (38°35′N). _____ (c) Vega practically crosses your zenith. _____ (d) Acrux is always above your horizon. _____ (e) Polaris appears about 30° above your horizon.  UNDERSTANDING THE STARRY SKY  29 5. Why do the stars appear to move along arcs in the sky during the night? ____________________________________________________________________ 6. Why do some different constellations appear in the sky each season? ____________________________________________________________________ 7. What is the zodiac? ___________________________________________________ ____________________________________________________________________ 8. Where on Earth would you have to be to have the Sun pass directly across your zenith at the time of the (a) vernal equinox? ________________________ (b) summer solstice? __________ (c) winter solstice? ______________________ 9. If a star rises at 8 P.M. tonight, at approximately what time will it rise a month from now? __________________________________________________________ 10. Why is a solar day about 4 minutes longer than a sidereal day? _____________ ____________________________________________________________________ 11. Arrange the following stars in order of decreasing brightness: Antares (magni- tude 1); Canopus (magnitude –1); Polaris (magnitude 2); Vega (magnitude 0).__________________________________________________________________ 12. Why will the polestar and the location of the vernal equinox on the celestial sphere be different thousands of years from now, causing your star maps finally to go out of date? ______________________________________________ ____________________________________________________________________ 30  ASTRONOMY ANSWERS Compare your answers to the questions on the self-test with the answers given below. If all of your answers are correct, you are ready to go on to the next chapter. If you missed any questions, review the sections indicated in parentheses following the answer. If you missed several questions, you should probably reread the entire chapter carefully. 1. (a) Celestial equator. (d) Declination. (b) North celestial pole. (e) Right ascension. (c) South celestial pole. (f) Vernal equinox. (Sections 1.1, 1.8, 1.9) 2. Above: Arcturus, Vega. Below: Sirius, Canopus, Rigil Kentaurus. (Sections 1.9, 1.10) 3. Canopus, Rigil Kentaurus. (Sections 1.10, 1.13, 1.14) 4. (a) 4; (b) 2; (c) 5; (d) 1; (e) 3. (Sections 1.10, 1.13 through 1.15) 5. Because of Earth’s rotation. (Sections 1.1, 1.12, 1.14) 6. Because of Earth’s revolution around the Sun. (Section 1.16) 7. A belt about 16° wide around the sky centered on the ecliptic, containing 12 constellations. (Section 1.17) 8. (a) Equator; (b) 23.5°N (Tropic of Cancer); (c) 23.5°S (Tropic of Capricorn). (Sections 1.19 through 1.21) 9. 6 P.M. (Section 1.16) 10. Because, while Earth rotates on its axis, it also moves along in its orbit around the Sun. Earth must complete slightly more than one whole turn in space before the Sun reappears on your meridian. (Section 1.23) 11. Canopus, Vega, Antares, Polaris. (Section 1.7) 12. Because of the precession of Earth’s axis. (Section 1.24)  2 LIGHT AND TELESCOPES Curiosity is one of the permanent and certain characteristics of a vigorous mind. Samuel Johnson (1709–1784) The Rambler Objectives  Describe the wave nature of light, including how it is produced and how it travels.  Name the major regions of the electromagnetic spectrum from the shortest wavelength to the longest.  State the relationship between wavelength and frequency.  State the relationship between the color of a star and its temperature.  List the three windows (spectral regions) in Earth’s atmosphere in order of their importance to observational astronomy.  Explain how refracting and reflecting telescopes work.  Define light-gathering power, resolving power, and magnification with respect to a telescope.  State the two most important factors in telescope performance.  State the purpose of a spectrograph.  Explain how radio telescopes work, and list some interesting radio sources.  Explain why infrared telescopes are located in very high, dry sites, and list some objects they observe.  Explain why ultraviolet, X-ray, and gamma ray telescopes must operate above Earth’s atmosphere, and list some objects they study. 31 32  ASTRONOMY Figure 2.1. Visualizing a light wave. 2.1 WHAT IS LIGHT? Most of our information about the universe has been obtained through the analysis of starlight. To explain how starlight travels across trillions of kilome- ters of empty space to waiting telescopes, astronomers picture light as a form of wave motion. A wave is a rising and falling disturbance that transports energy from a source to a receiver without the actual transfer of material. Wave motion is clearly observable in the ocean. During storms, crashing ocean waves vividly reveal the energy they carry. A light wave is an electromagnetic disturbance consisting of rapidly varying electric and magnetic effects. Light waves transport energy from accelerating elec- tric charges in stars (the source) to electric charges in the retina of your eye (the receiver) (Figure 2.1). You become aware of that energy when you see starlight. What is a wave? ______________________________________________________ ________________________________________________________________________ Answer: A wave is a rising and falling disturbance that transports energy from a source to a receiver without the actual transfer of material. 2.2 WAVELENGTH Light waves are distinguished by their lengths. The distance from any point on a wave to the next identical point, such as from crest to crest, is called the wavelength (Figure 2.2). The human eye responds to waves that have extremely short wavelengths. Physicists and astronomers measure these waves in nanometers, nm, or the angstrom unit, Å, after Swedish physicist Anders J. Ångstrom (1814–1874), who first measured wavelengths of sunlight. One nm is 10–9 m, and one angstrom is 0.10 nm. The diameter of a human hair is about 50,000 nm (500,000 Å)! Visible light has wavelengths of 4000 Å to 7000 Å. The varying wave-  LIGHT AND TELESCOPES  33 Figure 2.2. Wavelength measured from crest to crest or trough to trough. lengths of visible light are perceived as different colors. The arrangement of the colors according to wavelength is called the visible spectrum. Refer to Figure 2.3. Which color light has (a) the shortest wavelength? __________ (b) the longest wavelength? __________ (c) To which wavelength (color) is the eye most sensitive? __________ Answer: (a) Violet; (b) red; (c) 5550 Å ( yellow–green). Figure 2.3. Relative sensitivity of the human eye to different colors and wavelengths of visible light. 34  ASTRONOMY 2.3 THE ELECTROMAGNETIC SPECTRUM Visible light is only one small part of all the electromagnetic radiation in space. Energy is also transmitted in the form of gamma rays, X-rays, ultravio- let radiation, infrared radiation, and radio waves. Figure 2.4. The electromagnetic spectrum includes all electromagnetic radiation from shortest, highest-frequency gamma rays to longest, lowest-frequency radio waves.  LIGHT AND TELESCOPES  35 Because we make such different uses of them, these forms of radiation seem very different from one another. Doctors use gamma rays in cancer treat- ment and X-rays for medical diagnosis. Ultraviolet rays give you a suntan, and infrared rays warm you up. Radio waves are used for communication. All of these forms of radiation are really the same basic kind of energy as visible light. They have different properties because they have different wave- lengths. The shortest waves have the most energy, whereas the longest waves are the least energetic. The whole family of electromagnetic waves, arranged according to wavelength, is called the electromagnetic spectrum. Electromagnetic waves of all wavelengths are important to astronomers because each type brings unique clues about its source. Refer to Figure 2.4. List six forms of electromagnetic radiation from the short- est waves (highest energy) to the longest waves (lowest energy). _______________ ________________________________________________________________________ Answer: Gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, radio waves. 2.4 RANGE OF WAVELENGTHS What is the range of wavelengths included in the whole electromagnetic spec- trum? ___________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: Wavelengths vary from less than a trillionth of a meter, 10–12 m, for the shortest gamma rays to longer than a kilometer, 103 m (a mile), for the longest radio waves. 2.5 SPEED OF LIGHT All kinds of electromagnetic waves move through empty space at the same speed—that is, at the speed of light. The speed of light in empty space, usually symbolized by the letter c, is practically 300,000 km/second (186,000 miles per second). The speed of light in empty space has been called the “speed limit of the universe,” because no known object can be accelerated to move faster. It is one of the most important and precisely measured numbers in astronomy (Appendix 2). 36  ASTRONOMY A light-year (ly) is the distance light travels through empty space in one year. How many kilometers (miles) does 1 light-year represent? Tips: (1) distance = speed × time. (2) A year is equal to 3.156 × 107 seconds. ______________________ Answer: Practically 9.5 trillion km (6 trillion miles). Solution: Multiply 300,000 km/second × 3.156 × 107 second/year (186,000 miles/second × 3.156 × 107 seconds/year) 2.6 WAVE FREQUENCY Wave motion can be described in terms of frequency as well as wavelength. The frequency of a wave motion is the number of waves that pass by a fixed point in a given time, measured in cycles per second (cps). The human eye responds to different-color light waves that have very high frequencies. Visible light waves vary in frequency from 4.3 × 1014 cps for red to 7.5 × 1014 cps for violet, with the other colors in between. For radio waves, one cycle per second is commonly called a hertz (Hz), after the German physicist Heinrich Hertz (1857–1894), who first produced radio waves in a laboratory. An AM radio receives radio waves with frequencies of 550 to 1650 KHz (kilohertz); 1 KHz is 1000 cycles per second. The FM band ranges from 88 to 108 MHz (megahertz); 1 MHz is a million cycles per second. Refer to the electromagnetic spectrum shown in Figure 2.4. Which waves have (a) a higher frequency than the visible light waves? ______________________ _________________________________ (b) a lower frequency than the visible light waves? _________________________________________________________________ Answer: (a) Higher frequency: gamma rays, X-rays, ultraviolet radiation. (b) Lower fre- quency: infrared radiation, submillimeter waves, microwaves, radio waves. 2.7 WAVELENGTH AND FREQUENCY Can you deduce a general relationship between wavelength and frequency for these electromagnetic waves? ______________________________________________ ________________________________________________________________________  LIGHT AND TELESCOPES  37 Answer: The wavelength is inversely proportional to the frequency. The shorter waves have a relatively higher frequency, and the longer waves have a relatively lower frequency. 2.8 WAVE PROPAGATION The relationship you have just found is an example of a formula that holds true for all kinds of wave motion: Speed of wave = Frequency × Wavelength You can use this formula to calculate the frequency of any kind of electro- magnetic wave in empty space if you know its wavelength (or the wavelength if you know the frequency). Explain why. Tip: Review Section 2.5. ________________ ________________________________________________________________________ ________________________________________________________________________ Answer: All electromagnetic waves have the same speed in empty space—that is, the speed of light, or about 300,000 km/second (186,000 miles per second). 2.9 WAVE EQUATION Be sure you understand the relationship between speed (c), frequency (f ), and wavelength (λ) for electromagnetic waves. The formula is: c = fλ Calculate the wavelength of a radio wave whose frequency is 100 KHz (100,000 cycles per second). ___________________ Answer: 3 km (1.86 miles). Solution: Speed = Frequency × Wavelength Thus, Speed 300,000 km/second Wavelength = = Frequency 100,000 cycles/second 186,000 miles/second = 100,000 cycles/second 38  ASTRONOMY 2.10 RADIATION LAWS Stars, like other hot bodies, radiate electromagnetic energy of all different wavelengths. Energy due to temperature is called thermal radiation. The temperature of a star determines which wavelength is brightest. Stars radiate energy practically as a blackbody, or theoretical perfect radiator. The intensity of radiation emitted over a range of wavelengths depends only on the blackbody’s temperature. Wien’s law of radiation states that the wavelength, λmax, at which a blackbody emits the greatest amount of radiation is inversely proportional to its temperature (T). The formula is λmax = 0.3 T where λmax is in centimeters and T is in kelvin (K). Thus the hotter a star, the shorter the wavelength at which it emits its maximum radiation. Some stars are thousands of degrees hotter than others. You can judge how hot a star is by its color (wavelength). The hottest stars look blue-white (short wavelength), and the coolest stars look red (long wavelength). Look in the sky for the examples cited in Table 2.1. Figure 2.5. The Sun’s thermal radiation spectrum. All blackbody radiation spectrums have the same shape. Hotter stars emit more energy at all wavelengths, and the peak shifts to shorter wavelengths.  LIGHT AND TELESCOPES  39 TABLE 2.1 Four Hot and Cool Stars Surface Temperature Season Star Constellation Color (K) Summer Vega Lyra Blue-white 10,000 Summer Antares Scorpius Red 3,000 Winter Sirius Canis Major Blue-white 10,000 Winter Betelgeuse Orion Red 3,400 The Stefan-Boltzmann radiation law states that the total energy (E), emitted by a blackbody is proportional to the fourth power of its absolute temperature (T). Thus a star that is twice as hot as our Sun radiates 24, or 16, times more energy than the Sun. A radiation spectrum shows how much energy a body radiates at differ- ent wavelengths, which wavelengths it radiates most intensely, and the total amount of energy it radiates at all wavelengths (indicated by the area under the curve). Examine Figure 2.5. (a) The Sun radiates most intensely in the __________ wavelengths. (b) The total amount of energy that the Sun radiates as visible light is (more, less) __________ than the amount radiated outside the visible region. Answer: (a) Visible; (b) less. 2.11 ASTRONOMICAL OBSERVATIONS Today astronomers have tools to observe and analyze all forms of electromag- netic radiation from space. The main function of a telescope—whatever type of radiation is being detected—is to gather sufficient radiation for analysis. Earth’s atmosphere stops most radiation from space and permits only cer- tain wavelengths to shine through to telescopes on the ground. Ground-based astronomers look out at the universe through two atmospheric windows, or spectral ranges within which air is largely transparent to radiation. These are the optical/visible light including some infrared, and radio windows. An astronomical observatory is a place equipped for the observation of sky objects. For ground-based observations, astronomers choose dark sites where the air is dry, thin, and steady, on mountaintops far from city lights and pollution (Figure 2.6). 40  ASTRONOMY Figure 2.6. Mauna Kea, a 4200-m (13,800-ft.)-high site on the Island of Hawaii, U.S., hosts the world’s largest group of optical, infrared, and submillimeter telescopes. Mauna Kea Visitors Information Station http://www.ifa.hawaii.edu/info/vis is at 3000 m (9200 ft.). What would you suggest to astronomers who want to observe the universe in the gamma ray, X-ray, and ultraviolet ranges? ________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: Locate their instruments beyond Earth’s atmosphere. Space age technology makes space-based observations in these wavelength bands possible from rockets, space- craft, or even Moon-based observing stations. 2.12 OPTICAL TELESCOPES An optical telescope forms images of faint and distant stars. It can collect much more light from space than the human eye can. Optical telescopes are built in two basic designs—refractors and reflectors. The heart of a telescope is its objective, a main lens (in refractors) or a mirror (in reflectors). Its function is to gather light from a sky object and  LIGHT AND TELESCOPES  41 focus this light to form an image. The ability of a telescope to collect light is called its light-gathering power. Light-gathering power is proportional to the area of the collecting sur- face, or to the square of the aperture (clear diameter of the main lens or mir- ror). The size of a telescope, such as 150-mm or 8-m (6-inch or 26-foot), refers to the size of its aperture. You can look at the image directly through an eyepiece, which is essen- tially a magnifying glass. Or you can photograph the image or record and process it electronically. Your eye lens size is about 5 mm (0.2 inch). A 150- mm (6-inch) telescope has an aperture over 30 times bigger than your eye lens. Its light-gathering power is 302, or 900 times greater than that of your eye. So a star appears over 900 times brighter with a 150-mm (6-inch) tele- scope than it does to your unaided eye. Astronomers build giant telescopes to detect ever fainter and more distant objects. All stars appear brighter with telescopes than they do to the eye alone. The extra starlight gathered by the telescope is concentrated into a single point. Using time exposure, a giant 10-m (400-inch) telescope can image very faint stars down to about magnitude 28, which is the same apparent bright- ness as a candle viewed from the Moon! How much brighter would a star appear with the 10-m (33-foot) telescope than to your unaided eye? Explain. _________________________________________ ________________________________________________________________________ ________________________________________________________________________ Answer: Over 4 million times brighter. The 10-m (33-foot) telescope is over 2000 times bigger than your eye lens, so it gathers over 20002, or 4 million, times more light. 2.13 BINOCULARS Binoculars are a practical first instrument for stargazing because they are easy to use and portable. A pair labeled 7 × 50 has an aperture of 50 mm. The 7× specifies the magnification. Why do binoculars and telescopes reveal many more sky objects than you can see with your unaided eye? ____________________________________________ ________________________________________________________________________ 42  ASTRONOMY Answer: They can collect much more light than your eye can. (Light-gathering power is proportional to the square of the aperture.) 2.14 REFRACTING TELESCOPES A refracting telescope has a main, objective lens permanently mounted at the front end of a tube. Starlight enters this lens and is refracted, or bent, so that it forms an image near the back of the tube. The distance from this lens to the image is its focal length. You may look at the image through a removable magnifying lens called the ocular, or eye- piece. The tube keeps out scattered light, dust, and moisture. Italian astronomer Galileo Galilei (1564–1642) first pointed a refracting telescope skyward in 1609. The largest instrument he made was smaller than 50 mm (2 inches). Today refracting telescopes range in size from a beginner’s 60-mm (2.4-inch) to the largest ever built, the 1-m (40-inch) telescope at the Yerkes Observatory in Williams Bay, Wisconsin, U.S., which was completed in 1897. Refer to Figure 2.7. Identify the refracting telescope’s (a) objective lens; (b) the eyepiece; and (c) the focal length of the objective lens. State the purpose of (a) and (b). (a) ___________________________________________________________ Figure 2.7. A refracting telescope with a long focal length objective lens and a short focal length eyepiece.  LIGHT AND TELESCOPES  43 (b) ____________________________________________________________________ (c) ____________________________________________________________________ Answer: (a) Objective lens: to gather light and form an image. (b) Eyepiece: to magnify the image formed by the objective. (c) Focal length of objective lens. 2.15 REFLECTING TELESCOPES A reflecting telescope has a highly polished curved-glass mirror, the primary mirror, mounted at the bottom of an open tube. When starlight shines on this mirror, it is reflected back up the tube to form an image at the prime focus. You can record the image at the prime focus, or you can use additional mirrors to reflect the light to another spot. The Newtonian telescope, origi- nated by British scientist Sir Isaac Newton in 1668, uses a small, flat mirror to reflect the light through the side of the tube to an eyepiece (Figure 2.8). The Cassegrain telescope uses a small convex mirror, a secondary mir- ror, to reflect the light back through a hole cut in the primary mirror at the bottom end of the tube (Figure 2.9). It is more compact than a refractor or Newtonian reflector of the same aperture. The Schmidt-Cassegrain telescope combines an extremely short-focus spherical primary mirror at the back end of a sealed tube with a thin lens at the front. Reflecting telescopes range in size from a beginner’s 76-mm (3-inch) Newtonian reflector to the world’s largest, the 10.4-m (34-foot) Gran Telescopio Canarias atop a peak on the Canary Island La Palma, Spain. Figure 2.8. A Newtonian reflecting telescope with a primary mirror, a small diagonal secondary mirror, and an eyepiece. 44  ASTRONOMY Figure 2.9. A Cassegrain reflecting telescope with a concave primary mirror, a small con- vex secondary mirror, and an eyepiece. Refer to Figures 2.8 and 2.9. Identify the reflecting telescope’s primary mirror; eyepiece; and prime focus. (a) __________ ; (b) __________ ; (c) __________ Answer: (a) Eyepiece; (b) prime focus; (c) primary mirror. 2.16 REFLECTORS VERSUS REFRACTORS What is the essential difference between a reflecting telescope and a refracting tel- escope? Explain.__________________________________________________________ ________________________________________________________________________ Answer: The main optical part (objective). A reflecting telescope uses a mirror, whereas a refracting telescope uses a lens to collect and focus starlight. 2.17 f NUMBER Telescopes are often described by both their aperture size and f number. The f number is the ratio of the focal length of the main lens or mirror to the aper- ture. These specifications are important because the brightness, size, and clar- ity of the image produced by a telescope depend on the aperture and focal length of its main lens or mirror. For example, a “150-mm (6-inch), f/8 reflector” means the primary mirror is 150 mm (6 inches) in diameter and has a focal length of 1200 mm (8 × 150), or 48 inches (8 × 6).  LIGHT AND TELESCOPES  45 What is the focal length of the 5-m (200-inch), f/3.3 mirror on Mount Palomar in California, U.S.? ________________________________________________ Answer: 16.5 m (660 inches, or 55 feet). 2.18 IMAGES All stars except our Sun are so far away that they appear as dots of light in a telescope. The Moon and planets appear as small disks. Image size is propor- tional to the focal length of the telescope’s main lens or mirror. For example, a mirror with a focal length of 2.5 m (100 inches) produces an image of the Moon that measures about 2.5 cm (1 inch) across. You know that the 5-m (200-inch), f/3.3 mirror has a focal length of 16.5 m (660 inches), which is over six times as long. Hence, it produces an image of the Moon that is about six times as big, or 15 cm (6 inches) across. Lenses and mirrors form real images that are upside down. (A real image is formed by the actual convergence of light rays.) Since inverted images do not matter in astronomical work, and righting them would require additional light-absorbing optics, nothing is done to turn images upright in telescopes. What determines the size of the image formed by a telescope? _____________ ________________________________________________________________________ Answer: The focal length of the main lens or mirror. 2.19 RESOLVING POWER Even if a telescope were of perfect optical quality, it would not produce perfectly focused images because of the nature of light itself. A telescope’s resolving power is its ability to produce sharp, detailed images under ideal observing conditions. Resolving power depends directly on the size of the aperture and inversely on the wavelength of the incoming light. For the same light, a 150- mm (6-inch) telescope has twice the resolving power of a 75-mm (3-inch) tele- scope. Starlight travels in straight lines through empty space, but when waves of starlight pass close to the edge of a lens or mirror, they spread out, in an effect called diffraction, and come to a focus at different spots. Because of diffraction, the image of a star formed by a lens or mirror appears under magnification as 46  ASTRONOMY Figure 2.10. Diffraction pattern (image of a star). a tiny, blurred disk surrounded by faint rings, called a diffraction pattern, instead of as a single point of light (Figure 2.10). Diffraction limits resolving power. If two stars are close together, the