• Author / Uploaded
• Simon_98

• Categories
• Estrellas
• Telescopio
• Corona
• Radiación electromagnética
• Espectroscopia

Citation preview

Fundamental Physical Constants1 Constant

SI Units

Gravitational con tant, G Velocity of light in vacuum, c Magnetic field constant, JJ-o

6.673 · 10- I I 2.9979· 108 1.2566 · Io- 6

Electric field con tant, eo

8.8542·10- 12 = Asv - 1 m 6.6261 .w- 34 1.0546· Io- 34 1.6022. 10- 19 1.6605. 10- 27

Planck's constant. h h = h/2rr

Elementary (electron· ) charge. e Atomic ma unit. mu = u Ret ma ofth proton lllp neutron mn electron me hydrogen atom ntH tnp/llle

pecific charge of the electron, efme Avogadro's number, NA Univer al gas con tant, :R Boltzmann' con tant. k Fine- tructure con tant, a Compton wavelength of the proton Ap electron Ae Classical electron radiu , rc Thomson cro ection, aT Rydberg's con tant, R I st Bohr radiu . ao Planck' radiation constant. tefan-Boltzmann radiation con tant. a

c2

=

Gaus ian Units m3 kg-

w-

I -2

m -I m kg - 2 A-2

8 cm 3 g- 1 s-2 6.67 . 2.9979· 10 10 cms- 1

V A- 1 m- 1

1.6726. 1o- 27 1.0073 1.6749. w- 27 1.0087 9.1094· 10- 3 1 5 .4858 . w-.; 1.6736. 10- 27 1.0078 1.8362. 103 1.7588-10 11 6.0221· 1023 8.3145 1.3807·10- 23 7.2974· w- 3 =

m- 3 kg- 1

4

A2

1

m2 kgs- 1 = J s

w-

kg

21 ergs 6.6261 . 1.0546. w- 27 erg 4. 032. w - IO esu 1.6605. w- 24 g

kg

1.6726. lo-24 g

Js

A =

c

u

kg u kg u kg u Ckg-1 mol - 1 Jmol-tK-1 JK- 1

1.6749.

w- 24 g

9.1094·10- 28 g 1.6736. 1o- 24 g

5.2728 · 10 17 e u g-

1

8.3145· 107 ergmol - 1 K- 1 1.3807 · I0- 16 ergK-I

1/137.036

1.3214·10- 15 2.4263. 10- 12 2.4263. 1o- 2 2.8179·10- 15 6.6525. 1o- 29 1.0974 · 107 1/911.27 5.2918· 10- 11 1.4388 . w- 2

m mK

5.2918. 1.4388

5.6704. 1o- 8

wm- 2 K- 4

5.6704·10- 5 ergcm- 2 s- 1 K- 4

m m

1.3214·10- 13 cm 2.4263. w-IO em

m m2 m- 1

2. 179 ·10- 13 em 6.6525 · 10- 25 cm 2 1.0974 · lo5 cm- 1

-I

w- 9

em

em K

1 All numerical values were rounded to 5 significant digits. For more precise values (with error limits) see P.J. Mohr, B.N. Taylor: CODATA recommended values of the fundamental physical constants: 1998. Rev. Modem Phys. 72, 351

(2000)

Astronomical Constants and Units I AU= 1.496· 10 11 m = 499.0 light second. I pc = 3.086 · 10 16 m = 2.063. 105 A = 3.261ight years 365.256d = 3. 1558. 107 s 365.242d = 3.1557. 107 s I mag corrc. ponds to a brightness ratio of 2.512 = I0°..1

A tronomical unit Parsec idereal year Tropical year Magnitude

Earrlz: RE = 6.378 ·106 m ME=5.97 3 · 1024 kg S = 1.37kwm - 2

Equatorial radius Mas olar onstant

Sun: Radius Mas Luminosit y Effective temperatu re urface gravity Ab olute vi ual magnitude

R0 .M.0 L'0

Rotational period Oort con ·tants

Ro

= 8.5 kpc

CtJQ

=26kms

Vt> = 220 km s -

1)

Critical density Temperatu re of the microwav e backgroun d radiation Planck time

~

I

1 kpc- 1

to

= 2JT / CtJQ = 2.4- 108 yr

A

14kms= - 12 km s

8

=

o

=3.846 · I0~6 w

Tell..,= 5780 K =2.736 · 102 m g~Z. Mv.0 = 4.87 mag

Milky Way: Distance un -galactic center Rotational velocity

Universe: Hubble con tant h Ho/(50km s- 1 MpcHubble time

= 6.960 · 10M m = 1. 989 . 1 30 kg

=

1 kpc 1

1 kpc 1

Ho =50hkm s- 1 Mpc~ 1.4 ,-, ro Qc.O

= 1/ Ho = 6.2·10 17 ii- 1 s = 19.6· 109 1! - I yr = 4 .7 · I 0

27

llc.o

= 3 ii 2 m - 3

To

= 2.73K

Tp

1

= 5.4 · 10

.j.j

ii~ kg m- 3

Albrecht Unsold Bodo Baschek

The New Cosmos

Albrec ht Unsold Bodo Baschek

The New

cosmos An lntroduc tion to Astrono my and Astrophysics Translated by William D. Brewer Fifth Edition With 278 Figures lncluding 20 Color Figures

Professor Dr. Albrecht Unsold t Professor Dr. Bodo Baschek Institut fiir Theoretische Astrophysik Universitat Heidelberg Albert-Oberie-StraBe 2 69120 Heidelberg, Germany

Translator: Professor William D. Brewer, Ph. D. Fachbereich Physik, Freie Universitat Berlin, Arnimallee 14, 14195 Berlin, Germany

Corrected 2nd printing ISBN 978-3-642-08746-2

Library of Congress Cataloging-in-Publication Data. Unsold, Albrecht, 1905- [Neue Kosmos. English] The new cosmos: an introduction to astronomy and astrophysics.- 5th ed./ Albrecht UnsOld, Bodo Baschek; translated by William D. Brewer. p. em. Includes bibliographical references and index. ISBN 978-3-662-04356-1 (eBook) ISBN 978-3-642-08746-2 DOI 10.1007/978-3-662-04356-1 I. Astronomy. I. Baschek, B., 1935- II. Title QB43.3 .U5713 2001 520-dc21 2001034462 Cover: Optical image of the radio galaxy NGC5128 = Centaurus A. Recorded by the Anglo-Australian Telescope. (© Anglo-Australian Observatory, photograph by David Malin) Frontispiece: The galaxy group (NGC 6769-71) in Pavo with indications of gravitational interaction between the galaxies: faint haloes and connecting arcs, deforming of spiral arms and central regions. (By S. Laustsen with the 3.6 m-telescope of the European Southern Observatory)

Title of the German edition: A. Unsold, B. Baschek: Der neue Kosmos. Siebte Auflage © Springer-Verlag Berlin Heidelberg 1967, 1974, 1981, 1988, 1991, 1999, 2002 Originally published by Springer-Verlag Berlin Heidelberg New York in 2002 Softcover reprint of the hardcover 5th edition 2002 ISBN 978-3-642-08746-2

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag Berlin Heidelberg GmbH. Violations are liable for prosecution under the German Copyright Law. springeronline.com

©Springer-Verlag New York Inc. 1969, 1977, 1983 © Springer-Verlag Berlin Heidelberg 1991, 2001

The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Production and Typesetting: LE-TeX, Jelonek, Schmidt & Vockler GbR, Leipzig Layout: Schreiber VIS, Seeheim Cover Design: Erich Kirchner, Heidelberg

Printed on acid-free paper 55/3141/yl 543210

v

Preface to the Corrected Reprint In the preparation of this corrected and updated reprint of the 5th edition, Prof. Wolfgang J. Duschl once more provided active help with many contributions and exciting discussions. My dearest thanks to him. I would also like to thank Prof. W. Beiglbt:ick and Claus-Dieter Bachem from Springer most sincerely for their constructive collaboration in the preparation of this reprint.

I take this opportunity to note that, to my great pleasure, Wolfgang J. Dusch! is prepared to work together with me as author of all further, new editions of "The New Cosmos".

Heidelberg, December 2004

Bodo Baschek

Preface to the 5th Edition Albrecht Unsold's Der Neue Kosmos, in which he gave an overview of the whole field of astronomy that was intended to be accessible to all students and practitioners of the natural sciences, first appeared in 1967. The title was deliberately chosen with reference to Alexander von Humboldt's Kosmos and expressed the author's intention of "making our new understanding of the Universe clear" to a wide group of readers and of "allowing the basic ideas of the various areas of astronomical research with their factual and historical-humanistic connections to come into the foreground" . After the third edition, new editions of the book were prepared with the collaboration of B. Baschek, who gradually took over responsibility for it. Prof. Dr. Dr. h.c. mult. Albrecht Unsold died in 1995 at the age of 90; he was a pioneer and an Old Master of astrophysics. Although he announced his retirement from active research "officially" in 1988 in the Foreword to the 4th German edition of this book, he remained keenly interested in the further development of this introduction to astronomy and astrophysics and continued to contribute to its revisions through discussions and observations. Now, ten years after the appearance of the 4th edition, a completely revised and updated version of the New Cosmos, translated from the 7th German edition, has been completed; it takes into account the wealth of new results from astronomical research which have appeared in recent years, ranging from our Solar Sys-

tern to the most distant galaxies. In this new edition, the organization of the material has been made clearer by introducing some changes in the order of presentation as well as by a finer subdivision of the topics covered. Within our Solar System, space probes have investigated the Moon and Mars, the Jupiter system, some of the asteroids, and the solar wind from close up. New satellites have brought an enormous increase in observation power ranging from the gamma-ray region down to the radiofrequency spectral range; especially noteworthy is the Hubble Space Telescope with its incomparable angular resolution in the optical and near-ultraviolet, as well as the two large X-ray satellites, Chandra and XMM Newton. Furthermore, in the past decade, a new generation of large earthbound telescopes based on active and adaptive optics has come into use. Among the discoveries and events of this period we mention the following: the numerous planetoids found outside the orbit of Neptune, the impact of a comet onto Jupiter, and the appearance of the bright comets Hyakutake and Hale-Bopp; advances in solar seismology, the resolution of very detailed structures in the regions of star formation and in planetary nebulae, the evidence for black holes in the centers of our Milky Way and of other galaxies; the observation of very distant supernova explosions, and the precise determination of the fluctuations in the 3 K cosmic background radiation, giving indications of a flat Universe with a nonzero cosmo-

VI

logical constant; the view of most distant galaxies in the Hubble Deep Field, obtained with the Hubble Space Telescope; the new evidence for neutrino oscillations, the explanation of the origin of the gamma-ray bursts which had remained a riddle for decades; and finally the discoveries of numerous planets orbiting nearby stars. My thanks go to all those readers of the earlier editions who have contributed to the improvement of this book through their suggestions, criticisms and detection of errors. Furthermore, I wish to thank my colleagues W.J. Duschl, D. Fiebig, B. Fuchs, H. Holweger, G. Klare, M. Scholz, A. Schwope, P. Ulmschneider, C. van de Bruck, R. Wehrse, and G. Weigelt for critical readings of various sections and for their comments

and suggestions on numerous topics. In particular, I owe sincere thanks to Prof. Wolfgang J. Duschl for his aid in the choice of new illustrations and for their acquisition and electronic image processing. It was a great asset for the preparation of this book that Prof. William D. Brewer was once again willing to undertake the translation. I wish to express my heartfelt thanks for his excellent job as well as for very agreeable and constructive collaboration. Prof. W. Beiglbock, Dr. H. Lotsch, and Mr. C.-D. Bachem of SpringerVerlag are due my sincere gratitude for their excellent cooperation, as always, in the completion of this edition. Heidelberg, June 2001

Bodo Baschek

VII

Contents 1. Introduction...................................................................................................

1

I. Classical Astronomy and the Solar System .................................................

5

Humanity and the Stars: Observing and Thinking Historical Introduction to Classical Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Classical Astronomy

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.2 2.2.1 2.2.2 2.2.3

Spatial Coordinates and Time; the Motions of the Sun, the Earth, and the Moon . . . . . . . . . . The Celestial Sphere and Astronomical Coordinate Systems . . . . . . . . . . . . . . . . . . . . . . . . The Motions of the Earth. Seasons and the Zodiac . . . . . . . . . . . . Time: Days, Years, and the Calendar . . . . . . . . . . . . . . . . . . . . . . . . . . The Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eclipses of the Sun and the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

10 10 12 15 17 19

Orbital Motions and Distances in the Solar System . . . . . . . Planetary Motions and Orbital Elements . . . . . . . . . . . . . . . . . . . . . . Comets and Meteors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distance Determination, the Doppler Effect and Aberration of Light . . . . . . . . . . . . . . . . . . .

20 20 23

2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6

Mechanics and Gravitational Theory . . . . . . . . . . . . . . . . . . . . . . . Newton's Laws of Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . The Conservation of Linear Momentum . . . . . . . . . . . . . . . . . . . . . . . Conservation of Angular Momentum: the Area Theorem . . . . . Conservation of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Virial Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Law of Gravitation. Gravitational Energy . . . . . . . . . . . . . . . .

27 28 29 29 30 31 31

2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7

Celestial Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kepler's First and Second Laws: Planetary Orbits . . . . . . . . . . . . Kepler's Third Law: Determination of Masses . . . . . . . . . . . . . . . . Conservation of Energy and the Escape Velocity . . . . . . . . . . . . . . Rotation and the Moment of Inertia . . . . . . . . . . . . . . . . . . . . . . . . . . . Precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Tides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Ptolemaic and the Copernican Worldviews . . . . . . . . . . . . . . .

33 33 34 35 35 36 36 37

2.5 2.5.1 2.5.2 2.5.3 2.5.4

Space Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Orbits of Artificial Satellites and Space Vehicles . . . . . . . . . Astronomical Observations from Space . . . . . . . . . . . . . . . .. . .. . .. The Exploration of the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Space Probe Missions in the Solar System . . . . . . . . . . . . . . . . . . . .

38 39 40 42 43

25

ICooteot< VIII

3. The Physical Structure of the Objects in the Solar System

3.1 3.1.1 3.1.2 3.1.3 3.1.4

Global Properties of the Planets and Their Satellites . . . . . . Ways of Studying the Planets and Their Satellites . . . . . . . . . . . . The Global Energy Balance of the Planets . . . . . . . . . . . . . . . . . . . . Interior Structure and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Structures of Planetary Atmospheres . . . . . . . . . . . . . . . . . . . . .

46 46 48 48 50

3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6

The Earth, the Moon, and the Earthlike Planets . . . . . . . . . . . The Internal Structures of the Earthlike Planets . . . . . . . . . . . . . . . Radioactive Dating. The Earth's History . . . . . . . . . . . . . . . . . . . . . . Magnetic Fields. Plate Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Lunar Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Surfaces of the Earthlike Planets . . . . . . . . . . . . . . . . . . . . . . . . . . The Atmospheres of the Earthlike Planets . . . . . . . . . . . . . . . . . . . . .

51 52 53 55 57 60 66

3.3 3.3.1 3.3.2

Asteroids or Small Planets (Planetoids) . . . . . . . . . . . . . . . . . . . . . 69 The Orbits of the Asteroids . ... ... .. .. . .. . .. .. .. . .. . .. .. . .. .. . . 70 Properties of the Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

3.4 3.4.1 3.4.2 3.4.3 3.4.4

The Major Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uranus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neptune . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.5 3.5.1 3.5.2

Pluto and the Transneptunian Objects . . . . . . . . . . . . . . . . . . . . . . 84 Pluto and Charon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 The Transneptunian Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.6 3.6.1 3.6.2

Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Structure, Spectra, and Chemical Composition . . . . . . . . . . . . . . . . 87 The Evolution of Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

3.7 3.7.1 3.7.2 3.7.3

Meteors and Meteorites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meteorites and Impact Craters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meteors in the Earth's Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and Origins of Meteorites . . . . . . . . . . . . . . . . . . . . . . . . . .

3.8

Interplanetary Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

II. Radiation, Instruments, and Observational Techniques .............................

72 72 77 80 83

89 90 91 91

97

The Development of Astronomical Observation Methods Historical Introduction to Our Knowledge of the Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . 98 4. Radiation and Matter

4.1 4.2

Electromagnetic Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

4.2.1 4.2.2

The Theory of Special Relativity ............................ 102 The Lorentz Transformation. The Doppler Effect . . . . . . . . . . . . . 103 Relativistic Mechanics ......................................... 103

4.3 4.3.1 4.3.2 4.3.3

The Theory of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenomenological Description of Radiation . . . . . . . . . . . . . . . . . . Emission and Absorption. The Radiation Transport Equation Thermodynamic Equilibrium and Black-Body Radiation .....

104 104 107 109

Contents IX

5. Astronomical and Astrophysical Instruments

4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5

Matter in Thermodynamic Equilibrium ................... . Boltzmann Statistics ........................................... . The Velocity Distribution ..................................... . Thermal Excitation ............................................ . Thermal Ionization ............................................ . The Law of Mass Action ...................................... .

Ill 111 112 112 112 113

4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5 4.5.6

The Interaction of Radiation with Matter ................. . Mean Free Paths ............................................... . Interaction Cross Section and Reaction Rate ................. . Collision and Radiation Processes: Kinetic Equations ....... . Elementary Atomic Processes ................................ . Extinction and Emission Coefficients ......................... . Energetic Photons and Particles ............................... .

114 114 114 115 116 119 119

5.1

Telescopes and Detectors for the Optical and the Ultraviolet Regions ................ . Conventional Telescopes ...................................... . Resolving Power and Light Gathering Power. Optical Interferometers ........................................ . Adaptive and Active Optics. Large Telescopes ............... . Optical Detectors .............................................. . Spectrographs ................................................. . Space Telescopes .............................................. .

5.1.1 5.1.2

5.1.3 5.1.4 5.1.5 5.1.6

5.2

123 123 128 131 135 138 141

5.2.3

Telescopes and Detectors for Radiofrequencies and the Infrared ..................... . Radiotelescopes ............................................... . Receivers and Spectrometers for the Radiofrequency Region ............................... . Observation Methods in the Infrared ......................... .

5.3 5.3.1 5.3.2 5.3.3

Instruments for High-Energy Astronomy ................. . Particle Detectors .............................................. . Telescopes for Cosmic Radiation ............................. . Gamma-ray Telescopes ....................................... .

152 152 154 155

5.4 5.4.1 5.4.2 5.4.3

Instruments for X-rays and the Extreme Ultraviolet ..... . Detectors and Spectrometers for the X-ray Region ........... . Telescopes and Satellites for the X-ray Region ............... . Telescopes for the Extreme Ultraviolet ....................... .

157 158 158 160

Ill. The Sun and Stars: The Astrophysics of Individual Stars ............................ .

161

5.2.1 5.2.2

143 143 149 150

Astronomy + Physics = Astrophysics Historical Introduction ...................................................................... ............... . 162 6. The Distances and Fundamental Properties of the Stars

6.1 6.1.1 6.1.2 6.1.3

TheSun ....................................................... . The Spectrum of the Photosphere. Center-Limb Variation .. . The Energy Distribution ...................................... . Luminosity and Effective Temperature ....................... .

167 168 169 169

Contents X

6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.5 6.5.1 6.5.2 6.5.3 6.5.4 6.5.5 6.5.6 6.5.7 7. The Spectra and Atmospheres of Stars

7.1 7.1.1

7.1.2 7.1.3 7.1.4 7.1.5

7.2

Distances and Velocities of the Stars . . . . . . . . . . . . . . . . . . . . . . . . 171 171 172 172 173 Magnitudes, Colors and Radii of the Stars ................. 174 Apparent Magnitudes .......................................... 174 Color Index and Energy Distribution .......................... 175 Absolute Magnitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Bolometric Magnitudes and Luminosities . . . . . . . . . . . . . . . . . . . . . 178 Stellar Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Classification of the Stellar Spectra. The Hertzsprung-Russell Diagram ......................... 180 The Spectral Type .............................................. 180 The Hertzsprung-Russell Diagram. Luminosity Classes ..... 181 The MK Classification ......................................... 183 Two-Color Diagrams ........................................... 185 Rotation of the Stars ............................................ 185 Binary Star Systems and the Masses of Stars . . . . . . . . . . . . . . 186 Visual Binary Stars ............................................. 186 Spectroscopic Binary Stars and Eclipsing Variables .......... 187 Periods and Rotation in Binary Systems ....................... 188 The Stellar Masses ............................................. 188 Close Binary Star Systems ..................................... 189 Pulsars in Binary Star Systems ................................ 190 Trigonometric Parallaxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radial Velocities and Proper Motions . . . . . . . . . . . . . . . . . . . . . . . . . Stream Parallaxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Star Positions and Catalogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Companions of Substellar Mass: Brown Dwarfs and Exoplanets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Spectra and Atoms ............................................ 195 Basic Concepts of Atomic Spectroscopy ...................... 195 Excitation and Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Line Absorption Coefficients .................................. 201 Broadening of Spectral Lines .................................. 202 Remarks on Molecular Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 204

The Physics of Stellar Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . 205 205 207 209 211

7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.2.7

The Structure of Stellar Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . Absorption Coefficients in Stellar Atmospheres . . . . . . . . . . . . . . . Model Atmospheres. The Spectral Energy Distribution . . . . . . . Radiation Transport in the Fraunhofer Lines .................. The Curve of Growth .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . Quantitative Analysis of Stellar Spectra ....................... Element Abundances in the Sun and in Other Stars ...........

7.3

The Sun: Its Chromosphere and Corona. Flow Fields, Magnetic Fields and Activity . . . . . . . . . . . . . . . . . . 219 Granulation and Convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Magnetic Fields and Magnetohydrodynamics . . . . . . . . . . . . . . . . . 220

7.3.1 7.3.2

213

215 216

XI

7.3.3 7.3.4 7.3.5 7.3.6 7.3.7 7.3.8

Sunspots and the Activity Cycle. Magnetic Flux Tubes ....... The Chromosphere and the Corona ............................ Prominences .................................................... Solar Eruptions or Flares ....................................... The Solar Wind ................................................. Oscillations: Helioseismology .................................

7.4

Variable Stars. Flow Fields, Magnetic Fields and Activity of the Stars . . . . Pulsating Stars. R Coronae Borealis Stars . . . . . . . . . . . . . . . . . . . . . Magnetic or Spectrum Variables. Ap Stars and Metallic-Line Stars .............................. Activity, Chromospheres, and Coronas of Cool Stars ......... Coronas, Stellar Winds and Variability of Hot Stars .......... Cataclysmic Variables: Novae and Dwarf Novae .............. X-ray Binary Systems: Accretion onto Neutron Stars ......... Supernovae and Pulsars ........................................ Stellar Gamma-ray Sources .................................... Gamma Bursters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 .4.1 7.4.2 7.4.3 7.4.4 7.4.5 7.4.6 7.4.7 7.4.8 7 .4.9

8. The Structure and Evolution of Stars

8.1

222 225 231 233 236 240 243 244 247 248 251 253 255 259 266 267

The Fundamental Equations of Stellar Structure . . . . . . . . . . Hydrostatic Equilibrium and the Equation of State of Matter . Temperature Distribution and Energy Transport . . . . . . . . . . . . . . Energy Production Through Nuclear Reactions ............... Gravitational Energy and Thermal Energy . . . . . . . . . . . . . . . . . . . . Stability of the Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The System of Fundamental Equations and Their General Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

272 272 273 274 278 280

Stellar Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main Sequence Stars: Central Hydrogen Burning . . . . . . . . . . . . . The Internal Structure of the Sun. Solar Neutrinos . . . . . . . . . . . . From Hydrogen to Helium Burning . . . . . . . . . . . . . . . . . . . . . . . . . . . Late Phases of Stellar Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nucleosynthesis in Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Close Binary Star Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Physics of Accretion Disks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

282 282 283 286 289 293 296 298

8.3.1 8.3.2 8.3.3 8.3.4

The Final Stages of Stellar Evolution . . . . . . . . . . . . . . . . . . . . . . . Brown Dwarfs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Equation of State of Matter with Degenerate Electrons .. The Structure of the White Dwarfs ............................ Neutron Stars ...................................................

300 300 300 302 302

8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.4.5

Strong Gravitational Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Theory of General Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spherically Symmetric Fields in Vacuum . . . . . . . . . . . . . . . . . . . . . Light Deflection and Gravitational Lenses . . . . . . . . . . . . . . . . . . . . Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gravity Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

304 304 305 306 309 309

8.1.1 8.1.2 8.1.3 8.1.4 8.1.5 8.1.6

8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6 8.2.7

8.3

280

ICooteot< XII

IV. Stellar Systems. Cosmology and Cosmogony .............................................

311

The Advance into the Universe Historical Introduction to the Astronomy of the 20th Century ......................................... 312 9. Star Clusters

9.1

Open Star Clusters and Stellar Associations . . . . . . . . . . . . . . . Open (Galactic) Star Clusters .................................. Stellar Associations ............................................ Color-Magnitude Diagrams and the Age of Open Clusters ...

318 318 319 319 322 322 323

9.2.4

Globular Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Globular Clusters in the Milky Way . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Abundances and Two-Color Diagrams . . . . . . . . . . . . . . . . . Color-Magnitude Diagrams and the Ages of the Globular Clusters . . . . . . . . . . . . . . . . . . . . . . . . . Globular Clusters in Other Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . .

10.1 10.1.1 10.1.2 10.1.3 10.1.4 10.1.5

Interstellar Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dark Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interstellar Extinction and Reddening . . . . . . . . . . . . . . . . . . . . . . . . . Polarization of Starlight by Interstellar Dust . . . . . . . . . . . . . . . . . . Properties of the Dust Grains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffuse Interstellar Absorption Bands . . . . . . . . . . . . . . . . . . . . . . . . .

328 330 330 332 332 334

10.2 10.2.1 10.2.2 10.2.3

Neutral Interstellar Gas ...................................... Atomic Interstellar Absorption Lines . . . . . . . . . . . . . . . . . . . . . . . . . . The 21 em Line of Neutral Hydrogen. HI Clouds ............ Interstellar Molecular Lines. Molecular Clouds . . . . . . . . . . . . . . .

335 335 337 339

10.3 10.3.1 10.3.2 10.3.3 10.3.4

Ionized Gas: Luminous Gaseous Nebulae .................. H II Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Planetary Nebulae .............................................. Supernova Remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hot Interstellar Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

341 342 346 347 354

10.4 10.4.1 10.4.2 10.4.3

High-Energy Components .................................... Interstellar Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cosmic Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Galactic Gamma Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

354 355 355 360

10.5 10.5.1 10.5.2 10.5.3 10.5.4 10.5.5 10.5.6

Early Evolution and Formation of the Stars . . . . . . . . . . . . . . . . Pre-Main-Sequence Stars ...................................... Regions of Star Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gravitational Instability and Fragmentation . . . . . . . . . . . . . . . . . . . The Evolution of Protostars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Matter Flows in the Vicinity of Protostars . . . . . . . . . . . . . . . . . . . . . Stellar Statistics and the Star Formation Rate .................

363 364 366 368 369 371 373

9.1.1 9.1.2 9.1.3

9.2 9.2.1 9.2.2 9.2.3

10. Interstellar Matter and Star Formation

11. The Structure and Dynamics of the Milky Way Galaxy

324 326

11.1 Stars and the Structure of the Milky Way . . . . . . . . . . . . . . . . . . 377 11.1.1 Galactic Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 11.1.2 Star Gauging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

Cooteoh I XIII

11.1.3 Spatial Velocities of the Stars .................................. 378 11.1.4 Star Clusters: Distance Determinations and the Structure of the Milky Way ............................ 380

12. Galaxies and Clusters of Galaxies

11.2 11.2.1 11.2.2 11.2.3 11.2.4 11.2.5 11.2.6

The Dynamics and Distribution of Matter .................. The Rotation of the Galactic Disk ............................. The Distribution of the Interstellar Matter ..................... The Galactic Orbits of the Stars. Local Mass Density ......... The Mass Distribution in the Milky Way Galaxy ............. The Dynamics of the Spiral Arms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stellar Populations and Element Abundances .................

381 382 383 387 389 390 392

11.3 11.3.1 11.3.2 11.3 .3 11.3.4

The Central Region of the Milky Way . . . . . . . . . . . . . . . . . . . . . . The Galactic Bulge (R s 3 kpc) ............................... The Nuclear Region of the Galactic Bulge (R s 300 pc) ..... The Circumnuclear Disk and the Mini spiral (R s 10 pc) . . . . . The Innermost Region (R s 1 pc) and Sgr A* .................

395 395 396 398 399

12.1 12.1.1 12.1.2 12.1.3 12.1.4 12.1.5 12.1.6 12.1.7

Normal Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distance Determination ........................................ Classification and Absolute Magnitudes ....................... The Luminosity Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brightness Profiles and Diameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamics and Masses .......................................... Stellar Populations and Element Abundances ................. The Distribution of Gas and Dust ..............................

402 402 405 409 410 411 415 418

12.2 Infrared and Starburst Galaxies ............................. 422 12.2.1 Infrared Galaxies ............................................... 423 12.2.2 Starburst Activity ............................................... 424

12.3.3 12.3.4 12.3.5 12.3.6

Radio Galaxies, Quasars, and Activity in Galactic Nuclei .............................. Synchrotron Radiation ......................................... The Non-thermal Radiofrequency Emissions of Normal Galaxies ............................................. Radio Galaxies ................................................. Quasars (Quasistellar Objects) ................................. Seyfert Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Activity in Galactic Nuclei .....................................

428 429 434 441 443

12.4 12.4.1 12.4.2 12.4.3 12.4.4 12.4.5

Clusters and Superclusters of Galaxies . . . . . . . . . . . . . . . . . . . . . The Local Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Classification and the Masses of the Galaxy Clusters . . . . The Gas in Galaxy Clusters .................................... Interacting Galaxies. The Evolution of Galaxy Clusters ...... Superclusters of Galaxies ......................................

446 447 448 450 452 452

12.3 12.3.1 12.3.2

425 425

12.5 The Formation and Evolution of the Galaxies . . . . . . . . . . . . . . 455 12.5.1 The Formation of the Galaxies and the Clusters of Galaxies .. 455 12.5.2 The Intergalactic Medium and Lyman a Systems ............. 458

XIV

12.5.3 Interacting Galaxies ............................................ 460 12.5.4 Evolution of the Galaxies ...................................... 461 12.5.5 Galaxies in the Early Universe ................................. 466 13. Cosmology: the Cosmos as a Whole

13.1 13.1.1 13.1.2 13.1.3 13.1.4

13.2.1 13.2.2 13.2.3 13.2.4 13.2.5 13.2.6

Radiation and Observations. Element Synthesis in the Universe ........................... The Propagation of Radiation .................................. The Microwave Background Radiation . . . . . . . . . . . . . . . . . . . . . . . . The Radiation Cosmos ......................................... Element Synthesis in the Universe ............................. Observed Parameters of the Present-Day Universe ............ Olbers' Paradox ................................................

13.3 13.3.1 13.3.2 13.3.3 13.3.4 13.3.5

The Planck Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elementary Particles and Fundamental Interactions . . . . . . . . . . . Cosmic Evolution According to the Standard Model . . . . . . . . . The Inflationary Universe ...................................... Other Cosmologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14.1 14.1.1 14.1.2 14.1.3 14.1.4 14.1.5

The Formation of the Sun and of the Solar System ........ A Survey of the Solar System .................................. The Protoplanetary Disk and the Formation of the Planets . . . The Origin of the Meteorites .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . The Formation of the Earth-Moon System .................... Planetary Systems Around Other Stars ........................

494 494 496 498 500 503

14.2 14.2.1 14.2.2 14.2.3 14.2.4

The Evolution of the Earth and of Life ..................... The Development of the Atmosphere and of the Oceans . . . . . . Fundamentals of Molecular Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . Prebiotic Molecules ............................................ The Development ofLifeforms ................................

505 505 506 509 510

A.1

Units: the International and the Gaussian Unit Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517

A.2

Names of the Constellations .................................. 521

13.2

14. The Cosmogony of the Solar System

Appendix

Models of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468 468 469 471 474

The Expanding Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Newtonian Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relativistic Cosmology ........................................ The Matter Cosmos ............................................

475 475 4 77 479 481 482 485

The Evolution of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 486 487 489 491 492

Selected Exercises ............................................................................................... 523 Literature and Sources of Data ................................................................................. 531 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 Subject Index .................................................................................................... 543 Fundamental Physical Constants (Inside back cover) Astronomical Constants and Units (Inside back cover)

1

L Introduction Astronomy, the study of the stars and other celestial objects, is one of the exact sciences. It deals with the quantitative investigation of the cosmos and the physical laws which govern it: with the motions, the structures, the formation, and the evolution of the various celestial bodies. Astronomy is among the oldest of the sciences. The earliest human cultures made use of their knowledge of celestial phenomena and collected astronomical data in order to establish a calendar, measure time, and as an aid to navigation. This early astronomy was often closely interwoven with magical, mythological, religious, and philosophical ideas. The study of the cosmos in the modem sense, however, dates back only to the ancient Greeks: the determination of distances on the Earth and of positions of the celestial bodies in the sky, together with knowledge of geometry, led to the first realistic estimates of the sizes and distances of the objects in outer space. The complex orbits of the Sun, the Moon, and the planets were described in a mathematical, kinematical picture, which allowed the calculation of the positions of the planets in advance. Greek astronomy attained its zenith, and experienced its swan song, in the impressive work of Ptolemy, about 150 a.D. The name of the science, astronomy, is quite appropriately derived from the Greek word "aa7:7Jp" = star or "aar:pov" =constellation or heavenly body. At the beginning of the modem period, in the 16th and 17th centuries, the Copernican view of the universe became generally accepted. Celestial mechanics received its foundation in Newton's Theory of Gravitation in the 17th century and was completed mathematically in the period immediately following. Major progress in astronomical research was made in this period, on the one hand through the introduction of new concepts and theoretical approaches, and on the other through observations of new celestial phenomena. The latter were made possible by the development of new instruments. The invention of the telescope at the beginning of the 17th century led to a nearly unimaginable increase in the scope of astronomical knowledge. Later, new eras in astronomical research were opened up by the development of photography, of the spectrograph, the radio

telescope, and of space travel, allowing observations to be made over the entire range of the electromagnetic

spectrum. In the 19th and particularly in the 20th centuries, physics assumed the decisive role in the elucidation of astronomical phenomena; astrophysics has steadily increased in importance over "classical astronomy". There is an extremely fruitful interaction between astrophysics/astronomy and physics: on the one hand, astronomy can be considered to be the physics of the cosmos, and there is hardly a discipline in physics which does not find application in modem astronomy; on the other hand, the cosmos with its often extreme states of matter offers the opportunity to study physical processes under conditions which are unattainable in the laboratory. Along with physics, and of course mathematics, applications of chemistry and the Earth and biological sciences are also of importance in astronomy. Among the sciences, astronomy is unique in that no experiments can be carried out on the distant celestial objects; astronomers must content themselves with observations. "Diagnosis from a distance", and in particular the quantitative analysis of radiation from the cosmos over the widest possible spectral range, thus play a central role in astronomical research. The rapid development of many branches of astronomy has continued up to the present time. With this revised edition of The New Cosmos, we have tried to keep pace with the rapid expansion of astronomical knowledge while maintaining our goal of providing a comprehensive - and comprehensible - introductory survey of the whole field of astronomy. We have placed emphasis on observations of the manifold objects and phenomena in the cosmos, as well as on the basic ideas which provide the foundation for the various fields within the discipline. We have combined description of the observations as directly as possible with the theoretical approaches to their elucidation. Particular results, as well as information from physics and the other natural sciences which are required for the understanding of astronomical phenomena, are, however, often simply stated without detailed explanations. The complete bibliography, together with a list of important reference works, journals, etc., is intended to

2

I l.lotmductloo

help the reader to gain access to the more detailed and specialized literature. We begin our study of the cosmos, its structure and its laws, "at home" by considering our Solar System in Part I, along with classical astronomy. This part, like the three following parts, starts with a historical summary which is intended to give the reader an overview of the subject. We first become acquainted with observations of the heavens and with the motions of the Earth, the Sun, and the Moon, and introduce celestial coordinates and sidereal time. The apparent motions of the planets and other objects are then explained in the framework of the Newtonian Theory of Gravitation. Before considering the planets and other objects in the Solar System in detail, we give a summary of the development of space research, which has contributed enormously to knowledge of our planetary system. Part I ends with a discussion of the individual planets, their moons, and other smaller bodies such as asteroids, comets and meteors. Prior to taking up the topic of the Sun and other stars, it is appropriate to describe the basic principles of astronomical observation methods, and we do this in Part II. An impressive arsenal of telescopes and detectors is available to today's astronomer; with them, from the Earth or from space vehicles, he or she can investigate the radiation emitted by celestial bodies over the entire range of the electromagnetic spectrum, from the radio and microwave regions through the infrared, the visible, and the ultraviolet to the realm of highly energetic radiations, the X-rays and gamma rays. The use of computers provides an essential tool for the modem astronomer in these observations. Part III is devoted to stars, which we first treat as individual objects. We give an overview of the different types of stars such as those of the main sequence, giants and supergiants, brown dwarfs, white dwarfs and neutron stars, as well as the great variety of variable stars (Cepheids, magnetic stars, novas, supernovas, pulsars, gamma sources ... ) and of stellar activity, and become acquainted with their distances, magnitudes, colors, temperatures, luminosities, and masses. In this part, the Sun plays a particularly important role: on the one hand, as the nearest star, it offers us the possibility of making incomparably more detailed observations than of any other star; on the other, its properties are those of an "average" star, and their study thus yields important information about the physical state of stars in general.

The treatment of the physics of individual stars occupies an important place in Part III. Along with the theory of radiation, atomic spectroscopy in particular forms the basis for quantitative investigation of the radiation and the spectra of the Sun and other stars, and for the understanding of the physical-chemical structure of their outer layers, the stellar atmospheres. Understanding of the mechanism of energy release by thermonuclear reactions and by gravitation is of decisive importance for the study of stellar interiors, their structures and evolution. We then discuss the development of the stars of the main sequence, which includes the phase of intensive stellar hydrogen burning, continuing to their final stages (white dwarf, neutron star or black hole). The formation of stars and their earliest development are treated in the following sections in connection with the interstellar material in our galaxy. At the end of Part III, we deal with strong gravitational fields, which we describe in the framework of Einstein's General Relativity theory; here, we concentrate in particular on black holes, gravitational lenses, and gravitational waves. In Part IV, we take up stellar systems and the macroscopic structure of the universe. Making use of our knowledge of individual stars and their distances from the Earth, we first develop a picture of stellar clusters and stellar associations. We then discuss the interstellar matter which consists of tenuous gas and dust clouds, and treat star formation. Finally, we develop a picture of our own Milky Way galaxy, to which the Sun belongs together with about 100 million other stars. We treat the distribution and the motions of the stars and star clusters and of the interstellar matter. After making the acquaintance of methods for the determination of the enormous distances in intergalactic space, we tum to other galaxies, among which we find a variety of types: spiral and elliptical galaxies, infrared and starburst galaxies, radio galaxies, and the distant quasars. In the centers of many galaxies, we observe an "activity" involving the appearance of extremely large amounts of energy, whose origins are still a mystery. Galaxies, as a rule, belong to larger systems, called galactic clusters. These are in tum ordered in clusters of galactic clusters, the superclusters, which finally form a "lattice" enclosing large areas of empty intergalactic space and defining the macroscopic structure of the Universe. Like individual stars, the galaxies and galactic clusters evolve with the passage of time. The

mutual gravitational influence of the galaxies plays an important role in their development. At the conclusion of Part IV, we consider the Universe as a whole, its content of matter, radiation, and energy, and its structure and evolution throughout the expansion which has taken place over the approximately 13.5 · 109 years from the "big bang" to the present time.

Finally, after pressing out to the far reaches of the cosmos, we return at the end of Part IV to our Solar System and take up the problems of the formation and evolution of the Sun and the planets as well as the existence of planetary systems around other stars. In this section, we give particular attention to the development of the Earth and of life on Earth.

Classical

Astro nomy and the Solar Syste m

6

Humanity and the Stars: Observing and Thinking Historical Introduction to Classical Astronomy Unaffected by the evolution and the activities of mankind, the objects in the heavens have moved along their paths for millenia. The starry skies have thus always been a symbol of the "Other" - of Nature, of deities - the antithesis of the "Self' with its world of inner experience, striving and activity. The history of astronomy is at the same time one of the most exciting chapters in the history of human thought. Again and again, there has been an interplay between the appearance of new concepts and ways of thinking on the one hand and the discovery of new phenomena on the other, the latter often with the aid of newly-developed observational instruments. We cannot treat here the great achievements of the ancient Middle Eastern peoples, the Sumerians, Babylonians, Assyrians, and the Egyptians; nor do we have the space to describe the astronomy of the the Far Eastern cultures in China, Japan, and India, which was highly developed by the standards of the time. The concept of the Universe and its investigation in the modem sense dates back to the ancient Greeks, who were the first to dare to shake off the fetters of black magic and mythology and, aided by their enormously flexible language, to adopt forms of thinking which allowed them, bit by bit, to "comprehend" the phenomena of the cosmos. How bold were the ideas of the pre-Socratic Greeks! Thales of Milet, about 600 B.C., had already clearly understood that the Earth is round, and that the Moon is illuminated by the Sun, and he predicted the Solar eclipse of the year 585 B.C. But is it not just as important that he attempted to reduce understanding of the entire universe to a single principle, that of "water"? The little that we know of Pythagoras (in the middle of the 6th century B.C.) and of his school seems surprisingly modem. The spherical shapes of the Earth, the Sun, and the Moon, the Earth's rotation, and the revolution of at least the inner planets, Venus and Mercury, were already known to the Pythagorans. After the collapse of the Greek states, Alexandria became the center of ancient science; there, the quantitative investigation of the heavens made rapid progress

with the aid of systematic measurements. The numerical results are less important for us today than the happy realization that the great Greek astronomers made the bold leap of applying the laws of geometry to the cosmos! Aristarchus of Samos, who lived in the first half of the 3rd century B.C., attempted to compare the distances of the Earth to the Sun and the Earth to the Moon with the diameters of the three bodies by making the assumption that when the Moon is in its first and third quarter, the triangle Sun-Moon-Earth makes a right angle at the Moon. In addition to carrying out these first quantitative estimates of dimensions in space, Aristarchus was the first to teach the heliocentric system and to recognize its important consequence that the distances to the fixed stars must be incomparably greater than that from the Earth to the Sun. How far he was ahead of his time with these discoveries can be seen from the fact that by the following generation, they had already been forgotten. Soon after Aristarchus' important achievements, Eratosthenes carried out the first measurement of a degree of arc on the Earth's surface, between Alexandria and Syene: he compared the difference in latitude between the two places with their distance along a much-traveled caravan route, and thereby determined the circumference and diameter of the Earth fairly precisely. However, the greatest observer of ancient times was Hipparchus (about 150 B.C.), whose stellar catalog was still nearly unsurpassed in accuracy in the 16th century A.D. Even though the means at his disposal naturally did not allow him to make significantly better determinations of the basic dimensions of the Solar System, he was able to make the important discovery of precession, i.e. the yearly shift of the equinoxes and thus the difference between the tropical and the sidereal years. The theory of planetary motion, which we shall treat next, was necessarily limited in Greek astronomy to a problem in geometry and kinematics. Gradual improvements and extensions of observations on the one hand, and new mathematical approaches on the other, formed the basis for the attempts of Philolaus, Eudoxus, Heracleides, Appollonius, and others to describe the observed motions of the planets; their attempts employed

Humanity and the Stars: Observing and Thinking

J 7

the superposition of ever more complicated circular motions. Ancient astronomy and planetary theory attained its final development much later, in the work of Claudius Ptolemy, who wrote his 13-volume Handbook of Astronomy (Mathematics), Ma81Jf.LCXTLX1J