astronomy - openstax cnx · · 2016-07-211.5 numbers in astronomy ... 8.5 life and chemical...

Astronomy

By:OpenStax Astronomy

Astronomy

By:OpenStax Astronomy

Online:< http://cnx.org/content/col11992/1.2/ >

OpenStax-CNX

This selection and arrangement of content as a collection is copyrighted by OpenStax Astronomy. It is licensed under

the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).

Collection structure revised: June 17, 2016

PDF generated: July 21, 2016

For copyright and attribution information for the modules contained in this collection, see p. 95.

Table of Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Science and the Universe: A Brief Tour

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2 The Nature of Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3 The Nature of Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.4 The Laws of Nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.5 Numbers in Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.6 Consequences of Light Travel Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.7 A Tour of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.8 The Universe on the Large Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281.9 The Universe of the Very Small . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321.10 A Conclusion and a Beginning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2 Observing the Sky: The Birth of Astronomy

2.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.2 The Sky Above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.3 Ancient Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.4 Astrology and Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.5 The Birth of Modern Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3 Orbits and Gravity

3.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2 The Laws of Planetary Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3 Newton's Great Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.4 Newton's Universal Law of Gravitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.5 Orbits in the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.6 Motions of Satellites and Spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.7 Gravity with More Than Two Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4 Earth, Moon, and Sky

4.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.2 Earth and Sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.3 The Seasons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.4 Keeping Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.5 The Calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.6 Phases and Motions of the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.7 Ocean Tides and the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.8 Eclipses of the Sun and Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5 Radiation and Spectra

5.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.2 The Behavior of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.3 The Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.4 Spectroscopy in Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.5 The Structure of the Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.6 Formation of Spectral Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.7 The Doppler E�ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6 Astronomical Instruments

6.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.2 Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

iv

6.3 Telescopes Today . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.4 Visible-Light Detectors and Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.5 Radio Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.6 The Future of Large Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7 Other Worlds: An Introduction to the Solar System

7.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.2 Overview of Our Planetary System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.3 Composition and Structure of Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.4 Dating Planetary Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.5 Origin of the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 47

8 Earth as a Planet

8.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498.2 The Global Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498.3 The Crust of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498.4 The Earth's Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498.5 Life and Chemical Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 498.6 Cosmic In�uences on the Evolution of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

9 Cratered Worlds

9.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.2 General Properties of the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.3 The Lunar Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.4 Impact Craters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.5 The Origin of the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.6 Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

10 Earth-Like Planets: Venus and Mars

10.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.2 The Nearest Planets: An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.3 The Geology of Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.4 The Massive Atmosphere of Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.5 The Geology of Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.6 Water and Life on Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.7 Divergent Planetary Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

11 The Giant Planets

11.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5511.2 Exploring the Outer Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5511.3 The Giant Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5511.4 Atmospheres of the Giant Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

12 Rings, Moons, and Pluto

12.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5712.2 Rings and Moons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5712.3 The Galilean Moons of Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5712.4 Triton and Pluto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5712.5 Planetary Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

13 Solar System Debris: Comets, Asteroids, and Other Small Bodies

13.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5913.2 Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5913.3 Asteroids and Planetary Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5913.4 The "Long-Haired" Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5913.5 The Origin and Fate of Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

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14 Cosmic Samples and the Origin of the Solar System

14.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6114.2 Meteors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6114.3 Meteorites: Stones from Heaven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6114.4 Formation of the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6114.5 Comparison with Other Planetary Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6114.6 Planetary Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

15 The Sun: The Star That Really Matters

15.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6315.2 The Sun From the Inside Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6315.3 Solar Magnetism and Solar Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6315.4 Space Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6315.5 Where Does the Sun Go From Here? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

16 The Sun: A Nuclear Powerhouse

16.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6516.2 Thermal and Gravitational Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6516.3 Mass, Energy, and the Theory of Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6516.4 The Interior of the Sun: Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6516.5 The Solar Interior: Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

17 Analyzing Starlight

17.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6717.2 The Brightness of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6717.3 Colors of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6717.4 The Spectra of Stars (and Brown Dwarves) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6717.5 Using Spectra to Measure Stellar Radius, Composition, and Motion . . . . . . . . . . . . . . . . . . . . . . . 67

18 The Stars: A Celestial Census

18.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6918.2 A Stellar Census . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6918.3 Measuring Stellar Masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6918.4 Diameters of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6918.5 The H-R Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

19 Celestial Distances

19.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7119.2 Fundamental Units of Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7119.3 Surveying the Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7119.4 Variable Stars: One Key to Cosmic Distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7119.5 The H-R Diagram and Cosmic Distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

20 Between the Stars: Gas and Dust in Space

20.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.2 The Interstellar Medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.3 Interstellar Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.4 Cosmic Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.5 Cosmic Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.6 Recycling Cosmic Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7320.7 Interstellar Matter around the Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

21 The Birth of Stars and the Discovery of Planets outside the Solar System

21.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7521.2 Star Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75


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21.3 The H-R Diagram and the Study of Stellar Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7521.4 Evidence That Planets Form Around Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7521.5 Planets beyond the Solar System; Search and Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

22 Stars from Adolescence to Old Age

22.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7722.2 Evolution from the Main Sequence to Red Giants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7722.3 Star Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7722.4 Checking Out the Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7722.5 Further Evolution of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

23 The Death of Stars

23.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.2 The Death of Low-Mass Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.3 Evolution of Massive Stars: An Explosive Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.4 Supernova Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.5 Pulsars and the Discovery of Neutron Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.6 The Evolution of Binary Star Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7923.7 The Mystery of the Gamma-Ray Bursts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

24 Black Holes and Curved Spacetime

24.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.2 Introducing General Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.3 The Principle of Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.4 Spacetime and Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.5 Tests of General Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.6 Time in General Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.7 Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.8 Evidence for Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8124.9 Gravitational Wave Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

25 The Milky Way Galaxy

25.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.2 The Architecture of the Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.3 Spiral Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.4 The Mass of the Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.5 The Center of the Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.6 Stellar Populations in the Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8325.7 The Formation of the Galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

26 Galaxies

26.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8526.2 The Great Nebula Debate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8526.3 Types of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8526.4 Properties of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8526.5 The Extragalactic Distance Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

27 Active Galaxies, Quasars, and Giant Black Holes

27.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8727.2 The Quasars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8727.3 Giant Black Holes at the Centers of Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8727.4 Lighting up Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8727.5 Codependence of Black Holes and Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8727.6 Quasars as Lighthouses to See in the Dark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87


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28 The Evolution and Distribution of Galaxies

28.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8928.2 Observations of Distant Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8928.3 Galaxy Mergers and Active Galactic Nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8928.4 The Distribution of Galaxies in Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8928.5 A Universe of (Mostly) Dark Matter and Dark Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8928.6 The Formation and Evolution of Galaxies and Structure in the Universe . . . . . . . . . . . . . . . . . . . 89

29 The Big Bang

29.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.2 The Age of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.3 A Model of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.4 The Beginning of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.5 The Cosmic Background Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.6 What is the Universe Really Made of? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.7 The In�ationary Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9129.8 The Anthropic Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

30 Life in the Universe

30.1 Thinking Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9330.2 The Cosmic Context for Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9330.3 Astrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9330.4 Searching for Life beyond Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9330.5 The Search for Extraterrestrial Intelligence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Attributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95


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Preface1

Welcome to Astronomy, an OpenStax resource. This textbook was written to increase student access to highquality learning materials, maintaining highest standards of academic rigor at little to no cost.

About OpenStax

OpenStax is a nonpro�t based at Rice University, and it's our mission to improve student access to education.Our �rst openly licensed college textbook was published in 2012 and has since scaled to over 20 books used byhundreds of thousands of students across the globe. Our adaptive learning technology, designed to improvelearning outcomes through personalized educational paths, is currently being piloted for K-12 and college.The OpenStax mission is made possible through the generous support of philanthropic foundations. Throughthese partnerships and with the help of additional low-cost resources from our OpenStax partners, OpenStaxis breaking down the most common barriers to learning and empowering students and instructors to succeed.

About OpenStax Resources

Customization

Astronomy is licensed under a Creative Commons Attribution 4.0 International (CC BY) license, whichmeans that you can distribute, remix, and build upon the content, as long as you credit OpenStax for theoriginal creation.

Because our books are openly licensed, you are free to use the entire book or pick and choose the sectionsthat are most relevant to the needs of your course. Feel free to remix the content by assigning your studentsselect certain chapters and sections in your syllabus, in the order that you prefer. You can even provide adirect link in your syllabus to the sections in the web view of your book.Errata

All OpenStax textbooks undergo a rigorous review process. However, like any professional-grade textbook,errors sometimes occur. Since our books are web-based, we can make updates periodically when deemedpedagogically necessary. If you have a correction to suggest, submit it through the link on your book pageon openstax.org. Subject matter experts review all errata suggestions. OpenStax is committed to remainingtransparent about all updates, so you will also �nd a list of past errata changes on your book page onopenstax.org.Format

You can access this textbook for free in web view or PDF through openstax.org, and for a low cost in print.

About Astronomy

Astronomy is written to meet standard scope and sequence requirements and is �exible enough to be adaptedfor either one- or two-semester introductory courses. The book communicates in accessible, non-technical

1This content is available online at <http://cnx.org/content/m62004/1.1/>.


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language, with a wide range of clarifying illustrations and digital applications to enrich students' understand-ing of the concepts. Astronomy draws analogies from everyday life to help non-science majors appreciate,on their own terms, the discoveries of our modern exploration of the universe.Coverage and Scope

Astronomy was written, updated, and reviewed by a broad range of astronomers and astronomy educators ina strong community e�ort. It is designed to meet scope and sequence requirements of introductory astronomycourses nationwide, and subject-matter experts have reviewed all content for accuracy and currency. It isdesigned to be as �exible as possible to accommodate the variety of formats of introductory astronomyclasses. Each chapter section addresses a di�erent aspect of the topic at hand and math examples arepresented in separate sections throughout, so that instructors can include or omit portions to �t the needsof their curriculum. The language is accessible and inviting, with helpful diagrams and summary tables tohelp students understand the big picture.

It is up to date with information and images from the New Horizons exploration of Pluto, the discovery ofgravitational waves, the Rosetta Mission to Comet C-G, and many other recent projects in space science. Thediscussion of exoplanets has been updated with recent information�indicating not just individual examples,but trends in what types of planets seem to be most common. Black holes receive their own chapter, andthe role of super-massive black holes in active galaxies and galaxy evolution is clearly explained.

• Chapter 1: Science and the Universe: A Brief Tour• Chapter 2: Observing the Sky: The Birth of Astronomy• Chapter 3: Orbits and Gravity• Chapter 4: Earth, Moon, and Sky• Chapter 5: Radiation and Spectra• Chapter 6: Astronomical Instruments• Chapter 7: Other Worlds: An Introduction to the Solar System• Chapter 8: Earth as a Planet• Chapter 9: Cratered Worlds• Chapter 10: Earth-like Planets: Venus and Mars• Chapter 11: The Giant Planets• Chapter 12: Rings, Moons, and Pluto• Chapter 13: Solar System Debris: Comets, Asteroids, and Other Small Bodies• Chapter 14: : Cosmic Samples and the Origin of the Solar System• Chapter 15: The Sun: The Star That Really Matters• Chapter 16: The Sun: A Nuclear Powerhouse• Chapter 17: Analyzing Starlight• Chapter 18: The Stars: A Celestial Census• Chapter 19: Celestial Distances• Chapter 20: Between the Stars: Gas and Dust in Space• Chapter 21: The Birth of Stars and the Discovery of Planets outside the Solar System• Chapter 22: Stars from Adolescence to Old Age• Chapter 23: The Death of Stars• Chapter 24: Black Holes and Curved Spacetime• Chapter 25: The Milky Way Galaxy• Chapter 26: Galaxies• Chapter 27: Active Galaxies, Quasars, and Giant Black Holes• Chapter 28: The Evolution and Distribution of Galaxies• Chapter 29: The Big Bang• Chapter 30: Life in the Universe• Appendix A: How to Study for Your Introductory Astronomy Course• Appendix B: Astronomy Websites, Pictures, and Apps• Appendix C: Scienti�c Notation• Appendix D: Units Used in Science


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• Appendix E: Some Useful Constants for Astronomy• Appendix F: Data for the Planets• Appendix G: Selected Moons of the Planets• Appendix H: Upcoming Total Eclipses• Appendix I: The Nearest Stars• Appendix J: The Brightest Stars• Appendix K: The Chemical Elements• Appendix L: The Constellations

Engaging Feature Boxes

Throughout Astronomy, you will �nd features that connect astronomy to students' other subjects andhighlight the men and women who have been key to its progress. Our features include:

• Making Connections. This feature connects the chapter topic to students' experiences with other�elds, from poetry to engineering, popular culture, and natural disasters.

• Voyagers in Astronomy. This feature presents brief and engaging biographies of the people behindhistorically signi�cant discoveries and emerging research.

• Astronomy Basics. This feature covers basic science concepts to make sure that students have thefoundational knowledge they need to understand the text.

• Seeing for Yourself. This feature provides practical ways that students can make astronomicalobservations on their own.

End-of-Chapter Materials That Reinforce Key Concepts

• Chapter Summaries. Summaries at the end of each chapter distill key information for easy review.• For Further Exploration. This section o�ers a list of suggested articles, websites, and videos

on particular topics of interest. Students can use For Further Exploration for their own learning andinterest, or instructors can harness these suggestions for homework, extra credit, or writing assignments.

• Review Questions. Review Questions provide opportunities to assess how well students have under-stood the chapter concepts.

• Thought Questions. Thought Questions help students assess their learning by prompting them tothink critically about principles presented in the chapter.

• Figuring For Yourself. Mathematical questions, using only basic algebra and arithmetic, allowstudents to apply the math principles that were presented in example boxes throughout the chapter.

• Collaborative Group Activities. This section provides suggestions for group discussion, research,or reports.

Comprehensive Art Program

Our art program is designed to enhance students' understanding of concepts through clear and e�ectiveillustrations, diagrams, and photographs.


4

How a Pulsar Beam Sweeps Over the Earth

Figure 1


5

The Structure of the Milky Way Galaxy

Figure 2


6

Two Aspects of Plate Tectonics

Figure 3


7

Pluto Close up

Figure 4

Additional Resources

Student and Instructor Resources

We've compiled additional resources for both students and instructors, including Getting Started Guides,PowerPoint slides, and an instructor answer guide. Instructor resources require a veri�ed instructor account,which can be requested on your openstax.org log-in. Take advantage of these resources to supplement yourOpenStax book.Partner Resources

OpenStax partners are our allies in the mission to make high-quality learning materials a�ordable andaccessible to students and instructors everywhere. Their tools integrate seamlessly with our OpenStax titlesat a low cost. To access the partner resources for your text, visit your book page on openstax.org.

About the Authors

Content Leads

Andrew Fraknoi, Foothill CollegeAndrew Fraknoi is the Chair of the Astronomy Department at Foothill College near San Francisco, andserved as the Executive Director of the Astronomical Society of the Paci�c (ASP) from 1978 to 1992. Hiswork with the Society included editing Mercury Magazine, the Universe in the Classroom Newsletter, andAstronomy Beat. He founded and directed Project ASTRO and Family ASTRO at the ASP, bringingastronomy to classrooms and homes across the country. He has taught astronomy and physics at SanFrancisco State University, Canada College, and the University of California Extension Division. He iseditor and co-author of The Universe at Your Fingertips 2.0, a widely used collection of astronomy teaching


8

activities and resources, and co-author of Solar Science, a book for middle-school teachers. For �ve years hewas the lead author of a nationally syndicated newspaper column on astronomy, and he appears regularlyon local and national radio, explaining astronomical developments. With Sidney Wol�, he was founder andco-editor of Astronomy Education Review, an online journal for those working in astronomy education. Heserves on the Board of Trustees of the SETI Institute and on the Lick Observatory Council. In addition, hehas organized six national symposia on teaching introductory astronomy at the college level. He receivedthe Klumpke-Roberts Prize of the ASP, the Gemant Award of the American Institute of Physics, and theFaraday Award of the National Science Teachers Association for his contributions to astronomy educationand outreach.

David Morrison, National Aeronautics and Space AdministrationDavid Morrison is a Senior Scientist at NASA Ames Research Center in Silicon Valley. Dr. Morrisonreceived his PhD in astronomy from Harvard University, where he was one of Carl Sagan's �rst graduatestudents. He is one of the founders of the �eld of astrobiology and is also known for his research on smallbodies in the solar system, including the defense of our planet against asteroid impacts. Before joining thesta� at NASA Ames, he spent 17 years at the University of Hawaii as a tenured professor in the Institutefor Astronomy and the Department of Physics and Astronomy. In Hawaii, he worked with the telescopes ofMauna Kea Observatory, where he was also Director of the IRTF (Infrared Telescope Facility) for four years.Since moving to NASA, Morrison has held a variety of senior science management positions, including Chiefof the Ames Space Science Division and founding Director of the NASA Lunar Science Institute. He hasbeen a member of science teams for the Voyager, Galileo, and Kepler space missions. Morrison receivedNASA Outstanding Leadership Medals for his work dealing with the hazard of cosmic impacts and forhis contributions to the Galileo mission, and the NASA Exceptional Achievement Medal for using scienceto counteract public fears of the end of the world. He was awarded the AAS Carl Sagan medal and theASP Klumpke-Roberts prize recognizing his work in public outreach. He committed to the struggle againstpseudoscience, and he serves as Contributing Editor of Skeptical Inquirer and is a member of the AdvisoryCouncil of the National Center for Science Education.

Sidney C. Wol�, National Optical Astronomy Observatories (Emeritus)After receiving her PhD from the University of California, Berkeley, Dr. Wol� moved to Hawaii, where shewas involved with the astronomical development of Mauna Kea, now recognized as the best astronomicalobserving site in the Northern Hemisphere. In 1984, she became the Director of Kitt Peak NationalObservatory, and in 1987 was named director of National Optical Astronomy Observatory (NOAO), whereshe led the design and development phases for the twin 8-meter Gemini telescopes and the 4-meter SouthernObservatory for Astronomical Research. Most recently, she led the design and development phase of the8.4-meter Large Synoptic Survey Telescope, which is designed to survey the entire observable sky everyfew nights. Gemini South, LSST, and SOAR are all located on Cerro Pachón in Chile; a view point thatlooks toward Cerro Pachón has been named �Mirador Sidney Wol�� in recognition of her leadership rolein developing these facilities. Dr. Wol� has published over ninety scienti�c papers on star formationand stellar atmospheres in refereed journals. She has served as President of the two leading professionalastronomy societies in the United States�the American Astronomical Society and the Astronomical Societyof the Paci�c, and was for 24 years on the Board of Trustees of Carleton College. Her recently publishedbook, The Boundless Universe: Astronomy in the New Age of Discovery, won the 2016 IPPY (IndependentPublisher Book Awards) Silver Medal in Science.

All three senior contributing authors have received the Education Prize of the American Astronomi-cal Society and have had an asteroid named after them by the International Astronomical Union. Theyhave worked together on a series of astronomy textbooks over the past two decades.Contributing Authors

John Beck, Stanford UniversitySusan Benecchi, Planetary Science Institute


9

John Bochanski, Rider UniversityHoward Bond, Pennsylvania State University, Emeritus, Space Telescope Science InstituteJennifer Carson, Occidental CollegeBryan Dunne, University of Illinois at Urbana-ChampaignMartin Elvis, Harvard-Smithsonian Center for AstrophysicsDebra Fischer, Yale UniversityHeidi Hammel, Association of Universities for Research in AstronomyTori Hoehler, NASA Ames Research CenterDouglas Ingram, Texas Christian UniversitySteven Kawaler, Iowa State UniversityLloyd Knox, University of California, DavisMark Krumholz, Australian National UniversityJames Lowenthal, Smith CollegeSiobahn Morgan, University of Northern IowaDaniel Perley, California Institute of TechnologyDeborah Scherrer, retired, Stanford UniversityPhillip Scherrer, Stanford UniversitySanjoy Som, Blue Marble Space Institute of Science, NASA Ames Research CenterWes Tobin, Indiana University EastWilliam Waller, retired, Tufts University, Rockport (MA) Public SchoolsTodd Young, Wayne State College

Reviewers

Elisabeth Adams, Planetary Science InstituteAlfred Alaniz, San Antonio CollegeCharles Allison, Texas A&M University�KingsvilleDouglas Arion, Carthage CollegeTimothy Barker, Wheaton CollegeMarshall Bartlett, The Hockaday SchoolCharles Benesh, Wesleyan CollegeGerald Cleaver, Baylor UniversityKristi Concannon, King's CollegeScott Engle, Villanova UniversityMatthew Fillingim, University of California, BerkeleyCarrie Fitzgerald, Montgomery CollegeChristopher Fuse, Rollins CollegeShila Garg, Emeritus, The College of WoosterRichard Gelderman, Western Kentucky UniversityBeth Hufnagel, Anne Arundel Community CollegeFrancine Jackson, Brown UniversityJoseph Jensen, Utah Valley UniversityJohn Kielkopf, University of LouisvilleJames Lombardi, Jr., Allegheny CollegeAmy Lovell, Agnes Scott CollegeCharles Niederriter, Gustavus Adolphus CollegeRichard Olenick, University of DallasMatthew Olmstead, King's CollegeZoran Pazameta, Eastern Connecticut State UniversityDavid Quesada, Saint Thomas UniversityValerie Rapson, Dudley ObservatoryJoseph Ribaudo, Utica College


10

Dean Richardson, Xavier University of LouisianaAndrew Rivers, Northwestern UniversityMarc Sher, College of William & MaryChristopher Sirola, University of Southern MississippiRan Sivron, Baker UniversityJ. Allyn Smith, Austin Peay State UniversityJason Smolinski, Calvin CollegeMichele Thornley, Bucknell UniversityRichard Webb, Union CollegeTerry Willis, Chesapeake CollegeJared Workman, Colorado Mesa UniversityKaisa Young, Nicholls State University


Chapter 1

Science and the Universe: A Brief Tour

1.1 Introduction1

Distant Galaxies.

Figure 1.1: These two interacting islands of stars (galaxies) are so far away that their light takes millionsof years to reach us on Earth (photographed with the Hubble Space Telescope). (credit: modi�cationof work by NASA, ESA, the Hubble Heritage (STScl/AURA)-ESA/Hubble Collaboration, and K. Noll(STScl))

We invite you to come along on a series of voyages to explore the universe as astronomers understand ittoday. Beyond Earth are vast and magni�cent realms full of objects that have no counterpart on our home



11

12 CHAPTER 1. SCIENCE AND THE UNIVERSE: A BRIEF TOUR

planet. Nevertheless, we hope to show you that the evolution of the universe has been directly responsiblefor your presence on Earth today.

Along your journey, you will encounter:

� a canyon system so large that, on Earth, it would stretch from Los Angeles to Washington, DC(Figure 1.2 (Mars Mosaic.)).

Mars Mosaic.

Figure 1.2: This image of Mars is centered on the Valles Marineris (Mariner Valley) complex of canyons,which is as long as the United States is wide. (credit: NASA)


13

� a crater and other evidence on Earth that tell us that the dinosaurs (and many other creatures) diedbecause of a cosmic collision.

� a tiny moon whose gravity is so weak that one good throw from its surface could put a baseball intoorbit.

� a collapsed star so dense that to duplicate its interior we would have to squeeze every human being onEarth into a single raindrop.

� exploding stars whose violent end could wipe clean all of the life-forms on a planet orbiting a neighboringstar (Figure 1.3 (A Stellar Corpse.)).

� a �cannibal galaxy� that has already consumed a number of its smaller galaxy neighbors and is not yet�nished �nding new victims.

� a radio echo that is the faint but unmistakable signal of the creation event for our universe.



A Stellar Corpse.

Figure 1.3: We observe the remains of a star that was seen to explode in our skies in 1054 (and was,brie�y, bright enough to be visible during the daytime). Today, the remnant is called the Crab Nebulaand its central region is seen here. Such exploding stars are crucial to the development of life in theuniverse. (credit: NASA, ESA, J. Hester (Arizona State University))

Such discoveries are what make astronomy such an exciting �eld for scientists and many others�but youwill explore much more than just the objects in our universe and the latest discoveries about them. We willpay equal attention to the process by which we have come to understand the realms beyond Earth and thetools we use to increase that understanding.


15

We gather information about the cosmos from the messages the universe sends our way. Because the starsare the fundamental building blocks of the universe, decoding the message of starlight has been a centralchallenge and triumph of modern astronomy. By the time you have �nished reading this text, you will knowa bit about how to read that message and how to understand what it is telling us.

1.2 The Nature of Astronomy2

Astronomy is de�ned as the study of the objects that lie beyond our planet Earth and the processes by whichthese objects interact with one another. We will see, though, that it is much more. It is also humanity'sattempt to organize what we learn into a clear history of the universe, from the instant of its birth in the BigBang to the present moment. Throughout this book, we emphasize that science is a progress report�onethat changes constantly as new techniques and instruments allow us to probe the universe more deeply.

In considering the history of the universe, we will see again and again that the cosmos evolves; it changesin profound ways over long periods of time. For example, the universe made the carbon, the calcium, andthe oxygen necessary to construct something as interesting and complicated as you. Today, many billions ofyears later, the universe has evolved into a more hospitable place for life. Tracing the evolutionary processesthat continue to shape the universe is one of the most important (and satisfying) parts of modern astronomy.

1.3 The Nature of Science3

The ultimate judge in science is always what nature itself reveals based on observations, experiments, models,and testing. Science is not merely a body of knowledge, but a method by which we attempt to understandnature and how it behaves. This method begins with many observations over a period of time. From thetrends found through observations, scientists can model the particular phenomena we want to understand.Such models are always approximations of nature, subject to further testing.

As a concrete astronomical example, ancient astronomers constructed a model (partly from observationsand partly from philosophical beliefs) that Earth was the center of the universe and everything moved aroundit in circular orbits. At �rst, our available observations of the Sun, Moon, and planets did �t this model;however, after further observations, the model had to be updated by adding circle after circle to representthe movements of the planets around Earth at the center. As the centuries passed and improved instrumentswere developed for keeping track of objects in the sky, the old model (even with a huge number of circles)could no longer explain all the observed facts. As we will see in the chapter on the birth of astronomy, anew model, with the Sun at the center, �t the experimental evidence better. After a period of philosophicalstruggle, it became accepted as our view of the universe.

When they are �rst proposed, new models or ideas are sometimes called hypotheses. You may thinkthere can be no new hypotheses in a science such as astronomy�that everything important has alreadybeen learned. Nothing could be further from the truth. Throughout this textbook you will �nd discussionsof recent, and occasionally still controversial, hypotheses in astronomy. For example, the signi�cance thatthe huge chunks of rock and ice that hit Earth have for life on Earth itself is still debated. And while theevidence is strong that vast quantities of invisible �dark energy� make up the bulk of the universe, scientistshave no convincing explanation for what the dark energy actually is. Resolving these issues will requiredi�cult observations done at the forefront of our technology, and all such hypotheses need further testingbefore we incorporate them fully into our standard astronomical models.

This last point is crucial: a hypothesis must be a proposed explanation that can be tested. The moststraightforward approach to such testing in science is to perform an experiment. If the experiment isconducted properly, its results either will agree with the predictions of the hypothesis or they will contradictit. If the experimental result is truly inconsistent with the hypothesis, a scientist must discard the hypothesisand try to develop an alternative. If the experimental result agrees with predictions, this does not necessarily

2This content is available online at <http://cnx.org/content/m59747/1.2/>.3This content is available online at <http://cnx.org/content/m59755/1.2/>.



prove that the hypothesis is absolutely correct; perhaps later experiments will contradict crucial parts of thehypothesis. But, the more experiments that agree with the hypothesis, the more likely we are to accept thehypothesis as a useful description of nature.

One way to think about this is to consider a scientist who was born and lives on an island where onlyblack sheep live. Day after day the scientist encounters black sheep only, so he or she hypothesizes that allsheep are black. Although every observed sheep adds con�dence to the theory, the scientist only has to visitthe mainland and observe one white sheep to prove the hypothesis wrong.

When you read about experiments, you probably have a mental picture of a scientist in a laboratoryconducting tests or taking careful measurements. This is certainly the case for a biologist or a chemist, butwhat can astronomers do when our laboratory is the universe? It's impossible to put a group of stars into atest tube or to order another comet from a scienti�c supply company.

As a result, astronomy is sometimes called an observational science; we often make our tests by observingmany samples of the kind of object we want to study and noting carefully how di�erent samples vary. Newinstruments and technology can let us look at astronomical objects from new perspectives and in greaterdetail. Our hypotheses are then judged in the light of this new information, and they pass or fail in the sameway we would evaluate the result of a laboratory experiment.

Much of astronomy is also a historical science�meaning that what we observe has already happenedin the universe and we can do nothing to change it. In the same way, a geologist cannot alter what hashappened to our planet, and a paleontologist cannot bring an ancient animal back to life. While this canmake astronomy challenging, it also gives us fascinating opportunities to discover the secrets of our cosmicpast.

You might compare an astronomer to a detective trying to solve a crime that occurred before the detectivearrived at the scene. There is lots of evidence, but both the detective and the scientist must sift throughand organize the evidence to test various hypotheses about what actually happened. And there is anotherway in which the scientist is like a detective: they both must prove their case. The detective must convincethe district attorney, the judge, and perhaps ultimately the jury that his hypothesis is correct. Similarly,the scientist must convince colleagues, editors of journals, and ultimately a broad cross-section of otherscientists that her hypothesis is provisionally correct. In both cases, one can only ask for evidence �beyonda reasonable doubt.� And sometimes new evidence will force both the detective and the scientist to revisetheir last hypothesis.

This self-correcting aspect of science sets it o� from most human activities. Scientists spend a great dealof time questioning and challenging one another, which is why applications for project funding�as well asreports for publication in academic journals�go through an extensive process of peer review, which is acareful examination by other scientists in the same �eld. In science (after formal education and training),everyone is encouraged to improve upon experiments and to challenge any and all hypotheses. New scientistsknow that one of the best ways to advance their careers is to �nd a weakness in our current understandingof something and to correct it with a new or modi�ed hypothesis.

This is one of the reasons science has made such dramatic progress. An undergraduate science majortoday knows more about science and did math than Sir Isaac Newton, one of the most renowned scientistswho ever lived. Even in this introductory astronomy course, you will learn about objects and processes thatno one a few generations ago even dreamed existed.

1.4 The Laws of Nature4

Over centuries scientists have extracted various scienti�c laws from countless observations, hypotheses, andexperiments. These scienti�c laws are, in a sense, the �rules� of the game that nature plays. One remarkablediscovery about nature�one that underlies everything you will read about in this text�is that the samelaws apply everywhere in the universe. The rules that determine the motion of stars so far away that your



17

eye cannot see them are the same laws that determine the arc of a baseball after a batter has hit it out ofthe park.

Note that without the existence of such universal laws, we could not make much headway in astronomy.If each pocket of the universe had di�erent rules, we would have little chance of interpreting what happenedin other �neighborhoods.� But, the consistency of the laws of nature gives us enormous power to understanddistant objects without traveling to them and learning the local laws. In the same way, if every region of acountry had completely di�erent laws, it would be very di�cult to carry out commerce or even to understandthe behavior of people in those di�erent regions. A consistent set of laws, though, allows us to apply whatwe learn or practice in one state to any other state.

This is not to say that our current scienti�c models and laws cannot change. New experiments andobservations can lead to new, more sophisticated models�models that can include new phenomena and lawsabout their behavior. The general theory of relativity proposed by Albert Einstein is a perfect example ofsuch a transformation that took place about a century ago; it led us to predict, and eventually to observe, astrange new class of objects that astronomers call black holes. Only the patient process of observing natureever more carefully and precisely can demonstrate the validity of such new scienti�c models.

One important problem in describing scienti�c models has to do with the limitations of language. Whenwe try to describe complex phenomena in everyday terms, the words themselves may not be adequate to dothe job. For example, you may have heard the structure of the atom likened to a miniature solar system.While some aspects of our modern model of the atom do remind us of planetary orbits, many other of itsaspects are fundamentally di�erent.

This problem is the reason scientists often prefer to describe their models using equations rather thanwords. In this book, which is designed to introduce the �eld of astronomy, we use mainly words to discusswhat scientists have learned. We avoid complex math, but if this course piques your interest and you go onin science, more and more of your studies will involve the precise language of mathematics.

1.5 Numbers in Astronomy5

In astronomy we deal with distances on a scale you may never have thought about before, with numberslarger than any you may have encountered. We adopt two approaches that make dealing with astronomicalnumbers a little bit easier. First, we use a system for writing large and small numbers called scienti�c

notation (or sometimes powers-of-ten notation). This system is very appealing because it eliminates themany zeros that can seem overwhelming to the reader. In scienti�c notation, if you want to write a numbersuch as 500,000,000, you express it as 5 × 108. The small raised number after the 10, called an exponent,keeps track of the number of places we had to move the decimal point to the left to convert 500,000,000to 5. If you are encountering this system for the �rst time or would like a refresher, we suggest you lookat Appendix C6 and Example 1.1 for more information. The second way we try to keep numbers simple isto use a consistent set of units�the metric International System of Units, or SI (from the French Système

International d'Unités). The metric system is summarized in Appendix D7 (see Example 1.2).

note: Watch this brief PBS animation8 that explains how scienti�c notationworks and why it's useful.

A common unit astronomers use to describe distances in the universe is a light-year, which is the distancelight travels during one year. Because light always travels at the same speed, and because its speed turns

5This content is available online at <http://cnx.org/content/m59751/1.2/>.6"Scienti�c Notation" <http://cnx.org/content/m59995/latest/>7"Units Used in Science" <http://cnx.org/content/m59996/latest/>8https://openstaxcollege.org/l/30scinotation



out to be the fastest possible speed in the universe, it makes a good standard for keeping track of distances.You might be confused because a �light-year� seems to imply that we are measuring time, but this mix-up oftime and distance is common in everyday life as well. For example, when your friend asks where the movietheater is located, you might say �about 20 minutes from downtown.�

So, how many kilometers are there in a light-year? Light travels at the amazing pace of 3 × 105 kilometersper second (km/s), which makes a light-year 9.46 × 1012 kilometers. You might think that such a large unitwould reach the nearest star easily, but the stars are far more remote than our imaginations might lead us tobelieve. Even the nearest star is 4.3 light-years away�more than 40 trillion kilometers. Other stars visibleto the unaided eye are hundreds to thousands of light-years away (Figure 1.4 (The Orion Nebula.)).


19

The Orion Nebula.

Figure 1.4: This beautiful cloud of cosmic raw material (gas and dust from which new stars and planetsare being made) called the Orion Nebula is about 1400 light-years away. That's a distance of roughly1.34 × 1016 kilometers�a pretty big number. The gas and dust in this region are illuminated by theintense light from a few extremely energetic adolescent stars. (credit: NASA, ESA, M. Robberto (SpaceTelescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)



1.6 Consequences of Light Travel Time9

1.7 A Tour of the Universe10

We can now take a brief introductory tour of the universe as astronomers understand it today to get ac-quainted with the types of objects and distances you will encounter throughout the text. We begin at homewith Earth, a nearly spherical planet about 13,000 kilometers in diameter (Figure 1.5 (Humanity's HomeBase.)). A space traveler entering our planetary system would easily distinguish Earth from the other plan-ets in our solar system by the large amount of liquid water that covers some two thirds of its crust. If thetraveler had equipment to receive radio or television signals, or came close enough to see the lights of ourcities at night, she would soon �nd signs that this watery planet has sentient life.

9This content is available online at <http://cnx.org/content/m59752/1.1/>.10This content is available online at <http://cnx.org/content/m59753/1.2/>.


21

Humanity's Home Base.

Figure 1.5: This image shows the Western hemisphere as viewed from space 35,400 kilometers (about22,000 miles) above Earth. Data about the land surface from one satellite was combined with anothersatellite's data about the clouds to create the image. (credit: modi�cation of work by R. Stockli, A.Nelson, F. Hasler, NASA/ GSFC/ NOAA/ USGS)

Our nearest astronomical neighbor is Earth's satellite, commonly called the Moon. Figure 1.6 (Earthand Moon, Drawn to Scale.) shows Earth and the Moon drawn to scale on the same diagram. Notice howsmall we have to make these bodies to �t them on the page with the right scale. The Moon's distance fromEarth is about 30 times Earth's diameter, or approximately 384,000 kilometers, and it takes about a month



for the Moon to revolve around Earth. The Moon's diameter is 3476 kilometers, about one fourth the sizeof Earth.

Earth and Moon, Drawn to Scale.

Figure 1.6: This image shows Earth and the Moon shown to scale for both size and distance. (credit:modi�cation of work by NASA)

Light (or radio waves) takes 1.3 seconds to travel between Earth and the Moon. If you've seen videosof the Apollo �ights to the Moon, you may recall that there was a delay of about 3 seconds between thetime Mission Control asked a question and the time the astronauts responded. This was not because theastronomers were thinking slowly, but rather because it took the radio waves almost 3 seconds to make theround trip.

Earth revolves around our star, the Sun, which is about 150 million kilometers away�approximately 400times as far away from us as the Moon. We call the average Earth�Sun distance an astronomical unit (AU)because, in the early days of astronomy, it was the most important measuring standard. Light takes slightlymore than 8 minutes to travel 1 astronomical unit, which means the latest news we receive from the Sun isalways 8 minutes old. The diameter of the Sun is about 1.5 million kilometers; Earth could �t comfortablyinside one of the minor eruptions that occurs on the surface of our star. If the Sun were reduced to the sizeof a basketball, Earth would be a small apple seed about 30 meters from the ball.

It takes Earth 1 year (3 × 107 seconds) to go around the Sun at our distance; to make it around, wemust travel at approximately 110,000 kilometers per hour. (If you, like many students, still prefer miles tokilometers, you might �nd the following trick helpful. To convert kilometers to miles, just multiply kilometersby 0.6. Thus, 110,000 kilometers per hour becomes 66,000 miles per hour.) Because gravity holds us �rmlyto Earth and there is no resistance to Earth's motion in the vacuum of space, we participate in this extremelyfast-moving trip without being aware of it day to day.

Earth is only one of eight planets that revolve around the Sun. These planets, along with their satellitesand swarms of smaller bodies such as dwarf planets, make up the solar system (Figure 1.7 (Our SolarFamily.)). A planet is de�ned as a body of signi�cant size that orbits a star and does not produce its ownlight. (If a large body consistently produces its own light, it is then called a star.) Later in the book thisde�nition will be modi�ed a bit, but it is perfectly �ne for now as you begin your voyage.


23

Our Solar Family.

Figure 1.7: The Sun, the planets, and some dwarf planets are shown with their sizes drawn to scale.The orbits of the planets are much more widely separated than shown in this drawing. Notice the sizeof Earth compared to the giant planets. (credit: modi�cation of work by NASA)

We are able to see the nearby planets in our skies only because they re�ect the light of our local star,the Sun. If the planets were much farther away, the tiny amount of light they re�ect would usually notbe visible to us. The planets we have so far discovered orbiting other stars were found from the pull theirgravity exerts on their parent stars, or from the light they block from their stars when they pass in front ofthem. We can't see most of these planets directly, although a few are now being imaged directly.

The Sun is our local star, and all the other stars are also enormous balls of glowing gas that generate vastamounts of energy by nuclear reactions deep within. We will discuss the processes that cause stars to shinein more detail later in the book. The other stars look faint only because they are so very far away. If wecontinue our basketball analogy, Proxima Centauri, the nearest star beyond the Sun, which is 4.3 light-yearsaway, would be almost 7000 kilometers from the basketball.

When you look up at a star-�lled sky on a clear night, all the stars visible to the unaided eye are part of asingle collection of stars we call the Milky Way Galaxy, or simply the Galaxy. (When referring to the MilkyWay, we capitalize Galaxy ; when talking about other galaxies of stars, we use lowercase galaxy.) The Sun isone of hundreds of billions of stars that make up the Galaxy; its extent, as we will see, staggers the humanimagination. Within a sphere 10 light-years in radius centered on the Sun, we �nd roughly ten stars. Withina sphere 100 light-years in radius, there are roughly 10,000 (104) stars�far too many to count or name�butwe have still traversed only a tiny part of the Milky Way Galaxy. Within a 1000-light-year sphere, we �ndsome ten million (107) stars; within a sphere of 100,000 light-years, we �nally encompass the entire MilkyWay Galaxy.

Our Galaxy looks like a giant disk with a small ball in the middle. If we could move outside our Galaxyand look down on the disk of the Milky Way from above, it would probably resemble the galaxy in Figure 1.8



(A Spiral Galaxy.), with its spiral structure outlined by the blue light of hot adolescent stars.

A Spiral Galaxy.

Figure 1.8: This galaxy of billions of stars, called by its catalog number NGC 1073, is thought to besimilar to our own Milky Way Galaxy. Here we see the giant wheel-shaped system with a bar of starsacross its middle. (credit: NASA, ESA)

The Sun is somewhat less than 30,000 light-years from the center of the Galaxy, in a location with nothingmuch to distinguish it. From our position inside the Milky Way Galaxy, we cannot see through to its far rim(at least not with ordinary light) because the space between the stars is not completely empty. It containsa sparse distribution of gas (mostly the simplest element, hydrogen) intermixed with tiny solid particlesthat we call interstellar dust. This gas and dust collect into enormous clouds in many places in the Galaxy,becoming the raw material for future generations of stars. Figure 1.9 (The Milky Way.) shows an image ofthe disk of the Galaxy as seen from our vantage point.


25

The Milky Way.

Figure 1.9: Because we are inside the Galaxy, we see its disk in cross-section �ung across the sky likea great milky white avenue of stars with dark �rifts� of dust. In this dramatic image, part of it is seenabove Trona Pinnacles in the California desert. (credit: Ian Norman)

Typically, the interstellar material is so extremely sparse that the space between stars is a much bettervacuum than anything we can produce in terrestrial laboratories. Yet, the dust in space, building up overthousands of light-years, can block the light of more distant stars. Like the distant buildings that disappearfrom our view on a smoggy day in Los Angeles, the more distant regions of the Milky Way cannot be seen



behind the layers of interstellar smog. Luckily, astronomers have found that stars and raw material shinewith various forms of light, some of which do penetrate the smog, and so we have been able to develop apretty good map of the Galaxy.

Recent observations, however, have also revealed a rather surprising and disturbing fact. There appearsto be more�much more�to the Galaxy than meets the eye (or the telescope). From various investigations,we have evidence that much of our Galaxy is made of material we cannot currently observe directly withour instruments. We therefore call this component of the Galaxy dark matter. We know the dark matter isthere by the pull its gravity exerts on the stars and raw material we can observe, but what this dark matteris made of and how much of it exists remain a mystery. Furthermore, this dark matter is not con�ned toour Galaxy; it appears to be an important part of other star groupings as well.

By the way, not all stars live by themselves, as the Sun does. Many are born in double or triple systemswith two, three, or more stars revolving about each other. Because the stars in�uence each other in suchclose systems, multiple stars allow us to measure characteristics that we cannot discern from observing singlestars. In a number of places, enough stars have formed together that we recognized them as star clusters(Figure 1.10 (Star Cluster.)). Some of the largest of the star clusters that astronomers have cataloged containhundreds of thousands of stars and take up volumes of space hundreds of light-years across.


27

Star Cluster.

Figure 1.10: This large star cluster is known by its catalog number, M9. It contains some 250,000stars and is seen more clearly from space using the Hubble Space Telescope. It is located roughly 25,000light-years away. (credit: NASA, ESA)

You may hear stars referred to as �eternal,� but in fact no star can last forever. Since the �business� ofstars is making energy, and energy production requires some sort of fuel to be used up, eventually all starsrun out of fuel. This news should not cause you to panic, though, because our Sun still has at least 5 or 6billion years to go. Ultimately, the Sun and all stars will die, and it is in their death throes that some of themost intriguing and important processes of the universe are revealed. For example, we now know that many



of the atoms in our bodies were once inside stars. These stars exploded at the ends of their lives, recyclingtheir material back into the reservoir of the Galaxy. In this sense, all of us are literally made of recycled�star dust.�

1.8 The Universe on the Large Scale11

In a very rough sense, you could think of the solar system as your house or apartment and the Galaxy asyour town, made up of many houses and buildings. In the twentieth century, astronomers were able to showthat, just as our world is made up of many, many towns, so the universe is made up of enormous numbersof galaxies. (We de�ne the universe to be everything that exists that is accessible to our observations.)Galaxies stretch as far into space as our telescopes can see, many billions of them within the reach of moderninstruments. When they were �rst discovered, some astronomers called galaxies island universes, and theterm is aptly descriptive; galaxies do look like islands of stars in the vast, dark seas of intergalactic space.

The nearest galaxy, discovered in 1993, is a small one that lies 75,000 light-years from the Sun in thedirection of the constellation Sagittarius, where the smog in our own Galaxy makes it especially di�cult todiscern. (A constellation, we should note, is one of the 88 sections into which astronomers divide the sky,each named after a prominent star pattern within it.) Beyond this Sagittarius dwarf galaxy lie two othersmall galaxies, about 160,000 light-years away. First recorded by Magellan's crew as he sailed around theworld, these are called the Magellanic Clouds (Figure 1.11 (Neighbor Galaxies.)). All three of these smallgalaxies are satellites of the Milky Way Galaxy, interacting with it through the force of gravity. Ultimately,all three may even be swallowed by our much larger Galaxy, as other small galaxies have been over the courseof cosmic time.



29

Neighbor Galaxies.

Figure 1.11: This image shows both the Large and Small Magellanic Clouds above the telescopes ofthe Atacama Large Millimeter/Submillimeter Array (ALMA) in the Atacama Desert of northern Chile.(credit: ESO, C. Malin)

The nearest large galaxy is a spiral quite similar to our own, located in the constellation of Andromeda,and is thus called the Andromeda galaxy; it is also known by one of its catalog numbers, M31 (Figure 1.12(Closest Spiral Galaxy.)). M31 is a little more than 2 million light-years away and, along with the MilkyWay, is part of a small cluster of more than 50 galaxies referred to as the Local Group.



Closest Spiral Galaxy.

Figure 1.12: The Andromeda galaxy (M31) is a spiral-shaped collection of stars similar to our ownMilky Way. (credit: Adam Evans)

At distances of 10 to 15 million light-years, we �nd other small galaxy groups, and then at about 50 millionlight-years there are more impressive systems with thousands of member galaxies. We have discovered thatgalaxies occur mostly in clusters, both large and small (Figure 1.13 (Fornax Cluster of Galaxies.)).


31

Fornax Cluster of Galaxies.

Figure 1.13: In this image, you can see part of a cluster of galaxies located about 60 million light-yearsaway in the constellation of Fornax. All the objects that are not pinpoints of light in the picture are galax-ies of billions of stars. (credit: ESO, J. Emerson, VISTA. Acknowledgment: Cambridge AstronomicalSurvey Unit)

Some of the clusters themselves form into larger groups called superclusters. The Local Group is partof a supercluster of galaxies, called the Virgo Supercluster, which stretches over a diameter of 110 millionlight-years. We are just beginning to explore the structure of the universe at these enormous scales and arealready encountering some unexpected �ndings.

At even greater distances, where many ordinary galaxies are too dim to see, we �nd quasars. These arebrilliant centers of galaxies, glowing with the light of an extraordinarily energetic process. The enormousenergy of the quasars is produced by gas that is heated to a temperature of millions of degrees as it fallstoward a massive black hole and swirls around it. The brilliance of quasars makes them the most distantbeacons we can see in the dark oceans of space. They allow us to probe the universe 10 billion light-yearsaway or more, and thus 10 billion years or more in the past.



With quasars we can see way back close to the Big Bang explosion that marks the beginning of time.Beyond the quasars and the most distant visible galaxies, we have detected the feeble glow of the explosionitself, �lling the universe and thus coming to us from all directions in space. The discovery of this �afterglowof creation� is considered to be one of the most signi�cant events in twentieth-century science, and we arestill exploring the many things it has to tell us about the earliest times of the universe.

Measurements of the properties of galaxies and quasars in remote locations require large telescopes,sophisticated light-amplifying devices, and painstaking labor. Every clear night, at observatories around theworld, astronomers and students are at work on such mysteries as the birth of new stars and the large-scalestructure of the universe, �tting their results into the tapestry of our understanding.

1.9 The Universe of the Very Small12

The foregoing discussion has likely impressed on you that the universe is extraordinarily large and extraor-dinarily empty. On average, it is 10,000 times more empty than our Galaxy. Yet, as we have seen, even theGalaxy is mostly empty space. The air we breathe has about 1019 atoms in each cubic centimeter�and weusually think of air as empty space. In the interstellar gas of the Galaxy, there is about one atom in everycubic centimeter. Intergalactic space is �lled so sparsely that to �nd one atom, on average, we must searchthrough a cubic meter of space. Most of the universe is fantastically empty; places that are dense, such asthe human body, are tremendously rare.

Even our most familiar solids are mostly space. If we could take apart such a solid, piece by piece, wewould eventually reach the tiny molecules from which it is formed. Molecules are the smallest particles intowhich any matter can be divided while still retaining its chemical properties. A molecule of water (H2O),for example, consists of two hydrogen atoms and one oxygen atom bonded together.

Molecules, in turn, are built of atoms, which are the smallest particles of an element that can still beidenti�ed as that element. For example, an atom of gold is the smallest possible piece of gold. Nearly 100di�erent kinds of atoms (elements) exist in nature. Most of them are rare, and only a handful account formore than 99% of everything with which we come in contact. The most abundant elements in the cosmostoday are listed in Table 1.1; think of this table as the �greatest hits� of the universe when it comes toelements.

The Cosmically Abundant Elements

Element16 Symbol Number of Atoms perMillion Hydrogen Atoms

Hydrogen H 1,000,000

Helium He 80,000

Carbon C 450

Nitrogen N 92

Oxygen O 740

Neon Ne 130

Magnesium Mg 40

Silicon Si 37

Sulfur S 19

Iron Fe 32

Table 1.1



33

All atoms consist of a central, positively charged nucleus surrounded by negatively charged electrons. Thebulk of the matter in each atom is found in the nucleus, which consists of positive protons and electricallyneutral neutrons all bound tightly together in a very small space. Each element is de�ned by the number ofprotons in its atoms. Thus, any atom with 6 protons in its nucleus is called carbon, any with 50 protons iscalled tin, and any with 70 protons is called ytterbium. (For a list of the elements, see Appendix K18.)

The distance from an atomic nucleus to its electrons is typically 100,000 times the size of the nucleusitself. This is why we say that even solid matter is mostly space. The typical atom is far emptier than thesolar system out to Neptune. (The distance from Earth to the Sun, for example, is only 100 times the sizeof the Sun.) This is one reason atoms are not like miniature solar systems.

Remarkably, physicists have discovered that everything that happens in the universe, from the smallestatomic nucleus to the largest superclusters of galaxies, can be explained through the action of only fourforces: gravity, electromagnetism (which combines the actions of electricity and magnetism), and two forcesthat act at the nuclear level. The fact that there are four forces (and not a million, or just one) has puzzledphysicists and astronomers for many years and has led to a quest for a uni�ed picture of nature.

note: To build an atom, particle by particle, check out this guided animation19

for building an atom.

1.10 A Conclusion and a Beginning20

If you are new to astronomy, you have probably reached the end of our brief tour in this chapter with mixedemotions. On the one hand, you may be fascinated by some of the new ideas you've read about and youmay be eager to learn more. On the other hand, you may be feeling a bit overwhelmed by the number oftopics we have covered, and the number of new words and ideas we have introduced. Learning astronomyis a little like learning a new language: at �rst it seems there are so many new expressions that you'll nevermaster them all, but with practice, you soon develop facility with them.

At this point you may also feel a bit small and insigni�cant, dwarfed by the cosmic scales of distanceand time. But, there is another way to look at what you have learned from our �rst glimpses of the cosmos.Let us consider the history of the universe from the Big Bang to today and compress it, for easy reference,into a single year. (We have borrowed this idea from Carl Sagan's 1997 Pulitzer Prize-winning book, TheDragons of Eden.)

On this scale, the Big Bang happened at the �rst moment of January 1, and this moment, when you arereading this chapter would be the end of the very last second of December 31. When did other events in thedevelopment of the universe happen in this �cosmic year?� Our solar system formed around September 10,and the oldest rocks we can date on Earth go back to the third week in September (Figure 1.14 (ChartingCosmic Time.)).

17This list of elements is arranged in order of the atomic number, which is the number of protons in each nucleus.18"The Chemical Elements" <http://cnx.org/content/m60004/latest/>19https://openstaxcollege.org/l/30buildanatom20This content is available online at <http://cnx.org/content/m59762/1.2/>.



Charting Cosmic Time.

Figure 1.14: On a cosmic calendar, where the time since the Big Bang is compressed into 1 year,creatures we would call human do not emerge on the scene until the evening of December 31. (credit:February: modi�cation of work by NASA, JPL-Caltech, W. Reach (SSC/Caltech); March: modi�cationof work by ESA, Hubble and NASA, Acknowledgement: Giles Chapdelaine; April: modi�cation of workby NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), A. Mahdavi (San FranciscoState University); May: modi�cation of work by NASA, JPL-Caltech; June: modi�cation of work byNASA/ESA; July: modi�cation of work by NASA, JPL-Caltech, Harvard-Smithsonian; August: mod-i�cation of work by NASA, JPL-Caltech, R. Hurt (SSC-Caltech); September: modi�cation of work byNASA; October: modi�cation of work by NASA; November: modi�cation of work by Dénes Em®ke)

Where does the origin of human beings fall during the course of this cosmic year? The answer turns outto be the evening of December 31. The invention of the alphabet doesn't occur until the �ftieth second of11:59 p.m. on December 31. And the beginnings of modern astronomy are a mere fraction of a second beforethe New Year. Seen in a cosmic context, the amount of time we have had to study the stars is minute, andour success in piecing together as much of the story as we have is remarkable.

Certainly our attempts to understand the universe are not complete. As new technologies and new ideasallow us to gather more and better data about the cosmos, our present picture of astronomy will very likelyundergo many changes. Still, as you read our current progress report on the exploration of the universe,take a few minutes every once in a while just to savor how much you have already learned.

Example 1.1

Scienti�c Notation

In 2015, the richest human being on our planet had a net worth of $79.2 billion. Some might saythis is an astronomical sum of money. Express this amount in scienti�c notation.


35

Solution

$79.2 billion can be written $79,200,000,000. Expressed in scienti�c notation it becomes $7.92 ×1010.

Example 1.2

Getting Familiar with a Light-Year

How many kilometers are there in a light-year?Solution

Light travels 3 × 105 km in 1 s. So, let's calculate how far it goes in a year:

� There are 60 (6 × 101) s in 1 min, and 6 × 101 min in 1 h.� Multiply these together and you �nd that there are 3.6 × 103 s/h.� Thus, light covers 3 × 105 km/s × 3.6 × 103 s/h = 1.08 × 109 km/h.� There are 24 or 2.4 × 101 h in a day, and 365.24 (3.65 × 102) days in 1 y.� The product of these two numbers is 8.77 × 103 h/y.� Multiplying this by 1.08 × 109 km/h gives 9.46 × 1012 km/ly.

That's almost 10,000,000,000,000 km that light covers in a year. To help you imagine how longthis distance is, we'll mention that a string 1 light-year long could �t around the circumference ofEarth 236 million times.

1.10.1 For Further Exploration

1.10.1.1 Books

Miller, Ron, and William Hartmann. The Grand Tour: A Traveler's Guide to the Solar System. 3rd ed.Workman, 2005. This volume for beginners is a colorfully illustrated voyage among the planets.

Sagan, Carl. Cosmos. Ballantine, 2013 [1980]. This tome presents a classic overview of astronomy by anastronomer who had a true gift for explaining things clearly. (You can also check out Sagan's television seriesCosmos: A Personal Voyage and Neil DeGrasse Tyson's current series Cosmos: A Spacetime Odyssey.)

Tyson, Neil DeGrasse, and Don Goldsmith. Origins: Fourteen Billion Years of Cosmic Evolution. Norton,2004. This book provides a guided tour through the beginnings of the universe, galaxies, stars, planets, andlife.

1.10.1.2 Websites

If you enjoyed the beautiful images in this chapter (and there are many more fabulous photos to come inother chapters), you may want to know where you can obtain and download such pictures for your ownenjoyment. (Many astronomy images are from government-supported instruments or projects, paid for bytax dollars, and therefore are free of copyright laws.) Here are three resources we especially like:

� Astronomy Picture of the Day: apod.nasa.gov/apod/astropix.html. Two space scientists scour theInternet and select one beautiful astronomy image to feature each day. Their archives range widely,from images of planets and nebulae to rockets and space instruments; they also have many photos ofthe night sky. The search function (see the menu on the bottom of the page) works quite well for�nding something speci�c among the many years' worth of daily images.

� Hubble Space Telescope Images: www.hubblesite.org/newscenter/archive/browse/images. Starting atthis page, you can select from among hundreds of Hubble pictures by subject or by date. Note thatmany of the images have supporting pictures with them, such as diagrams, animations, or comparisons.Excellent captions and background information are provided. Other ways to approach these imagesare through the more public-oriented Hubble Gallery (www.hubblesite.org/gallery) and the Europeanhomepage (www.spacetelescope.org/images).



� National Aeronautics and Space Administration's (NASA's) Planetary Photojournal: photojour-nal.jpl.nasa.gov. This site features thousands of images from planetary exploration, with captionsof varied length. You can select images by world, feature name, date, or catalog number, and down-load images in a number of popular formats. However, only NASA mission images are included. Notethe Photojournal Search option on the menu at the top of the homepage to access ways to search theirarchives.

1.10.1.3 Videos

Cosmic Voyage: www.youtube.com/watch?v=qxXf7AJZ73A. This video presents a portion of Cosmic Voy-age, narrated by Morgan Freeman (8:34).

Powers of Ten: www.youtube.com/watch?v=0fKBhvDjuy0. This classic short video is a much earlierversion of Powers of Ten, narrated by Philip Morrison (9:00).

The Known Universe: www.youtube.com/watch?v=17jymDn0W6U. This video tour from the AmericanMuseum of Natural History has realistic animation, music, and captions (6:30).

Wanderers: apod.nasa.gov/apod/ap141208.html. This video provides a tour of the solar system, withnarrative by Carl Sagan, imagining other worlds with dramatically realistic paintings (3:50).


Chapter 2

Observing the Sky: The Birth of

Astronomy

2.1 Thinking Ahead1

2.2 The Sky Above2

2.3 Ancient Astronomy3

2.4 Astrology and Astronomy4

2.5 The Birth of Modern Astronomy5

1This content is available online at <http://cnx.org/content/m59769/1.1/>.2This content is available online at <http://cnx.org/content/m59770/1.1/>.3This content is available online at <http://cnx.org/content/m59763/1.1/>.4This content is available online at <http://cnx.org/content/m59772/1.1/>.5This content is available online at <http://cnx.org/content/m59765/1.1/>.


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38 CHAPTER 2. OBSERVING THE SKY: THE BIRTH OF ASTRONOMY


Chapter 3

Orbits and Gravity

3.1 Thinking Ahead1

3.2 The Laws of Planetary Motion2

3.3 Newton's Great Synthesis3

3.4 Newton's Universal Law of Gravitation4

3.5 Orbits in the Solar System5

3.6 Motions of Satellites and Spacecraft6

3.7 Gravity with More Than Two Bodies7

1This content is available online at <http://cnx.org/content/m59773/1.1/>.2This content is available online at <http://cnx.org/content/m59775/1.1/>.3This content is available online at <http://cnx.org/content/m59776/1.1/>.4This content is available online at <http://cnx.org/content/m59777/1.1/>.5This content is available online at <http://cnx.org/content/m59778/1.1/>.6This content is available online at <http://cnx.org/content/m59779/1.1/>.7This content is available online at <http://cnx.org/content/m59780/1.1/>.


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40 CHAPTER 3. ORBITS AND GRAVITY


Chapter 4

Earth, Moon, and Sky

4.1 Thinking Ahead1

4.2 Earth and Sky2

4.3 The Seasons3

4.4 Keeping Time4

4.5 The Calendar5

4.6 Phases and Motions of the Moon6

4.7 Ocean Tides and the Moon7

4.8 Eclipses of the Sun and Moon8

1This content is available online at <http://cnx.org/content/m59781/1.1/>.2This content is available online at <http://cnx.org/content/m59783/1.1/>.3This content is available online at <http://cnx.org/content/m59785/1.1/>.4This content is available online at <http://cnx.org/content/m59786/1.1/>.5This content is available online at <http://cnx.org/content/m59787/1.1/>.6This content is available online at <http://cnx.org/content/m59788/1.1/>.7This content is available online at <http://cnx.org/content/m59789/1.1/>.8This content is available online at <http://cnx.org/content/m59790/1.1/>.


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42 CHAPTER 4. EARTH, MOON, AND SKY


Chapter 5

Radiation and Spectra

5.1 Thinking Ahead1

5.2 The Behavior of Light2

5.3 The Electromagnetic Spectrum3

5.4 Spectroscopy in Astronomy4

5.5 The Structure of the Atom5

5.6 Formation of Spectral Lines6

5.7 The Doppler E�ect7



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44 CHAPTER 5. RADIATION AND SPECTRA


Chapter 6

Astronomical Instruments

6.1 Thinking Ahead1

6.2 Telescopes2

6.3 Telescopes Today3

6.4 Visible-Light Detectors and Instruments4

6.5 Radio Telescopes5

6.6 The Future of Large Telescopes6

1This content is available online at <http://cnx.org/content/m59802/1.1/>.2This content is available online at <http://cnx.org/content/m59803/1.1/>.3This content is available online at <http://cnx.org/content/m59805/1.1/>.4This content is available online at <http://cnx.org/content/m59821/1.1/>.5This content is available online at <http://cnx.org/content/m59809/1.1/>.6This content is available online at <http://cnx.org/content/m59812/1.1/>.


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46 CHAPTER 6. ASTRONOMICAL INSTRUMENTS


Chapter 7

Other Worlds: An Introduction to the

Solar System

7.1 Thinking Ahead1

7.2 Overview of Our Planetary System2

7.3 Composition and Structure of Planets3

7.4 Dating Planetary Surfaces4

7.5 Origin of the Solar System5



47

48CHAPTER 7. OTHER WORLDS: AN INTRODUCTION TO THE SOLAR

SYSTEM


Chapter 8

Earth as a Planet

8.1 Thinking Ahead1

8.2 The Global Perspective2

8.3 The Crust of the Earth3

8.4 The Earth's Atmosphere4

8.5 Life and Chemical Evolution5

8.6 Cosmic In�uences on the Evolution of the Earth6



49

50 CHAPTER 8. EARTH AS A PLANET


Chapter 9

Cratered Worlds

9.1 Thinking Ahead1

9.2 General Properties of the Moon2

9.3 The Lunar Surface3

9.4 Impact Craters4

9.5 The Origin of the Moon5

9.6 Mercury6



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52 CHAPTER 9. CRATERED WORLDS


Chapter 10

Earth-Like Planets: Venus and Mars

10.1 Thinking Ahead1

10.2 The Nearest Planets: An Overview2

10.3 The Geology of Venus3

10.4 The Massive Atmosphere of Venus4

10.5 The Geology of Mars5

10.6 Water and Life on Mars6

10.7 Divergent Planetary Evolution7



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54 CHAPTER 10. EARTH-LIKE PLANETS: VENUS AND MARS


Chapter 11

The Giant Planets


11.2 Exploring the Outer Solar System2

11.3 The Giant Planets3

11.4 Atmospheres of the Giant Planets4

1This content is available online at <http://cnx.org/content/m59850/1.1/>.2This content is available online at <http://cnx.org/content/m59853/1.1/>.3This content is available online at <http://cnx.org/content/m59858/1.1/>.4This content is available online at <http://cnx.org/content/m59859/1.1/>.


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56 CHAPTER 11. THE GIANT PLANETS


Chapter 12

Rings, Moons, and Pluto


12.2 Rings and Moons2

12.3 The Galilean Moons of Jupiter3

12.4 Triton and Pluto4

12.5 Planetary Rings5



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58 CHAPTER 12. RINGS, MOONS, AND PLUTO


Chapter 13

Solar System Debris: Comets, Asteroids,

and Other Small Bodies


13.2 Asteroids2

13.3 Asteroids and Planetary Defense3

13.4 The "Long-Haired" Comets4

13.5 The Origin and Fate of Comets5



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60CHAPTER 13. SOLAR SYSTEM DEBRIS: COMETS, ASTEROIDS, AND

OTHER SMALL BODIES


Chapter 14

Cosmic Samples and the Origin of the

Solar System


14.2 Meteors2

14.3 Meteorites: Stones from Heaven3

14.4 Formation of the Solar System4

14.5 Comparison with Other Planetary Systems5

14.6 Planetary Evolution6



61

62CHAPTER 14. COSMIC SAMPLES AND THE ORIGIN OF THE SOLAR

SYSTEM


Chapter 15

The Sun: The Star That Really Matters


15.2 The Sun From the Inside Out2

15.3 Solar Magnetism and Solar Activity3

15.4 Space Weather4

15.5 Where Does the Sun Go From Here?5



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64 CHAPTER 15. THE SUN: THE STAR THAT REALLY MATTERS


Chapter 16

The Sun: A Nuclear Powerhouse


16.2 Thermal and Gravitational Energy2

16.3 Mass, Energy, and the Theory of Relativity3

16.4 The Interior of the Sun: Theory4

16.5 The Solar Interior: Observations5



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66 CHAPTER 16. THE SUN: A NUCLEAR POWERHOUSE


Chapter 17

Analyzing Starlight


17.2 The Brightness of Stars2

17.3 Colors of Stars3

17.4 The Spectra of Stars (and Brown Dwarves)4

17.5 Using Spectra to Measure Stellar Radius, Composition, andMotion5



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68 CHAPTER 17. ANALYZING STARLIGHT


Chapter 18

The Stars: A Celestial Census


18.2 A Stellar Census2

18.3 Measuring Stellar Masses3

18.4 Diameters of Stars4

18.5 The H-R Diagram5



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70 CHAPTER 18. THE STARS: A CELESTIAL CENSUS


Chapter 19

Celestial Distances


19.2 Fundamental Units of Distance2

19.3 Surveying the Stars3

19.4 Variable Stars: One Key to Cosmic Distances4

19.5 The H-R Diagram and Cosmic Distances5



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72 CHAPTER 19. CELESTIAL DISTANCES


Chapter 20

Between the Stars: Gas and Dust in

Space


20.2 The Interstellar Medium2

20.3 Interstellar Gas3

20.4 Cosmic Dust4

20.5 Cosmic Rays5

20.6 Recycling Cosmic Material6

20.7 Interstellar Matter around the Sun7



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74 CHAPTER 20. BETWEEN THE STARS: GAS AND DUST IN SPACE


Chapter 21

The Birth of Stars and the Discovery of

Planets outside the Solar System


21.2 Star Formation2

21.3 The H-R Diagram and the Study of Stellar Evolution3

21.4 Evidence That Planets Form Around Stars4

21.5 Planets beyond the Solar System; Search and Discovery5



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76CHAPTER 21. THE BIRTH OF STARS AND THE DISCOVERY OF

PLANETS OUTSIDE THE SOLAR SYSTEM


Chapter 22

Stars from Adolescence to Old Age


22.2 Evolution from the Main Sequence to Red Giants2

22.3 Star Clusters3

22.4 Checking Out the Theory4

22.5 Further Evolution of Stars5



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78 CHAPTER 22. STARS FROM ADOLESCENCE TO OLD AGE


Chapter 23

The Death of Stars


23.2 The Death of Low-Mass Stars2

23.3 Evolution of Massive Stars: An Explosive Finish3

23.4 Supernova Observations4

23.5 Pulsars and the Discovery of Neutron Stars5

23.6 The Evolution of Binary Star Systems6

23.7 The Mystery of the Gamma-Ray Bursts7



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80 CHAPTER 23. THE DEATH OF STARS


Chapter 24

Black Holes and Curved Spacetime


24.2 Introducing General Relativity2

24.3 The Principle of Equivalence3

24.4 Spacetime and Gravity4

24.5 Tests of General Relativity5

24.6 Time in General Relativity6

24.7 Black Holes7

24.8 Evidence for Black Holes8

24.9 Gravitational Wave Astronomy9

1This content is available online at <http://cnx.org/content/m59938/1.1/>.2This content is available online at <http://cnx.org/content/m59939/1.1/>.3This content is available online at <http://cnx.org/content/m59940/1.1/>.4This content is available online at <http://cnx.org/content/m59941/1.1/>.5This content is available online at <http://cnx.org/content/m59942/1.1/>.6This content is available online at <http://cnx.org/content/m59943/1.1/>.7This content is available online at <http://cnx.org/content/m59944/1.1/>.8This content is available online at <http://cnx.org/content/m59945/1.1/>.9This content is available online at <http://cnx.org/content/m59946/1.1/>.


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82 CHAPTER 24. BLACK HOLES AND CURVED SPACETIME


Chapter 25

The Milky Way Galaxy


25.2 The Architecture of the Galaxy2

25.3 Spiral Structure3

25.4 The Mass of the Galaxy4

25.5 The Center of the Galaxy5

25.6 Stellar Populations in the Galaxy6

25.7 The Formation of the Galaxy7



83

84 CHAPTER 25. THE MILKY WAY GALAXY


Chapter 26

Galaxies


26.2 The Great Nebula Debate2

26.3 Types of Galaxies3

26.4 Properties of Galaxies4

26.5 The Extragalactic Distance Scale5



85

86 CHAPTER 26. GALAXIES


Chapter 27

Active Galaxies, Quasars, and Giant

Black Holes


27.2 The Quasars2

27.3 Giant Black Holes at the Centers of Galaxies3

27.4 Lighting up Black Holes4

27.5 Codependence of Black Holes and Galaxies5

27.6 Quasars as Lighthouses to See in the Dark6



87

88CHAPTER 27. ACTIVE GALAXIES, QUASARS, AND GIANT BLACK

HOLES


Chapter 28

The Evolution and Distribution of

Galaxies


28.2 Observations of Distant Galaxies2

28.3 Galaxy Mergers and Active Galactic Nuclei3

28.4 The Distribution of Galaxies in Space4

28.5 A Universe of (Mostly) Dark Matter and Dark Energy5

28.6 The Formation and Evolution of Galaxies and Structure in theUniverse6



89

90 CHAPTER 28. THE EVOLUTION AND DISTRIBUTION OF GALAXIES


Chapter 29

The Big Bang


29.2 The Age of the Universe2

29.3 A Model of the Universe3

29.4 The Beginning of the Universe4

29.5 The Cosmic Background Radiation5

29.6 What is the Universe Really Made of?6

29.7 The In�ationary Universe7

29.8 The Anthropic Principle8

1This content is available online at <http://cnx.org/content/m59975/1.1/>.2This content is available online at <http://cnx.org/content/m59976/1.1/>.3This content is available online at <http://cnx.org/content/m59977/1.1/>.4This content is available online at <http://cnx.org/content/m59978/1.1/>.5This content is available online at <http://cnx.org/content/m59979/1.1/>.6This content is available online at <http://cnx.org/content/m59980/1.1/>.7This content is available online at <http://cnx.org/content/m59981/1.1/>.8This content is available online at <http://cnx.org/content/m59982/1.1/>.


91

92 CHAPTER 29. THE BIG BANG


Chapter 30

Life in the Universe


30.2 The Cosmic Context for Life2

30.3 Astrobiology3

30.4 Searching for Life beyond Earth4

30.5 The Search for Extraterrestrial Intelligence5



93

94 INDEX

Index of Keywords and Terms

Keywords are listed by the section with that keyword (page numbers are in parentheses). Keywordsdo not necessarily appear in the text of the page. They are merely associated with that section. Ex.apples, � 1.1 (1) Terms are referenced by the page they appear on. Ex. apples, 1

A Astronomical unit (AU), � 1.7(20)Astronomy, � 1.1(11), � 1.2(15)Atom, � 1.9(32)

C Cosmic calendar, � 1.10(33)

E Element, � 1.9(32)

G Galaxy, � 1.8(28)

H Historical science, � 1.3(15)Hypothesis, � 1.3(15)

L Light-year, � 1.5(17)

M Molecule, � 1.9(32)

O Observational science, � 1.3(15)

P Powers-of-ten notation, � 1.5(17)

Q Quasar, � 1.8(28)

S Scienti�c laws, � 1.4(16)Scienti�c method, � 1.3(15)Scienti�c notation, � 1.1(11), � 1.5(17)Speed of light, � 1.1(11)Supercluster, � 1.8(28)


ATTRIBUTIONS 95

Attributions

Collection: AstronomyEdited by: OpenStax AstronomyURL: http://cnx.org/content/col11992/1.2/License: http://creativecommons.org/licenses/by/4.0/

Module: "Preface"By: OpenStax AstronomyURL: http://cnx.org/content/m62004/1.1/Pages: 1-10Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/Based on: PrefaceBy: OpenStax ChemistryURL: http://cnx.org/content/m53245/1.25/

Module: "Introduction"By: OpenStax AstronomyURL: http://cnx.org/content/m59743/1.2/Pages: 11-15Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Nature of Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59747/1.2/Page: 15Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Nature of Science"By: OpenStax AstronomyURL: http://cnx.org/content/m59755/1.2/Pages: 15-16Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Laws of Nature"By: OpenStax AstronomyURL: http://cnx.org/content/m59750/1.2/Pages: 16-17Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Numbers in Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59751/1.2/Pages: 17-19Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


96 ATTRIBUTIONS

Module: "Consequences of Light Travel Time"By: OpenStax AstronomyURL: http://cnx.org/content/m59752/1.1/Page: 20Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "A Tour of the Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59753/1.2/Pages: 20-28Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Universe on the Large Scale"By: OpenStax AstronomyURL: http://cnx.org/content/m59754/1.2/Pages: 28-32Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Universe of the Very Small"By: OpenStax AstronomyURL: http://cnx.org/content/m59760/1.2/Pages: 32-33Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "A Conclusion and a Beginning"By: OpenStax AstronomyURL: http://cnx.org/content/m59762/1.2/Pages: 33-36Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Thinking Ahead"By: OpenStax AstronomyURL: http://cnx.org/content/m59769/1.1/Page: 37Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Sky Above"By: OpenStax AstronomyURL: http://cnx.org/content/m59770/1.1/Page: 37Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Ancient Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59763/1.1/Page: 37Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 97

Module: "Astrology and Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59772/1.1/Page: 37Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Birth of Modern Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59765/1.1/Page: 37Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Laws of Planetary Motion"By: OpenStax AstronomyURL: http://cnx.org/content/m59775/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Newton's Great Synthesis"By: OpenStax AstronomyURL: http://cnx.org/content/m59776/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Newton's Universal Law of Gravitation"By: OpenStax AstronomyURL: http://cnx.org/content/m59777/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Orbits in the Solar System"By: OpenStax AstronomyURL: http://cnx.org/content/m59778/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Motions of Satellites and Spacecraft"By: OpenStax AstronomyURL: http://cnx.org/content/m59779/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


98 ATTRIBUTIONS

Module: "Gravity with More Than Two Bodies"By: OpenStax AstronomyURL: http://cnx.org/content/m59780/1.1/Page: 39Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Earth and Sky"By: OpenStax AstronomyURL: http://cnx.org/content/m59783/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Seasons"By: OpenStax AstronomyURL: http://cnx.org/content/m59785/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Keeping Time"By: OpenStax AstronomyURL: http://cnx.org/content/m59786/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Calendar"By: OpenStax AstronomyURL: http://cnx.org/content/m59787/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Phases and Motions of the Moon"By: OpenStax AstronomyURL: http://cnx.org/content/m59788/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Ocean Tides and the Moon"By: OpenStax AstronomyURL: http://cnx.org/content/m59789/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 99

Module: "Eclipses of the Sun and Moon"By: OpenStax AstronomyURL: http://cnx.org/content/m59790/1.1/Page: 41Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Behavior of Light"By: OpenStax AstronomyURL: http://cnx.org/content/m59792/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Electromagnetic Spectrum"By: OpenStax AstronomyURL: http://cnx.org/content/m59798/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Spectroscopy in Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59793/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Structure of the Atom"By: OpenStax AstronomyURL: http://cnx.org/content/m59794/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Formation of Spectral Lines"By: OpenStax AstronomyURL: http://cnx.org/content/m59795/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Doppler E�ect"By: OpenStax AstronomyURL: http://cnx.org/content/m59799/1.1/Page: 43Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


100 ATTRIBUTIONS


Module: "Telescopes"By: OpenStax AstronomyURL: http://cnx.org/content/m59803/1.1/Page: 45Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Telescopes Today"By: OpenStax AstronomyURL: http://cnx.org/content/m59805/1.1/Page: 45Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Visible-Light Detectors and Instruments"By: OpenStax AstronomyURL: http://cnx.org/content/m59821/1.1/Page: 45Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Radio Telescopes"By: OpenStax AstronomyURL: http://cnx.org/content/m59809/1.1/Page: 45Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Future of Large Telescopes"By: OpenStax AstronomyURL: http://cnx.org/content/m59812/1.1/Page: 45Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Overview of Our Planetary System"By: OpenStax AstronomyURL: http://cnx.org/content/m59816/1.1/Page: 47Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 101

Module: "Composition and Structure of Planets"By: OpenStax AstronomyURL: http://cnx.org/content/m59819/1.1/Page: 47Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Dating Planetary Surfaces"By: OpenStax AstronomyURL: http://cnx.org/content/m59820/1.1/Page: 47Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Origin of the Solar System"By: OpenStax AstronomyURL: http://cnx.org/content/m59823/1.1/Page: 47Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Global Perspective"By: OpenStax AstronomyURL: http://cnx.org/content/m59825/1.1/Page: 49Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Crust of the Earth"By: OpenStax AstronomyURL: http://cnx.org/content/m59826/1.1/Page: 49Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Earth's Atmosphere"By: OpenStax AstronomyURL: http://cnx.org/content/m59827/1.1/Page: 49Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Life and Chemical Evolution"By: OpenStax AstronomyURL: http://cnx.org/content/m59828/1.1/Page: 49Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


102 ATTRIBUTIONS

Module: "Cosmic In�uences on the Evolution of the Earth"By: OpenStax AstronomyURL: http://cnx.org/content/m59829/1.1/Page: 49Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "General Properties of the Moon"By: OpenStax AstronomyURL: http://cnx.org/content/m59831/1.1/Page: 51Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Lunar Surface"By: OpenStax AstronomyURL: http://cnx.org/content/m59832/1.1/Page: 51Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Impact Craters"By: OpenStax AstronomyURL: http://cnx.org/content/m59833/1.1/Page: 51Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Origin of the Moon"By: OpenStax AstronomyURL: http://cnx.org/content/m59834/1.1/Page: 51Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Mercury"By: OpenStax AstronomyURL: http://cnx.org/content/m59835/1.1/Page: 51Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/



ATTRIBUTIONS 103

Module: "The Nearest Planets: An Overview"By: OpenStax AstronomyURL: http://cnx.org/content/m59845/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Geology of Venus"By: OpenStax AstronomyURL: http://cnx.org/content/m59838/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Massive Atmosphere of Venus"By: OpenStax AstronomyURL: http://cnx.org/content/m59840/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Geology of Mars"By: OpenStax AstronomyURL: http://cnx.org/content/m59843/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Water and Life on Mars"By: OpenStax AstronomyURL: http://cnx.org/content/m59856/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Divergent Planetary Evolution"By: OpenStax AstronomyURL: http://cnx.org/content/m59847/1.1/Page: 53Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Exploring the Outer Solar System"By: OpenStax AstronomyURL: http://cnx.org/content/m59853/1.1/Page: 55Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


104 ATTRIBUTIONS

Module: "The Giant Planets"By: OpenStax AstronomyURL: http://cnx.org/content/m59858/1.1/Page: 55Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Atmospheres of the Giant Planets"By: OpenStax AstronomyURL: http://cnx.org/content/m59859/1.1/Page: 55Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Rings and Moons"By: OpenStax AstronomyURL: http://cnx.org/content/m59861/1.1/Page: 57Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Galilean Moons of Jupiter"By: OpenStax AstronomyURL: http://cnx.org/content/m59862/1.1/Page: 57Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Triton and Pluto"By: OpenStax AstronomyURL: http://cnx.org/content/m59863/1.1/Page: 57Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Planetary Rings"By: OpenStax AstronomyURL: http://cnx.org/content/m59864/1.1/Page: 57Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/



ATTRIBUTIONS 105

Module: "Asteroids"By: OpenStax AstronomyURL: http://cnx.org/content/m59866/1.1/Page: 59Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Asteroids and Planetary Defense"By: OpenStax AstronomyURL: http://cnx.org/content/m59867/1.1/Page: 59Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The "Long-Haired" Comets"By: OpenStax AstronomyURL: http://cnx.org/content/m59868/1.1/Page: 59Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Origin and Fate of Comets"By: OpenStax AstronomyURL: http://cnx.org/content/m59869/1.1/Page: 59Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Meteors"By: OpenStax AstronomyURL: http://cnx.org/content/m59871/1.1/Page: 61Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Meteorites: Stones from Heaven"By: OpenStax AstronomyURL: http://cnx.org/content/m59872/1.1/Page: 61Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Formation of the Solar System"By: OpenStax AstronomyURL: http://cnx.org/content/m59873/1.1/Page: 61Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


106 ATTRIBUTIONS

Module: "Comparison with Other Planetary Systems"By: OpenStax AstronomyURL: http://cnx.org/content/m59874/1.1/Page: 61Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Planetary Evolution"By: OpenStax AstronomyURL: http://cnx.org/content/m59875/1.1/Page: 61Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Sun From the Inside Out"By: OpenStax AstronomyURL: http://cnx.org/content/m59877/1.1/Page: 63Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Solar Magnetism and Solar Activity"By: OpenStax AstronomyURL: http://cnx.org/content/m59878/1.1/Page: 63Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Space Weather"By: OpenStax AstronomyURL: http://cnx.org/content/m59879/1.1/Page: 63Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Where Does the Sun Go From Here?"By: OpenStax AstronomyURL: http://cnx.org/content/m59884/1.1/Page: 63Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/



ATTRIBUTIONS 107

Module: "Thermal and Gravitational Energy"By: OpenStax AstronomyURL: http://cnx.org/content/m59895/1.1/Page: 65Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Mass, Energy, and the Theory of Relativity"By: OpenStax AstronomyURL: http://cnx.org/content/m59886/1.1/Page: 65Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Interior of the Sun: Theory"By: OpenStax AstronomyURL: http://cnx.org/content/m59887/1.1/Page: 65Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Solar Interior: Observations"By: OpenStax AstronomyURL: http://cnx.org/content/m59888/1.1/Page: 65Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Brightness of Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59890/1.1/Page: 67Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Colors of Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59891/1.1/Page: 67Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


108 ATTRIBUTIONS

Module: "The Spectra of Stars (and Brown Dwarves)"By: OpenStax AstronomyURL: http://cnx.org/content/m59892/1.1/Page: 67Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Using Spectra to Measure Stellar Radius, Composition, and Motion"By: OpenStax AstronomyURL: http://cnx.org/content/m59896/1.1/Page: 67Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "A Stellar Census"By: OpenStax AstronomyURL: http://cnx.org/content/m59898/1.1/Page: 69Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Measuring Stellar Masses"By: OpenStax AstronomyURL: http://cnx.org/content/m59899/1.1/Page: 69Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Diameters of Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59900/1.1/Page: 69Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The H-R Diagram"By: OpenStax AstronomyURL: http://cnx.org/content/m59901/1.1/Page: 69Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 109


Module: "Fundamental Units of Distance"By: OpenStax AstronomyURL: http://cnx.org/content/m59904/1.1/Page: 71Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Surveying the Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59905/1.1/Page: 71Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Variable Stars: One Key to Cosmic Distances"By: OpenStax AstronomyURL: http://cnx.org/content/m59906/1.1/Page: 71Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The H-R Diagram and Cosmic Distances"By: OpenStax AstronomyURL: http://cnx.org/content/m59907/1.1/Page: 71Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Interstellar Medium"By: OpenStax AstronomyURL: http://cnx.org/content/m59909/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Interstellar Gas"By: OpenStax AstronomyURL: http://cnx.org/content/m59910/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


110 ATTRIBUTIONS

Module: "Cosmic Dust"By: OpenStax AstronomyURL: http://cnx.org/content/m59911/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Cosmic Rays"By: OpenStax AstronomyURL: http://cnx.org/content/m59912/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Recycling Cosmic Material"By: OpenStax AstronomyURL: http://cnx.org/content/m59913/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Interstellar Matter around the Sun"By: OpenStax AstronomyURL: http://cnx.org/content/m59914/1.1/Page: 73Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Star Formation"By: OpenStax AstronomyURL: http://cnx.org/content/m59923/1.1/Page: 75Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The H-R Diagram and the Study of Stellar Evolution"By: OpenStax AstronomyURL: http://cnx.org/content/m59918/1.1/Page: 75Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Evidence That Planets Form Around Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59920/1.1/Page: 75Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 111

Module: "Planets beyond the Solar System; Search and Discovery"By: OpenStax AstronomyURL: http://cnx.org/content/m59921/1.1/Page: 75Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Evolution from the Main Sequence to Red Giants"By: OpenStax AstronomyURL: http://cnx.org/content/m59925/1.1/Page: 77Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Star Clusters"By: OpenStax AstronomyURL: http://cnx.org/content/m59926/1.1/Page: 77Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Checking Out the Theory"By: OpenStax AstronomyURL: http://cnx.org/content/m59927/1.1/Page: 77Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Further Evolution of Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59928/1.1/Page: 77Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Death of Low-Mass Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59932/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


112 ATTRIBUTIONS

Module: "Evolution of Massive Stars: An Explosive Finish"By: OpenStax AstronomyURL: http://cnx.org/content/m59933/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Supernova Observations"By: OpenStax AstronomyURL: http://cnx.org/content/m59934/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Pulsars and the Discovery of Neutron Stars"By: OpenStax AstronomyURL: http://cnx.org/content/m59935/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Evolution of Binary Star Systems"By: OpenStax AstronomyURL: http://cnx.org/content/m59936/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Mystery of the Gamma-Ray Bursts"By: OpenStax AstronomyURL: http://cnx.org/content/m59937/1.1/Page: 79Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "Introducing General Relativity"By: OpenStax AstronomyURL: http://cnx.org/content/m59939/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Principle of Equivalence"By: OpenStax AstronomyURL: http://cnx.org/content/m59940/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 113

Module: "Spacetime and Gravity"By: OpenStax AstronomyURL: http://cnx.org/content/m59941/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Tests of General Relativity"By: OpenStax AstronomyURL: http://cnx.org/content/m59942/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Time in General Relativity"By: OpenStax AstronomyURL: http://cnx.org/content/m59943/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Black Holes"By: OpenStax AstronomyURL: http://cnx.org/content/m59944/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Evidence for Black Holes"By: OpenStax AstronomyURL: http://cnx.org/content/m59945/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Gravitational Wave Astronomy"By: OpenStax AstronomyURL: http://cnx.org/content/m59946/1.1/Page: 81Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Architecture of the Galaxy"By: OpenStax AstronomyURL: http://cnx.org/content/m59948/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


114 ATTRIBUTIONS

Module: "Spiral Structure"By: OpenStax AstronomyURL: http://cnx.org/content/m59954/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Mass of the Galaxy"By: OpenStax AstronomyURL: http://cnx.org/content/m59950/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Center of the Galaxy"By: OpenStax AstronomyURL: http://cnx.org/content/m59951/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Stellar Populations in the Galaxy"By: OpenStax AstronomyURL: http://cnx.org/content/m59952/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Formation of the Galaxy"By: OpenStax AstronomyURL: http://cnx.org/content/m59955/1.1/Page: 83Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Great Nebula Debate"By: OpenStax AstronomyURL: http://cnx.org/content/m59964/1.1/Page: 85Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Types of Galaxies"By: OpenStax AstronomyURL: http://cnx.org/content/m59958/1.1/Page: 85Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


ATTRIBUTIONS 115

Module: "Properties of Galaxies"By: OpenStax AstronomyURL: http://cnx.org/content/m59959/1.1/Page: 85Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Extragalactic Distance Scale"By: OpenStax AstronomyURL: http://cnx.org/content/m59960/1.1/Page: 85Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


Module: "The Quasars"By: OpenStax AstronomyURL: http://cnx.org/content/m59962/1.1/Page: 87Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Giant Black Holes at the Centers of Galaxies"By: OpenStax AstronomyURL: http://cnx.org/content/m59965/1.1/Page: 87Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Lighting up Black Holes"By: OpenStax AstronomyURL: http://cnx.org/content/m59966/1.1/Page: 87Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Codependence of Black Holes and Galaxies"By: OpenStax AstronomyURL: http://cnx.org/content/m59967/1.1/Page: 87Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Quasars as Lighthouses to See in the Dark"By: OpenStax AstronomyURL: http://cnx.org/content/m59968/1.1/Page: 87Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


116 ATTRIBUTIONS


Module: "Observations of Distant Galaxies"By: OpenStax AstronomyURL: http://cnx.org/content/m59970/1.1/Page: 89Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Galaxy Mergers and Active Galactic Nuclei"By: OpenStax AstronomyURL: http://cnx.org/content/m59971/1.1/Page: 89Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Distribution of Galaxies in Space"By: OpenStax AstronomyURL: http://cnx.org/content/m59972/1.1/Page: 89Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "A Universe of (Mostly) Dark Matter and Dark Energy"By: OpenStax AstronomyURL: http://cnx.org/content/m59973/1.1/Page: 89Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Formation and Evolution of Galaxies and Structure in the Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59974/1.1/Page: 89Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/



ATTRIBUTIONS 117

Module: "The Age of the Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59976/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "A Model of the Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59977/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Beginning of the Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59978/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Cosmic Background Radiation"By: OpenStax AstronomyURL: http://cnx.org/content/m59979/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "What is the Universe Really Made of?"By: OpenStax AstronomyURL: http://cnx.org/content/m59980/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The In�ationary Universe"By: OpenStax AstronomyURL: http://cnx.org/content/m59981/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Anthropic Principle"By: OpenStax AstronomyURL: http://cnx.org/content/m59982/1.1/Page: 91Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/



118 ATTRIBUTIONS

Module: "The Cosmic Context for Life"By: OpenStax AstronomyURL: http://cnx.org/content/m59988/1.1/Page: 93Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Astrobiology"By: OpenStax AstronomyURL: http://cnx.org/content/m59989/1.1/Page: 93Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "Searching for Life beyond Earth"By: OpenStax AstronomyURL: http://cnx.org/content/m59990/1.1/Page: 93Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/

Module: "The Search for Extraterrestrial Intelligence"By: OpenStax AstronomyURL: http://cnx.org/content/m59993/1.1/Page: 93Copyright: OpenStax AstronomyLicense: http://creativecommons.org/licenses/by/4.0/


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