The Milky Way Galaxy’s Massive Halo 40 Magnetic Field 40 Rotation of the Milky Way Galaxy 41 The Galaxy’s Mass 43 Chapter2 The Study of Galaxies 47 Galaxy Evolution 49 Andromeda Nebula Novae 55 Van Maanen Rotation 56 Shapley-Curtis Debate 59 Hubble and the Discovery Of Extragalactic Objects 61 The Distance to the Andromeda Nebula 65 Edwin Powell Hubble 66 Extragalactic Astronomy 70 Chapter3 Galaxies of Many Types 72 Elliptical Galaxies 73 Spiral Galaxies 75 S0 Galaxies 77
Sa Galaxies 79 Sb Galaxies 79 Andromeda Galaxy 80 Sc Galaxies 82 SB Galaxies 83 Irregular Galaxies 85 Other Types of Galaxies 87 Chapter4 Galaxies Other Than Our Own 89 Luminosity of External Galaxies 90 Age 91 Composition of External Galaxies 92 Galaxy Clusters 95 Distribution of Clusters 97 How Cluster Members Interact 99 Radio Galaxies 101 Virgo A 104 X-ray Galaxies 105 Radio and X-ray Sources 106 Quasars 107
Gamma-Ray Bursters 110 Coma Cluster 111 Cygnus A 112 Great Attractor 113 Magellanic Clouds 114 M81 Group 115 Maffei 1 and 2 116 Virgo A 117 Virgo Cluster 118 Conclusion 119 Glossary 120 Bibliography 124 Index 126
INTRODUCTION Virtually all galaxies appear to have been formed soon after the universe began approximately 13 billion years ago Measurements Measurements tend to suggest that most if not all galaxies galaxies have very nearly the same age and that they consist of stars and interstellar matter clouds of gas and particles of dust that move together through space Even small galaxies contain millions millions of stars while the largest may have trillions of stars And the universe is made up of trillions of galaxies which are found all over the sky even in the depths of the farthest reaches penetrated by powerful modern telescopes The Sun and the solar system including Earth are part of the Milky Way Galaxy The Milky Way Galaxy is a large spiral system consisting of several hundred billion stars It takes its name from the Milky Way the irregular luminous luminous band of stars and gas clouds that stretches across the sky as seen from Earth Every star that can be seen from Earth without a telescope belongs to the Milky Way Galaxy The Milky Way Galaxy is an example of a spiral galaxy which are shaped like pinwheels and have immense spiral arms The Sun as well as Earth is located on the inner edge of a spiral arm 7
