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
The Milky Way and OTher Galaxies some 27,000 light-years from the nucleus or centre centre of the Galaxy The Gal axy’s disk and spiral arms have a diameter of roughly 100,000 light-years The Sun and all the other stars of the Galaxy take about 200 million years to orbit about the nucleus This image the result of 10 days observation by the Hubble Space Telescope shows 1,500 galaxies in different stages of their evolution Galaxies vary greatly in size and grow by colliding colliding and merging with other galaxies Some extreme dwarf galaxies found near the Milky Way Galaxy are only 100 light-years across while large spiral galaxies typically have diameters of 100,000 to 500,000 light-years On the other hand giant radio galaxies which are very strong sources of radio waves extend more than 3,000,000 light-years Most galaxies move through the universe in clusters sometimes in small groups and sometimes in enormous complexes Isolated galaxies are quite rare The Milky Way Galaxy for example belongs to the Local Group which 8
inTrOducTiOn also includes the Magellanic Clouds the Andromeda Galaxy Galaxy and about 50 other galaxies Evidence suggests that at the centre of many galaxies galaxies lies a supermassive black hole a region of such intense gravity that nothing not even light can escape Additionally Additionally when two galaxies merge their nuclei and black holes collide generating an immense amount of energy At the centre of the Galaxy is a massive black hole named Sagittarius A and it has a mass about 4 3 million times larger than the Sun’s In the late 20th century a surprising finding was made most of the mass in galaxies is not in the form of stars or other visible matter Most galaxies have much more mass than can be accounted for by their stars This means that they must contain some dark matter from which no light is seen The nature of this dark matter is not yet known This text studies the nature of the various types of galaxies including our own Scientists have only begun to understand the complexities of these immense celestial networks 9
