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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Chapter 1Our Place in the Universe11 Our Modern View of the Universey The Sun Moon planets and stars appear to circle around our sky each day and we cannot feel the constant motion of Earth as it rotates on its axis and orbits the sun It therefore seems quite natural to assume that we live in an Earthcentered or geocentric universeWhat is our place in the universey Earth is a planet in our solar system which consists of the sun and all the objects that orbit it the planets and their moons and countless smaller objects including rocky asteroids and icy comets y The sun and all the stars we can see with the naked eye make up only a small part of a huge diskshaped collection of stars called the Milky Way galaxyy A galaxy is a great island of stars in space containing from a few hundred million to a trillion or more starsy The milky way galaxy contains over 100 billion stars y Our solar system is located a little more than halfway from the galactic centre to the edge of the galactic disk y Our milky way is one of the two largest among the 40 galaxies in the Local Group y Groups of galaxies with more than a few dozen members are often called galaxy clusters y The regions in which galaxies and galaxy clusters are most tightly packed are called superclusters y Together all these structures make up our universeHow did we come to be The Big Bang and the Expanding Universey Telescopic observations of distant galaxies show that the entire universe is expanding meaning that average distances between galaxies are increasing with time this fact implies that galaxies must have been closer together in the past and if we go back far enough we much reach the point at which the expansion began y We call this beginning the Big Bang and from the observed rate of expansion we estimate that it occurred about 14 billion years ago y Structures such as galaxies and galaxy clusters occupy regions where gravity has won out against the overall expansion That is while the universe as a whole continues to expand individual galaxies and galaxy clusters and objects within them such as planets and starts do not expand
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