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Astronomy & Astrophysics
Michael Reid

September 19, 2011 Chapter One: Our Place in the Universe 1.1 Our Modern View of the Universe - Earth is a planet in our solar system - The sun and all the stars we can see with the naked eye make up only a small part of a huge, disk-shaped collection of stars called the Milky Way Galaxy (more than 100 billion stars) - A galaxy is a great island of stars in space, containing from a few hundred million to a trillion or more stars - Milky Way is one of the two largest among about 40 galaxies in the Local Group - Groups of galaxies with more than a few dozen members are often called galaxy clusters - The regions in which galaxies and galaxy clusters are most tightly packed are called superclusters - The universe is the sum total of all matter and energy, encompassing the superclusters and voids and everything within them - Expansion of universe occurred about 14 million years ago - The point at which the expansion of universe began is called the Big Bang - Other chemical elements were made by stars - While the universe as a whole continues to expand, individual galaxies and galaxy clusters do not expand - A star is born when gravity compresses the material in a cloud to the point where the center becomes dense enough and hot enough to generate energy by nuclear fusion, the process in which lightweight atomic nuclei smash together and stick to make heavier nuclei - A star lives as long as it can shine with energy from fusion, and dies when it exhausts its usable fuel - In its final death throes, a star blows much of its content back out into space - Massive stars die in titanic explosions called supernovae - The early universe contained only the simplest chemical elements: hydrogen and helium - Elements such as carbon, nitrogen, oxygen were manufactured by stars - Most of the material from which we and our planet are made was created inside stars that lived and died before th
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