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Unformatted text preview: 1 Extraterrestrial Life: Spring 2008 How many civilizations are there in the Galaxy? Number of stars in the Milky Way galaxy ~10 11 (100 billion) Distance from the Sun to the galactic Center is about 25,000 light years Number of civilizations `alive now depends upon: fraction of stars with habitable planets fraction of habitable planets that develop life fraction of time that life becomes intelligent fraction of civilizations that still exist Extraterrestrial Life: Spring 2008 CodiFed as the Drake Equation Frank Drake N civ = N * " f HP " f life " f civ " f alive N civ is the number of civilizations alive today N * is the number of stars in the Galaxy (~10 11 ) f HP is the fraction of stars with habitable planets f life is the fraction of habitable planets that develop life f civ is the fraction of planets on which life becomes intelligent f alive is the fraction of once intelligent civilizations that are still alive Extraterrestrial Life: Spring 2008 Not so much a traditional `equation - more a framework for thinking about SETI f HP : astronomical question f life / f civ : biological / evolutionary questions...
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- Spring '08