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Key concepts and summary. A supernova occurs on average once every 25 to 100 years in the Milky Way Galaxy.
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A dichotomous key is a tool created by scientists to help scientists and laypeople identify objects and organisms. ... Dichotomous keys are very useful because they allow non-expert users to identify organisms by directing them to look at the known, important organisms.
Air pressure is caused by the air molecules around us. Air particles are always in random and continuos motion, bombarding each other in random direction. As air particles collide they create pressure.