S U M M A R Y O F K E Y C O N C E P T S16.1Stellar Nurseries◗Where do stars form?Stars form in cold, relatively dense molecular clouds. Solidgrains ofinterstellar dustprevent visible light frompassing through these clouds,but we can use infraredobservations to see what’sgoing on inside them.◗Why do stars form?A star can form in a molecular cloud only when gravity isstrong enough to overpowerthe outward push ofthermalpressure. Stars tend to formin clusters because gravity canmore easily overcome pressurein more massive molecularclouds. A large cloud fragmentsinto many smaller clumps ofgas as it contracts, because the advantage of gravity overpressure increases as a clump of gas shrinks in size.16.2Stages of Star Birth◗What slows the contraction of a star-formingcloud?A contracting cloud begins its transformationinto a star when its core starts trapping the thermal energyreleased by gravitational contraction. As pressure begins topush back harder, the contraction slows down and the
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