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ch16_summary - BENN6189_05_C16_PR4_V1_TT.QXD 1:01 PM Page...

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S U M M A R Y O F K E Y C O N C E P T S 16.1 Stellar Nurseries Where do stars form? Stars form in cold, relatively dense molecular clouds . Solid grains of interstellar dust prevent visible light from passing through these clouds, but we can use infrared observations to see what’s going on inside them. Why do stars form? A star can form in a molecular cloud only when gravity is strong enough to overpower the outward push of thermal pressure . Stars tend to form in clusters because gravity can more easily overcome pressure in more massive molecular clouds. A large cloud fragments into many smaller clumps of gas as it contracts, because the advantage of gravity over pressure increases as a clump of gas shrinks in size. 16.2 Stages of Star Birth What slows the contraction of a star-forming cloud? A contracting cloud begins its transformation into a star when its core starts trapping the thermal energy released by gravitational contraction. As pressure begins to push back harder, the contraction slows down and the
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