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Chapter 16

Chapter 16 Summary Cosmic Perspective

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Western University
Astronomy 1021
Chris Racknor

CHAPTER SIXTEEN Star Births Star births occurs when gravity squeezes a cloud of gas to the point that it forms a star with a core hot enough to sustain nuclear fusion the key to understanding this process and all other aspects of a stars life lies in analyzing the ever present battle between the inward pull of gravity and the outward push of pressure Stars form in molecular clouds because these clouds have the relatively high densities and low temperatures needed to give gravity the upper hand in its battle against pressure a single molecular cloud typically fragments into many pieces as it contracts leaving behind a star cluster with many more low mass stars than high mass stars Stars are born with a wide range of masses but lower mass stars are born in far greater numbers than higher mass stars the process of star formation also gives birth to many objects like brown dwarfs that are too small to shine with energy generated by nuclear fusion Where 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 is going on inside them Why 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 Slowing the Contraction of a StarForming 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 central part of the cloud becomes a protostar meanwhile matter from the surrounding cloud rains down upon the protostar increasing its mass The Role of Rotation in Star Birth Conservation of angular momentum ensures that a young protostar spins rapidly and much of the material falling inward tow
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