chapter17 - Chapter 17 Star Stuff 17.1 Lives in the Balance...

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Chapter 17 Star Stuff
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17.1 Lives in the Balance Our goals for learning How does a star’s mass affect nuclear fusion?
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How does a star’s mass affect nuclear fusion?
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Stellar Mass and Fusion The mass of a main sequence star determines its core pressure and temperature Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes
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High-Mass Stars > 8 M Sun Low-Mass Stars < 2 M Sun Intermediate- Mass Stars Brown Dwarfs
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Star Clusters and Stellar Lives Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations Star clusters are particularly useful because they contain stars of different mass that were born about the same time
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What have we learned? How does a star’s mass affect nuclear fusion? A star’s mass determines its core pressure and temperature and therefore determines its fusion rate Higher mass stars have hotter cores, faster fusion rates, greater luminosities, and shorter lifetimes
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17.2 Life as a Low-Mass Star Our goals for learning What are the life stages of a low-mass star? How does a low-mass star die?
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What are the life stages of a low- mass star?
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A star remains on the main sequence as long as it can fuse hydrogen into helium in its core
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Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins
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Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins
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Life Track after Main Sequence Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over
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Broken Thermostat As the core contracts, H begins fusing to He in a shell around the core Luminosity increases because the core thermostat is broken— the increasing fusion rate in the shell does not stop the core from contracting
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Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon
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Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A.
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This note was uploaded on 03/03/2011 for the course RSM 100 taught by Professor Oesch during the Spring '08 term at University of Toronto- Toronto.

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chapter17 - Chapter 17 Star Stuff 17.1 Lives in the Balance...

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