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Evolution of Stars

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Evolution of Stars Evolution of Stars Part II – MASSIVE STARS 1 Closer look at stars in Orion: 2 Betelgeuse ~20 x solar mass Can actually SEE this star’s surface! 3 How bright will Betelgeuse be when it goes supernova? m − M = 5log(d) − 5 m = M + 5log(d) − 5 m 15 log 1 5 sn = − + ( 0.005) − msn = −15 + log(200) − 5 msn ≈ −17 m full −moon ≈ −13 € 4 € 5 •Carve out cavity in giant molecular cloud •Contract faster to the main sequence •Live for a shorter time there •Core pressures and temperatures so high that other fusion cycles occur •Most massive start blowing themselves apart. 100+ times as massive Eta Carinae as the Sun. It is so massive that it can barely hold itself together. Some astronomers think eta Carinae might die as a supernova blast within our lifetime. 6 http://www.spitzer.caltech.edu/Media/releases/ssc2005-12/ssc2005-12a.shtml Learning Goals: Be able to …. ! Explain why the much higher mass of some stars causes their evolution to be so different from the Sun. !Outline the basic stages of the evolution of a massive star 7 Apply the basics of the conservation of energy and the battle between gravity and pressure to what “drives” a star to evolve at each major stage of evolution. Give examples of where either gravity or pressure overwhelm the other and describe the temporary result. Summarize briefly the events happening in the core of a massive star between the collapse of the iron core into a neutron star and the supernova explosion. 8 Some perspective CNO cycle 9 Frame of reference Evolution for a star > 5 < 8 times Sun’s mass ! No helium flash. ! Helium " carbon fusion starts when temperature gets high enough (>100,000,000 K) ! Core does NOT become degenerate ! Rest of star expands as heat increases from core and shell fusion Triple-alpha fusion of He to C needs extremely high T’s to operate. C to O fusion requires T > 800,000,000 K 10 Summary for Stages 7 - 9 for massive stars Main sequence star - what is happening in the core? How does the star support itself? Core runs out of hydrogen, what happens in the core? Core runs out of helium, what happens in the core? Core runs out of carbon, what happens in the core? What’s happening to the rest of the star? Massive main sequence stars fuse hydrogen to helium via the CNO cycle (in addition to the proton-proton cycle). When the core runs out of hydrogen, the core shrinks, heats up and gives off heat starting hydrogen to helium shell fusion. Core does not go degenerate as pressures and temperatures reach high enough values to start helium to carbon fusion. When core runs out of helium, it shrinks, heats up and gives off heat, starting a helium to carbon fusion shell inside the H —> He shell. Core does not go degenerate because pressures and temperatures reach high enough values to start carbon to oxygen fusion, and so on through the various fusion cycles. Rest of the star is supported by thermal and radiative pressure from shell and core fusion. 12 ✗ Core shrinks, heats ✗ Carbon " oxygen fusion starts when temperature high enough ✗ Fusion continues to heavier and heavier nuclei 13 Mass > 8 solar masses 14 Time frames: each successive fusion cycle occurs faster and faster. What is the outer parts of the star doing? Dust clouds from ejected material. Fusion to iron marks irreversible doom for the once glorious star 15 Summary for high-mass stars - Stage 10 What happens in the core? When does fusion stop, and why? What supports the star? 16 Core contracts, heats, temperature rises. No helium flash; fusion of He to C starts. Shell fusion of H to He continues. Rest of star responds to increase in luminosity and expands. Out of helium, core contacts gives of thermal energy. Carbon to oxygen fusion starts. H to He shell fusion continues. He to C shell fusion continues. Rest of star expands. Cycle continues until iron core exists. Multiple shell fusion occurring. No support for core because iron most strongly bound nucleus. Rest of star is not supported. 17 What happens to the star? 18 NO SUPPORT! Gravity takes over! Star implodes -- atoms and nuclei torn apart Energy disappears Protons/electrons merge, neutrinos formed More energy carried away Electron degeneracy cannot act Core overshoots to huge density 19 NO SUPPORT! Gravity takes over! Star implodes -- atoms and nuclei torn apart Energy disappears Protons/electrons merge neutrinos formed More energy carried away Electron degeneracy cannot act Core overshoots to huge density Core collapses and its temperature rises. Photodisintegration and neutrino cooling reduce core pressure =>collapse accelerates. Core collapses until it reaches nuclear densities. Core collapse stops, bounces back, driving a shock wave through star. Shock wave takes a mere few hours to rip through the star. Summary for high-mass stars - Stage 11 What happens to the star? What does it become? What do we observe first? Second? 21 22 23 Learning Goals: Be able to …. ! Explain why the much higher mass of some stars causes their evolution to be so different from the Sun. Learning Goals: Be able to …. !Outline the basic stages of the evolution of a massive star 26 FOR WEDNESDAY! https://thetrappedelectron.files.wordpress.com/2015/07/download.jpg .
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