Chapter 12 Star Stuff Copyright 2009 Pearson Education Inc What have we learned What are the life stages of a low mass star H fusion in core main sequence H fusion in shell around contracting core red giant He fusion in core horizontal branch Double shell burning red giant How does a low mass star die Ejection of H and He in a planetary nebula leaves behind an inert white dwarf Copyright 2009 Pearson Education Inc 12 3 Life as a High Mass Star Our goals for learning What are the life stages of a high mass star How do high mass stars make the elements necessary for life How does a high mass star die Copyright 2009 Pearson Education Inc What are the life stages of a high mass star Copyright 2009 Pearson Education Inc CNO Cycle High mass mainsequence stars fuse H to He at a higher rate using carbon nitrogen and oxygen as catalysts A greater core temperature enables H nuclei to overcome greater repulsion Copyright 2009 Pearson Education Inc Life Stages of High Mass Stars Late life stages of high mass stars are similar to those of low mass stars Hydrogen core fusion main sequence Hydrogen shell burning supergiant Helium core fusion supergiant Copyright 2009 Pearson Education Inc How do high mass stars make the elements necessary for life Copyright 2009 Pearson Education Inc Big Bang made 75 H 25 He stars make everything else Copyright 2009 Pearson Education Inc Helium fusion can make carbon in low mass stars Copyright 2009 Pearson Education Inc The CNO cycle can change C into N and O Copyright 2009 Pearson Education Inc Helium Capture High core temperatures allow helium to fuse with heavier elements Copyright 2009 Pearson Education Inc Helium capture builds C into O Ne Mg Copyright 2009 Pearson Education Inc Advanced Nuclear Burning Core temperatures in stars with 8MSun allow fusion of elements as heavy as iron Copyright 2009 Pearson Education Inc Advanced reactions in stars make elements like Si S Ca and Fe Copyright 2009 Pearson Education Inc Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells Copyright 2009 Pearson Education Inc Iron is a dead end for fusion because nuclear reactions involving iron do not release energy Fe has lowest mass per nuclear particle Copyright 2009 Pearson Education Inc Evidence for helium capture Higher abundances of elements with even numbers of protons Copyright 2009 Pearson Education Inc How does a high mass star die Insert TCP 5e Chapter 17 opener pg 564 Copyright 2009 Pearson Education Inc Iron builds up in the core until degeneracy pressure can no longer resist gravity The core then suddenly collapses creating a supernova explosion The Death Sequence of a High Mass Star Copyright 2009 Pearson Education Inc Supernova Explosion Core degeneracy pressure goes away because electrons combine with protons making neutrons and neutrinos Neutrons collapse to the center forming a neutron star Copyright 2009 Pearson Education Inc Energy and neutrons released in a supernova explosion enable elements heavier than iron to form including Au and U Copyright 2009 Pearson Education Inc Supernova Remnant Energy released by the collapse of the core drives outer layers into space The Crab Nebula is the remnant of the supernova seen in A D 1054 Multiwavelength Crab Nebula Copyright 2009 Pearson Education Inc Supernova 1987A The closest supernova in the last four centuries was seen in 1987 Copyright 2009 Pearson Education Inc What have we learned What are the life stages of a high mass star They are similar to the life stages of a low mass star How do high mass stars make the elements necessary for life Higher masses produce higher core temperatures that enable fusion of heavier elements How does a high mass star die The iron core collapses leading to a supernova Copyright 2009 Pearson Education Inc 12 4 Summary of Stellar Lives Our goals for learning How does a star s mass determine its life story How are the lives of stars with close companions different Copyright 2009 Pearson Education Inc How does a star s mass determine its life story Copyright 2009 Pearson Education Inc Role of Mass A star s mass determines its entire life story because it determines its core temperature High mass stars have short lives eventually becoming hot enough to make iron and end in supernova explosions Low mass stars have long lives never become hot enough to fuse carbon nuclei and end as white dwarfs Copyright 2009 Pearson Education Inc 1 2 3 4 5 Low Mass Star Summary Main Sequence H fuses to He in core Red Giant H fuses to He in shell around He core Helium Core Burning He fuses to C in core while H fuses to He in shell Double Shell Burning H and He both fuse in shells Planetary Nebula leaves white dwarf behind Not to scale Copyright 2009 Pearson Education Inc Reasons for Life Stages Core shrinks and heats until it s hot enough for fusion Nuclei with larger charge require higher temperature for fusion Core thermostat is broken while core is not hot enough for fusion shell burning Core fusion can t happen if degeneracy pressure keeps core from shrinking Not to scale Copyright 2009 Pearson Education Inc Life Stages of High Mass Star 1 Main Sequence H fuses to He in core 2 Red Supergiant H fuses to He in shell around He core 3 Helium Core Burning He fuses to C in core while H fuses to He in shell 4 Multiple Shell Burning many elements fuse in shells 5 Supernova leaves neutron star behind Not to scale Copyright 2009 Pearson Education Inc How are the lives of stars with close companions different Copyright 2009 Pearson Education Inc Thought Question The binary star Algol consists of a 3 7MSun mainsequence star and a 0 8MSun subgiant star What s strange about this pairing How did it come about Copyright 2009 Pearson Education Inc Stars in Algol are close enough that matter can flow from the subgiant onto the main sequence star Copyright 2009 Pearson Education Inc The star that is now a subgiant was originally more massive As it reached the end of its life and started to grow it began to transfer mass to its companion mass exchange Now the companion star is more massive Copyright 2009 Pearson Education Inc What have we learned How does a star s mass determine its life story Mass determines how high a star s core temperature can rise and therefore determines how quickly a star uses its fuel and what kinds of elements it can make How are the lives of stars with close companions different Stars with close companions can exchange mass altering the usual life stories
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