Copyright © 2009 Pearson Education, Inc.Chapter 12Star StuffCopyright © 2009 Pearson Education, Inc.What have we learned?• What are the life stages of a low-massstar?—H fusion in core (main sequence)—H fusion in shell around contracting core (redgiant)—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 nebulaleaves behind an inert white dwarf.Copyright © 2009 Pearson Education, Inc.12.3 Life as a High-Mass StarOur goals for learning:• What are the life stages of a high-mass star?• How do high-mass stars make the elementsnecessary for life?• How does a high-mass star die?Copyright © 2009 Pearson Education, Inc.What are the life stages of ahigh-mass star?Copyright © 2009 Pearson Education, Inc.CNO Cycle• High-mass main-sequence stars fuseH to He at a higherrate using carbon,nitrogen, andoxygen as catalysts.• A greater coretemperature enablesH nuclei toovercome greaterrepulsion.Copyright © 2009 Pearson Education, Inc.Life Stages of High-Mass Stars• Late life stages of high-mass stars are similar tothose 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 theelements 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 tofuse 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 >8MSunallow 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 nuclearburning proceeds ina series of nestedshells.Copyright © 2009 Pearson Education, Inc.Iron is a deadend for fusionbecause nuclearreactionsinvolving irondo not releaseenergy.(Fe has lowestmass pernuclearparticle.)Copyright © 2009 Pearson Education, Inc.Evidencefor heliumcapture:Higherabundancesof elementswith evennumbers ofprotonsCopyright © 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 upin the core untildegeneracypressure can nolonger resistgravity.The core thensuddenlycollapses,creating asupernovaexplosion.The Death Sequence of a High-Mass StarCopyright © 2009 Pearson Education, Inc.Supernova Explosion• Core degeneracypressure goes awaybecause electronscombine withprotons, makingneutrons andneutrinos.• Neutrons collapse tothe center, forming aneutron star.Copyright © 2009 Pearson Education, Inc.Energy and neutrons released in a supernova explosion enableelements heavier than iron to form, including Au and U.Copyright © 2009 Pearson Education, Inc.Supernova Remnant• Energy released bythe collapse of thecore drives outerlayers into space.• The Crab Nebula isthe remnant of thesupernova seen inA.D. 1054.Multiwavelength Crab NebulaCopyright © 2009 Pearson Education, Inc.Supernova 1987A• The closest supernova in the last fourcenturies 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 elementsnecessary for life?— Higher masses produce higher core temperatures thatenable 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 LivesOur goals for learning:• How does a star’s mass determine its lifestory?• How are the lives of stars with closecompanions different?Copyright © 2009 Pearson Education, Inc.How does a star’s massdetermine its life story?Copyright © 2009 Pearson Education, Inc.Role of Mass• A star’s mass determines its entire life storybecause it determines its core temperature.• High-mass stars have short lives, eventuallybecoming hot enough to make iron, and end insupernova explosions.• Low-mass stars have long lives, never become hotenough to fuse carbon nuclei, and end as whitedwarfs.Copyright © 2009 Pearson Education, Inc. Low-Mass Star Summary1. Main Sequence: H fuses to He in core2. Red Giant: H fuses to He in shell around He core3. Helium Core Burning:He fuses to C in core while H fuses to He in shell4. Double Shell Burning:H and He both fuse in shells5. Planetary Nebula: leaves white dwarf behindNot 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 fromshrinking.Not to scale!Copyright © 2009 Pearson Education, Inc.Life Stages of High-Mass Star1. Main Sequence: H fuses to He in core2. Red Supergiant: H fuses to He in shell around He core3. Helium Core Burning:He fuses to C in core while H fuses to He in shell4. Multiple Shell Burning:many elements fuse in shells5. Supernova leaves neutron star behindNot to scale!Copyright © 2009 Pearson Education, Inc.How are the lives of stars withclose companions different?Copyright © 2009 Pearson Education, Inc.Thought QuestionThe binary star Algol consists of a 3.7MSun main-sequence 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 closeenough that matter canflow from the subgiantonto the main-sequencestar.Copyright © 2009 Pearson Education, Inc.The star that is now asubgiant was originallymore massive.As it reached the endof its life and started togrow, it began totransfer mass to itscompanion (massexchange).Now the companionstar is more massive.Copyright © 2009 Pearson Education, Inc.What have we learned?• How does a star’s mass determine its lifestory?—Mass
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