Chapter 12 Star Formation Death Copyright 2009 Pearson Education Inc 12 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 Copyright 2009 Pearson Education Inc What are the life stages of a low mass star Copyright 2009 Pearson Education Inc A star remains on the main sequence as long as it can fuse hydrogen into helium in its core Main Sequence Lifetimes and Stellar Masses Copyright 2009 Pearson Education Inc Thought Question What happens when a star can no longer fuse hydrogen to helium in its core A Its core cools off B Its core shrinks and heats up C Its core expands and heats up D Helium fusion immediately begins E It does a hissy fit Copyright 2009 Pearson Education Inc Thought Question What happens when a star can no longer fuse hydrogen to helium in its core A B C D Copyright 2009 Pearson Education Inc Its core cools off Its core shrinks and heats up Its core expands and heats up Helium fusion immediately begins 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 Copyright 2009 Pearson Education Inc 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 Copyright 2009 Pearson Education Inc Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion larger charge leads to greater repulsion The fusion of two helium nuclei doesn t work so helium fusion must combine three He nuclei to make carbon Copyright 2009 Pearson Education Inc Thought Question What happens in a low mass star when core temperature rises enough for helium fusion to begin A Helium fusion slowly starts up B Hydrogen fusion stops C Helium fusion rises very sharply Hint Degeneracy pressure is the main form of pressure in the inert helium core Copyright 2009 Pearson Education Inc Thought Question What happens in a low mass star when core temperature rises enough for helium fusion to begin A Helium fusion slowly starts up B Hydrogen fusion stops C Helium fusion rises very sharply Hint Degeneracy pressure is the main form of pressure in the inert helium core Copyright 2009 Pearson Education Inc Helium Flash The thermostat is broken in a low mass red giant because degeneracy pressure supports the core The core temperature rises rapidly when helium fusion begins The helium fusion rate skyrockets until thermal pressure takes over and expands the core again Copyright 2009 Pearson Education Inc Helium burning stars neither shrink nor grow because the core thermostat is temporarily fixed Copyright 2009 Pearson Education Inc Life Track After Helium Flash Models show that a red giant should shrink and become less luminous after helium fusion begins in the core Copyright 2009 Pearson Education Inc Life Track After Helium Flash Observations of star clusters agree with those models Helium burning stars are found in a horizontal branch on the H R diagram Copyright 2009 Pearson Education Inc Combining models of stars of similar age but different mass helps us to age date star clusters Using the H R Diagram to Determine the Age of a Star Cluster Copyright 2009 Pearson Education Inc How does a low mass star die Copyright 2009 Pearson Education Inc Thought Question What happens when a star s core runs out of helium A B C D Copyright 2009 Pearson Education Inc The star explodes Carbon fusion begins The core cools off Helium fuses in a shell around the core Thought Question What happens when a star s core runs out of helium A B C D Copyright 2009 Pearson Education Inc The star explodes Carbon fusion begins The core cools off Helium fuses in a shell around the core Double Shell Burning After core helium fusion stops He fuses into carbon in a shell around the carbon core and H fuses to He in a shell around the helium layer This double shell burning stage never reaches equilibrium the fusion rate periodically spikes upward in a series of thermal pulses With each spike convection dredges carbon up from the core and transports it to the surface Copyright 2009 Pearson Education Inc Planetary Nebulae Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a white dwarf Copyright 2009 Pearson Education Inc Planetary Nebulae Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a white dwarf Copyright 2009 Pearson Education Inc Planetary Nebulae Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a white dwarf Copyright 2009 Pearson Education Inc Planetary Nebulae Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a white dwarf Copyright 2009 Pearson Education Inc End of Fusion Fusion progresses no further in a low mass star because the core temperature never grows hot enough for fusion of heavier elements some He fuses to C to make oxygen Degeneracy pressure supports the white dwarf against gravity Copyright 2009 Pearson Education Inc Life stages of a lowmass star like the Sun The Death Sequence of the Sun Copyright 2009 Pearson Education Inc Life Track of a Sun Like Star 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
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