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Chapter 17

Chapter 17 Summary Cosmic Perspective

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Western University
Astronomy 1021
Chris Racknor

CHAPTER SEVENTEEN Star Stuff Virtually all elements other than hydrogen and helium were forged in the nuclear furnaces of stars and released into space as they died we and our planet are therefore made of stuff produced in stars that lived and died long ago Low mass stars like our sun live long lives and die by producing planetary nebulae that leave behind white dwarfs High mass stars live fast and die young exploding dramatically as supernovae and leaving behind neutron stars or black holes Close binary stars can exchange mass thus altering the usual course of stellar evolutionStar Masses Affecting Nuclear Fusion Stars of greater mass have hotter core temperatures causing fusion to proceed more quickly and enabling fusion of heavier elements to take place a stars mass at birth therefore determines almost every aspect of its life and death to understand the general characteristics of stellar lives we divide stars into three groups by mass low mass stars mass less than 2M intermediate mass stars mass between 2 M8 M and high mass stars masses SunSunSunabove 8 M SunLife Stages of a Low Mass Star A low mass star spends most of its life generating energy by fusing hydrogen in its corevia the protonproton chain when core hydrogen is exhausted the core begins to shrink while the star as a whole expands to become a red giant with hydrogen shell burning around an inert helium core when the core becomes hot enough a helium flash initiates helium fusion in the core which fuses helium into carbon this phase lasts until core helium is exhausted low mass stars never become hot enough for carbon fusion so at this point their lives must come to an end Death of Low Mass Stars The core again shrinks after core helium burning ceases helium shell burning begins around the inert carbon core beneath the hydrogen burning shell the outer layers expand again making the star into a double shell burning red giant the stars energy generation never reaches equilibrium during this time instead the st
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