Chapter 17 summary.docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Chapter 17 summary: 17.1 LIVES IN THE BALANCE  How does a star’s mass affect nuclear fusion:  Stars of greater mass have hotter core temperatures, causing fusion to process more quickly and enabling fusion of heavier elements to take place  A star’s 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, with masses less than 2Msun  Intermediate mass stars, with masses between 2Msun and 8Msun  High mass stars, with masses above 8Msun 17.2 LIFE AS A LOW MASS STAR  What are the life stages of a low mass star:  A low mass star spends most of its life generating energy by fusing hydrogen in its core via the proton-proton 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, with 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 much come to an end  How does a low mass star die:  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 star’s energy generation never reaches equilibrium during this time  Instead, the star experiences a series of thermal pulses and ultimately expels its outer layers into space as a planetary nebula  The remaining “dead” stellar core is a white dwarf 17.3 LIFE AS A HIGH-MASS STAR  What are the life stages of a high-mass star:
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