Lecture 9-Chapter 17-Star Stuff

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University of Toronto St. George
Astronomy & Astrophysics
Stefan Mochnacki

Chapter 17 Star Stuff +4Z,$9,78,88110.9:.O0,7:8L43 N All main-sequence stars are in gravitational equilibrium outward push of gas pressure balancing the inward pull of gravity. N Because the force of gravity is greater within more massive stars, the pressure inside must also be greater. N To provide this greater pressure, the cores must be hotter and denser, leading to a greater fusion rate. N With higher core temperatures and higher fusion rates, massive stars burn through their hydrogen gas much faster, explaining why they live shorter. N Low-mass stars are stars born with less than about the mass of our Sun. N Intermediate-mass stars have birth weights between about 2 and 8 solar masses. N High-mass stars are those stars born with masses greater than about 8 solar masses. Low-Mass Star DIAGRAM 17.2 Red Giant Stage N :K03,89,78.470K74J03L81L3,OO05O0903:.O0,71:8L43.0,808L39K089,78 core. The star becomes out of balance, gravity shrinking its core size. N %K089,784:907O,078K4Z0;07will expand outward, becoming a subgiant. N %K089,78O:2L348L9ZLOO9K03L3.70,80,31:79K070[5,3L38L]0-0.42L3J, red giant. DIAGRAM 17.4 why stars expand in size. N 89K0K74J03L39K0.470-:7384:9K0OL:2L8O019-0KL3 +4Z0;079K0.4708 vicinity still holds some hydrogen. N Gravity then shrinks both the non-burning helium core and the surrounding shell of hydrogen, allowing for hydrogen to sustain. N Hydrogen burning shell will proceed at a greater fusion rate, creating a greater luminosity. N Size increase because energy transport within its interior cannot keep pace with this larger energy-generation rate, so thermal pressure will build up and push the pressure outward. Helium Burning N Helium fusion occurs only when nuclei slam into one another at much higher speeds than those needed for hydrogen fusion, meaning that a higher temperature is needed. N The helium fusion process converts three helium nuclei into one carbon nucleus. N Degeneracy pressure takes part in counteracting gravity, so the core does not inflate. N The rising temperature causes the helium fusion to soar upward, called helium flash, releasing an enormous amount of energy into the core, allowing thermal pressure to dominate. N As a helium-burning star, size is smaller and is hotter, dropping its life track. DIAGRAM 17.5 DIAGRAM 17.6 www.notesolution.com
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