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Transcript
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? What are the life stages of a low mass star? High-Mass Stars > 8 MSun IntermediateMass Stars Low-Mass Stars < 2 MSun Brown Dwarfs A star remains on the main sequence as long as it can fuse hydrogen into helium in its core Life stages of a lowmass star like the Sun Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. B. C. D. Core cools off Core shrinks and heats up Core expands and heats up Helium fusion immediately begins Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. B. C. D. Core cools off Core shrinks and heats up Core expands and heats up Helium fusion immediately begins Helium fusion requires higher temperatures than hydrogen fusion because larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon Thought Question What happens as a star’s inert helium core starts to shrink? A. B. C. D. Hydrogen fuses in shell around core Helium fusion slowly begins Helium fusion rate rapidly rises Core pressure sharply drops Thought Question What happens as a star’s inert helium core starts to shrink? A. B. C. D. Hydrogen fuses in shell around core Helium fusion slowly begins Helium fusion rate rapidly rises Core pressure sharply drops Broken thermostat: rising fusion rate in shell does not expand core, so luminosity continues to rise. 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 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 Helium Flash • Thermostat is broken in low-mass red giant because degeneracy pressure supports core • Core temperature rises rapidly when helium fusion begins • Helium fusion rate skyrockets until thermal pressure takes over and expands core again Helium burning stars neither shrink nor grow because thermostat is temporarily fixed. Thought Question What happens when the star’s core runs out of helium? A. B. C. D. The star explodes Carbon fusion begins The core cools off Helium fuses in a shell around the core Thought Question What happens when the star’s core runs out of helium? A. B. C. D. The star explodes Carbon fusion begins The core cools off Helium fuses in a shell around the core Life stages of a lowmass star like the Sun Star clusters help us test models of stellar evolution because they contain stars of same age but at different life stages How does a low mass star die? A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind Thought Question What happens to Earth’s orbit as Sun loses mass late in its life? A. Earth’s orbit gets bigger B. Earth’s orbit gets smaller C. Earth’s orbit stays the same Thought Question What happens to Earth’s orbit as Sun loses mass late in its life? A. Earth’s orbit gets bigger B. Earth’s orbit gets smaller C. Earth’s orbit stays the same What have we learned? • 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. Then it becomes a red giant, with a hydrogen shell burning around an inert helium core. Next comes helium core burning, followed by doubleshell burning of hydrogen and helium shells around an inert carbon core. • c In the late What have we learned? • How does a low-mass star die? • A low-mass star like the Sun never gets hot enough to fuse carbon in its core. It expels its outer layers into space as a planetary nebula, leaving behind a white dwarf.
