Astrophysics of Life - The Lives and Deaths of Stars

Ascension up the red giant branch takes 100 million

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Ascension up the red giant branch takes ~100 million years When there is no energy source in the middle, core will contract and outer layers will expand When there is an energy source like the sun, things are nice and stable What happens in the core as it continues to contract and get hotter? Helium begins to fuse into Carbon at >10^8 K. This reaction is called the “triple alpha” = 3 He C. Remember why Hydrogen burning requires 10^ K? Protons repel each other. Helium nuclei (2 protons) repel each other even more . . . The Helium Flash : After the core reaches 10^8 K, Helium “ignites” to make Carbon. The onset of this burning causes the temperature to rise sharply in a runaway explosion – Helium Flash. Main sequence Red giant Helium burning star Stage 10 – Helium-to-Carbon burning occurs stably in core, with Hydrogen-to-Helium burning in shell . . . until the core Helium runs out . . . in just 20 – 50 million years During this phase the carbon core contracts and heats up Helium is burning to carbon in a shell around the carbon core while H-to-He burning occurs in an outer shell What do you suppose happens next in the core? Nothing because it doesn’t burn carbon. Solar mass stars cannot squeeze and heat the core enough to ignite Carbon
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