ASTR 1020 1st Edition Lecture 11Pressure OUT Gravity IN- A star’s life is a struggle between gravity, wanting to crush it, and thermal pressure, wanting to expand itSun-Mass Star Evolution 1. Main Sequence -90% of its life is spent on the Main Sequence a. fusing hydrogen into helium in the coreb. Stellar thermostat keeps luminosity and temperature stable for 10 billion of years Q: What will happen first when the hydrogen in the core runs out?A: The core will start to collapse2. Red Giant –helium “ash” in the corea. Temperature not hot enough to fuse helium (100 million K needed)b. With fusion no longer occurring in the core, gravity causes core collapsec. Core temperature starts to heat up along with the layers above the core/must collapsed. Now hot enough for hydrogen fusione. Hydrogen “shell burning” pushes outer layers of the star outMore pressure, less gravity –star becomes larger, cooler, but brighter!Core is continuing to shrink getting denser and hotter“The Sun as a Red Giant/A bad day for Earth”o Hotter core, puffs up outer hydrogeno Sun swells 100x /1000x luminous /temperatures drop to ~3000 K (red) 5 billion years: earth is toast!“Helium Fusion” –when core contracts enough to heat to 100 million K, helium starts to fuse into carbon. o Triple-alpha fusion: He + He + He C + energy / Helium “flash”These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Q: What happens after a helium flash?A: The core quickly heats up and expands3. Horizontal Branch –happily fusing helium to carbon as a Horizontal Branch Stara. Outer layers stopped pushing out, luminosity decreases/gravitational equilibrium is restoredb. R = 10 Rsun 50 million years4. Double-shell Red Giant –carbon core starts collapsing and heats upa. Two fusing shellsi. Inner shell fusing He C Outer shell fusing H Heo Outer layers lift and cool againo Star becomes very luminous Double-Shell fusion Red GiantCarbon fusion should start at 600 million K: Electron degeneracy pressure becomes a factor before the core reaches that temperatureElectron Degeneracy Pressure- Particles can’t have the same position and energyo Restricts how tightly packed particles can become o If ALL available positions are filled Degenerate - A new kind of pressure that depends on density only!5. Planetary Nebulaa. Temperature never get high enough for carbon fusionb. Shell fusion becomes sensitive to temperature; explosive reactionsi. Outer hydrogen layers blown off: Planetary nebula formed6. White Dwarfa. Small hot carbon “rock” life over: White dwarfb. Tiny (earth-sized), hot /density is ~1 ton/cm^3c. Energy source is gone and cools over billions of years black dwarf(not found yet)How Long Does it Take? Cloud collapse / Protostar 10^8 yr H-core fusion / Main Sequence 10^10 yrInactive He core, H shell fusion / Red Giant 10^8 yrHe-core fusion, +H shell / Helium Fusion 10^7 yrC-core, He+H shells / Double Shell Red Giant 10^4 yrEnvelope ejected / Planetary Nebula 10^5 yrCooling C-core / White Dwarf >10^10 yrCold C-core / Black Dwarf ∞Q: The Big Bang only produced hydrogen and helium. Suppose the universe contained only non-binary low-mass stars. Would elements heavierthan carbon exist?A:
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