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Star evolution

Star evolution. Chapters 17 & 18 (Yes, we skip chap. 16, star birth). 3 star groups (p. 549). 3 categories of stars: Low mass (<2 M sun ) Intermediate mass (2  8 M sun ) High mass (>8 M sun )

leigh-harper
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Star evolution

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  1. Star evolution Chapters 17 & 18 (Yes, we skip chap. 16, star birth)

  2. 3 star groups (p. 549) • 3 categories of stars: • Low mass (<2 Msun) • Intermediate mass (2  8 Msun) • High mass (>8 Msun) • Intermediate lives similar life to both high and low mass. Book focuses more on similarities with high mass (in section 17.1). • One difference: high mass stars die very differently!

  3. Which star group has the highest core pressure? • Low mass • Intermediate mass • High mass

  4. Which star group has the hottest core temperature? • Low mass • Intermediate mass • High mass So what can you conclude about the fusion rate? Luminosity? Which stars live longer? Why?

  5. The end of the Sun • Eventually core runs out of hydrogen. • What did the core need fusion for? • What will happen to it as a result of losing fusion? • What happens to gas balls when they shrink? • What happens to the temperature of the material surrounding the core? • CLICKER QUESTION. • What are the surrounding layers made of? • What can happen if they get hot enough? • For Sun, this takes hundreds of millions of years.

  6. Is there Hydrogen outside the Sun’s core? • Yes • No

  7. Shell “burning” • In fact, it gets hotter than 15 million K. • What does that tell us about fusion rate? • What should we observe as a result? CLICKER • The light “gets stuck” and pushes the outer layers out. • What happens to gas when you expand it? • What kind of star do we have? • What is the core made of? • What is the structure? • See fig. 17.4 page 552

  8. Star becomes ______ luminous • More • Less

  9. What’s happening to the mass of the core as the shell “burns”? • Increasing • Decreasing • Staying the same

  10. Inside the core… • Core shrinks • Core gets hotter • More hot helium dumped onto core • Something must stop the core from shrinking. • Low mass stars: degeneracy pressure • Read section 16.3, pp. 541-542 and S4.4 pp. 468-469 • Mosh pit • Intermediate & High mass: fusion causing thermal & gas pressure. • Fusion turns on at 100 million K • Low mass: whole core starts fusing simultaneously: helium “flash” • Intermediate & high mass: “regular” fusion

  11. Next phase • Structure of the star now? • Figure 17.5 • This lasts until … • What happens to the core? • Low & intermediate mass: core shrinks until degeneracy pressure stops it. Focus on that now. • [for High mass: next fusion turns on] • Back to low mass: What’s the core made of? • Shrinks to size of Earth. • What happens outside the core? • Temp, composition

  12. Double shell burning • Not stable • Outer layers pulsate • Outer layers come off • See pictures around the planetarium • Cat’s eye, Butterfly, Ring: all “planetary nebula” • See also figure 17.7 – more examples • NOT related to planets • What’s in the center of a planetary nebula? • End of low & intermediate mass stars… • Show interactive figure 17.4

  13. If the universe contained only low mass stars, would there be elements heavier than carbon? • Yes • No

  14. High mass star differences • Degeneracy pressure never turns on • Gas & thermal pressure always stronger • Can fuse carbon with helium into Oxygen • Can fuse Oxygen with helium into neon • Etc. (magnesium, silicon, sulfur) • When core hot enough, can fuse carbon with carbon, carbon with oxygen … • Etc. • Big picture: carbon and stuff fuses until you get to a core made of … • Iron (Fe on the periodic table, #26)

  15. Iron • Most stable nucleus • Can’t release energy by fusing it • Fusion USES energy • True for everything heavier than iron, too. • Fission USES energy • True for most things lighter than iron, too. • Iron is the last element made in stable reactions in stars • Look at the periodic table on page A-13 • Find iron • Gold = Au. Mercury = Hg. Xenon = Xe. Are these made in stable stars?

  16. What we see • See figure 17.12 for onion skin model • See HR diagram on p. 559 (fig. 17.13)

  17. After the Iron core forms • Iron core shrinks • Gravity is stronger than Electron degeneracy pressure • Electrons squeezed more than they can tolerate • Electrons merge with protons • Result: neutrons • And neutrinos! (Fly straight out!) • No more degeneracy pressure support. • Rapidly shrinks: Earth-size to town-size in 1 second! • Lots of energy released • Core bounces. Demo • Supernova explosion. Leaves behind core • Core is made of … Called … • Interactive figure 17.12 & 17.17 (crab nebula in 1054)

  18. Stellar remnants • End states for stars • Low mass stars become … • Intermediate mass & high mass stars become … • The highest mass stars (O & B) become …

  19. Which stars should begin with the most heavy elements inside them? • The stars that formed earliest • The most recently formed stars

  20. Summary of star death • When fusion runs out, core ____ & _____ • Shell fusing occurs. Many shells possible. • Core fusion can turn on. • What’s different for low mass & high mass? • Which elements get made in low & high? • What’s special about iron? • Degeneracy pressure (electron & neutron) • What, where, why • Possible end states; which stars make them • RG  PN  WD, RG  SN  NS or BH

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