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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 )

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star evolution

Star evolution

Chapters 17 & 18

(Yes, we skip chap. 16, star birth)

3 star groups p 549
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!
which star group has the highest core pressure
Which star group has the highest core pressure?
  • Low mass
  • Intermediate mass
  • High mass
which star group has the hottest core temperature
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?

the end of the sun
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.
shell burning
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
what s happening to the mass of the core as the shell burns
What’s happening to the mass of the core as the shell “burns”?
  • Increasing
  • Decreasing
  • Staying the same
inside the core
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
next phase
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
double shell burning
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
high mass star differences
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)
slide15
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?
what we see
What we see
  • See figure 17.12 for onion skin model
  • See HR diagram on p. 559 (fig. 17.13)
after the iron core forms
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)
stellar remnants
Stellar remnants
  • End states for stars
    • Low mass stars become …
    • Intermediate mass & high mass stars become …
    • The highest mass stars (O & B) become …
which stars should begin with the most heavy elements inside them
Which stars should begin with the most heavy elements inside them?
  • The stars that formed earliest
  • The most recently formed stars
summary of star death
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