Transcript Star evolution Chapters 17 & 18

Star evolution
Chapters 17 & 18
(Yes, we skip chap. 16, star birth)
3 star groups (p. 549)
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3 categories of stars:
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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?
1. Low mass
2. Intermediate mass
3. High mass
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Which star group has the hottest
core temperature?
1. Low mass
2. Intermediate mass
3. High mass
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So what can you conclude about the fusion rate? Luminosity?
Which stars live longer? Why?
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The end of the Sun
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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.
Is there Hydrogen outside the
Sun’s core?
1. Yes
2. No
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Shell “burning”
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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
Star becomes ______ luminous
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2. Less
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What’s happening to the mass of
the core as the shell “burns”?
1. Increasing
2. Decreasing
3. Staying the same
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Inside the core…
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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
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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
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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
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Not stable
Outer layers pulsate
Outer layers come off
See pictures around the planetarium
– Cat’s eye, Butterfly, Ring: all “planetary nebula”
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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
If the universe contained only low mass stars,
would there be elements heavier than carbon?
1. Yes
2. No
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High mass star differences
• Degeneracy pressure never turns on
– Gas & thermal pressure always stronger
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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 …
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• Big picture: carbon and stuff fuses until you get
to a core made of …
• Iron (Fe on the periodic table, #26)
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
• See figure 17.12 for onion skin model
• See HR diagram on p. 559 (fig. 17.13)
After the Iron core forms
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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!)
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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
• 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?
1. The stars that formed earliest
2. The most recently formed stars
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Summary of star death
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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