Star evolution
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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

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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 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?

  • Low mass

  • Intermediate mass

  • High mass


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

  • 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?

  • Yes

  • No


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


Star becomes ______ luminous

  • More

  • Less


What’s happening to the mass of the core as the shell “burns”?

  • Increasing

  • Decreasing

  • Staying the same


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

  • 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

  • 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


If the universe contained only low mass stars, would there be elements heavier than carbon?

  • Yes

  • No


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)


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

  • 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

  • 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?

  • The stars that formed earliest

  • The most recently formed stars


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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