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

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.


Is there hydrogen outside the sun s core

Is there Hydrogen outside the Sun’s core?

  • Yes

  • No


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


Star becomes luminous

Star becomes ______ luminous

  • More

  • Less


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


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

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

  • Yes

  • No


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)


Star evolution

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


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