Stellar Formation

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Chapter 19

• Stellar Formation
• (Read Chapter Summary Only)

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Chapter 20

• Stellar Evolution

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H-R Diagram Questions

• 1. What property is measure along the horizontal
  axis?
• 2. along the vertical axis?
• 3. Where are the red giants?
• 4. the white dwarfs?
• 5. the hottest stars?
• 6. the coolest stars?
• 7. the largest stars?
• 8. the smallest stars?

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H-R Diagram Questions

• 9. Where are O class stars?
• 10. M class stars?
• 11. G class stars?
• 12. Where is the Sun?
• 13. Where are the high-mass main-
• sequence stars?
• 14. Where are the low-mass main-sequence stars?
• 15. Where are the oldest stars?
• 16. Which stars along the main-sequence live the
  longest?

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Evolution of Low-Mass Stars

• 1. The Sun began its life like all stars as an
  intersteller cloud.
• 2. This cloud collapses due to gravity into a
  dense core.
• 3. In about a million years a small, hot, dense
  core called a protostar forms.

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• 4. When the temperature reaches 10,000,000K in
  the core, fusion begins and transforms the
  protostar into a main-sequence star.
• 5. Low mass stars like the Sun remain on the
  main-sequence for about 10 billion years.
• Note Massive stars stay on the main-sequence
  for about 1 billion years.

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• 6. Hydrogen fusion begins in a shell around the
  core and the star expands into a Red Giant.
• 7. After most of the hydrogen is fused into
  helium, helium fusion begins in an event called
  the Helium Flash.
• 8. Stars can then become unstable and turn into
  pulsating stars like RR Lyrae Variables or
  Cephied Variables.

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• 9. As a star burns helium into carbon the
  radiation pressure pushes the star's outer
  atmosphere away from the core creating a
  Planetary Nebula.
• 10. This leaves an exposed core called a White
  Dwarf. These have about the same diameter as the
  Earth.

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Evolution of High-Mass Stars

• 1 to 5. Same as before
• intersteller cloud
• dense core
• protostar
• main-sequence star
• 6. When a high-mass star exhausts the hydrogen
  fuel in its core the star leaves the main
  sequence and begins to burn helium.

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• 7. The star becomes a Red Supergiant after
  millions of years of helium fusion.
• 8. When helium is depleted, fusion of heavier
  elements begins. This process is called
  nucleosynthesis.
• H -gt He -gt C -gt O -gt Si -gt Fe

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• 9. A star with an iron core is out of fuel.
• Reason Iron atoms cannot fuse and release
  energy.
• 10. The core collapses due to reduced pressure
  converting the iron core into mostly neutrons.
• 11. The core pressure then surges and lifts the
  outer layers from the star in a titanic explosion
  - a supernova!

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End of Chapter 20
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