Giant Stars

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Giant Stars
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Energy Reserves

• When stars reach the main sequence they are in
  equilibrium between gravity and radiation.
• Hydrogen to helium for 90 of lifetime
• Too cool for helium fusion
• Helium builds up in the core.

Hydrogen-rich zones
Helium-rich core
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Red Giant

• The helium in the core stops fusion except right
  along a hydrogen shell.
• The helium core compresses to degenerate matter.
• Density 1000 kg/cm3
• The hydrogen expands and cools to orange and red.

Expanded hydrogen-rich zones
Helium core
Hydrogen-burning shell
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The Future Sun

• The Sun will spend about 10 billion years on the
  main sequence.
• Then expansion to red giant
• Cool to class K or M
• The Sun will expand to engulf at least Mercury
  and Venus.

Sun
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Stellar Evolution

• Other mass stars will also run out of fuel and
  become giants.
• Low mass stars expand a little
• High mass stars grow more after less time

10 M?
3 M?
0.5 M?
0.02 M?
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Helium Fusion

• Three helium nuclei can fuse into one carbon
  nucleus.
• Temperature over 108 K (100 million)
• High density to collide within 10-8 s

photon
photon
helium-4
carbon-12
beryllium-8
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Helium Flash

• In a red giant, helium starts fusing and more
  energy is produced.
• Mass over 2 M? goes to slow helium burning
• Mass from 0.5 M? to 2 M?, less stable equilibrium
• Lasts a few hours
• The rapid helium burning is called a helium
  flash.
• Star slightly contracts after explosion
• Horizontal branch of HR diagram

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Cluster Evolution

• The M3 Globular cluster shows stars going giant.
• Main sequence
• Red giant branch
• Horizontal branch
• The largest giants are on an asymptotic giant
  branch.
• Degenerate carbon core

Asymptotic giant branch
horizontal branch
red giant branch
main sequence