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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-rich zones
– Hydrogen to helium for 90%
of lifetime
– Too cool for helium fusion
• Helium builds up in the core.
Helium-rich core
Red Giant
• The helium in the core stops
fusion except right along a
hydrogen shell.
Expanded hydrogen-rich
zones
• The helium core compresses
to degenerate matter.
– Density 1000 kg/cm3
• The hydrogen expands and
cools to orange and red.
Hydrogen-burning
shell
Helium core
The Future Sun
• The Sun will spend about
10 billion years on the
main sequence.
-20
-15
Abs. Magnitude
-10
– Then expansion to red giant
– Cool to class K or M
-5
0
5
Sun
10
15
20
O B A F G K M
Spectral Type
• The Sun will expand to
engulf at least Mercury
and Venus.
Stellar Evolution
• Other mass stars will also
run out of fuel and
become giants.
-20
-15
Abs. Magnitude
-10
-5
10 M
3 M
0
5
0.5 M
10
0.02 M
15
20
O B A F G K M
Spectral Type
– Low mass stars expand a
little
– High mass stars grow
more after less time
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
beryllium-8
carbon-12
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
Cluster Evolution
• The M3 Globular cluster
shows stars going giant.
horizontal branch
Asymptotic
giant
branch
red giant branch
main sequence
– Main sequence
– Red giant branch
– Horizontal branch
• The largest giants are on
an asymptotic giant
branch.
– Degenerate carbon core