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Annoucements
• Next test is in one week
• Review session will be next Wednesday
– Please hand-in or e-mail questions
• Practice test will be available by Monday
How Stars Evolve
• Pressure and temperature
– Normal gases
– Degenerate gases
• Evolution of the Sun after the main sequence
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Red giant phase
Helium flash
Horizontal branch
Asymptotic branch
Planetary nebula
White dwarf
Pressure and Temperature
• Pressure is the force exerted by atoms in a gas
• Temperature is a measure of how fast the atoms
in a gas move
• Hotter  atoms move
faster  higher pressure
• Cooler  atoms move
slower  lower pressure
Degenerate gas
• Very high density
• Motion of atoms is not due to kinetic
energy, but instead due to quantum
mechanical motions
• Pressure no longer depends on temperature
• This type of gas is sometimes found in the
cores of stars
Movement on HR diagram
Helium Flash
• He core
– Eventually the core gets
hot enough to fuse Helium
into Carbon.
– The Helium in the core is
so dense that it becomes
a degenerate gas.
• H layer
• Envelope
Red Giant after Helium Ignition
• He burning core
– Fusion burns He into C, O
• He rich core
– No fusion
• H burning shell
– Fusion burns H into He
• Envelope
– Expands because of
increased energy
production
Sun moves onto horizontal branch
Sun burns He
into Carbon
and Oxygen
Sun becomes
hotter and
smaller
What happens
next?
Helium burning in the core stops
H burning is continuous
He burning happens in
“thermal pulses”
Core is degenerate
Sun moves
onto
Asymptotic
Giant
Branch
(AGB)
Sun looses mass via winds
• Creates a “planetary nebula”
• Leaves behind core of carbon and oxygen
surrounded by thin shell of hydrogen
• Hydrogen continues to burn
Planetary nebula
Planetary nebula
Planetary nebula
Hourglass
nebula
White dwarf
• Star burns up rest of hydrogen
• Nothing remains but degenerate core of Oxygen
and Carbon
• “White dwarf” cools but does not contract
because core is degenerate
• No energy from fusion, no energy from
gravitational contraction
• White dwarf slowly fades away…
Evolution on HR diagram
Time line for Sun’s evolution
Transport of energy through the
radiative zone
It takes about
200,000 years
for photons
made in the
core to make it
through the
radiative zone
Radiative zone game
1. Balloons start at center of room (in core of the
Sun).
2. Everyone needs to randomly tap the balloons
– gentle taps in random directions.
3. The balloons exit the Sun when they leave the
center seats.
4. We’ll time how long it takes for half the
balloons to leave the Sun.
Radiative zone game
1. Guess how long it will take half the
balloons to reach the edge of the room.
2. Do the experiment compare with guesses.
3. How does this game relate to how photons
carry energy from the core to the surface
of the sun?