Annoucements

1
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

2
How Stars Evolve

• Pressure and temperature
• Normal gases
• Degenerate gases
• Evolution of the Sun after the main sequence
• Red giant phase
• Helium flash
• Horizontal branch
• Asymptotic branch
• Planetary nebula
• White dwarf

3
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

4
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

5
Movement on HR diagram
6
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

7
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

8
Sun moves onto horizontal branch
Sun burns He into Carbon and Oxygen Sun becomes
hotter and smaller What happens next?
9
Helium burning in the core stops
H burning is continuous He burning happens in
thermal pulses Core is degenerate
10
Sun moves onto Asymptotic Giant Branch (AGB)
11
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

12
Planetary nebula
13
Planetary nebula
14
Planetary nebula
15
Hourglass nebula
16
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

17
Evolution on HR diagram
18
Time line for Suns evolution
19
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
20
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. Well time how long it takes for half the
   balloons to leave the Sun.

21
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?