1. Annoucements Next test is in one week
Review session will be next Wednesday
Please hand-in or 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 H layer Envelope
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 Sun’s 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
Balloons start at center of room (in core of the Sun).
Everyone needs to randomly tap the balloons – gentle taps in random directions.
The balloons exit the Sun when they leave the center seats.
We’ll time how long it takes for half the balloons to leave the Sun.

21. Radiative zone game
Guess how long it will take half the balloons to reach the edge of the room.
Do the experiment compare with guesses.
How does this game relate to how photons carry energy from the core to the surface of the sun?