1. How Stars Evolve Pressure and temperature The fate of the Sun
Normal gases
Degenerate gases
The fate of the Sun
Red giant phase
Horizontal branch
Asymptotic branch
Planetary nebula
White dwarf
Reading: sections ,

2. 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
Do cold balloon demo

3. 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

4. The Fate of the Sun How will the Sun evolve over time?
What will be its eventual fate?

5. Sun’s Structure Core Envelope Where nuclear fusion occurs
Supplies gravity to keep core hot and dense

6. Main Sequence Evolution
Core starts with same fraction of hydrogen as whole star
Fusion changes H  He
Core gradually shrinks and Sun gets hotter and more luminous

7. Gradual change in size of Sun
Now 40% brighter, 6% larger, 5% hotter

8. Main Sequence Evolution
Fusion changes H  He
Core depletes of H
Eventually there is not enough H to maintain energy generation in the core
Core starts to collapse

9. Red Giant Phase He core H layer Envelope No nuclear fusion
Gravitational contraction produces energy
H layer
Nuclear fusion
Envelope
Expands because of increased energy production
Cools because of increased surface area

10. Sun’s Red Giant Phase

11. HR diagram
Giant phase is when core has been fully converted to Helium

12. Helium Flash He core H layer Envelope
Eventually the core gets hot enough to fuse Helium into Carbon.
This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once.
We don’t see this flash because it’s buried inside the Sun.
H layer
Envelope

13. Movement on HR diagram

14. Movement on HR diagram

15. 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

16. 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

17. Sun moves onto horizontal branch
Sun burns He into Carbon and Oxygen
Sun becomes hotter and smaller
What happens next?

18. Helium burning in the core stops
H burning is continuous
He burning happens in “thermal pulses”
Core is degenerate

19. Sun moves onto Asymptotic Giant Branch (AGB)

20. 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

22. Planetary nebula

23. Planetary nebula

24. Planetary nebula

25. Hourglass nebula