1. Chapter 16: Evolution of Low-Mass Stars

2. While on the Main Sequence stars of all mass burn hydrogen into helium

3. How long a star lives on the Main Sequence depends on its mass

4. As the star burns its hydrogen, it accumulates a helium ash
Because energy flow in the central regions of the star is by radiation, the helium ash isn’t being stirred out.

5. As time goes by, the helium ash gets in the way
To continue to burn hydrogen with all that helium in the way, the star gets a little hotter, a little bigger and a little brighter .

6. At the center of the star, a dead helium core starts to form
The central helium core is not fusing. It’s just being squeezed by gravity and added to by the hydrogen fusing above it

7. Once the hydrogen runs out, the helium ash gets compressed until it becomes degenerate
When something is degenerate all the low energy states are filled.
Only the highest energy states are left for new electrons

8. What does it mean to be Degenerate?
Electron energy levels crowded together almost continuous
All low energy levels are full according to the Pauli Exclusion Principle
Only place for additional electrons to go is in high energy levels which means they must move very fast
Adding more mass decreases the volume
Temperature is same everywhere

9. If you add mass to a degenerate object it shrinks

10. When the helium core shrinks, it heats up
When the helium core shrinks, it heats up. This causes hydrogen to start fusing in a shell around the core
Because the core is shrinking as more mass is added to it, it heats up. As it heats up that causes the shell fusion around it to speed up and the star stars to expand

11. The expansion to a red giant is all due to the battle between gravity and pressure

12. Evolution off the Main Sequence is the reverse of forming a protostar

13. The most massive stars become red supergiants

14. Once fusion in the core stops the core shrinks and heats up while the outer surface expands and cools

15. The core doesn’t completely collapse due to degeneracy

16. Helium Fusion starts when the core reaches 100,000,000°

17. In stars with low mass the helium ignition is explosive
In the degenerate core the temperature is the same everywhere

18. Once helium fusion settles down the star resides on the “horizontal branch” for a while
The energy production will stabilize so the star will shrink in size (some). It will then start a second life burning helium into carbon

19. For low mass stars: a second red giant stage when the helium in the core runs out

20. Internal Structure of AGB star

21. Thermal Pulses cause whole layers of star to lift off
Near the end, shell fusion becomes unstable resulting in thermal pulses which push layers of the star into space

22. Planetary nebulae recycle most of a stars’ matter back out into space
By this time, convection is starting to reach farther down into the interior of the star and dredge up the products of fusion

23. The Death of a Low Mass Star <8 solar masses
Planetary nebulae can have very complex forms. The details of how they create those forms is not well understood but probably has something to do with magnetic fields or if the star is in a binary system.

24. The death of a low mass star on the H-R Diagram

25. After planetary nebula dissipates only a white dwarf is left
X-ray light
Visible light

26. White Dwarf Stars are degenerate matter
Chandrasekhar Limit 1.4 Msun
They are composed mostly of carbon with some oxygen

27. Many stars live and die in binary systems

28. The most massive will form a white dwarf first

29. Eventually, the other star evolves off the main sequence

30. The white dwarf can become very active as it gains mass
White dwarfs in binary systems are called cataclysmic variables because they can vary cataclysmically

31. As hydrogen builds up on the white dwarf it can ignite
If the white dwarf mass exceeds the Chandrasekhar limit it explodes in a supernova. If not, it can undergo an ordinary nova outburst