1. Homework #6: due Friday, March 23, 5pm
Exam #2: Monday, April 2.

2. Stellar Mass and Fusion
The mass of a main sequence star determines its core pressure and temperature
Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived
Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes

3. High-Mass Stars
> 8 MSun
Intermediate-Mass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs

4. Star Clusters and Stellar Lives
Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations
Star clusters are particularly useful because they contain stars of different mass that were born about the same time

5. What are the life stages of a low-mass star?
How does a low-mass star die?

6. A star remains on the main sequence as long as it can fuse hydrogen into helium in its core
Main-seq_lifetime_and_mass.swf

7. Eventually, the core runs out of hydrogen
Main-seq_lifetime_and_mass.swf

8. Broken Thermostat
As the core contracts, H begins fusing to He in a shell around the core

9. Broken Thermostat
As the core contracts, H begins fusing to He in a shell around the core
Luminosity increases because the core thermostat is broken - the increasing fusion rate in the shell does not stop the core from contracting

10. Broken Thermostat
As the core contracts, H begins fusing to He in a shell around the core
Luminosity increases because the core thermostat is broken - the increasing fusion rate in the shell does not stop the core from contracting
Increased luminosity causes shell to expand – star becomes a giant

11. Triple-Alpha Process (He nuclei = alpha particle)
Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion
Helium fusion must combine three He nuclei to make one carbon nucleus

12. Beryllium-8 is VERY unstable
In order for a third helium-4 nucleus to smash into the beryllium-8 before it decays, the density of the stellar core must be very high: roughly 7700 times that of water.

13. Helium Flash
Thermostat is broken in low-mass red giant because electron degeneracy pressure supports core
Core temperature rises rapidly when helium fusion begins
Helium fusion rate skyrockets until thermal pressure takes over and expands core again

14. Helium burning stars neither shrink nor grow because core thermostat is temporarily fixed.

15. Life Track after Helium Flash
Models show that a red giant should shrink and become less luminous after helium fusion begins in the core

16. Life Track after Helium Flash
Observations of star clusters agree with those models
Helium-burning stars are found in a horizontal branch on the H-R diagram

17. Combining models of stars of similar age but different mass helps us to age-date star clusters
Hr_diagr_and_age_of_cluster.swf

18. Activity Time! 1. Gather into your group and select today’s scribe.
2. Consider the following:
The Sun is in mid-life as a Main Sequence star. It will remain on the Main Sequence for approximately another 5 billion years. Shortly after that time it will become a red giant.
3. Address the following questions as a group.

19. Describe what sunrise and sunset will be like when the Sun becomes a red giant.
About how long will these last?
Do you expect the color of the sky to change? Explain.
Although the Sun will be cooler than today (it will be a “red” giant), it will have 3000 times the luminosity of today’s sun. How is this possible given its color.
Will be Earth be warmer or cooler?
What do you expect will happen to the inner planets (Mercury & Venus)
Do you expect that humans will survive until that time? Explain why or why not. If not, what do you expect will have happened to them. If so, do you expect them to be any different than today’s humans.

20. How does a low-mass star die?

21. Double Shell Burning
After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer
This double-shell burning stage never reaches equilibrium—fusion rate periodically spikes upward in a series of thermal pulses
With each spike, convection dredges carbon up from core and transports it to surface
Some of this carbon flows from the star’s surface, forming carbon rich dust – this is where most of the carbon on Earth originated!

23. Planetary Nebulae
Double-shell burning ends with a pulse that ejects the H and He into space as a planetary nebula
The core left behind becomes a white dwarf

24. Planetary Nebulae
Double-shell burning ends with a pulse that ejects the H and He into space as a planetary nebula
The core left behind becomes a white dwarf

25. Planetary Nebulae
Double-shell burning ends with a pulse that ejects the H and He into space as a planetary nebula
The core left behind becomes a white dwarf

26. Planetary Nebulae
Double-shell burning ends with a pulse that ejects the H and He into space as a planetary nebula
The core left behind becomes a white dwarf

27. Planetary Nebulae

28. Hour Glass Nebula

29. Ring Nebula

30. Planetary Nebulae

31. Planetary Nebulae

32. End of Fusion
Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements
Degeneracy pressure supports the white dwarf against gravity

33. Life Track of a Sun-Like Star

34. Earth’s Fate
Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

35. Earth’s Fate
Sun’s radius will grow to near current radius of Earth’s orbit

36. What are the life stages of a low-mass star?
H fusion in core (main sequence)
H fusion in shell around contracting core (red giant)
He fusion in core (horizontal branch)
Double-shell burning (red giant)
How does a low-mass star die?
Ejection of H and He in a planetary nebula leaves behind an inert white dwarf

37. High-Mass Stars What are the life stages of a high-mass star?
How do high-mass stars make the elements necessary for life?
How does a high-mass star die?

38. CNO Cycle
High-mass main sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts
Greater core temperature enables H nuclei to overcome greater repulsion

39. Life Stages of High-Mass Stars
Late life stages of high-mass stars are similar to those of low-mass stars:
Hydrogen core fusion (main sequence)
Hydrogen shell burning (supergiant)
Helium core fusion (supergiant)

40. How do high-mass stars make the elements necessary for life?

41. Big Bang made 75% H, 25% He – stars make everything else

42. Helium fusion can make carbon in low-mass stars

43. CNO cycle can change C into N and O

44. Helium Capture
High core temperatures allow helium to fuse with heavier elements

45. Helium capture builds C into O, Ne, Mg, …

46. Advanced Nuclear Burning
Core temperatures in stars with >8MSun allow fusion of elements as heavy as iron