1. Death of stars Final evolution of the Sun
Determining the age of a star cluster
Evolution of high mass stars
Where were the elements in your body made?
Reading 21.3,

2. 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…

3. Evolution on HR diagram

4. Time line for Sun’s evolution

5. When hydrogen burning in the core stops, a star like the Sun begins to evolve
toward the upper right in the H-R diagram.
toward the zero age main sequence stage.
up the main sequence to become an O star.
toward the lower left in the H-R diagram.

6. Higher mass protostars contract faster
Hotter

7. Higher mass stars spend less time on the main sequence

8. Determining the age of a star cluster
Imagine we have a cluster of stars that were all formed at the same time, but have a variety of different masses
Using what we know about stellar evolution is there a way to determine the age of the star cluster?

9. Turn-off point of cluster reveals age

10. The HR diagram for a cluster of stars shows stars with spectral types A through K on the main sequence and stars of type O and B on the (super) giant branch. What is the approximate age of the cluster?
1 Myr
10 Myr
100 Myr
1 Gyr

11. Higher mass stars do not have helium flash

12. Nuclear burning continues past Helium
1. Hydrogen burning: 10 Myr
2. Helium burning: 1 Myr
3. Carbon burning: 1000 years
4. Neon burning: ~10 years
5. Oxygen burning: ~1 year
6. Silicon burning: ~1 day
Finally builds up an inert Iron core

13. Why does nuclear fusion stop at Iron?

14. Fusion versus Fission

15. Core collapse Iron core is degenerate
Core grows until it is too heavy to support itself
Core collapses, density increases, normal iron nuclei are converted into neutrons with the emission of neutrinos
Core collapse stops, neutron star is formed
Rest of the star collapses in on the core, but bounces off the new neutron star (also pushed outwards by the neutrinos)

16. If I drop a ball, will it bounce higher than it began?
Do 8B Supernova Core Bounce

17. Supernova explosion

18. Crab nebula

19. Cas A

20. In 1987 a nearby supernova gave us a close-up look at the death of a massive star

22. Neutrinos from SN1987A

23. Where do the elements in your body come from?
Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets
Supernova produce all of the heavier elements
Elements up to Iron can be produced by fusion
Elements heavier than Iron are produced by the neutrons and neutrinos interacting with nuclei in the supernova explosion

24. Review Questions
How does the evolution of a high mass star differ from that of a low mass star?
How can the age of a cluster of stars, all formed at the same time, be determined?
Why does fusion stop at Iron?
How are heavy elements produced?