Element Building in Large Stars

1. Element Building in Large Stars Life Cycle of Stars - Part 2 Elements 2002 Workshop

2. Re: Principles of Stellar Evolution • The bigger it is, the hotter and the faster a star burns its life away. • Energy released from nuclear fusion counter-acts inward force of gravity. • Throughout its life, these two forces determine the stages of a star’s life.

3. Annie J Cannon (1863-1941) All Types of Stars Oh Big And Ferocious Gorilla, Kill My Roommate Next Saturday !

4. Nuclear Fusion ! • At 15 million degrees Celsius in the center of the star, fusion ignites ! • 4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy • Where does the energy come from ? • Mass of four 1H > Mass of one 4He E = mc2

5. Stellar Winds in High Mass Stars • Wolf-Rayet stars may have winds up to thousands of km/s • These stars are hot, massive O stars • But are only 1-2% of normal stars in galaxy • Mass loss equivalent to ~1/3 mass of earth per year. • Elements pushed by intense photon radiation emitted by the star. • We observe He, N, C, O, Si, Fe

6. Reprise: the Life Cycle Sun-like Stars Massive Stars

7. The beginning of the end: Red Giants

8. Red Giants • After Hydrogen is exhausted in core, • Core collapses, releasing energy to the outer layers • Outer layers expand • Meanwhile, as core collapses, • Increasing Temperature and Pressure ...

9. More Fusion ! • At 100 million degrees Celsius, Helium fuses: • Triple Alpha Process • 4He + 4He --> 8Be + energy • 4He + 8Be --> 12C + energy • 3 (4He) --> 12C + energy • (Only 7.3 MeV produced) • Energy sustains the expanded outer layers • of the Red Giant

10. Fate of high mass stars • After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen. • 12C + 12C --> 24Mg OR 12C + 4H --> 16O • Through a combination of processes, successively heavier elements are formed and burned.

11. Light Elements Heavy Elements 4 (1H) 4He + energy C-N-O Cycle 3(4He) 12C + energy 4He + 16O 20Ne + energy 16O + 16O 32S + energy 28Si + 7(4He) 56Ni + energy 56Fe 4He + 12C 16O + energy 12C + 12C 24Mg + energy Periodic Table

12. The End of the Line for Massive Stars • Massive stars burn a succession of elements. • But the process stops at Iron.

13. The Stop Light at Iron • Iron is the most stable element and cannot be fused further. • Instead of releasing energy, it uses energy. • Why ? Binding energy • Mass of atomic nucleus < Sum of masses of individual neutrons and protons • Difference is energy which holds nucleus together. • (= amount of energy to break nucleus apart)

14. Fission Fusion Binding Energy/Nucleon Fusion or Fission results in more tightly bound nuclei (I.e. less stable --> more stable + energy)

15. S-Processes - Going beyond Iron • In Asymptotic Giant Branch Stars, He burning occurs in a shell around Carbon core. • Here reactions occur which release neutrons. • These neutrons help build heavy isotopes: • Fe56 + n --> Fe57 • Fe57 + n --> Fe58 • Fe58 + n --> Fe59 • This process is slow - thousands of years between captures.

16. S-Process: Creating Heavy Elements • Beta Decay of unstable nuclei produce new elements: • 26Fe59 --> 27Co59 + e- + neutrino • (Decay occurs before another neutron can be captured) • Then Co59 + n --> Co60 --> Ni60 + e- • All the way up to 82Pb208 and 83Bi209

17. Dispersal of S-Process Elements • Heavy elements enter convection layers of star. • Stellar winds then disperse them.

18. S-Process Elements from Large Stars Slow Addition of Neutrons

19. All Elements from Large Stars Dispersed via Stellar Winds

20. The Next Step ... • Once Iron core forms, • Energy Source disappears • Gravity takes over …