Evolution of High Mass Stars

1. Evolution of High Mass Stars AST 112

2. High Mass Stars • So… what exactly do high mass stars do? • The same thing as low mass stars: they get on the Main Sequence and convert H to He. • Then they blow up!

3. Life From Stars • Need low mass stars for life • They live long enough to allow life to flourish • Need high mass stars for life • They produce the elements heavier than carbon

4. High Mass Stars: Main Sequence • Low mass stars fuse H into He through the proton – proton chain • Slow! • High mass stars fuse H into He through the CNO cycle • Fast!

5. The CNO Cycle • Recall that nuclear reactions happen when nuclei have enough kinetic energy to overcome electric repulsion • High mass stars heat the cores to a higher temperature • H nuclei can now react with carbon, oxygen and nitrogen

6. The CNO Cycle • Carbon, nitrogen and oxygen act as catalysts • C, N, and O don’t get consumed; they just “help out” • This is why high mass stars shine bright and die young.

7. The CNO Cycle • Text, Page 574: Did the first high-mass stars in the history of the universe produce energy through the CNO cycle?

8. Hydrogen Exhaustion • 25 MSun star uses up its hydrogen in a few million years • Quickly develops a hydrogen burning shell, outer layers expand • Helium gradually begins to burn (no helium flash)

9. Supergiant • Core collapses, outer layers swell • At this point, it’s a supergiant star.

10. Burning Helium • Star burns He for few hundred thousand years • Runs out of He • Inert carbon core begins collapse • Similar to low-mass star thus far

11. Burning Carbon • High-mass stars: HOT! • Easily reach 1,200,000,000 oF for carbon fusion • Fuses carbon for a few hundred years, runs out

12. He-Capture Reactions • Helium nucleus fuses with heavier nuclei • Carbon to Oxygen • Oxygen to Neon • Neon to Magnesium Helium Capture Reactions

13. Heavy Nucleus Reactions • In the core: • Carbon + Oxygen -> Silicon • Oxygen + Oxygen -> Sulfur • Silicon + Silicon -> Iron Heavy-Nucleus Reactions

14. What can you think of that camefrom the inside of a dying high-massstar?

15. Advanced Nuclear Burning • The core fuses elements, runs out, shrinks, heats, and fuses new elements • This results in layers of heavy elements

16. High Mass Stars: Advanced Nuclear Burning • These sequential shells result in a zig-zag path about the HR diagram • Most massive stars: outer layers don’t have time to respond!

17. High Mass Stars: Advanced Nuclear Burning • Iron starts to accumulate in the central core • Elements lighter than iron release energy when fused • Elements heavier than iron release energy when split

18. High Mass Stars: Advanced Nuclear Burning • Not energetically advantageous for iron to fuse / split …so it doesn’t.

19. High Mass Stars: Advanced Nuclear Burning • Iron is not undergoing nuclear reactions • Doesn’t collapse • Electron degeneracy pressure (cramming too much stuff together) • Iron keeps on piling up…

20. Death of a High Mass Star A good way to remove electron degeneracy pressure: Get rid of the electrons!

21. Death of a High Mass Star • … and piling up and piling up… • Conditions such that electrons combine with protons • Forms neutrons, releases neutrinos • Degeneracy pressure vanishes instantly

22. Death of a High Mass Star • In a split second, an iron core the size of Earth collapses into a sphere of neutrons 5-10 miles across and releases a torrent of neutrinos. • This releases 100x the energy released by our Sun in its entire lifetime!

23. Supernova

24. Supernova • Outer layers of the star get blown away • Mostly due to neutrinos • 6000 miles / second • (3% speed of light!) • The leftover core is either: • A neutron star if it’s small enough • A black hole if it’s large enough

25. Supernova • A supernova is so bright it can briefly outshine an entire galaxy! • Bright for about a week, fades over months

26. Neutron-Capture Reactions • Where do elements heavier than iron come from? • Rare reactions that capture a neutron • Neutron changes to proton • Repeats • Requires high energy • Only happens close to and during supernova

27. Nuclear Reactions: Observational Evidence • Look at composition of stars, gas, dust in Milky Way • Look at C, O, or Ne • Even number of protons • Come from He capture (+2 protons) • These can fuse together • Elements heavier than iron are rare

28. Notorious Supernova Remnants • Messier 1, The Crab Nebula (in Taurus) • Growing several thousand miles per second! • Neutron star lives inside

29. Notorious Supernova Remnants • Re-tracing the Crab Nebula’s expansion puts the supernova at 1100 A.D. In the first year of the period Chih-ho,the fifth moon, the day chi-ch’ou, a guest star appeared approximately several[degrees] southeast of Thien-kuan. After more than a year it gradually becameinvisible. July 4, 1054 Taurus

30. Notorious Supernova Remnants • Supernova 1987A occurred in the Large Magellanic Cloud • 150k LY away • Did the star explode in 1987?

31. Milky Way Supernovas • Four in the last 1000 years: • 1006 (So bright it cast shadows at night!) • 1054 (Just did that one) • 1572 (Tycho Brahe saw it) • 1604 (Kepler saw it)

32. Betelgeuse • The size of the star extends out past the orbit of Mars • Its shape is pulsating • 600 LY away… it’s safe.