1. 12 Stellar Evolution Where do gold earrings come from?

2. 12 Goals Explain why stars evolve off the main sequence. What happens when they leave the main sequence? How does mass affect what happens? How do stars die?

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4. 12 Concept Test Order the clusters from youngest to oldest. a. DBCA b. ACBD c. DCBA d. ADBC

5. 12 The Main Sequence A star is a delicate balance between the force of gravity pulling in, and pressure pushing out. Stars on the main sequence fuse hydrogen in their core to produce thermal pressure. Longest phase of a star’s life.

6. 12 What then? When the hydrogen in the core is almost consumed the balance between gravity thermal pressure pushing out and gravity pushing in is disturbed. The structure and appearance of the star changes dramatically. What happens then, depends on the star’s mass. Two cases: –Low-mass (< 8 x mass of Sun) –High-mass (> 8 x mass of Sun)

7. 12 Low-Mass Stars Where are low-mass stars? Longer lived or shorter than high- mass stars?

8. 12 Helium Ash Heavier elements, sink to the “bottom.” After 10 billion years, core is “choked” with helium “ash”. H  He continues in shell around non-burning core.

9. 12 The Red Giant Branch Without fusion pressure in core: –Helium core collapses (no counter to gravity) –Density in core increases. 3He  C + Energy in core 4H  He + Energy in shell Extra energy results in extra pressure. Star expands. The star gets bigger while its outside gets cooler.

10. 12 The Onion Sun Red Giant Stars Layers of: –Non-fusing H –Fusing H –Fusing He –Non-fusing C “ash”

11. 12 …And the Solar System? A few million years from now: –Sun becomes slightly brighter –Ocean’s begin to evaporate –“Hot House” Earth A few billion years from now: –Sun swells up –Engulfs the inner Solar System –Certain death for terrestrial planets –Possible “spring” on the Jovian ocean-moons!

12. 12 Red Supergiant What happens when the Sun runs out of helium in its core? Same as before. Core shrinks, surface expands. Radius ~ 3 AU!

13. 12 Death Core is contracting and heating. –Surface is cooling and expanding. Will it finally become hot enough in core for Carbon to fuse? For the Sun: No. Gravity keeps contracting the core: 1000 kg/cm 3 ! What stops it? Electron degeneracy pressure!

14. 12 Electron Degeneracy Pressure from motion of atoms

15. 12 Electron Degeneracy Pressure from electron shells

16. 12 Where are we now? Core dead – nothing happening. Shells – burning H and He, but soon stop too. Outside atmosphere of star still cooling and expanding. …and expanding Force of radiation from burning shells blows the atmosphere away.

17. 12 NGC3242 – HST – Bruce Balick

18. 12 M57 – Ring Nebula

19. 12 M27 – Dumbbell Nebula – copyright VLT, ESO

20. 12 Cat’s Eye

21. 12 Eskimo Nebula

22. 12 Hourglass Nebula

23. 12 NGC2440 – HST – Bruce Balick White Dwarf Mass of Sun Radius of Earth Hot as Sun’s core A million times denser than lead Slowly cool off

24. 12

25. 12 High-Mass Stars Think back to the first carbon core. How they get from main sequence to the carbon core stage is a little different. Now however, there is enough mass that it becomes hot enough to fuse carbon? Hot enough to eventually fuse lots of elements.

26. 12 The Iron Core 4H  He + Energy 3He  C + Energy C + He  O + Energy The ash of one reaction, becomes the fuel of the next. Fusion takes place in the core as long as the end result also yields energy. This energy causes pressure which counters gravity. But Iron doesn’t fuse.

27. 12 Core-Collapse Iron core – no outward pressure. Gravity wins! Star collapses rapidly! Electron degeneracy can’t stop it. Atomic structure can’t stop it. Electrons and protons crushed together to produce neutrons. Neutrons pushed together by force of gravity.

28. 12 Supernova

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32. 12 The result of the catastrophic collapse is the rebound and explosion of the core. From start of collapse to now: 1 second! Matter thrown back into the interstellar medium. Matter rushing outwards, fuses with matter rushing inwards. Every element after Fe is made in the instant of a supernova!

33. 12 M1 – Crab Nebula – copyright VLT

34. 12 NGC 4526 – 6 Million parsecs away

35. 12 Concept Test We should thank a high mass star for the carbon atoms in the sugar molecules in a candy bar because: a.Only in high mass stars is pressure great enough for carbon to be made by nuclear fusion. b.Only in high mass stars do we find the C-N-O cycle of nuclear fusion. c.In low mass stars the pressure due to gravity isn’t as high as the pressure in the core of a high mass star. d.Only a high mass star ends in an explosion that spreads carbon back into the gas and dust between stars. e.None of the above. Carbon is formed in both low mass and high mass stars.

36. 12 Concept Test Which of the following lists, in the correct order, a possible evolutionary path for a star? a.Red Giant, Neutron Star, White Dwarf, Nothing b.Red Giant, White Dwarf, Black Hole c.Red Giant, Supernova, Planetary Nebula, Neutron Star d.Red Giant, Planetary Nebula, White Dwarf e.Red Giant, Planetary Nebula, Black Hole

37. 12 Homework #12 For 2/20: Read B18.3-18.5 Do: Problems 16 and 18.