The Lives of Stars

1. The Lives of Stars The Deaths of Giants

2. Last Time • Last time, we traced the evolution of a star, starting with the cloud of gas from which it forms • Denser patches of these molecular clouds begin to grow, as their gravity attracts more mass

3. Last Time • These dense clumps continue to grow • As gravity increases the pressure, they heat up • Eventually, they start to radiate energy from gravitational collapse • These are called pre-main sequence stars

4. Last Time • The lower mass limit for a star is about 0.08 solar masses (80 Jupiter masses) • Anything less massive cannot kick start fusion • The upper limit is about 150 solar masses • Smaller proto-stars join together to form these ultra-massive stars • They can only survive for a few hundred thousand years before they blow themselves apart

5. Last Time • When the temperature gets high enough, they start to fuse Hydrogen into Helium, and a new start officially begins

6. Last Time • The main sequence on the H-R diagram is defined by stars that are steadily fusing Hydrogen into Helium in their cores • Remember the properties of luminosity and main sequence lifetime, and how this relates to mass

7. Last Time • While on the main sequence, stars are fairly normal • Things are well balanced, and the stars are happy • Some stars may still have interesting properties, like strong magnetic fields and starspots

8. Last Time • When stars run out of Hydrogen in their cores, things start to get interesting • We followed the end-stages of life for a star with a mass 0.08 – 2 times the mass of the Sun

9. Last Time • Once Hydrogen fusion is done in the core, all that is left is an inert ball of Helium • Hydrogen fusion now moves out into a shell around the core • The energy generated through this shell burning is greater than what was generated on the main sequence

10. Last Time • This increased energy production increases the luminosity of the star, and causes the outer regions of the star to puff up and expand • This happens even though the shell and the core shrink

11. Last Time • The temperature of the outer regions also goes down, and the star turns orange or red • This is called a sub-giant

12. Last Time • Helium produced in the Hydrogen shell burning falls onto the inert Helium core • Eventually, the temperatures in the core reach about 100 million Kelvin • Helium now begins to fuse into Carbon

13. Last Time • But the core is not supported by normal gas pressure • Instead, it is supported by degeneracy pressure • Increasing the temperature does not counteract gravity • The temperature and fusion rate start to increase in a runaway reaction

14. Last Time • This is called a Helium flash • It is a fast, huge increase in the energy production of the star • Eventually, the temperatures get so high that normal gas pressure takes over again • The star balances itself out

15. Last Time • Now, we have a core that is fusing Helium into Carbon, and a shell that is still fusing Hydrogen into Helium

16. Last Time • Eventually, the Helium in the core is all turned into Carbon • Now, Helium is fused in a shell around the core • Hydrogen is burned in another shell • The star grows again, just as it did when it first burned Hydrogen in a shell

17. Last Time • This is the Red Giant phase • It only lasts about 1 million years • The Sun will probably engulf the Earth

18. Last Time • There is no hope of fusing Carbon • The star begins its final march towards death • The star blows away its outer layers, forming a planetary nebula

19. Last Time • Finally, all that is left is a hot, but dead core of Carbon • It glows white hot, and is supported by degeneracy pressure • The electrons don’t want to get to close, so they resist gravity

20. Last Time • As the white dwarf cools, it changes color, from white to yellow, red, and finally black • It is now just a dead ball of Carbon

22. This Time • We will look at a high mass star, and trace its evolution

23. High Mass Stars • Higher mass stars go through a slightly different process • The results are…explosive

24. High Mass Stars • High mass stars spend their lives on the main sequence in a similar way to low mass stars • They burn hotter and brighter, and don’t live as long, but the same processes are at work

25. High Mass Stars • High mass stars begin to differ from their low mass companions when Helium fusion begins • Rather than the rapid Helium flash of low mass stars, high mass stars begin to fuse Helium gradually

26. Helium Fusion • High mass stars can make this switch smoothly because the temperatures are already high enough when Hydrogen fusion ends • No need for degeneracy pressure yet

27. Helium Fusion • The stars will once again fuse Hydrogen in shells around the core • The star swells to become a super giant, just like the low mass stars become sub-giants and Red Giants

28. Helium Fusion • Once the Helium in the core has been used up, Helium is fused in a shell around the core • This Helium came from the earlier Hydrogen shell • Hydrogen is now burned in the next shell up

29. Carbon Fusion • Unlike in the low mass stars, the crush of gravity can raise temperatures high enough to fuse Carbon in the core • This occurs at about 600 million Kelvin

30. Carbon Fusion • It is not long until the Carbon is used up • Now, the shell around the core begins fusing Carbon • Helium and Hydrogen are fused in successive shells

31. Fusion of Heavier Elements • The temperatures in high mass stars can get so high that elements heaver than Carbon can be fused • Next in line is Oxygen

32. Fusion of Heavier Elements • Once the Oxygen in the core is used up, Oxygen in a shell is fused • The core now starts to fuse Neon

33. Fusion of Heavier Elements • This process continues, as heavier and heavier elements undergo fusion • Each time an element runs out in the core, it is burned in a shell • More shells build on one another

34. Fusion of Heavier Elements • Next comes Magnesium, then Silicon

35. 0 of 5 What is the heaviest element we can make in a star? • Chromium • Iron • Uranium

36. Onions and Stars • Finally, the core turns into a lump of Iron • The star now resembles an onion, with many layers

38. Onions and Stars • As we move from the core out towards the surface of the star, the density of each layer decreases

39. Going Out With a Bang • When the core becomes Iron, the star has a problem • Iron cannot be fused • It has the highest binding energy of all the elements

40. Going Out With a Bang

41. Going Out With a Bang • If energy cannot come from the fusion of Iron, where will it come from? • Nowhere…the star is out of time

42. Going Out With a Bang • The iron core begin to collapse, until degeneracy pressure comes to the rescue... • Or not…electrons can only do so much • Gravity is just too strong, electron degeneracy pressure cannot stop the collapse

43. Going Out With a Bang • The overwhelming crush of gravity actually forces the electrons and protons together • They combine to make neutrons • Now all that remains are neutrons

44. Going Out With a Bang • Without the electron degeneracy pressure, there is no support for the core • It collapses, going from something the size of the Earth to something the size of Charlottesville in seconds

45. 0 of 5 Will anything stop the collapse of the core? • Yes, the fusion of iron • Yes, neutron degeneracy pressure • Nope

46. Going Out With a Bang • Finally, something stops the collapse of the core • It is neutron degeneracy pressure • It is capable of supporting more weight than electron degeneracy pressure • It seems like the day is saved, but…

47. Going Out With a Bang • What about all those outer layers? • When the core collapsed, there was nothing hold them up either • They started collapsing as well…

48. Going Out With a Bang • You can imagine their surprise when they slam into that super-dense iron core at 70,000 km/s (156,000,000 mph) • What will happen?

49. Endgame • The densest, inner layers slam into the core, followed by the less dense outer layers • Energy is transferred back through these layers, and a tremendous shock wave forms

50. Endgame • The star rips itself apart, in an ultra-violent explosion called a supernova • This particular type of supernova is called a Type II, or core collapse supernova (Yes, there are Type I…)