The Life and Death of Stars

1. The Life and Death of Stars

2. Nucleosynthesis • Remember that the Big-Bang created the H and He in the universe, but not much more. • H fusion in stars makes more He • But where do we get the stuff we are made of? Z N

3. We are “star stuff” because the elements necessary for life were made in stars

5. Stars are born in molecular clouds consisting mostly of hydrogen molecules • Stars form in places where gravity can overcome thermal pressure in a cloud

6. Orion Nebula is one of the closest star-forming clouds Infrared light from Orion

7. Solar-system formation is a good example of star birth

8. As gravity forces a cloud to become smaller, it begins to spin faster and faster Conservation of angular momentum

9. Protostar to Main Sequence • Protostar contracts and heats until core temperature is sufficient for hydrogen fusion. • Contraction ends when energy released by hydrogen fusion balances energy radiated from surface. • Takes 50 million years for star like Sun (less time for more massive stars)

10. Stars more massive than 100 MSun blow apart How massive are newborn stars? Luminosity Stars less massive than 0.08 MSun can’t sustain fusion Temperature

11. Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in same place

12. Pressure Gravity If M > 0.08 MSun, then gravitational contraction heats core until fusion begins If M < 0.08 MSun, degeneracy pressure stops gravitational contraction before fusion can begin

13. Life as a Low-Mass Star • What are the life stages of a low-mass star? • How does a low-mass star die?

14. High-Mass Stars > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs

15. For a solar-mass star most of life is relatively boring • Things get interesting after about 9 billion years…. • A star remains on the main sequence as long as it can fuse hydrogen into helium in its core

16. Table of Nuclides and their half-lives if unstable • Gray is stable • White is unstable • Hatched is long-lived unstable

17. Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins

18. Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins

19. So the core is contracting and heating up… • The core is inert He, but outside the core, in the radiation-zone, there is still plenty of H. • As the core contracts it, of course, brings some of the rest of the star with it. A layer of H in the radiation-zone gets sufficiently hot to start fusing! • We call this hydrogen-shell burning when you have a shell of H (around the He core) fusing into He.

20. But don’t forget -- the core is still shrinking, even though there is some fusion in the shell going on. So the thermostat is broken. Shell burning doesn’t do anything for the core. But it does fight back against the gravity of the rest of the star.

21. That means that the star poofs out and expands into a Red SubGiant. Radius and Luminosity are bigger.

22. Stage 1: H-shell burning SubGiant

23. But what about the core? Eventually the contraction of the core heats it up high enough for Helium Fusionto start Helium fusion requires higher temperatures than hydrogen fusion because larger charge in bigger atoms leads to greater repulsion. Fusion of two helium nuclei doesn’t work (the beryllium barrier), so helium fusion must combine three He nuclei to make carbon.

24. The beryllium barrier Helium Burning Hydrogen Burning

25. BUT! Note that Electron Degeneracy Pressure is supporting the core when He-burning begins. • That means that He-burning won’t immediately cause the core to expand back outward. • Instead there is a Helium Flash where a huge amount of fusion occurs quickly and a lot of energy is released. The Degeneracy Pressure keeps the core’s temperature hot so there’s no lessening of the fusion rate.

26. Fortunately, this lasts only a short time. • Thermal Pressure eventually does become larger than Degeneracy Pressure, and so lets the core expand. • Core expansion means the H-burning shell expands too and cools off a bit. • The thermostat is fixed, and the star goes back into equilibrium for a while, burning He in the core and some H in the shell. • The star shrinks back out of Red Giant phase, but not all the way back to the main sequence.

27. Helium burning stars neither shrink nor grow because thermostat is temporarily fixed.

28. …except a solar-mass star will never get hot enough to fuse Carbon into something else. No more fusion can happen! You’ll have two shells burning around the core -- a H shell and a He shell, but no more fusion in the core. Again, the shells do nothing for the core, but they poof out the star even larger than before. It is at this point that probably our Sun will engulf Earth!

29. Stage 3: He and H shell burning Red Giant

30. A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind

31. We have lots of great pictures of nebula being shed

34. Stage 4: Outer layers lost to planetary nebula Stage 5: White Dwarf

35. A white dwarf is about the same size as Earth

36. White dwarfs shrink when you add mass to them because their gravity gets stronger

37. Shrinkage of White Dwarfs • Quantum mechanics says that electrons in the same place cannot be in the same state • Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space • In order to avoid being in the same state some of the electrons need to move faster • Is there a limit to how much you can shrink a white dwarf?

38. The White Dwarf Limit • Einstein’s theory of relativity says that nothing can move faster than light • When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can no longer support it • Chandrasekhar found (at age 20!) that this happens when a white dwarf’s mass reaches 1.4 MSun • He actually puzzled this out on the boat from India to England before he started his grad studies in physics. (Once at Cambridge his advisor told him he was crazy and to drop this work…..it won him the Nobel Prize) S. Chandrasekhar

39. Hydrogen that accretes onto a while dwarf builds up in a shell on the surface (this happens with binary star systems) When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova

40. Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space

41. Thought Question What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit? A. It explodes B. It collapses into a neutron star C. It gradually begins fusing carbon in its core

42. Thought Question What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit? A. It explodes B. It collapses into a neutron star C. It gradually begins fusing carbon in its core

43. Two Types of Supernova Massive star supernova: Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing explosion White dwarf supernova: Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing total explosion These two types have different patterns of luminosity, so we can tell them apart….

44. Nova or Supernova? • Supernovae are MUCH MUCH more luminous!!! (about 10 million times) • Nova: H to He fusion of a layer, white dwarf left intact • Supernova: complete explosion of white dwarf, nothing left behind

45. Low-Mass Star Summary • Main Sequence: H fuses to He in core • Red Giant: H fuses to He in shell around He core • Helium Core Burning: He fuses to C in core while H fuses to He in shell • Double Shell Burning: H and He both fuse in shells 5. Planetary Nebula leaves white dwarf behind Not to scale!

46. Reasons for Life Stages • Core shrinks and heats until it’s hot enough for fusion • Nuclei with larger charge require higher temperature for fusion • Core thermostat is broken while core is not hot enough for fusion (shell burning) • Core fusion can’t happen if degeneracy pressure keeps core from shrinking Not to scale!

47. High-Mass Stars Life as a High-Mass Star > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs

48. High-Mass Star’s Life Early stages are similar to those of low-mass star: • Main Sequence: H fuses to He in core • Red Supergiant: H fuses to He in shell around inert He core. But the extra mass soon produces the temperatures and pressures necessary to start He fusion. • Helium Core Burning: He fuses to C in core (no flash)

49. High-mass stars become supergiants after core H runs out Luminosity doesn’t change much but radius gets far larger

50. The fusion in the high-mass star is a sequence of similar events that repeat themselves: • X is fusing in the core, making Y, but the core eventually runs out of X. • Core contracts, allowing layers around the core to heat up, initiating an X-burning shell around the core. • The shell burning does nothing for the core, but does change the star’s overall radius. • Core continues to contract, eventually getting hot enough to let Y start fusing into Z.