1. Chapter 11 The Death of High Mass Stars

2. a star’s mass determines its life story
1 Msun
25 Msun

3. Life Stages of High-Mass Stars
high-mass stars are similar to low-mass stars:
Hydrogen core fusion (main sequence)
Hydrogen shell burning (supergiant)
Helium core fusion (subgiant)
They are also different..
H-->He via CNO cycle not p-p chain
Core much hotter
Eventually fuse C, O into heavier elements
He core is not degenerate
no He flash!
Lose a lot of mass

4. High-mass stars make the elements necessary for life!

5. Big Bang made 90% H, 10% He – stars make everything else

6. Helium fusion can make only carbon in low-mass stars

7. Helium Capture occurs only in high-mass stars
High core T, P allow helium to fuse with heavier elements

8. Total # of P+N = Multiples of 4!
Helium capture builds C into O, Ne, Mg, …
Total # of P+N = Multiples of 4!

9. Evidence for helium capture:
Higher abundances of elements with even numbers of protons

10. Advanced Nuclear Burning
Core temperatures in stars with >8MSun allow fusion of elements up to iron

11. Si, S, Ca, Fe, etc. can only be made in high-mass stars

12. 9 7

13. Structure of massive stars

14. Fusion releases energy only when the mass of the products < mass of the reactants
Iron is “ash” of fusion: nuclear reactions involving iron do not release energy
Iron-56 has lowest mass per nuclear particle
Highest “binding energy” of all the elements

16. How does a high-mass star die?
Iron builds up in core until degeneracy pressure can no longer resist gravity

17. Supernova Explosion
Core degeneracy pressure cannot support degenerate core of > 1.4 Msun
electrons forced into nucleus, combine with protons
making neutrons, neutrinos and LOTS of energy!

18. Collapse only takes very short amount of time (~seconds)
Supernova!

19. Energy and neutrons released in supernova explosion cause elements heavier than iron to form, including Au and U

20. Neutron Stars & Supernova Remnants
Energy released by collapse of core drives outer layers into space
The Crab Nebula is the remnant of the supernova seen in A.D. 1054

21. Supernova 1987A
The first visible supernova in 400 years

22. Tycho’s supernova of 1572

23. Expanding at 6 million mph

24. Kepler’s supernova of 1609

26. Supernovae are 10,000 times more luminous than novae!
Massive star supernova: (Type II)
Massive star builds up 1.4 Msun core and collapses into a neutron star, gravitational PE released in explosion
White dwarf supernova: (Type I)
White dwarf near 1.4 Msun accretes matter from red giant companion, causing supernova explosion

27. light curve shows how luminosity changes with time

28. A neutron star:
A few km in diameter, supported against gravity by degeneracy pressure of neutrons

29. Discovery of Neutron Stars
Using a radio telescope in 1967, Jocelyn Bell discovered very rapid pulses of radio emission coming from a single point on the sky
The pulses were coming from a spinning neutron star—a pulsar

30. Pulsar at center of Crab Nebula pulses 30 times per second
Crab1b.mov

31. Why does a neutron star spin so rapidly?
Conservation of angular momentum!!

32. X-rays
Visible light

33. Pulsars

34. What happens if the neutron star has more mass than can be supported by neutron degeneracy pressure?
There is nothing to prevent it from collapsing infinitely:
BLACK HOLE!!
Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass is > about 3 Msun

35. Black Holes: Gravity’s Ultimate Victory
A black hole is an object whose gravity is so powerful that not even light can escape it.

36. Escape Velocity Final Gravitational Potential Energy Initial Kinetic =
Where m is your mass,
M is the mass of the object that you are trying to escape from, and
r is your distance from that object

37. “Surface” of a Black Hole
The “surface” of a black hole is the distance at which the escape velocity equals the speed of light.
This spherical surface = event horizon.
The radius of the event horizon is known as the Schwarzschild radius.

38. How does the radius of the event horizon change when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same

39. The event horizon of a 3 MSun black hole is a few km
Neutron star
Fill in your favorite city map here.
The event horizon of a 3 MSun black hole is a few km

40. A black hole’s mass strongly warps space and time in vicinity of event horizon

41. Light waves take extra time to climb out of a deep hole in spacetime, leading to a gravitational redshift

42. Time passes more slowly near the event horizon

43. Tidal forces near the event horizon of a
3 MSun black hole would be lethal to humans
Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

44. Do black holes really exist?

45. Black Hole Verification
Need to measure mass
Use orbital properties of companion
Measure velocity and distance of orbiting gas
It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

46. Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes

47. Cygnus X-1: Black hole candidate

48. If the Sun shrank into a black hole, its gravity would be different only near the event horizon

49. Some extra slides follow…
The end
Some extra slides follow…

51. High mass stars : CNO Cycle
H fusion is faster because C, N and O act as catalysts
Same net result: 4 H become 1 He.
No total gain or loss of C, N, O

52. How does the total energy produced during one CNO cycle compare to that of the proton-proton chain?