Chapter 11 The Death of High Mass Stars

1. Chapter 11The 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. 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 9 7 9 http://physics.gmu.edu/~jevans/astr103/CourseNotes/Text/Lec05/Lec05_pt5_txt_stellarPostMSEvol.htm

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

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? • Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass is > about 3 Msun There is nothing to prevent it from collapsing infinitely: BLACK HOLE!!

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 Energy = 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. Neutron star 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. The end Some extra slides follow…