1. Chapter 10 The Bizarre Stellar Graveyard

2. The Products of Star Death White Dwarfs Neutron Stars Black Holes

3. What is a white dwarf?

4. White Dwarfs White dwarfs are the remaining cores of dead stars. Electron degeneracy pressure supports them against the crush of gravity.

5. White Dwarfs White dwarfs are the remaining cores of dead stars. Electron degeneracy pressure supports them against the crush of gravity.

6. Size of a White Dwarf White dwarfs with same mass as Sun are about same size as Earth. Higher-mass white dwarfs are smaller.

7. The White Dwarf Limit Quantum mechanics says that electrons must move faster as they are squeezed into a very small space. As a white dwarf’s mass approaches 1.4M Sun, its electrons must move at nearly the speed of light. Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4M Sun, the white dwarf limit (or Chandrasekhar limit).

8. White dwarfs cool off and grow dimmer with time.

9. What is a neutron star?

10. A neutron star is the ball of neutrons left behind by a massive-star supernova. Degeneracy pressure of neutrons supports a neutron star against gravity.

11. Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star.

12. A neutron star is about the same size as a small city.

14. Discovery of Neutron Stars Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. The pulses were coming from a spinning neutron star—a pulsar.

15. Pulsars A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis.

16. Pulsars The radiation beams sweep through space like lighthouse beams as the neutron star rotates.

18. Why Pulsars Must Be Neutron Stars Circumference of NS = 2  (radius) ~ 60 km Spin rate of fast pulsars ~ 1000 cycles per second Surface rotation velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS Anything else would be torn to pieces!

19. What is a black hole? Insert TCP 6e Figure 18.12c

20. A black hole is an object whose gravity is so powerful that not even light can escape it.

21. Neutron Star Limit Quantum mechanics says that neutrons in the same place cannot be in the same state. Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3M sun. Some massive star supernovae can make a black hole if enough mass falls onto core.

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

23. The event horizon of a 3M Sun black hole is also about as big as a small city.

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

25. No Escape Nothing can escape from within the event horizon because nothing can go faster than light. No escape means there is no more contact with something that falls in. It increases the hole mass, changes the spin or charge, but otherwise loses its identity.

26. Singularity Beyond the neutron star limit, no known force can resist the crush of gravity. As far as we know, gravity crushes all the matter into a single point known as a singularity.

27. What would it be like to visit a black hole? Insert TCP 6e Figure 18.12c

28. If the Sun became a black hole, its gravity would be different only near the event horizon. Black holes don’t suck! Insert TCP 6e Figure 18.12

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

30. Time passes more slowly near the event horizon.

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

32. Do black holes really exist?

33. Black Hole Verification We need to measure mass by: —Using orbital properties of a companion —Measuring the 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 (~3M Sun )

34. Some X-ray binaries contain compact objects of mass exceeding 3M Sun, which are likely to be black holes.

35. One famous X-ray binary with a likely black hole is in the constellation Cygnus.

36. What have we learned? How does a star’s mass affect nuclear fusion? – A star’s mass determines its core pressure and temperature and therefore determines its fusion rate. – Higher mass stars have hotter cores, faster fusion rates, greater luminosities, and shorter lifetimes.

37. What have we learned? What are the life stages of a low-mass star? – Hydrogen fusion in core (main sequence) – Hydrogen fusion in shell around contracting core (red giant) – Helium fusion in core (horizontal branch) – Double shell burning (red giant) How does a low-mass star die? – Ejection of hydrogen and helium in a planetary nebula leaves behind an inert white dwarf.

38. What have we learned? What are the life stages of a high-mass star? – They are similar to the life stages of a low- mass star. How do high-mass stars make the elements necessary for life? – Higher masses produce higher core temperatures that enable fusion of heavier elements. How does a high-mass star die? – Its iron core collapses, leading to a supernova.

39. What have we learned? How does a star’s mass determine its life story? – Mass determines how high a star’s core temperature can rise and therefore determines how quickly a star uses its fuel and what kinds of elements it can make. How are the lives of stars with close companions different? – Stars with close companions can exchange mass, altering the usual life stories of stars.

40. What have we learned? What is a black hole? – A black hole is a massive object whose radius is so small that the escape velocity exceeds the speed of light. What would it be like to visit a black hole? – You can orbit a black hole like any other object of the same mass—black holes don’t suck! – Near the event horizon, time slows down and tidal forces are very strong.

41. What have we learned? Do black holes really exist? – Some X-ray binaries contain compact objects too massive to be neutron stars—they are almost certainly black holes.

42. What have we learned? Where do gamma-ray bursts come from? – Most gamma-ray bursts come from distant galaxies. – They must be among the most powerful explosions in the universe, probably signifying the formation of black holes. What causes gamma-ray bursts? – At least some gamma-ray bursts come from supernova explosions.