Chapter 13 The Bizarre Stellar Graveyard

1. Chapter 13The Bizarre Stellar Graveyard

2. 13.1 White Dwarfs • Our Goals for Learning • What is a white dwarf? • What can happen to a white dwarf in a close binary system?

3. What is a white dwarf?

4. White dwarfs are the cores of dead stars exposed after they shed their outer layers in planetary nebulae. Which type of stars turn into white dwarfs at the end of their lives? What are the white dwarfs made of? Sirius A, the brightest star on our sky, and Sirius B, its binary companion, which is a white dwarf star. X ray image (which of the two stars is more luminous in X ray?).

5. What supports them from collapsing under the pressure of gravity? White dwarfs cool off and grow dimmer with time.

6. What supports them from collapsing under the pressure of gravity? Electron degeneracy pressure. Because of the laws of Quantum Mechanics, electrons can be compressed only up to a some point. Compressing them, one makes them move faster and faster. White dwarfs cool off and grow dimmer with time.

7. A white dwarf of Suns mass is about the same size as Earth. And the earth is of the size of a typical Sun Spot – matter in white dwarfs is very dense. One teaspoon of this matter would weigh several tons on earth.

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

9. 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?

10. 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. 1.4 Msun is the maximal mass of white dwarf! S. Chandrasekhar

11. What can happen to a white dwarf in a close binary system?

12. In a close binary system, gas from a companion star can spill toward a white dwarf, forming a swirling accretion disk around it.

13. Hydrogen that accretes onto a white dwarf star builds up in a shell on the surface. When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova.

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

15. 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

16. What have we learned? What is a white dwarf? • A white dwarf is the core left over from a low-mass star, supported against the crush of gravity by electron degeneracy pressure. • What can happen to a white dwarf in a close binary system? • A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk. As hydrogen builds up on the white dwarf’s surface, it may ignite with nuclear fusion to make a nova.

17. 13.2 Neutron Stars • Our Goals for Learning • What is a neutron star? • How were neutron stars discovered? • What can happen to a neutron star in a close binary system?

18. What is a neutron star?

19. A neutron star is the ball of neutrons left behind by a massive-star supernova What supports a neutron star against gravity?

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

21. A neutron star is about the same size as a small city (10 km in size). Neutron stars are essentially giant atomic nuclei made almost entirely of neutrons and held together by gravity.

22. How were neutron stars discovered?

23. The first neutron star was discovered by Bell Burnell in 1967. • Using a radio telescope she noticed very regular pulses of radio emission coming from a single part of the sky.

24. Pulsars are neutron stars that give off very regular pulses of radiation.

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

26. Pulsars are rotating neutron stars that act like lighthouses Beams of radiation coming from poles look like pulses as they sweep by Earth

27. A pulsar’s rotation is not aligned with magnetic poles. Neutron stars rotate fast (why?). They also have strong magnetic fields. The collapse of an iron core bunches the magnetic field lines running through the core far more tightly, which intensifies the magnetic fields. What is the main cause of magnetic field? What could it be in a neutron star?

28. Why Pulsars must be Neutron Stars Circumference of NS = 2π (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!

29. Pulsars spin fast because core’s spin speeds up as it collapses into neutron star Conservation of angular momentum

30. Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No

31. Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No

32. What happens to a neutron star in a close binary system?

33. Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary

34. Accreting matter adds angular momentum to a neutron star, increasing its spin Episodes of fusion on the surface lead to X-ray bursts

35. Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? • The neutron star’s rotation would speed up. • The neutron star’s rotation would slow down. • Nothing, the directions would cancel each other out.

36. Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? • The neutron star’s rotation would speed up. • The neutron star’s rotation would slow down. • Nothing, the directions would cancel each other out.

37. Thought Question If you dropped a mountain onto the surface of a neutron star, what would happen? • It would heat up (conservation of energy) • It would flatten out (gravitational compression) • It would bounce off. • It would turn into pure neutrons and emit neutrinos.

38. Thought Question If you dropped a mountain onto the surface of a neutron star, what would happen? • It would heat up (conservation of energy) • It would flatten out (gravitational compression) • It would bounce off. • It would turn into pure neutrons and emit neutrinos.

39. What have we learned? What is a neutron star? • A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova. • How were neutron stars discovered? • Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the existence of neutron stars.

40. What have we learned? • What can happen to a neutron star in a close binary system? • Neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot accretion disks. The hot gas emits strongly in X rays, so we see these systems as X-ray binaries. In some of these systems, frequent bursts of helium fusion ignite on the neutron star’s surface, emitting X-ray bursts.

41. 13.3 Black Holes: Gravity’s Ultimate Victory • Our Goals for Learning • What is a black hole? • What would it be like to visit a black hole? • Do black holes really exist?

42. What is a black hole?

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

44. Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same

45. Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same Hint:

46. Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases B. It decreases C. It stays the same Hint:

47. Escape Velocity Initial Kinetic Energy Final Gravitational Potential Energy = (escape velocity)2 G x (mass) = 2 (radius)

48. Light would not be able to escape Earth’s surface if you could shrink it to < 1 cm

49. 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.

50. A black hole’s mass (or any other mass according to Einstein!) warps space and time in vicinity of event horizon. We use the term hole, because nothing exits from it – it is really a hole in the observable universe.