1. Chapter 13 Neutron Stars and Black Holes

2. Optical, Infrared and X-ray Image of Cassiopeia A

3. Neutron Stars Type I supernovae - star destroyed Type II supernovae (core collapse) - part of core could survive Ball of neutrons remain - neutron star 20 km or so across (size of major city) Mass greater than sun Density 10 17 to 10 18 kg/m 3 Thimbleful would weigh 100 million tons 150 lb human on surface would weigh 1 million tons

4. Figure 13.1 Neutron Star

5. Rotation and Magnetism Newly formed neutron star rotates rapidly Period of fraction of a second Strong magnetic field Trillions of times stronger than earth’s Collapsing core concentrates magnetic field of original star

6. Predicted by theory Neutron stars (and black holes) predicted by theory before discovered Fast rotation and strong magnetic fields allowed them to be detected

7. Pulsars Graduate student Jocelyn Bell at Cambridge University discovered rapid radio pulses in 1967 More than 1500 now known, as pulsars Bell’s thesis advisor, Anthony Hewish, reasoned it was a small rotating source of radiation

8. Figure 13.2 Pulsar Radiation

9. Pulsar explanation Accelerated charged particles near magnetic poles of neutron star Emit radiation along magnetic axis Rotation and magnetic axes not aligned Beam sweeps through space - lighthouse model

10. Figure 13.3 Pulsar Model

11. Analogy 13.1 A lighthouse beacon

12. Figure 13.4 Crab Pulsar

13. Pulsar radiation Most pulsars emit radio wavelengths Some emit visible, X- and gamma-rays

14. Figure 13.5 Gamma-Ray Pulsars

15. Pulsars and Neutron stars All pulsars are neutron stars Not all neutron stars are pulsars: Must be aligned just right to be visible from earth Lose rapid rotation and magnetic field over tens of millions of years

16. Figure 13.6 Isolated Neutron Star

17. Neutron star binaries Some neutron star binaries Can measure mass - close to 1.4 M 

18. X-ray bursters Neutron star tears matter from surface of binary companion Accretion disk heats up - emits X-rays Heats enough to fuse H - rapid burst of burning Similar to nova, but much more violent

19. Figure 13.7 X-Ray Burster

20. Millisecond pulsars Spin hundreds of times per second Period is several milliseconds Mass of sun, several km in size, spinning almost 1000 times per second Many are old (and should be slow) Are spun-up by infalling matter from binary companion

21. Figure 13.8 Millisecond Pulsar

22. Figure 13.9 Cluster X-Ray Binaries

23. Pulsar planets Several millisecond pulsars have variations in their periods Explained by Doppler shift due to interaction with orbiting planets Planets captured or formed from debris of companion star

24. Gamma-Ray bursts Gamma-rays are very high energy photons Bursts first discovered in 1960’s by military satellites Made public in 1970’s Bright irregular bursts lasting few seconds Compton Gamma-Ray Observatory - CGRO

25. Figure 13.10 Gamma-Ray Bursts

26. Gamma-Ray burst distances Some optical counterparts measured Distances of billions of parsecs If energy emitted in all directions, then hundreds of times more energetic than supernovae, all in seconds

27. Figure 13.11 Gamma-Ray Burst Counterpart

28. Gamma-Ray burst explanation millisecond variation in intensity Light travels 300 km in 1 millisecond Must be small Relativistic fireball - gases traveling at speeds approaching speed of light Probably jets

29. Two GRB models Merging stars - binary pair of neutron stars merge Hypernova - collapsing star forming black hole High temperature accretion disk in both cases Relativistic outflow, perhaps in jets

30. Figure 13.12 Gamma-Ray Burst Models

31. Figure 13.13 Hypernova?

32. Gravity waves Predicted by Einstein’s theory of gravity Very difficult to detect - wave amplitude smaller than atomic nucleus Most likely candidates: Merger of binary stars Collapse of star into a black hole

33. Discovery 13.1 Gravity Waves—A New Window on the Universe

34. Black holes Neutron stars can exist up to about 3 M  Above that, even tightly packed neutrons can’t prevent further gravitational collapse Any main-sequence star above 25 M  will collapse beyond neutron star Gravity so great not even light escapes

35. Two key facts from Einstein’s Relativity Nothing can travel faster than the speed of light, 300,000 km/s Even light is attracted by gravity

36. Escape speed How fast do you have to toss an object upward so that it never falls back? 11 km/s ignoring the atmosphere Depends on mass and radius of earth If crushed earth to 1 cm radius, escape speed would be 300,000 km/s so not even light would escape - black hole

37. Schwarzschild radius Radius for a given mass at which escape speed is speed of light 1 cm for mass of earth 3 km for mass of sun 9 km for 3 M  stellar core Surface of sphere of this radius is called event horizon

38. Figure 13.14 Speed of Light

39. More Precisely 13-1.1 Special Relativity

40. More Precisely 13-1.2 Special Relativity

41. Figure 13.15 Einstein’s Elevator

42. Gravity and Curved Space General relativity predicts gravity curves or warps space Objects follow curvature of space At black hole, curvature so great that space folds over and closes off Can picture as a rubber sheet

43. Figure 13.16 Curved Space

44. Figure 13.17 Space Warping

45. Tests of General Relativity Deflection of starlight by gravity Observable during solar eclipse Planetary orbits should deviate from ellipses Greatest for Mercury

46. More Precisely 13-2.1 Tests of General Relativity

47. More Precisely 13-2.2 Tests of General Relativity

48. Space travel near a black hole Beyond event horizon, objects orbit normally, and can escape Once inside event horizon, no escape Tidal forces very strong Objects ripped and stretched apart into pieces Frictional heating

49. Figure 13.18 Black Hole Heating

50. Figure 13.19 Robot-Astronaut

51. Robot probe with clock and light source As probe approaches event horizon, light from it more redshifted Gravitational redshift Redshift becomes infinite at event horizon Robot’s clock slows down as approaches event horizon - time dilation

52. Figure 13.20 Gravitational Redshift

53. Singularity General relativity predicts collapse to a point with infinite density - a singularity Probably incomplete theory Quantum gravity in future might properly explain

54. Black hole observational evidence Black holes in binary systems Large black holes at centers of galaxies (more in later chapters) Intermediate size black holes forming from tight clusters

55. Figure 13.21 Cygnus X-1 - likely black hole

56. Figure 13.22 Black Hole

57. Figure 13.23 Intermediate-Mass Black Holes?