1. 1. black hole - region of space where the pull of gravity is so great that even light cannot escape. Possible end of a very massive star.

2. 2. event horizon - imaginary spherical surface surrounding a collapsing star within which no event can be detected by an outside observer

3. 3. gamma-ray burster - object that radiates huge amounts of energy in the form of gamma rays, believed to be due to the accretion of matter onto a neutron star.

4. 4. gravitational red shift - A prediction of Einstein’s theory of relativity. Photons lose energy when they escape the huge gravity of a massive object. This energy loss decreases the frequency, correspondingly increasing the wavelength.

5. 5. lighthouse model - Leading explanation for pulsars. Region near magnetic pole of pulsar emits a steady stream of radiation which sweeps past Earth each time the star rotates.

6. 6. millisecond pulsar - Pulsar with a period indicating that the neutron start is rotating nearly 1000 times a second. Probably ‘spun up’ by drawing in matter from a companion star.

7. 7. neutron star - Dense ball of neutrons that remains at core of star after a supernova explosion has destroyed the rest of the star.

8. 8. photon sphere - Imaginary sphere surrounding a black hole at 1.5 times the Schwarzschild radius where photons move in a circular orbit.

9. 9. Principle of Cosmic Censorship - Nature always hide any singularity, such as a black hole, inside an event horizon, which prevents the universe from seeing inside a singularity.

10. 10. pulsar - Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Charged particles flow along the magnetic field lines of a rapidly rotating neutron star, producing a beam of radiation.

11. 11. pulsar glitch - Small drop in teh period of a pulsar. Occur when teh thin crust on a neutron stars surface cracks and settles slightly atop the superfluid interior.

12. 12. Schwarzschild radius - Distance from the center of an object such that, if all the mass were compressed within that region, the escape velocity would equal the speed of light. An object smaller than this radius is a black hole.

13. 13. singularity - A point in the universe where the density of matter and the gravitational field are infinite, such as at the center of a black hole.

14. 14. starquake - A cracking and resettling of the crust of a neutron star, slightly reducing the radius of the star. The spin rate increases, causing a pulsar glitch.

15. 15. superconductor - A material that conducts electricity without resistance. The interior of a neutron star may be a superconductive material.

16. 16. superfluid - A material that flows without friction. The interior of a neutron star may be a superfluid material.

17. 17. theory of relativity - Einstein’s theory, forms the basis of most of modern physics. Two essential facts: 1) nothing travels faster than light and 2) everything is affected by gravity.

18. 18. time dilation - To an outside observer, a clock lowered into a strong gravitational field will appear to run slow. A prediction of the theory of relativity.

19. 19. x-ray burster - A neutron star accretes matter onto its surface until temperatures reach the level needed to cause hydrogen fusion. Results in a sudden period of rapid nuclear burning.

20. 1. How does the way in which a neutron star forms determine some of its basic properties? The amount of matter and amount of collapse determines the spin rate of the neutron star. This determines its pulse rate and pulse period if it forms a pulsar.

21. 2. How can we observe objects as small as neutron stars? By observing pulsars.

22. 3. Why arent all neutron stars seen as pulsars? The beam that is emitted by the neutron star must pass over the Earth to be detected as a pulsar.

23. 4. What are x-ray bursters? A neutron binary that accretes material from its companion. As the gas builds up, it eventually becomes hot enough to fuse hydrogen. At this point a sudden period of rapid nuclear burning releases a great deal of energy in a brief, intense flash of x-rays.

24. 5. What is the favored explanation for the fast spin rates of millisecond pulsars? That the neutron star has been ‘spun up’ by drawing in matter from a companion star.

25. 6. How might some neutron stars have come to be members of binary systems? The supernova progenitor must have lost a lot of mass before the explosion, or a neutron star may displace one member of a preexisting binary system.

26. 7. How might a pulsar acquire planets? One possibility is that the binary companion may have been destroyed by the energy and gravity of the neutron star. The resulting matter may have cooled into planets.

27. 8. Use your knowledge of escape velocity to explain why black holes are said to be ‘black’? A black hole is so collapsed that the escape velocity has exceeded the speed of light. Therefore, nothing, not even light, can escape.

28. 9. Why is it so difficult to test the predictions of general relativity? Because its effects on the Earth and the solar system are so small.

29. 10. Describe two tests of general relativity. Light from a star is deflected as it passes close to the Sun. Planetary orbits deviate from the perfect ellipses of Kepler’s laws.

30. 11. What would happen to someone falling into a black hole? Why? The person would be vertically stretched and horizontally squeezed. They would be literally torn to pieces. The tidal forces are very different even over the length of a body. Spaghettification.

31. 12. Explain how a black hole can evaporate? I don’t want to go into this. It was suggested by Stephen Hawking years ago, but now even he doesn’t believe it occurs

32. 13. What makes Cygnus X-1 a good black hole candidate? It releases x-rays, probably from an accretion disk. The unseen companion exceeds the mass normally accepted for neutron stars, which means it is probably a black hole.