Chapter 13

1) In a neutron star, the protons and electrons are fused together, leaving only neutrons.

2) Neutron stars are 100,000 times denser than white dwarfs.

3) Stars of less than 8 solar masses will not go supernova.

4) A neutron star is what remains after a Type II supernova explosion has destroyed the rest of the star.

5) In a neutron star, the electrons in the core are all in contact with each other.

6) Newly-formed neutron stars start with weak magnetic fields, but they strengthen over time into pulsars.

7) The ʺpulseʺ from a pulsar is due to the rapidly expanding and contracting outer shell of the star.

8) Most pulsars are known only as radio sources, but a few of the younger ones, like the Crab and Vela, also give off visible and even X-ray energy.

9) Pulsars are created in a Type I supernova.

10) The pulses from a pulsar are most likely coming from localized areas near the magnetic poles.

11) All neutron stars seen from Earth are pulsars.

12) Planet-sized bodies have been detected around pulsars.

13) While most pulsars slow down over time, millisecond pulsars spin faster due to mass transfer from a close companion.

14) No planets have ever been found around any pulsars.

15) Planets similar to our terrestrials have been found orbiting pulsars.

16) Millisecond pulsars are never members of close binary systems.

17) An X-ray burster is similar to a nova.

18) A hypernova is a gamma-ray burster that forms a black hole as well.

19) Gamma-ray bursters are all found to be within our galaxy.

20) Gamma-ray bursters seem scattered randomly over the entire sky.

21) No optical traces have been found of the gamma-ray bursters.

22) All the gamma-ray bursters measured to date have distances of millions or even billions of light years.

23) All gamma-ray bursters have the same fundamental driving mechanism, although we donʹt yet know whether it is a hypernova or a merger of two neutrons stars.

24) Some gamma-ray bursters may be due to the merger of a neutron star and a black hole.

25) Only high-energy gamma rays can escape the event horizon of a black hole.

26) Any main-sequence star over 25 solar masses will probably retain enough matter in its core after its supernova to make a black hole.

27) The escape speed at the event horizon of a black hole is c, 300,000 km/sec.

28) All Type II supernovae produce neutron stars when they collapse.

29) The Schwarzschild radius of a black hole is about 3 km per solar mass.

30) Special relativity says that c, the speed of light, is the maximum velocity for both matter and energy in our universe.

31) Both space and time are warped near the strong gravitational fields of neutron stars and black holes.

32) No communication is possible across an event horizon.

33) Einsteinʹs prediction of the curvature of space was confirmed by the 1919 total solar eclipse.

34) Because of their immense size, black holes moving through space consume huge quantities of interstellar matter along their paths.

35) Even if a probe were in a stable orbit near a black hole, we would still observe its signals red shifted by the gravitational field of the black hole.

36) The presently known laws of physics clearly describe the conditions inside a black holeʹs event horizon.

37) The closer the observer comes to the event horizon, the slower his watch will appear to run to a distant observer.

38) In a neutron star, the core is A) made of compressed neutrons in contact with each other. B) electrons and protons packed so tightly they are in contact. C) constantly expanding and contracting. D) primarily iron and silicon. E) no longer rotating.

39) Two important properties of young neutron stars are A) extremely slow rotation and a strong magnetic field. B) extremely rapid rotation and a weak magnetic field. C) extremely rapid rotation and a strong magnetic field. D) no rotation and a weak magnetic field. E) no rotation and no magnetic field.

40) An object more massive than the Sun, but roughly the size of a city, is a A) black dwarf. B) white dwarf. C) brown dwarf. D) neutron star. E) red dwarf.

41) The mass range for neutron stars is A) 0.08 to 0.4 solar masses. B) 0.4 to 3 solar masses. C) 1.4 to 3 solar masses. D) 3 to 8 solar masses. E) 6 to 11 solar masses.

42) Which of these does not exist? A) a million solar mass black hole B) a 6 solar mass black hole C) a 1.8 solar mass neutron star D) a 1.5 solar mass white dwarf E) a 0.06 solar mass brown dwarf

43) Most pulsars are observed only as ________ sources. A) gamma-ray burster B) visible lighthouse C) ultraviolet repeating D) radio E) X-ray

44) Neutron stars have A) very strong bipolar magnetic fields. B) weak or non-existent magnetic fields. C) periods of days or weeks. D) monopolar fields that switch polarity every rotation. E) no relation to pulsars.

45) What makes the Crab Nebula pulsar unusual among other pulsars? A) It is the fastest spinning known pulsar. B) It is a magnetar, with far more intense magnetic fields than any other. C) It is relatively bright in shorter wavelengths, like visible and X-rays. D) Its period is much less regular than other pulsars. E) It is the oldest known pulsar.

46) Who discovered the first four pulsars? A) Anthony Hewish B) Stephen Hawking C) Carl Sagan D) Jocelyn Bell E) Martin Schwartzschild

47) Pulsars A) spin very rapidly when theyʹre young. B) are the cause of gamma-ray bursts. C) spin very slowly when theyʹre young, and gradually spin faster as they age. D) generally form from 25-solar-mass stars. E) emit radio in all directions.

48) In the Lighthouse Model, A) the star literally turns on and off like a lighthouse beacon. B) all pulsars must have their poles pointed directly toward us. C) if the beam sweeps across us, we will detect a pulse of radiation. D) the period of pulsation must speed up as the neutron star continues collapsing. E) the period of pulsation slows down due to the drag of the remnant on its field.

49) Neutron stars do not have A) masses greater than 1.4 solar masses. B) sizes comparable to large cities. C) strong magnetic fields. D) large surface gravities, compared to the Sun. E) rotation periods comparable to the Sunʹs.

50) The supernova of 1054 AD produced A) a remnant still visible to the naked eye. B) a pulsar with a period of 33 milliseconds, visible optically. C) the closest known neutron star to our Sun. D) the most famous black hole. E) no remaining visible trace, as it was a Type I supernova.

51) Three terrestrial-sized planets in orbits of a fraction of an AU have been found near A) Cygnus X-1. B) a magnetar. C) Supernova 1987A. D) a millisecond pulsar. E) a white dwarf.

52) Two-thirds of all known millisecond pulsars are found in what type of object? A) giant molecular clouds B) open clusters C) globular clusters D) emission nebulae E) extremely distant galaxies

53) X-ray bursters occur in binary star systems. The two types of stars that must be present to make up such an object are A) a white dwarf and a neutron star. B) a contact binary system of two red giants. C) a white dwarf and a main-sequence star. D) a main-sequence or giant star and a neutron star in a mass transfer binary. E) two neutron stars in a mass transfer binary.

54) Most pulsars have a measured mass of A) about 1.4 solar masses. B) less than 1.0 solar masses. C) between 2 and 4 solar masses. D) 5.2 solar masses. E) greater than 10 solar masses.

55) You would expect millisecond pulsars to be A) part of a binary system. B) isolated in space. C) rotating slowly. D) most common in open clusters. E) collapsing rapidly.

56) A proposed explanation for gamma-ray bursters is A) hypernova-making black holes and bipolar jets. B) coalescence of a neutron star binary. C) collisions between two white dwarfs. D) Both A and B are possible. E) All three are possible.

57) What would happen if more mass was added to a 1.4 -solar-mass neutron star? A) It would erupt as a Type I supernova. B) It could eventually become a black hole, via a hypernova explosion. C) It would grow larger, temporarily becoming a red giant again. D) All of its protons and electrons would turn into quarks. E) It would blow off mass as an X-ray burster.

58) In a hypernova, a very energetic supernova creates a A) very visible supernova remnant. B) millisecond pulsar. C) set of planets to orbit their neutron star host. D) black hole. E) white dwarf and its planetary nebula.

59) A hypernova creates A) a black hole. B) a pulsar. C) a neutron star. D) millisecond-duration gamma-ray bursts. E) Both B and C are correct.

60) Which statement about gamma ray bursters is not correct? A) They seem to be coming from far beyond our own Milky Way. B) They are scaled up X-ray bursters, with more massive objects involved. C) In seconds, they radiate enormous amounts of energy. D) Millisecond flickering implies they are tiny in size. E) The beams may be bi-polar ejections from the hypernova formation of black holes.

61) Which are the two most popular candidates for gamma-ray bursters? A) hypernova making pulsars, and mergers of two white dwarfs B) mergers of two black holes, and merger of a neutron star and a white dwarf C) hypernova making a black hole, and merger of two neutron stars D) collisions between a white dwarf and a giant, and merger of two neutron stars E) formation of uranium in the core of a supergiant, and collisions of white dwarfs

62) Which statement about black holes is true? A) Their escape velocity is greater than the speed of light. B) They form from 1.4 solar mass stars. C) They form an event horizon at twice the Schwartzschild radius. D) Their main-sequence mass was 5-10 solar masses. E) Their event horizon is a physical surface boundary.

63) The densely packed neutrons of a neutron star cannot balance the inward pull of gravity if the total mass is A) less than 1.0 solar masses. B) greater than Schwartzschildʹs limit of 3 solar masses. C) Chandrasekharʹs limit of 1.4 solar masses. D) between 1.4 and 2.0 solar masses. E) not at least 25 solar masses.

64) The Schwartzschild radius for a 12 solar mass star is A) 4 km. B) 15 km. C) 36 km. D) 100 km. E) 3000 km.

65) What explanation does general relativity provide for gravity? A) Gravity is the result of curved spacetime. B) Gravity is directly proportional to the mass of the attracting body. C) Gravity is inversely proportional to the radius of the body. D) Gravity is the opposite of the electromagnetic force. E) Gravity can affect only massive particles, not massless photons.

66) An observer on a planet sees a spaceship approaching at 0.5c. A beam of light projected by the ship would be measured by this observer to travel at A) 0.25c. B) 0.5c. C) c. D) 1.5c. E) 2.5c.

67) As a spaceshipʹs velocity gets closer to the speed of light with respect to an external observer, that observer will report that A) its length will increase and its clock will run more slowly. B) its length will decrease and its clock will run faster. C) its length will increase and its clock will run faster. D) its length will decrease and its clock will run more slowly. E) None of these will happen.

68) If light from a distant star passes close to a massive body, the light beam will A) bend towards the star due to gravity. B) continue moving in a straight line. C) change color to a shorter wavelength. D) slow down. E) accelerate due to gravity.

69) What can we detect from matter that has crossed an event horizon? A) gamma-ray bursts B) radio waves if the matter was traveling fast enough C) visible light D) X-rays if the matter was dense E) nothing

70) As a spaceship nears an event horizon, a clock on the spaceship will be observed A) to run faster. B) to stop. C) to run slowly. D) to run backwards. E) to run the same as one on Earth.

71) If the Sun were replaced by a one-solar-mass black hole, A) all terrestrial planets would fall in immediately. B) we would still orbit it in a period of one year. C) we would immediately escape into deep space, driven out by its radiation. D) our clocks would all stop. E) life here would be unchanged.

72) What is Cygnus X-1? A) the brightest star in the constellation Cygnus B) the leading candidate for an observable black hole binary system C) the strongest X-ray eclipsing binary system in the sky D) a millisecond pulsar with three Earth-like planets around it E) the first gamma-ray burster to be spotted in other wavelengths as well

73) A method for identifying a black hole is to A) look for voids in the star fields. B) look for its effects on a nearby companion. C) locate a visible star that disappears when the black hole passes in front of it. D) search for radio waves from the accretion disk. E) search for their pulsar signal.

74) The largest known black holes A) create the dark nebulae in the plane of the Milky Way. B) can be no more than 1.4 solar masses, according to Chandrasekhar. C) lie in the cores of the most massive galaxies. D) can be no bigger than a small city, just like neutron stars. E) can be no bigger than the Earth, like white dwarfs.

75) Which of these is not an argument for Cygnus X-1 being a black hole? A) It is the third strongest source of X-rays in the sky. B) Spectroscopic data suggests hot gas is flowing toward the X-ray source. C) The mass of the visible star is greater than that of the X-ray source. D) The mass of the X-ray source is about 10 solar masses. E) The X-rays from the compact source vary in as little as a millisecond.