Question 1

Suppose an observer were far from a planet that had a very strong gravitational field, and a light wave reaches the observer from a source of hydrogen (H $\alpha$ ) light on the surface of the planet. For an H $\alpha$  light source in the observer's own spaceship, the wavelength is 656.3 nm. What wavelength does the observer see when looking at the light source on the planet?

Accepted Answer

A)  shorter than 656.3 nm 
B)  longer than 656.3 nm 
C)  infinite, since the source is in a gravitational field 
D)  656.3 nm, the same as from the spaceship's light source

Question 2

Which of these statements correctly describes "cosmic censorship"?

Accepted Answer

A)  Black holes cannot have magnetic fields. 
B)  All properties of the matter inside a black hole are hidden by the event horizon, except for the total mass of the matter. 
C)  The amount of mass in a black hole can never be measured. 
D)  Nothing can leave a local region of space that contains a singularity.

Question 3

One prominent feature recently identified within many energetic close binary star systems as a result of their mutual interaction and mass exchange is

Accepted Answer

A)  two oppositely directed high-speed jets of matter leaving the system. 
B)  planetary formation between the stars, emitting IR radiation from molecular constituents and dust. 
C)  the beginnings of spiral arms, showing the possible origin of spiral galaxies. 
D)  a cool dust cloud surrounding the whole star system, hiding it from visible view.

Question 4

The formation of supermassive black holes

Accepted Answer

A)  occurred only during the Big Bang. 
B)  began during the early era that saw the formation of galaxies. 
C)  occurred at the instant dark energy became dominant in the universe. 
D)  only occurs in globular clusters.

Question 5

What is the event horizon of a black hole?

Accepted Answer

A)  "surface" at which any object passing through it will leave with greater energy than when it entered 
B)  "surface" at which all events happen 
C)  infinitesimally small volume at the center of the black hole that contains all of the black hole's mass 
D)  "surface" from inside of which nothing can escape

Question 6

According to Einstein's theory of general relativity, if one watches a clock from a distant location as it is moved closer to a source of gravity, one will see the clock

Accepted Answer

A)  maintain the same rate because time is unaffected by gravity. 
B)  change its rate if it is moving rapidly but maintain its standard rate if it is stationary in a gravity field. 
C)  slow down. 
D)  run faster.

Question 7

Why do astronomers seek to measure gravitational waves simultaneously with at least three detectors?

Accepted Answer

A)  in order to measure their amplitude 
B)  because the waves evolve as they pass through Earth 
C)  because the waves have wavelengths much larger than Earth 
D)  in order to pinpoint the direction of their source

Question 8

When particle-antiparticle pairs are created just outside the event horizon of a black hole, one member can escape while the other enters the black hole. What is the name of this stream of particles leaving a black hole?

Accepted Answer

A)  Schwarzschild radiation 
B)  Kerr radiation 
C)  Planck radiation 
D)  Hawking radiation

Question 9

Suppose an astronaut is aboard a spaceship that is passing Earth at 80% of the speed of light. The astronaut sees a clock on Earth tick off 5 seconds. How much time elapses on the astronaut's own clock while this is happening?

Accepted Answer

A)  5 seconds-the same as on the ship's clock 
B)  more than 5 seconds 
C)  more than 5 seconds if the astronaut is approaching Earth and less than 5 seconds if the astronaut is moving away from Earth 
D)  less than 5 seconds

Question 10

What is believed to be the maximum mass for a neutron star?

Accepted Answer

A)  150 solar masses 
B)  12 solar masses 
C)  1.4 solar mass 
D)  3 solar masses

Question 11

What is believed to be the solar mass of the black hole candidate at the center of the galaxy M87?

Accepted Answer

A)  3 million 
B)  3 billion 
C)  300,000 
D)  300

Question 12

The Schwarzschild radius is

Accepted Answer

A)  half the diameter of the singularity in a black hole. 
B)  the distance to which gas is ejected in a planetary nebula. 
C)  half the diameter of a neutron star. 
D)  the distance from the center of a black hole to the point at which the escape velocity becomes equal to the speed of light.

Question 13

How does the diameter of a black hole (size of the event horizon) depend on the mass inside the black hole?

Accepted Answer

A)  The diameter does not depend on the mass. 
B)  The greater the mass, the greater is the diameter-until the mass becomes relatively large and then the diameter decreases with increasing mass. 
C)  The greater the mass, the greater is the diameter. 
D)  The greater the mass, the smaller is the diameter.

Question 14

One day, while straying dangerously close to a black hole, an astronaut notices that no matter how hard he tries to remain at rest, he is inevitably drawn into the black hole unless he keeps moving. What does this tell an observer about the black hole?

Accepted Answer

A)  The black hole is evaporating. 
B)  The black hole is supermassive. 
C)  The black hole is electrically charged. 
D)  The black hole is rotating.

Question 15

Suppose a virtual particle-antiparticle pair appears just outside the event horizon of a black hole, and that each particle is the same distance from the center of the black hole. What will happen in this case?

Accepted Answer

A)  Gravity from the black hole will attract the particle and repel the antiparticle, thus separating them. They will become real, and neither of them needs to enter the black hole. 
B)  Gravity from the black hole will attract the particle and repel the antiparticle, thus separating them. They will become real, and both of them will enter the black hole. 
C)  Gravity from the black hole will attract the particle and repel the antiparticle, thus separating them. They will become real, and the particle will enter the black hole. 
D)  The gravitational force from the black hole will cause the particles to move toward each other, and they will recombine without affecting the black hole.

Question 16

Where should an astronomer look for a supermassive black hole?

Accepted Answer

A)  in the center of a galaxy 
B)  at the center of the universe 
C)  orbiting a normal star in the Milky Way Galaxy 
D)  at the center of a supernova remnant

Question 17

Which of these sources sometimes has an afterglow visible at optical and radio wavelengths?

Accepted Answer

A)  gamma-ray burst 
B)  pulsar 
C)  coronal mass ejection 
D)  nova

Question 18

Which of these has NOT been identified as the progenitor of at least some gamma-ray bursts?

Accepted Answer

A)  magnetar 
B)  a collision of two neutron stars 
C)  Wolf-Rayet star 
D)  a collision of two black holes

Question 19

In the context of black holes, what is a wormhole?

Accepted Answer

A)  "tunnel" of undistorted space through an event horizon allowing objects to enter and leave a black hole without being torn apart 
B)  direct connection from any black hole to another part of spacetime 
C)  small, black hole through a solid object such as a planet 
D)  direct connection from a rotating black hole to another part of spacetime

Question 20

Where is the event horizon of a black hole located?

Accepted Answer

A)  position of maximum X-ray emission 
B)  singularity 
C)  outer surface of the ergoregion 
D)  Schwarzschild radius away from its center

Exam 15: Black Holes: Matters of Gravity