Black Holes Quiz 3 (All PollEverywhere Qs + All HW Qs)

What are the main pieces of evidence we use to differentiate a black hole binary from a neutron star binary?

A- Black hole accretion disks emit in the X-rays but neutron star disks are cooler.
B- We assume it is likely to be a black hole if its mass is larger than 3 Msun
C- We assume it is a neutron star if it shows pulsations or thermonuclear bursts.

B and C

Which type of observations do astronomers worry will be most affected by satellite constellations?

A- Narrow-field observations that focus on a relatively small part of the sky.
B- Wide-field observations that focus on a relatively large part of the sky.
C- Observations taken from other satellites in orbit around the Earth.
D- Observations take late at night, well after the Sun is below the horizon.

B

A 60 Msun star has a luminosity of about 3 × 10^32Watts. The total energy available for fusion of H toHe is the solar value (1.3 × 10^45 Joules) multipliedby 20 (1/3 of the star's H). How long will this starlive?
a) 30 billion years
b) 3 billion years
c) 300 million years
d) 30 million years
e) 3 million years

e

If L is proportional to (=?) M^3.5, what happens to the luminosity if we increase M by a factor of 5?

a) increases by factor 17.5
b) decreases by factor of 17.5
c) increases by factor 79
d) increases by factor of 280
e) decreases by factor of 79

D
This is a strong dependence on mass. Modestincreases in mass lead to very large increases inluminosity.

If we confine a particle to a very small volume,
what happens to its velocity uncertainty? If we
force a particle to have a very specific velocity,
what happens to its position uncertainty?

a) Velocity uncertainty gets smaller. Position uncertainty gets smaller.
b) Velocity uncertainty gets larger. Position uncertainty gets smaller.
c) Velocity uncertainty gets smaller. Position uncertainty gets larger.
d) Velocity uncertainty gets larger. Position uncertainty gets larger.

D

A more massive white dwarf has stronger gravityand needs larger pressure support in its center.Since white dwarfs are supported by degeneracypressure, this means the centers of massive whitedwarfs must be what?
a) Denser and hotter
b) Hotter but not much denser
c) Denser but not much hotter
d) Neither denser nor hotter

C
Degeneracy pressure only depends on density. Temperature is notrelevant. This allows white dwarfs to exist for an extremely long time(possibly forever) even though they are gradually cooling and no newheat is supplied.

If the mass transferred to a white dwarf ever approaches the limit 1.4 solar masses, what happens?
a) The star can begin undergoing unstable nuclear reactionsand explode
b) The exploding star can emit as much light as many galaxiesfor several days
c) The star will form a black hole
d) All of the above.
e) a and b

E

If electron degeneracy is not able to support the
core of a massive star against gravity, what do you
suppose happens?
a) The core collapses and forms a black hole.
b) The core is supported by degeneracy pressure of neutrons.
c) The star will explode.
d) All of the above can happen.

D

How could we tell observationally if a supernova is a white dwarf
supernova or a massive star supernova?
a) White dwarf supernova leave no remnant but core collapse
supernova do.
b) Massive star supernova would show evidence of significant
hydrogen in their emission but white dwarf supernova would not.
c) Massive star supernova have much longer durations.
d) All of the above
e) Only a and b

E

At a density of 5 × 1017 kg/m3, what volume would one need to have a mass equivalent to the largest supertankers ≈ 500,000,000 kg
a) 1 mm3
b) 1 cm3
c) 10 cm3
d) 1 m3
e) 10 m3

A
These are incredibly huge densities - samedensity as the nucleus of an atom!

Assume you had an unimpeded view of a spherical neutron star, What fraction of the surface should you be able to see at any moment?
a) 50%
b) More than 50%
c) Less that 50%
d) None at all

B
Light can be bent around the neutron star, just like near a black hole

Your Asgardian friend wants you to come with him on his
journey to a neutron star to help him forge a giant axe a few
10s of kilometers away from the star. Is this a good idea?

a) Yes. Nothing bad would happen and you could help save the galaxy.
b) No. The light from the neutron star would be dangerous due to the large redshift.
c) No. The tidal forces from the neutron star's gravity would kill you if you got that close.
d) No. The time dilation effects you would encounter would cause you to age so slowly that everyone you know would be dead upon return.

C
Remember a neutron star is almost a small solar mass blackhole and solar mass sized blackholes have very strong tidal gravity forces

What happens to thermal radiation if you make the source hotter?
a) More energy comes out at all wavelengths
b) The peak of the spectrum (wavelength at which most energy
is emitted) shifts redward
c) The peak of the spectrum shifts blueward
d) a and b
e) a and c

E

What is the temperature of this star?
a) 600 K
b) 1000 K
c) 5000 K
d) 10,000 K
e) 50,000 K

C

When the Sun becomes a red giant both its luminosity and surface area will increase. Knowing that its color will be redder than its current color, what must be true?
a) Its surface area increases more than its luminosity
b) Its luminosity increases more than its surface area
c) Both its luminosity and surface area increase by the same
amount

A
A redder star must have a lower temperature and there for a lower flux. Since flux is luminosity / area, a lower area requires area to increase more than luminosity.

Young neutron stars are expected to have a temperature of 107 K. In what part of the electromagnetic spectrum should we see their thermal emission?
a) radio
b) infrared
c) ultraviolet
d) X-rays

D

If the beam from a spinning neutron star hit the Earth:
a) We would all die.
b) We would call it a pulsar.
c) We would call it a quasar.
d) We would call it a blazar

B

Why were pulsars first discovered in radio rather than in X-rays?
a) Most of their emission comes out in radio.
b) Astronomers did not think neutron stars would emit X-
ray radiation.
c) Sensitive radio telescopes existed before sensitive X-
ray telescopes.
d) All of the above.

C

So, if even a tiny amount of angular momentum makes it hard for gas to fall directly into the black hole, how does anything end up there?
a) It does not. Black holes don't grow much after they are born.
b) The angular momentum of the gas will slowly decrease with time because angular momentum is not a conserved quantity.
c) Gas can interact with itself so that some gas loses and some gains angular momentum, allowing a fraction of the gas to fall in.

C

What is needed for gas to accrete?
a) aliens
b) pressure
c) friction
d) light

C

So gas can fall into a black hole. Why does this matter?
a) Gas falling into a black hole will cause it to grow and start radiating Hawking radiation that can be observed.
b) Gas falling into a black hole loses gravitational potential energy and some of that can be radiated as light.
c) Gas falling into a black hole loses kinetic energy and of that can be radiated as light.
d) Gas clouds falling into a black hole becomes very dense due to tidal forces and forms stars that radiate intensely.

B

Why do we think common envelope phases must occur?
a) We see binaries that appear to be in a common envelope phase.
b) The physics of the common envelope phase is well understood from theory.
c) The final separation of some binaries with black holes
or neutron stars are smaller than the radii of the original star.
d) A common envelope is the most common outcome in binaries due angular momentum losses in stellar winds.

C

What do astronomers need to measure masses of black holes in binaries?
a) orbital period, distance, and temperature
b) orbital period, Doppler shift, and distance
c) orbital period, Doppler shift, and viewing angle
d) Doppler shift, viewing angle, and distance
e) Doppler shift, distance, and luminosity

C

Why would these accretion disks be so hot that their thermal emission is in X-rays?
a) large luminosity and large emitting area
b) large luminosity and small emitting area
c) small luminosity and large emitting area
d) small luminosity and small emitting area

B

Black hole spin affects all of the following aspects of accretion in black hole X-ray binaries. Based on what we've discussed, which is the most important to astronomers studying black holes?
a) Spin affects the structure of the black hole inside the event horizon.
b) Spin determines the effective inner edge of the black hole accretion disk (the ISCO).
c) Spin determines amount of Hawking radiation.
d) Spin affects the vertical thickness of the accretion disk.

B
This is somewhat controversial. Numerical simulations:inner edge near ISCO but not exactly at ISCO.

Based on the above criteria, which of the following would be strong sources of gravitational waves?
a) spinning black hole
b) spinning neutron star
c) spinning neutron star with a mountain on it
d) binary star system
Choose all that apply.

C & D

Rotating black hole?
No (axisymmetric). Same
argument applies to neutron star

Rotating neutron star with a
"mountain" on surface?
Yes. non-axisymmetric due to
mountain

Binary stars?
Yes. non-axisymmetric.
Particularly strong if v ≈ c

Review: Why is this particular neutron star binary containing a pulsar so useful to astronomers?
a) The pulsar is an accurate clock that allows us to measure small changes in the orbital period of the binary.
b) The pulsar is an accurate clock that allows to measure small changes in the neutron star spin period.
c) We can directly measure the gravitational waves produced by this binary.
d) The mass of one of the neutron stars in this system is above 3 Msun, placing important constraints on the theory of general relativity.

A

Why would merging black hole binaries be stronger sources of gravitational waves than merging stars of the same mass?
a) The black holes themselves are non-axisymmetric
b) The black holes are compact enough that they can
get very close to each other before merging.
c) Gravity far from the black holes is stronger.
d) Merging stars of the same mass would produce more gravitational waves, but this is rarer than black hole mergers.

B
Since the black holes can get closer to each other, their orbital velocities get much closer to c.

How does the LIGO interferometer work?
a) It measures changes in the speed of light as the gravitational wave passes
b) It measures changes in the distance between mirrors as the gravitational wave passes
c) It measures changes in the Earth's gravitational acceleration as the gravitational wave passes
d) It measures changes in the Earth's radius at two different detectors as the gravitational wave passes

B

Why build two LIGO detectors separated by thousands of kilometers?
a) So one detector could be operating while the other is switched off.
b) So that geographically local sources of noise that mimic gravitational waves could be ruled out if they showed up in only one detector.
c) Each detector works in different ways to test different technologies.
d) Gravitational waves are absorbed inside the Earth so we can see a larger fraction of sky with two sites.

B

Where did the energy of the gravitational waves come from?
a) The gravitational potential energy of the black holes as their orbit shrank
b) The orbital energy of the black holes as their orbit shrank
c) The rest mass energy of the black holes that was lost when they merged
d) All of the above

D
All of the above are energy sources for the
gravitational wave. In fact, in GR, the distinction
between these types of energy is not clear!

What provides the primary pressure support against gravity for a white dwarf?
A)Electron degeneracy pressure
B)Neutron degeneracy pressure
C)Proton degeneracy pressure
D)Thermal pressure

A
Electron degeneracy pressure is what supports white dwarfs.

What is the name of the concept that says fermions cannot occupy the same quantum state?
A)The equivalence principle
B)The cosmic censorship hypothesis
C)The uncertainty principle
D)The exclusion principle

D
The exclusion principle (sometime called the Pauli exclusion principle) states that two fermions cannot occupy the same quantum state.

What makes white dwarf supernovae useful to astronomers?
A)They are all thought to have approximately the same average luminosity so they can be used measure distances to very distant galaxies.
B)Their light curves are very consistant and can be used as accurate clocks.
C)They are much brighter than massive star supernovae and can be used to locate distant galaxies.
D)Modeling of their light curves provides a precise test of general relativity

A
The primary use of white dwarfs is to measure distances to very distant galaxies. This method was used to infer that the expansion of the universe is accelerating.

What sort of remnants do massive star supernova leave behind after they explode?
A)Black holes
B)Neutron stars
C)White dwafs
D)All of the above
E) Only A and B

E
Massive star supernovae lead to neutron stars and black holes. White dwarfs are produce by low mass stars after their planetary nebula phase.

Which of the following statements about the maximum mass of neutron stars is true?
A)The maximum mass is not thought to be much bigger than 3 solar masses
B)We know that the maximum mass is greater than 2 solar masses from observations
C)The precise value of the maximum mass is unknown due to our limited understanding of the stong force
D)All of the above are true
E)Only A and B are true.

D

See the lecture 24 slides for a discussion of all of these points.

How do the effects of general relativity affect the emission from neutron stars?
A)The emission is gravitationally redshifted
B)We can receive light from more than half the star because of the bending of light by gravity.
C)Most of the light cannot reach us because it is trapped by the neutron star's gravity
D)All of the above.
E)Only A and B

E
The first two effects are both present so both A and B are true. These should be familiar from our discussion of black holes and general relativity. Even light from Earth's surface is redshifted, but the effect is much larger in neutron stars - almost as large as the redshift at the ISCO of a black hole. Similarly, the gravity of the star acts as a lens in much the same way the black hole's gravity does. However, unlike a black hole, there is no event horizon so any light radiated from the surface can escape to infinity so c is false.

The first calculations of stellar collapse to form a black hole were carried out in 1939, but most physicists were skeptical. By the early 1960s, many more physicists had come around to the idea that black holes might actually happen in nature. What changed?
A)They found evidence of black holes behind after a supernovae explosion.
B)The discovery of pulsars showed that neutron stars existed, lending weight to the earlier work which predicted their existence.
C) Computers had advanced to the stage that one could compute the collapse with realistic physical models for pressure and thermodynamics.
D) All of the above.

C
In the early 1960s, clear evidence of black holes formed from the collapse of massive stars did not exist and neutron stars were not discovered until 1967. So, the main breakthrough in the early 60s was theoretical rather than observational. Computers had advanced significantly due to the strong motivation and large amounts of funding provided by the development of nuclear weapons.

Assume that a star radiates as a thermal (blackbody) emitter. If we keep the luminosity of the star fixed but quadruple its radius, what would happen to its temperature?
A)It will increase by a factor of 4
B)It will increase by a factor of 2
C)It will remain unchanged
D)It will decrease by a factor of 2
E)It will decrease by a factor of 4

D
Remember that L=F A and A=4piR2 so if we quadruple R, we increase A by a factor of 16. Since luminosity stays fixed and A increases, the flux F must decrease by a factor of 16. Finally, we use the fact that F = sigma T4 to infer that if flux decreases by a factor of 16 than T must decrease by a factor 161/4 or by a factor of 2.

Human body temperature is 37 degrees C. Convert this to K (look it up!) and find the corresponding value in K. What wavelength does this correspond to?
A)94,000 nm
B)78,000 nm
C)9,400 nm
D)7,800 nm
E)940 nm
F) 780 nm

C
37 C corresponds to 310 K (we add 273 to conver C to K). Plugging this into Wien's law yields about 9400 nm. This is in the infrared part of the spectrum.

If we increase the temperature of a star that emits like a blackbody by a factor of 2, what happens?
A)Its flux goes up by a factor of 4 and its peak wavelength doubles
B)Its flux goes up by a factor of 4 and its peak frequency doubles
C) Its flux goes up by a factor of 16 and its peak wavelength doubles
D)Its flux goes up by a factor of 16 and its peak frequency doubles

D
Flux goes at temperature to the fourth power and 24 = 16. Wien's law says that if you double the temperature, you decrease the wavelength by a factor of 2. Since frequency is and wavelength of light are inversely related, that means that the frequency doubles.

Why were isolated neutron stars expected to be difficult to see in the early 1960s? Choose all that apply.
A)They cool very rapidly.
B)Before they cool, they have very small emitting area so they are dim.
C)Before they cool, they have a very small flux so they are dim.
D)We need space based telescopes to view X-rays and sensitive X-ray instruments had not yet been launched into space.
F) The light from their surface is nearly infinitely redshifted

A,B,D
Neutrons stars do cool rapidly due their large flux and there is not source of energy (e.g. no fusion) to replenish the lost energy. The emitting are is small due to their very small radii. C is incorrect because neutron star fluxes are huge due to the high initial temperatures - a trillion times larger than the Sun's flux. D is true. The first X-ray instruments were launched on rockets in 1962 but they were not sensitive enough to see the dim isolated neutron stars. E the redshift is large compared to normal stars but still less than a factor of a few so this is false.

How were the first neutron stars discovered?
A)Radio astronomers were looking for thermal emission from neutron stars.
B)Radio astronomers were looking for the radiation from electrons accelerated in the neutron star's strong magnetic field.
C)Radio astronomers were looking for pulsating sources and the ones they found were eventually shown to be neutron stars.
D)Radio astronomers were looking for other things and discovered sources that seemed to pulse by accident.

D
The discovery of neutrons stars was accidental. The graduate student who discovered them had a tough time convincing her senior colleagues that they were not from man made noise sources.

What is the main reason that pulsars spin down?
A)They are spun down by accretion.
B)Due to a Penrose like process in their ergospheres, just like black holes.
C)They are spun down by an electromagnetic wind.
D)Most pulsars generally spin up and only rarely spin down.

C
Pulsars predominantly spin down via an electromagnetic wind. Accretion tends to spin pulsars up rather than down. Pulsars due have a frame dragging effect on the spacetime around them but it is too small for a Penrose like process to work. Pulsars in accreting systems due appear to be spun up, but these are a small minority of cases.

Why do we think most pulsars begin with rapid spins (spins periods less than a second)?
A)As the star's radius decreases, any small rotation is amplified via conservation of angular momentum.
B)The star that forms the pulsar accretes lots of matter from a companion that makes it very rapidly spinning.
C)The electromagnetic radiation carries away negative angular momentum that acts to spin up the star.
D)Most pulsars don't begin with rapid spins - they must be spun up by accretion.

A
This is just like how a skater pulls her arms into increase her rotation rate. In the case of part B, it is not thought that the star that produces the neutron star is rapidly spinning. Electromagnetic radiation carries away positive angular momentum so C is false. D is incorrect because most pulsars are thought to be born rapidly spinning. This is true of most young pulsars that we observe.

What makes pulsars so useful to astronomers?
A)They are very bright and all have the same luminosity so we can use them to measure distances to other galaxies.
B)The absorption lines in their spectra allow us to make sensitive tests of general relativity in regions of strong gravity.
C)Their large inertia and rapid spins make their pulses very precise clocks.
D)We can measure their ages very accurately, putting lower limits on the age of the universe.

C
Pulsar do make very precise clocks. Pulsars are not particularly bright and we do not think they all have the same luminosity. Pulsars have no atomic features in their radio emission. Those that we can see in the X-ray also do not have features in their X-ray spectrum. We can not accurately measure the ages of pulsars. They can infer ages from spin down but these are not particularly reliable.

How do astronomers investigate astrophysical black holes if no light can escape from the event horizon of the black hole?
A)We observe light emitted by matter close to the black hole that has not yet crossed the event horizon.
B)We observe the black hole's gravitational effect on the motion of nearby stars.
C)We observe the Hawking radiation emitted by lack holes.
D)All of the above
E)Both a and b

E
Both a and b are true and were mentioned at the start of lecture 27. White holes are not generally thought to exist in nature and none have been found.

Once an accretion disk forms, how does matter primarily lose angular momentum in order to slowly move towards the black hole?
A)The matter does not need to lose angular momentum to move inwards.
B)The viscosity caused by collisions between particles provides friction between neighboring rings in the disk.
C)The turbulent magnetic fields provide friction between neighboring rings in the disk.
D)The radiation forces from light emitted by the accretion disk provide friction between neighboring rings in the disk.

C
Matter that is in a disk is rotating so it has angular momentum and must lose angular momentum to move toward smaller radii. Simulations show that magnetic turbulence generated by the magnetorotational instability allow matter to accrete on to the black hole. Both particle collision and radiation forces due provides some friction, but both are very small compared to the effects of magnetic turbulence.

Why are accreting black holes thought to form jets?
A)The jets are associated with the emission of Hawking radiation from the black hole's event horizon.
B)Accreting black holes are not thought to form jets. This phenomenon only occurs for accreting neutron stars.
C)Magnetic fields threading the accretion disk or black hole are wound up by rotation, which accelerates particles that then radiate.
D)The jets are thought to form because of the relativistic beaming of light emitted on the approaching side of the accretion disk.

C
Almost all theoretical models of jets attribute them to magnetic fields. Magnetic fields which are rotating at their base get wound up into a helical pattern. This helical structure collimates charge particles into a narrow structure while accelerating them and causing them to radiate.

Why can systems where matter accretes onto black holes be so luminous?
A)Each black hole coexists with a white hole that emits immense amounts of light.
B)Black holes radiate immense amounts of radiation via hawking radiation.
C)Black holes are very compact for their mass.
D)Black holes accrete more matter than other objects with the same mass because their gravitational force is so much stronger.

C
Astronomers have never seen any evidence for white holes. Hawking radiation has not yet been discussed in the course and is not very luminous. Black holes have the same gravitational force as other objects with the same mass. The main reason black holes are more luminous is that the potential energy lost by matter falling onto an object is proportional to its mass and inversely proportional to its radius. Since black holes have the smallest possible radius for a given mass (they are the most compact), they release the most gravitational potential energy.

How much energy is radiated by a non-spinning black hole that accretes 10-7 Msun per year?
3.4 x 10^29 W
3.4 x 10^30 W
3.4 x 10^31 W
3.4 x 10^32 W

C
We compute this using L=ηMc^2 (where M is the accretion rate). Putting in the numbers, we find L=0.06(10^-7 x 6.3 x 10^22 kg/s)(3 x 10^8 m/s)2 = 3.4 x 10^31 W.

What is the theoretical argument to explain why observable black hole binaries are rare?
A)Massive stars that form black holes are a small fraction of stars.
B)Because the stars had to have survived a common envelope phase to decrease the orbital separation.
C)Because most binaries are probably disrupted when the more massive star undergoes supernova.
D)All of the above.

D
All of these statements are true and all contribute to why we think observable black hole binaries are rare.

What do we primarily need to measure to infer a lower limit on the mass of a black hole in a binary star system?
A)The orbital period and Doppler shift.
B)The orbital period and the distance to the binary.
C)The distance to the binary and the Doppler shift.
D)The temperature of the companion star and the Doppler shift.

A
One needs to know the orbital period and the Doppler shift of lines from the companion as it orbits. Measuring the distance and the temperature could be helpful in identifying the mass of the companion, but this is a secondary concern.

What are the main pieces of evidence we use to differentiate a black hole binary from a neutron star binary? Choose all that apply.
A)Black hole accretion disks emit in the X-rays but neutron star disks are cooler.
B)We assume it is likely to be a black hole if its mass is larger than 3 Msun.
C)We assume it is a neutron star if it shows pulsations or thermonuclear bursts.

B & C
The accretion disk in black hole and neutron star X-ray binaries both emit in the X-rays and it is difficult to distinguish them solely on the X-ray spectrum. However, a mass estimate greater than 3 solar masses is strong evidence of a black hole and pulsations/bursts are strong evidence of a neutron star.

Why did it take astronomers so long to discover evidence for black holes using X-ray telescopes?
A)Astronomers had not considered that any objects would be hot enough to emit X-rays.
B)The Earth's atmosphere is opaque to X-rays so we had to wait until we could launch detectors into space.
C)The technology to detect X-rays was not available until the 1960s.
D)Black hole X-ray binaries are very faint X-ray sources.

B
The Earth's atmosphere is opaque to X-rays so X-ray astronomy had to wait until we could launch detectors into space on rockets. None of the other statements is true.

Why are accreting black holes so hot that they primarily emit in the X-rays?
A)They are very luminous and the emitting area is small.
B)They are very luminous and the emitting area is large.
C)They need to be hot for nuclear reactions to proceed.
D)The emitting area is large and the accretion rate is large.

A
A large temperature follows from a large flux which requires both a large luminosity and/or a small emitting area. Black holes are very luminous because they have large accretion rates. Black hole accretion flows also have a small emitting area, which explains why they are hot. Nuclear reactions are not very important in black hole X-ray binary emission.

How is black hole spin estimated in X-ray binaries?
Choose all that apply.
A)By taking an image of the accretion disk and finding the position of the inner edge of the disk.
B)By modeling the spectrum of the accretion disk continuum emission.
C)By modeling the relativistic effects on spectral emission lines.
D)Astrophysical black holes are not thought to have significant spin so spin estimates are not considered useful.

B & C
Both B and C are used. Accretion disks in X-ray binaries are too small to image. Both the spectral methods described can provide some constraint on the spin. Astrophysical black holes form from stars that are rotating and accrete gas with angular momentum so they are expected to have non-zero spin. (They are expected to have a charge that is very close to zero).

Why do astronomers think black holes produce jets?
A)We think that the jet is the material ejected from the white hole companion.
B)We think that the jet is the black hole's Hawking radiation.
C)We think that some of the material falling towards the hole is gravitationally deflected predominantly along the spin axis of the black hole.
D)We think that magnetic fields threading the accretion flow are twisted by the rotating black hole and/or accretion disk and that this twisted magnetic field accelerates charged particles.

D
Black holes (and possibly disks) will cause the magnetic field to rotate near the base, twisting them into a helical pattern that is ideal for accelerating particles.

Why do astrophysicists not expect black holes to have a gravitationally significant charge?
A)General relativity does not allow charged black hole solutions.
B)General relativity allows solutions that are either charge or spinning but not both and most astronomers expect black holes to spin.
C)Gravity is very weak compared to the electromagnetic force so if a strongly charged black hole was created, it would strongly attract particles of the opposite charge and become neutral.
D)Charged black holes would be gravitationally unstable.

C

Which type of observations do astronomers worry will be most affected by satellite constellations?
A)Narrow-field observations that focus on a relatively small part of the sky.
B)Wide-field observations that focus on a relatively large part of the sky.
C)Observations taken from other satellites in orbit around the Earth.
D)Observations take late at night, well after the Sun is below the horizon.

B
Because they observed large patches of the sky, wide-field observations are the most likely ones to be contaminated by satellites moving through them. See: https://www.space.com/megaconstellations-could-destroy-astronomy-no-easy-fix

If a powerful gravitational wave passed through you body, moving from your back to your chest, what would happen to your body?
A)You would experience an oscillation in your height only.
B)You would experience an oscillation in your width only.
C)Your width and height would both oscillate in such a way that you get taller and wider at the same time.
D)Your width and height would both oscillate in such a way that you would get taller as you got narrower and shorter as you got wider.

D
Gravitational waves always cause oscillations in two perpendicular directions. The oscillations are such that while one direction is increasing, the perpendicular direction is decreasing. So if your height is increasing, your width must be getting smaller at the same time.

Which of the following produce gravitational waves if general relativity is correct? Choose all that apply.
A)A spinning black hole.
B)A conductor directing a symphony.
C) The Earth orbiting the Sun.
D) A spinning neutron star with a mountain on it.

B,C,D
The only incorrect answer is A, because a spinning black hole only involves axisymmetric motion of mass. All of the other options involve non-axisymmetric motion and would produce gravitational waves. The gravitational waves produced by a conductor directing a symphony would be nearly impossible to measure, but are predicted by general relativity.

Why did observation of a pulsar in a binary allow astronomers to infer the existence of gravitational waves?
A)Gravitational waves from black hole mergers modified the period of the binary as they passed by.
B)We can measure the gradual shift in the stars' closest approach due to emission of gravitational waves.
C)We can measure the change in the Doppler shift caused by the emission of gravitational waves.
D)We observed the gravitational waves produced when the binary merged.

B
Emission of gravitational waves carries off energy at the expense of the orbital energy of the two neutron stars. Lower orbital energy requires the stars' semi-major axis to shrink and leads to a gradual shift in the time of the stars' closest approach, which can be measured because pulsars are very good clocks.

Why do astronomers expect observations of black hole mergers to require the measurement of incredibly small wave strain?
A)Mergers of black hole are thought to be incredibly rare so that a typical event will happen very far from Earth and the gravitational waves will be weak by the time they reach us.
B)Even the merger of black holes produces rather weak gravitational waves.
C)Black hole mergers are thought to have only occurred in the very early universe so the gravitational waves will be weak because they have travelled billions of light years to reach us.
D)The frequencies at which we can observe black holes with LIGO are very poorly matched to the incredibly low frequencies that gravitational waves will be emitted at.

A
Black hole mergers are thought to be incredibly rare. To have a decent chance of seeing an event over a several year baseline, we must be able to detect mergers over a huge volume, which means the typical source will be very far away. Since the gravitational waves are radiated in all directions, only a small fraction of the original energy will be directed toward the Earth.

Why would astronomers be excited to see a neutron star merger and/or its electromagnetic (light) counterpart signal?
A)Neutron star mergers are thought tp emit more gravitational waves than black hole mergers.
B)If detected along with electromagnetic counterpart, they might provide additional test of general relativity.
C)If detected along with electromagnetic counterpart, they can be used to constrain cosmological expansion
D)All of the above
E)Only B and C

E
Neutron stars are thought to generally to be weaker gravitational wave emitters than black holes because they have lower masses. Hence both A and D are incorrect. Detection of gravitational waves from a neutron star merger along with its electromagnetic counterpart could provide new tests of general relativity and measure the cosmological expansion. So both B and C are correct, making E the correct answer.

How does LIGO detect gravitational waves? Choose all that apply.
A)It looks for changes in the distance between sets of mirrors located at right angles to each other.
B)It uses lasers in an interferometer.
C)It measures variations in the Earth's gravitational acceleration.
D)It looks for the same signal nearly simultaneously in two different detectors located a few miles apart.

A & B
A is true because LIGO does measure the change is distance between mirrors arranged in perpendicular arms. B is true because the lasers bouncing between the mirrors are used as an interferometer. C is not true because LIGO is not trying to measure variations in the Earth's gravitational acceleration. D is not true because the two detectors are located more than a 1000 miles apart so that they experience different seismic noise.

How does LIGO identify a black hole merger event?
A)It simply looks for very large oscillations at the correct frequency.
B)It looks for very large oscillations coincident with the detection of a gamma ray burst.
C)It looks for specific waveforms that can be computed analytically.
D)It looks for specific waveforms that must be computed using numerical simulations.

D
The signal is generally does not stand out above the noise and is required to match a theoretically predicted waveform. Due to the complexities of solving Einstein's equations, this requires a full numerical solution of the black hole merger.

How luminous (in gravitational waves) was the first black hole merger detected by LIGO at its peak?
A)It was comparable to the luminosity of the Sun.
B)It was comparable to the luminosity of the entire Milky way galaxy.
C)It was comparable to the luminosity of a gamma ray burst.
D)It was comparable to the luminosity of all the stars in the observable universe.

D
The peak luminosity of the merger only lasted for about 1/100 of a second, but its luminosity was briefly larger than that of all the stars in the observable universe.

If at 15 Msun black hole merges with a 30 Msun black hole and they form a single black hole of 43 Msun, how much energy must be radiated as gravitational waves?
A)3.6 x 10^46 J
B)3.6 x 10^47 J
C)3.6 x 10^48 J
D)3.6 x 10^49 J

B
The remnant black hole is about 2 Msun lower than the sum of the initial black holes. Hence, an equivalent amount of rest mass energy must be carried of by the gravitational waves. Then we must have E=Mc^2=(2 x 2 x 10^30 kg)(3 x 10^8 m/s)^2 = 3.6 x 10^47 J.

What do astronomers infer from observations thant almost every galaxy is moving away from us and ones that are more distant are moving away faster?
A)We must be near the center of the universe.
B)The universe is expanding everywhere.
C)The universe is contracting everywhere.
D)The Big Bang originated somewhere close to our own galaxy.

B
If space is expanding everywhere, the effect we would perceive is that all galaxies (except gravitationally bound ones, like Andromeda) are moving away from us. The ones farther away would be moving faster. It does not tell us anything about our position in the universe. D is incorrect because the Big Bang happened simultaneous everywhere in the universe. A is incorrect because there does not seem to be anything special about the location of our Galaxy.

What observation of galaxies implies the existence of dark matter?
A)Average stellar velocities are too high near the galaxies centers.
B)Rotational velocities are too high in the outer regions of galaxies.
C)Rotational velocities are too low in the outer regions of galaxies.
D)Galaxies seem to be moving away from us too quickly.

B
The rotation velocities of galaxies are observed to increase or stay flat well beyond where we see most of the visible stars. If only light emitting matter were present, we would expect these rotation velocities to decrease with radius. The absence of a decrease implies that more unseen matter must be present.

What observation of galaxies implies the existence of a supermassive black hole at its center?
A)Average stellar velocities are too high near the galaxies centers.
B)Rotational velocities are too high in the outer regions of galaxies.
C)Rotational velocities are too low in the outer regions of galaxies.
D)Galaxies seem to be moving away from us too quickly.

A
The changes in the rotation curves as larger radii are used to infer the presence of dark matter. The high stellar velocities in the centers of galaxies where stars dominate over dark matter must be attributed to some other non-luminous mass and the only viable candidate is a supermassive black hole.

How do we think galaxies interact with each other over the course of the age of the universe? Choose all that apply.
A)We think that most galaxies undergo interactions with nearby galaxies that cause them to collide and merge over the age of the universe.
B)We think most galaxies start off large and gradually break apart into smaller galaxies as time passes.
C)We think most galaxies start off small and gradually assemble into larger galaxies as time passes.
D)We think that most galaxies are isolated and have evolved independently of their neighbors over the age of the universe.

A&C
Observational evidence and theory both indicate that galaxies begin as many, relatively small concentrations of gas, stars, and dark matter. But, due to the pull of gravity and the fact that galaxies are relatively large compared to their typical separations, most galaxies merge and grow into fewer, larger galaxies over the age of the universe.

Which of the following appears to be a common constituent of disk (spiral) galaxies?
A)Dark matter.
B)Interstellar gas.
C)Supermassive black holes.
D)All of the above.
E)Only b and c.

D

Which of the following are true statements about the first neutron star merger detected by LIGO? Choose all that apply.
A)It leads astronomers to believe that neutron star mergers may account for a significant fraction of the production of heavy elements, like gold, in the universe.
B)It was accompanied by the detection of a gamma ray burst by the Fermi telescope.
C)It was a surprise because no one thought that LIGO could detect neutron star mergers.
D)The addition of the Virgo detector in Italy aided in the localization of the direction from which the gravitational waves had come.

A,B,D
All but C are correct. C is incorrect because LIGO was expected to find neutron star mergers. The signal is weaker due to the lower masses, but it was also thought that neutron star mergers happened more frequently so some detections were expected.