Black Holes Quiz 3

C

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

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.

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

A

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

C and D

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.

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 insdie the event horizon. b. Spin determines the effective inner edge of the black hole accretion disk (the ISCO). c. Spin determines the amount of Hawking radiation. d. Spin affects the vertical thickness of the accretion disk.

D D is correct. We can measure this angle by looking at the ellipsoidal variations caused by the companions distorted teardrop geometry. The strength of these variations over the orbital periods can be fit for the viewing angle.

Given a measured Doppler shift, the associated line-of-sight velocity depends on our viewing angle. How could we measure this angle? a. We cannot measure this angle. b. We measure the angle dependent lensing from the black hole. c. We use two different estimates of the Doppler shift at different times in the stars orbit. d. We measure the changes in the observed flux from the companion star as it orbits.

E

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. Core collapse 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 Both a and b are true and were mentioned at the start of lecture 27. Black holes do emit hawking radiation, but the wavelengths are much too small and the radiation too faint for us to see such radiation.

How do astronomers see astrophysical black holes if no light can escape from the event horizon of the black hole? a. We look for light emitted by matter close to the black hole that has not yet crossed the event horizon. b. We look for the black hole's gravitational effect on the motion of nearby stars. c. We look for the emission from the black holes corresponding white hole. d. All of the above e. Both 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.

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. Some 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

A and 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 detect gravitational waves? 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. Choose all that apply.

D D is correct. 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 does LIGO identify at 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.

B

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

B and 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).

How is black hole spin estimated in X-ray binaries? 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. Choose all that apply.

D D is correct. 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.

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.

C The correct answer is C. We compute this using L=ηMc2 (where M is the accretion rate). Putting in the numbers, we find L=0.06(10-7 x 6.3 x 1022 kg/s)(3 x 108 m/s)2 = 3.4 x 1031 W.

How much energy is radiated by a non-spinning black hole that accretes 10-7 Msun per year? a. 3.4 x 1029 W b. 3.4 x 1030 W c. 3.4 x 1031 W d. 3.4 x 1032 W

D D is correct. 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.

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.

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.

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

D D is correct. 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.

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.

B The correct answer is 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=M c2=(2 x 2 x 1030 kg)(3 x 108 m/s)^2 = 3.6 x 1047 J.

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 1046 J b. 3.6 x 1047 J c. 3.6 x 1048 J d. 3.6 x 1049 J

E E is correct because 1003.5 = (102)3.5 = 107 or 10 million times more luminous.

If luminosity is proportional to mass to the 3.5 power (L∝M3.5) , how many times more luminous is a 100 solar mass star than the Sun? a. 100 times more luminous b. 350 times more luminous c. thousand times more luminous d. million times more luminous e. 10 million times more luminous

C

If the beam from a spinning neutron star hit the Earth: a. We would all die. b. We wouldn't notice it. c. We would call it a pulsar. d. We would call it a quasar. e. We would call it a "Crab Nebula".

E

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

D

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 D is correct. 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.

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

C

In the distant future astronauts decided they want to observe a neutron star up close - from an orbit a few neutron star radii away. Is this a good idea? a. Yes. Nothing bad would happen and they would learn a lot. 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 obliterate them. d. No. The time dilation effects they encounter would cause them to age so slowly that everyone they know would be dead upon return.

C The correct answer is 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.

Once an accretion disk forms, how does matter 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.

B Inserting 80K in to Wien's laws yields 2.9 x 106/80 = 3.6 x 104 nm. If we look at the electromagnetic spectrum, it is given in meters so we multiply by 109 to convert nanometers to meters to get 3.6 x 10-5 m, which lies in the infrared.

Our galaxy is filled with clouds of molecular gas which is can be quite cold compared to stars. If this gas has a temperature of 80 K, in what part of the spectrum will its emission peak? (Hint: a figure of the electromagnetic spectrum with wavelength ranges in meters can be found in lecture 6: Properties of Light .pdf on the collab site.) a. Radio b. Infrared c. Visible d. Ultraviolet e. X-rays

A

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 us 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.

B

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.

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 Only C is true. In the early 1960s, clear evidence of black holes 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.

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 started seeing clear observational evidence of black holes. 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.

B and C B and C are correct but not A. 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.

What are the main pieces of evidence we use to differentiate a black hole binary from a neutron star binary? a. Black hole accretion disk 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. Choose all that apply.

B B is the only correct statement. See the first few slides of lecture 21.

What causes stars to form? a. Interstellar gas is compressed by the motion of nearby stars. b. Interstellar gas collects and collapses under its mutual gravitational attraction c. Stars start off as planets that grow by accreting interstellar gas. d. Stars form primarily from the collision of other stars.

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

A A is correct. 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 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.

D D is correct. This is actually a somewhat uncertain questions. Some people claim the number is slightly less than half while others think it could be higher than 2/3. But it is definitely not any of the other options in this problem.

What fraction of stars in the stellar neighborhood are binary stars? a. Binaries are extremely rare. b. About 1/100 c. About 1/10 d. About 2/3 e. Almost all stars are in binaries. The Sun is a rare exception.

D D is correct. When the hydrogen is used up, the core is still not hot and dense enough for helium fusion to proceed. It first goes through a phase where it fuses more hydrogen to helium in a shell around the core. During this stage the outer envelope of the core expands, making it a giant star. (Red giant for low mass stars and blue supergiant for higher mass stars.)

What happens next after a star uses up all the hydrogen in its core? a. It starts fusing helium in its core and the star contracts. b. It starts fusing helium in its core and the star expands. c. It starts burning hydrogen in a shell and the star contracts. d. It starts burning hydrogen in a shell and the star expands.

E

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

C C is correct. 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.

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

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

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

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.

C C is the only correct statement. 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.

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

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

D Gas pressure is the main source of pressure support against gravity in a main sequence star.

What primarily supports a main sequence star against gravity? a. Electron degeneracy pressure. b. Neutron degeneracy pressure. c. Gas pressure maintained by nuclear fission. d. Gas pressure maintained by nuclear fusion. e. Gas pressure maintained by gravitational contraction.

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

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

C and D

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

B, C, and E Stars supported by degeneracy pressure have radii that decrease when their mass increases so A is incorrect and B is correct. The theoretical maximum mass of white dwarfs (Chandrasekhar mass) is 1.4 solar masses so C is correct. D is incorrect because neutron stars are denser than white dwarfs. E is correct -- the Sun will eventually become a white dwarf.

Which of the following are true about white dwarfs supported by degeneracy pressure? a. Their radius increases as their mass increases b. Their radius decreases as their mass increases c. They have a theoretical maximum mass of 1.4 solar masses d. They are the densest stars in the universe that are not black holes e. Our sun will eventually become a white dwarf Choose all that apply.

B, C, and 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.

Which of the following produce gravitational waves if general relativity is correct? 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. Choose all that apply.

D All of these statements are true. See the lecture 24 slides for a discussion of all of these points.

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 thant 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.

B B is correct. More massive stars are more luminous. Since the luminosity goes up very rapidly with increasing mass, they use up their greater energy budget (E=mc^2) in much less time, leading to shorter lifetimes.

Which of the following statements is correct? a. More massive stars live longer lives and are more luminous. b. More massive stars live shorter lives and are more luminous. c. More massive stars live longer lives and are less luminous. d. More massive stars live shorter lives and are less luminous.

A A is correct. 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.

Why are accreting black holes so hot that they primarily emit in the X-rays? a. They are very luminous and emitting area is small. b. They are very luminous and 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.

C C is correct. 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 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.

B

Why build two LIGO detectors separated by thousands of kilometers? a. So one detector could be operating while the other was 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.

C C is correct. 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.

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.

B B is correct. 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 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 B is correct. 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 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.

A A is correct. 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 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.

C C is correct. Long narrow structures are seen in the radio emission of quasars, which give rise to their radio emission. Althougt the radio emission of X-ray binaries can not generally be resolved, it is assumed that it arises for the same reason in both types of sources.

Why do astronomers think the accretion disks in X-ray binaries form a collimated jet? a. They observe long narrow structures in image of the X-ray emission from X-ray binaries. b. They observe long narrow structures in images of radio emission from X-ray binaries. c. They assume radio emission in X-ray binaries arises from the same mechanism seen in quasars. d. The spectral properties of the X-ray emission are difficult to understand without invoking a relativistically beamed jet.

E The correct answer is E. The primary distinction between the formation of disks and spheres is how much angular momentum the object has. When most objects form gravity pulls in objects from large distances. As it contracts, if it has lots of angular momentum, it will tend to form a disk.

Why do some astrophysical objects form spheres and others form disks? a. Disks form when the initial distribution of matter is created by an explosion of a spherical object. b. Disks form when the initial distribution of matter has low angular momentum. c. Disks form when the initial distribution of matter is very cold. d. Disks form when the initial distribution fo matter is very hot. e. Disks form when the initial distribution of matter has high angular momentum.

C

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. common envelope is the most common outcome in binaries due angular momentum losses in stellar winds.

A A is correct. 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.

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

A, B, and D A, B, and D are correct. 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.

Why were isolated neutron stars expected to be difficult to see in the early 1960s? 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. e. The light from their surface is nearly infinitely redshifted. Choose all that apply.

C

Why were neutron stars 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.

D Reread the article from you blog assignment which disucsses the risks: http://content.time.com/time/health/article/0,8599,1838947,00.html Scientists did belive the production of black holes was a remote possibility, but were excited that this might occur. They thought the threat to Earth was remote because Hawking radiaiton would cause the black holes to evaporate before they could accrete. Even if physicist were wrong about the evaporation, the energies acchieved in the CERN collisions are also routinely achieved by cosmic rays hitting the Earth's atmosphere and we are still here. So both B and C are correct.

Why were physicists not that worried about the large hadron collider (LHC) at CERN producing black holes that would destroy the Earth? a. They were certain that the LHC could not produce black holes. b. They were fairly certain that any black holes produced would evaporate before they could grow. c. Collisions of cosmic rays with the Earth's atmosphere routinely achieve the same energies so the threat of black holes from such collisions must be small or the Earth would have already been destroyed. d. Both 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.

Why would astronomers be excited to see a neutron star merger and/or its electromagnetic (light) counterpart signal? a. Neutron star mergers are thought 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

B

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. The gravity 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

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