Deck 20: Galaxies

Question 1

Any asymmetry in the gravity field of a spiral galaxy can create spiral arms.

Answer 1

True

Question 2

The 3-D shape of an elliptical galaxy depends on the average velocity of its stars along each of the three axes.

Answer 2

True

Question 3

Dark matter is distributed inside galaxies in exactly the same way as luminous matter,although the total mass of dark matter is much larger than that of luminous matter.

Answer 3

False

Question 4

Although elliptical and irregular galaxies have been observed in both dwarf and giant varieties,we find only giant spiral galaxies.

Question 5

We can determine the three-dimensional shape of any individual elliptical galaxy by photographing it.

Question 6

The Hubble diagram is a way of sorting galaxies by their visual appearance.

Question 7

Galaxies that are classified as quasars were much more numerous in the past.

Question 8

Seyfert galaxies are unique because they have large lobes of radio emission that extend well beyond the visible part of the galaxy.

Question 9

Astronomers have evidence that all giant galaxies probably contain supermassive black holes.

Question 10

Elliptical galaxies have the shapes they do because all the stars are orbiting in the same direction in the galaxy.

Question 11

Star formation in spiral galaxies is enhanced in the spiral arms because more hot gas is concentrated there.

Question 12

A galaxy can contain a supermassive black hole that remains invisible because no matter is falling onto the black hole.

Question 13

In most galaxies,the amount of dark matter and the amount of luminous matter is about equal.

Question 14

The type of AGN we see depends mostly on the angle at which we view the galaxy.

Question 15

We know AGN are about the size of our Solar System because of how fast they change in brightness over time.

Question 16

Dark matter consists of material that emits radiation at wavelengths other than visible light.

Question 17

Today in our local universe,approximately 30 percent of galaxies are AGNs.

Question 18

Elliptical galaxies contain hot gas that is heated by supernova explosions.

Question 19

The Hubble tuning fork diagram represents an evolutionary diagram of galaxies from elliptical to spiral.

Question 20

We know that spiral galaxies contain dark matter because the gas in the disk rotates with a velocity that decreases in a way that is proportional to r^-1/2.

Question 21

In spiral galaxies,the size of the central bulge is correlated with the:

A)tightness of the spiral arms
B)luminosity of the galaxy
C)age of the galaxy
D)thickness of the disk
E)presence of an active nucleus

Question 22

Spiral structure:

A)rotates at the same speed as the gas in the disk
B)rotates at a speed that is different than the gas in the disk
C)stays constant in time as the gas in disk rotates through it
D)rotates backward compared to the stellar orbits
E)must only last a short time,since few galaxies with disks show spiral patterns

Question 23

Which of the following is NOT a difference between spiral and elliptical galaxies?

A)Spiral galaxies still have ongoing star formation whereas elliptical galaxies do not.
B)Spiral galaxies are sometimes very thin in appearance whereas elliptical galaxies are always round or elliptical.
C)Spiral galaxy disks contain lots of cold gas and dust whereas elliptical galaxies do not.
D)Spiral galaxies tend to have bluish areas whereas elliptical galaxies tend to be all reddish.
E)Spiral galaxies have dark matter halos whereas elliptical galaxies do not.

Question 24

Stars in the disks of spiral galaxies have orbits that:

A)are randomly oriented
B)constantly are getting larger
C)mostly aligned in the same plane
D)spiral shaped
E)unaffected by dark matter

Question 25

The three-dimensional shape of __________ is set by the average velocities of the stars along each of the three axes.

A)spiral galaxies
B)only elliptical galaxies
C)spiral bulges
D)elliptical galaxies and spiral bulges
E)quasars

Question 26

We classify galaxies by:

A)their true shapes
B)how they appear on the sky
C)the amount of gas and dust in them
D)whether they are near or far from us
E)the amount of dark matter they contain

Question 27

Elliptical galaxies appear red because:

A)they are moving away from us
B)they contain mostly ionized hydrogen gas
C)they contain mostly old stars
D)they contain lots of dust
E)they contain a mix of old and young stars

Question 28

Stars in elliptical galaxies have velocities that:

A)change constantly
B)are very large in size
C)are equal in all directions
D)are constant in time
E)are orientated in random in directions

Question 29

What type of galaxy is this?

A)a giant elliptical
B)an ordinary spiral galaxy
C)an irregular galaxy
D)a barred spiral
E)a dwarf elliptical

Question 30

The Hubble classification scheme for galaxies sorts them by:

A)their evolutionary state
B)their mass
C)the amount of dust
D)the amount of dark matter
E)their visual appearance

Question 31

What type of galaxy is this?

A)a giant elliptical
B)a regular spiral galaxy
C)an irregular galaxy
D)a barred spiral
E)a dwarf elliptical

Question 32

Approximately __________ of spiral galaxies are barred.

A)10 percent
B)25 percent
C)35 percent
D)50 percent
E)75 percent

Question 33

Which of the following would NOT be found in an elliptical galaxy?

A)novae
B)Type I supernovae
C)Type II supernovae
D)planetary nebulae
E)hot gas

Question 34

Spiral structure can be triggered in a disk galaxy by:

A)the merger of close binary stars
B)a supernova explosion
C)the presence of an active nucleus
D)the gravity of distant galaxies
E)any disturbance to the disk

Question 35

Why is 21-cm radiation a good tracer of spiral arm structure?

A)Emitted by neutral hydrogen,it traces the location of high concentrations of gas.
B)Emitted by O and B stars,it traces the location of newly formed stars.
C)Emitted by ionized hydrogen,it traces the location of H II regions.
D)Emitted by molecular hydrogen,it traces the location of clouds in the process of collapsing into stars.
E)Emitted by red supergiants,it traces the presence of massive stars that are experiencing high rates of mass loss.

Question 36

Two elliptical galaxies have the same apparent size and brightness.Galaxy A appears lumpy,and it is possible to observe individual stars and star clusters.The other one,galaxy B,appears very smooth,with no fine detail.Which one is likely to be more luminous?

A)Galaxy A.It has to contain more luminous objects if we can see them individually,and so the galaxy as a whole has to be more luminous.
B)Galaxy A.The lumpiness means that there is a lot of dust in the galaxy,which absorbs some of the starlight.Therefore the total amount actually emitted must be larger,if it is to have the same brightness as the other galaxy.
C)Neither.If they have the same apparent size and brightness,they must be at equal distances.
D)Galaxy B.This galaxy appears smooth because it is farther away and the light from its stars all blends together.Because it is farther away,it has to be more luminous.
E)Galaxy B.The smooth light distribution means that the stars are moving around very quickly.Therefore the galaxy has more mass and is more luminous.

Question 37

Which of the following are NOT found mainly in the spiral arms of galaxies?

A)molecular clouds
B)H II regions
C)K- and M-type stars
D)Type II supernovae
E)O-type stars

Question 38

The disks of spiral galaxies appear blue because:

A)they are moving toward us
B)they contain a relatively high concentration of low-mass stars
C)they contain active regions of star formation
D)they contain more metals that,when ionized,emit blue light
E)stars collide with each other frequently in these dense regions and explode as Type I supernovae

Question 39

The image below was probably taken in ____________ wavelengths because _______________________.

A)radio;the disk probably contains many planets with intelligent life,who are beaming out radio and television signals into space
B)infrared;the central regions are bright,and cool stars in the bulges emit a lot of infrared light
C)visible;there is a lot dust in this galaxy that blocks light in a ring about halfway along the disk
D)ultraviolet;the light from hot young stars is mainly found along spiral arms
E)X-ray;the central nucleus is easily seen,and AGN emit a lot of X-rays

Question 40

Active star formation does not occur in elliptical galaxies because they:

A)rotate too fast
B)contain little molecular hydrogen
C)are too massive
D)are too far away
E)usually contain active nuclei

Question 41

If a galaxy had no dark matter,how would the velocity (v)of a star that orbited far outside the visible extent of the galaxy depend on its distance (r)from the center of the galaxy?

A)ν ∝ r⁻²
B)ν ∝ r⁻¹
C)ν ∝
D)ν ∝ r
E)ν ∝ r²

Question 42

What makes up the majority of the mass of an individual spiral galaxy?

A)a central supermassive black hole
B)dark matter
C)massive O- and B-type stars
D)cold molecular gas clouds
E)low-mass G-,K-,and M-type stars

Question 43

The dark matter in galaxies is __________ the luminous material.

A)distributed in roughly the same way as
B)much more extended in radius than
C)much more centrally concentrated than
D)much less uniform in density than
E)much clumpier than

Question 44

If you observed a gas cloud orbiting in a disk of a spiral galaxy that was located 10,000 pc from the center and it rotated with a speed of 300 km/s,what is the mass of the galaxy interior to gas cloud's orbit?

A)1 × 10¹¹ M_⨀
B)2 × 10¹¹ M_⨀
C)8 × 10¹¹ M_⨀
D)1 × 10¹² M_⨀
E)4 × 10¹² M_⨀

Question 45

Dust in galaxies does not count as dark matter because:

A)it only absorbs some wavelengths of light but not all wavelengths the way dark matter does
B)it interacts with light
C)it is only found in spiral disks,whereas dark matter is more widely distributed
D)dust is easily destroyed by dark matter
E)there is never enough to make all the gravity

Question 46

Long-lasting spiral density waves occur in the disk of a spiral galaxy when:

A)the spiral merges with another galaxy
B)the galaxy is young
C)the galaxy's disk is regularly disturbed
D)the galaxy absorbs a satellite galaxy
E)a supernova explosion takes place

Question 47

Quasars were first discovered in which region of the electromagnetic spectrum?

A)X-ray
B)gamma-ray
C)radio
D)infrared
E)visible

Question 48

Which of these would NOT be useful for measuring the mass of the dark matter halo in a spiral galaxy?

A)Hα emission from H II regions in the spiral disk
B)the velocities of satellite dwarf galaxies surrounding a spiral galaxy
C)absorption lines in the spectra of its globular clusters
D)the velocities of star clusters in the outer disk
E)all of these can be used to measure the mass of the dark matter

Question 49

If you measure the velocity of a cloud of gas that is rotating around the center of a galaxy in a circular orbit with radius R,you can determine:

A)the total mass of the galaxy
B)the mass of the stars and gas in the galaxy
C)the mass of the galaxy enclosed within the radius R
D)the mass of the galaxy located outside the radius R
E)the mass of luminous matter within the radius R

Question 50

Roughly what percentage of the total mass of a galaxy is made up of luminous,or normal,matter?

A)5 percent
B)25 percent
C)33 percent
D)50 percent
E)90 percent

Question 51

Dark matter is most likely made up of:

A)elementary particles that have mass but do not interact much with normal matter
B)supermassive black holes
C)faint stellar remnants such as white dwarfs and neutron stars
D)free-floating Jupiter-mass planets
E)cold concentrations of dust

Question 52

If the average quasar has a luminosity of 10¹⁴ L_⨀ and an absolute magnitude of −30,and you discover one that has an apparent magnitude of +10,then what is its distance?

A)10,000 Mpc
B)5,000 Mpc
C)1,000 Mpc
D)500 Mpc
E)0.1 Mpc

Question 53

Why do rotation curves of galaxies imply that dark matter exists?

A)The velocity of stars rises dramatically at large distances from the centers of galaxies,implying large amounts of dark matter cause these stars to move much faster than stars at smaller distances.
B)The velocity of stars stays approximately constant at distances well beyond the visible edges of galaxies,implying that galaxies contain much more matter than what we observe in stars and gas.
C)The velocity of stars is low in the inner regions of galaxies,suggesting that dark matter is impeding their motion.
D)The velocity of stars rises quickly in the inner regions of galaxies,suggesting that unseen matter is gravitationally moving this material at faster rates than expected.
E)The velocity of stars falls slowly in the outer regions of galaxies,showing that while there is a lot of dark matter it is distributed the same way as is the visible light.

Question 54

How have astronomers searched for evidence of MACHOs in the halo of the Milky Way?

A)They search for stars that are binary companions of MACHOs.
B)They search for rapidly moving,massive gas clouds.
C)They search for disturbances in the background of gravitational waves.
D)They search for stars whose light is briefly amplified by gravitational lensing.
E)They search for close binary stars undergoing mass transfer.

Question 55

How did astronomers determine that elliptical galaxies are composed mostly of dark matter?

A)They measured the velocities of stars in the inner regions of the galaxies.
B)They measured the rotation rates of gas as a function of distance from the centers of the galaxies.
C)They measured the amount of gravitational pull elliptical galaxies have on companion galaxies.
D)They assumed that the proportion of dark matter was roughly the same as in spiral galaxies.
E)They measured X-ray emission from hot gas gravitationally bound to the galaxies.

Question 56

If you measure the spectrum emitted by a cloud of gas that orbits a nearby spiral galaxy and the observed wavelength of a hydrogen emission line is 658.4 nm while the rest wavelength of that line is 656.3 nm,what is the velocity of this gas cloud along your line of sight (i.e. ,the radial velocity)? Note that the speed of light is 3 × 10⁵ km/s.

A)960 km/s
B)1,200 km/s
C)2,300 km/s
D)3,800 km/s
E)4,400 km/s

Question 57

The density of ordinary luminous matter can exceed the density of dark matter in which parts of galaxies?

A)everywhere inside galaxies
B)only in the outer regions of galaxies
C)only in star clusters
D)near the centers of galaxies
E)only around an AGN

Question 58

If elliptical galaxies do not have cold gas clouds for which we can measure the Doppler shift,then how can we measure the mass of an elliptical galaxy?

A)by measuring the speeds of stars in the nucleus
B)by observing the velocity of the hot,X-ray emitting gas that surrounds the galaxy
C)by measuring the number of H II regions in the galaxy
D)by measuring how many supernovae go off each century
E)by measuring the amount of blue light in the galaxy

Question 59

If the rotational speed of the gas in the disk of a spiral galaxy is 200 km/s at a distance of 25,000 pc from its center,then what is the mass enclosed within this radius?

A)7.5 billion M_⨀
B)18 billion M_⨀
C)70 billion M_⨀
D)230 billion M_⨀
E)400 billion M_⨀

Question 60

A galaxy rotation curve is a plot of the rotation speed as a function of the:

A)galaxy's luminosity
B)mass of the dark matter halo
C)brightness of the luminous matter in the galaxy
D)radius from the center
E)ages of star clusters

Question 61

Describe two ways by which spiral structure may be induced in a spiral galaxy's disk.

Question 62

How did Hubble use his tuning fork diagram to classify galaxies? Is there any significance to this organization?

Question 63

The unified model of AGN suggests that quasars,Seyfert galaxies,and radio galaxies:

A)are unrelated,although they are all very luminous galaxies at radio wavelengths
B)are powered by similar mass accretion rates onto supermassive black holes
C)are driven by violent mergers of two gas-rich spiral galaxies
D)are similar phenomena but viewed from different orientation angles
E)are all powered by high rates of star formation and supernova explosions

Question 64

How do we know that AGNs have sizes on the order of our Solar System?

A)Quasars and Seyfert galaxies are stellar-like sources.
B)Their brightness varies by factors of a few.
C)The emission lines in their spectra show gas rotating at speeds of thousands of km/s.
D)Their brightness varies on timescales ranging from hours to a day.
E)We can measure their angular size,which gives us the physical size when we know the distance.

Question 65

What is the approximate percentage of nearby galaxies that contain AGNs?

A)3 percent
B)15 percent
C)20 percent
D)50 percent
E)80 percent

Question 66

If a Seyfert galaxy's nucleus varies in brightness on the timescale of 10 hours,then approximately what is the size of the emitting region?

A)20 AU
B)30 AU
C)50 AU
D)70 AU
E)120 AU

Question 67

How do the current star formation rates of spiral and elliptical galaxies compare?

Question 68

What happens to active galaxies when their AGNs run out of fuel?

A)The black holes powering them collapse even further.
B)They simply become normal galaxies.
C)They slowly dissipate into clouds of gas and dust.
D)They eventually pull in material from farther out,starting the AGN process all over again.
E)They explode as gamma-ray bursts.

Question 69

We interpret the fact that we saw more AGNs in the past than we do today to indicate that:

A)there were fewer galaxy-galaxy interactions in the past
B)there were more galaxy-galaxy interactions in the past
C)supermassive black holes were larger in the past
D)today's AGNs are primarily hidden by large amounts of cold gas and dust
E)AGNs had bigger central black holes in the past

Question 70

What do the broad emission lines in quasar spectra tell astronomers about these objects?

A)They are very far away.
B)They are moving away from us.
C)They have rapid internal motions.
D)They are very dense.
E)They have few heavy elements.

Question 71

In some radio galaxies,we only see one side of the jet because:

A)the black hole only produces a jet in one direction
B)of relativistic beaming
C)the other side is inside the black hole's event horizon
D)of dust obscuration
E)of dark matter that blocks the light from one side of the jet

Question 72

How do we know that the stellar disks in spiral galaxies are flat? How do we know that elliptical galaxies are not flat?

Question 73

Assume that when a supermassive black hole accretes gas,approximately 50 percent of its mass is converted directly into energy,which is radiated away.If a quasar has a luminosity of 10¹⁴ L_⨀ then what must its mass accretion rate be?

A)1 M_⨀ per year
B)10 M_⨀per year
C)1 M_⨀ per century
D)10 M_⨀ per century
E)1 M_V per million years

Question 74

AGNs appear to be powered by:

A)extremely dense star clusters
B)supermassive black holes
C)ultradense molecular clouds
D)decaying dark matter
E)supernova explosion

Question 75

What determines the three-dimensional shapes of galaxies?

Question 76

What do the formation of the Solar System and the existence of AGNs have in common?

A)They both demonstrate the importance of how luminosity varies with time.
B)They both demonstrate the high efficiency of nuclear fusion.
C)They both demonstrate the importance of accretion disks.
D)They both demonstrate the importance of living in the center of a galaxy.
E)They both demonstrate the existence of dark matter.

Question 77

How do we measure directly that AGNs have black holes at their centers?

A)We know that massive stars make black holes,and most galaxies contain a lot of massive stars.
B)We know black holes are made of dark matter,and galaxies are mostly made up of dark matter.
C)We know that black holes emit radio waves,and many AGNs are bright at radio waves.
D)We measure that the mass of the black hole is increasing as material falls into it.
E)In some systems,we can see both redshifted and blueshifted emission lines from material orbiting around the black hole.

Question 78

What is plotted on a rotation curve,and what can it tell us about a galaxy?

Question 79

Which of these statements about quasars is FALSE?

A)Quasars are extremely luminous.
B)Quasars are rare in our local vicinity.
C)Quasars have high brightnesses.
D)Quasars are extremely massive.
E)Quasars are located at the centers of some active galaxies.

Question 80

More luminous giant galaxies tend to have __________ supermassive black holes at their centers.

A)more massive
B)radio quiet
C)more luminous
D)slower rotating
E)less massive