Deck 21: The Milky Waya Normal Spiral Galaxy

Question 1

A dwarf galaxy is currently being ripped apart and accreted onto the Milky Way.

Answer 1

True

Question 2

The Milky Way has lived out its life mainly in isolation and has not experienced many mergers with other galaxies.

Answer 2

False

Question 3

Stars in the halo are most easily identified passing by the Sun because of their distinctively redder colors.

Answer 3

False

Question 4

The Milky Way has two major spiral arms that emanate from the ends of its bar.

Question 5

We can estimate the mass of the central black hole in the Milky Way by measuring the orbital speeds of stars near it.

Question 6

Halo stars formed before the Milky Way developed a thin stellar disk.

Question 7

Our galaxy is classified as an Sd galaxy in the Hubble sequence.

Question 8

Globular clusters have much higher stellar densities than the local solar neighborhood.

Question 9

The Milky Way is like most other galaxies in that its mass is dominated by dark matter.

Question 10

The locations of globular clusters on the sky and their distances tell us that the Sun is located near the center of the Milky Way.

Question 11

Our galaxy is a typical barred spiral galaxy.

Question 12

Open clusters are only found in the disk of the Milky Way; globular clusters are only found in the halo of the Milky Way.

Question 13

The chemical abundance of a main-sequence star usually reflects how much nuclear burning has gone on inside it.

Question 14

There are many more individual stars residing in the Milky Way stellar halo than inside globular clusters.

Question 15

The youngest disk stars are found in a very thin plane in the disk.

Question 16

Stars in the Milky Way that are 12 to 13 billion years old are only found in globular clusters.

Question 17

There are no known processes that can change the orbits of disk stars over time.

Question 18

Cosmic rays come mostly from extragalactic sources and contribute little to the overall energy balance in the Milky Way disk.

Question 19

There is a 10⁹ M _$\odot$ black hole at the center of the Milky Way that is rapidly accreting stars and gas.

Question 20

Most of the gravity that the Sun feels in the galaxy is caused by the black hole at the center of the Milky Way.

Question 21

If a globular cluster were 1,000 pc away from us and an RR Lyrae star in it had an absolute magnitude of 0.5, what would its apparent magnitude be?

A) 4.0
B) 6.5
C) 8.0
D) 10.5
E) 15.5

Question 22

How many years does it take the Sun to complete one orbit around the Milky Way? Note that the Sun is traveling at approximately 220 km/s and is 8 kpc from the center of the Milky Way.

A) 180 thousand years
B) 7 million years
C) 35 million years
D) 230 million years
E) 620 million years

Question 23

Which of these galaxies is most like our own, and what is the name for the type of the galaxy we live in?

A) Galaxy A; giant elliptical
B) Galaxy A: nonbarred spiral
C) Galaxy B: barred spiral
D) Galaxy B: irregular
E) Galaxy B: lenticular

Question 24

The Sun is located approximately:

A) halfway out in the disk
B) one-third of the way out in the halo
C) one-quarter of the way out in the bulge
D) three-quarters of the way out in the disk
E) near the galactic center

Question 25

If RR Lyrae stars in a globular cluster had an apparent magnitude of 11.5 and an absolute magnitude of 0.5, what would the globular cluster's distance be?

A) 1,600 pc
B) 1,900 pc
C) 2,500 pc
D) 3,200 pc
E) 8,600 pc

Question 26

The Milky Way appears as __________ in the night sky because __________.

A) randomly distributed stars; the Sun lies near the center of the Milky Way
B) a faint band of light sprinkled with dark clouds; the Sun lies in the disk
C) a faint band of light; the Sun lies in the halo
D) an ellipse of light in the Southern sky; the Sun lies midway along the disk
E) a circular disk with spiral arms; the Sun lies far above the galaxy's plane

Question 27

If we found a star cluster at the edge of the disk of the Milky Way at a distance of 10 kpc from the center, which orbited with a velocity of 300 km/s, what would be the total mass enclosed within this radius?

A) 3 * 10¹¹ M _$\odot$ 0
B) 7 * 10¹¹ M _$\odot$
C) 2 *10¹¹ M _$\odot$
D) 4 * 10¹² M _$\odot$
E) 8 * 10¹² M _$\odot$

Question 28

What type of standard candle is used to determine distances to globular clusters?

A) O-type main-sequence stars
B) Cepheid variable stars
C) T Tauri stars
D) Type I supernovae
E) RR Lyrae stars

Question 29

The Milky Way is classified as an __________ galaxy.

A) Sa
B) SBbc
C) Sd
D) SBc
E) SBd

Question 30

Studying standard candles in globular clusters offered the first conclusive proof that our galaxy was much __________ than originally believed.

A) smaller
B) rounder
C) older
D) flatter
E) larger

Question 31

The Milky Way Galaxy is a(n) __________ galaxy.

A) irregular
B) elliptical
C) nonbarred spiral
D) barred spiral
E) lenticular

Question 32

By comparing globular clusters, you find that Cluster A's RR Lyrae stars are 100 times fainter than Cluster B's RR Lyrae stars. You know that Cluster A's stars are 1.5 times more luminous than Cluster B's stars, because Cluster A is more metal-rich and somewhat younger. How do the clusters' distances compare?

A) Cluster A is 65 times farther away than Cluster B.
B) Cluster A is 12 times farther away than Cluster B.
C) Cluster A is 10 times farther away than Cluster B.
D) Cluster A is seven times farther away than Cluster B.
E) Cluster A and Cluster B are at approximately the same distance.

Question 33

By comparing globular clusters, you find that Cluster A's RR Lyrae stars are 100 times fainter than Cluster B's RR Lyrae stars. You know that both clusters have approximately the same chemical composition and age, and thus their RR Lyrae stars should have the same luminosities. Which is true about the clusters' distances?

A) Cluster A is 10 times farther away than Cluster B.
B) Cluster A is 10 times closer to us than Cluster B.
C) Clusters A and B are approximately the same distances from us.
D) Cluster A is 100 times farther away than Cluster B.
E) Cluster A is 100 times closer to us than Cluster B.

Question 34

Imagine you discovered a barred spiral galaxy that was a "Milky Way twin" because the size of the bulge and disk, the arrangement of spiral arms, and other characteristics were just like those in our galaxy. Then you measured its rotation curve and plotted the figure shown below. What would you conclude about this galaxy?

A) This galaxy has much more dark matter than does the Milky Way.
B) This galaxy has about the same amount of dark matter as does the Milky Way.
C) This galaxy inexplicably has much less dark matter than does the Milky Way.
D) This galaxy probably has no stellar halo.
E) This galaxy probably has no central black hole.

Question 35

Why is 21-cm radiation the best way to map the spiral arms in the Milky Way?

A) The molecular hydrogen gas that produces this emission is concentrated in the spiral arms.
B) These photons, which are produced by neutral hydrogen, pass through the dense clouds of gas and dust in the disk.
C) The emission is produced by supernovae, which are concentrated in the spiral arms.
D) Radio telescopes are easier to operate than optical telescopes, and observations can be made even during the daytime.
E) Radio photons do not have Doppler shifts, so we can detect clouds of gas without having to worry about the orbital velocities.

Question 36

Our galaxy is __________ other galaxies because the dark matter in the Milky Way __________.

A) unlike; makes up a small fraction of the total mass than in other galaxies
B) unlike; makes up a larger fraction of the total mass than in other galaxies
C) like; contains most of the mass in the galaxy
D) like; is made up of black holes and other stellar remnants
E) like; is made up of dust and faint objects of planetary mass

Question 37

Most of the mass in our galaxy is in the form of:

A) stars
B) gas
C) dust
D) dark matter
E) globular clusters

Question 38

What observed property of the Milky Way suggests that it contains a large amount of matter not in the form of stars?

A) the rotation curve
B) the velocities of the open star clusters
C) the number and shape of the spiral arms
D) the thickness of the disk
E) the presence of a black hole at the galactic center

Question 39

Below are two images of the Milky Way at different wavelengths. Which one was taken at infrared wavelengths, and why do you think so?

A) A; there are many cool stars in the image, which emit a lot of infrared light.
B) A; the dust in the galaxy mainly blocks infrared light and lets visible light through.
C) B; we see mainly warm dust emitting thermal radiation.
D) B; infrared light passes through the dust so we can see the galaxy's structure more clearly.
E) B; stars in the central bulge shine only at infrared wavelengths.

Question 40

Globular clusters are found in which stellar subgroups of the galaxy?

A) the disk
B) the halo
C) the bulge
D) both the halo and the inner disk
E) the outer halo

Question 41

Open star clusters primarily inhabit which part of spiral galaxies?

A) disk
B) halo
C) bulge
D) nucleus
E) satellite galaxies

Question 42

Where are the most metal-rich stars found in the Milky Way?

A) in the disk near the Sun
B) near the center
C) in the halo
D) in globular clusters
E) in old open clusters

Question 43

We find that interstellar gas and the youngest disk stars are found in a very narrow distribution along the galactic plane, but that older disk stars can be found in a thicker disk. What might this mean?

A) Gravitational scattering off molecular clouds changes the orbits of disk stars as they get older.
B) The disk is still settling down from a thicker to a thinner state.
C) The galaxy has not absorbed any dwarf galaxies in the last several billion years.
D) Cosmic rays are responsible for causing star formation to happen in a narrow layer over time.
E) Most of the dark matter in the galaxy is in a thin layer along the disk.

Question 44

If you found a star cluster that was 13 billion years old and whose stars had fractions of heavy elements that were 100 times less than that in the Sun, what type of star cluster would this be and where would it most likely be located?

A) an open cluster that is likely to be in the disk
B) an open cluster that is likely to be in the stellar halo
C) a globular cluster that is likely to be in the thin disk
D) a globular cluster that is likely to be in the stellar halo
E) a globular cluster that is likely to be in the central bar

Question 45

The ratio of the diameter of the galaxy's disk to the thickness of its disk is about:

A) 10 to 1
B) 30 to 1
C) 100 to 1
D) 1,000 to 1
E) 10,000 to 1

Question 46

The location and motions of interstellar gas can be strongly affected by what?

A) Dark matter
B) Low-mass stars
C) Pulsars
D) Supernovae
E) Halo stars

Question 47

The stars in the disk of the Milky Way near the Sun have:

A) chemical abundances similar to the Sun
B) much higher velocities than the Sun
C) chemical abundances that are on average 10 times higher than the Sun
D) much lower velocities than the Sun
E) chemical abundances that are on average 10 times lower than the Sun

Question 48

In the Milky Way's outermost halo, the stars orbit:

A) mostly in the prograde direction
B) mostly in the retrograde direction
C) both in the prograde and retrograde directions
D) the same way the disk stars do
E) neither in the prograde nor the retrograde directions

Question 49

The Milky Way has both a thin and thick disk of stars. Which one of the following statements about them is FALSE?

A) The thin disk stars on average are younger than the thick disk stars.
B) Molecular clouds are distributed more like the thin disk stars rather than the thick disk stars.
C) The thin disk stars on average have lower abundances of heavy elements than the thick disk stars.
D) The thick disk stars may be older thin disk stars that have higher upward velocities because they have gravitationally interacted more with molecular clouds in the spiral arms.
E) The thick disk may have formed when a small dwarf galaxy merged with the Milky Way.

Question 50

In which part of the Milky Way would you find little or no neutral hydrogen, no current star formation, and stars that are all older than 10 billion years?

A) the inner disk
B) the outer disk
C) the galactic center
D) the solar neighborhood
E) the halo

Question 51

Halo stars near the Sun are distinguished from disk stars by what characteristics?

A) low chemical abundances and low relative velocities
B) high chemical abundances and high relative velocities
C) low chemical abundances and high relative velocities
D) high chemical abundances and low relative velocities
E) equal chemical abundances and high relative velocities

Question 52

The oldest stars in the galaxy are usually __________ in heavy elements because __________.

A) low; they have had time to accrete unprocessed gas from the interstellar medium
B) low; they were formed before much chemical enrichment had taken place
C) low; old stars must have low mass, and low-mass stars do not generate many heavy elements
D) high; they have turned a lot of their initial hydrogen and helium into heavier elements
E) high; stars with higher fractions of heavy elements have shorter lifetimes

Question 53

The chemical composition of a star's atmosphere tells us:

A) how much nuclear burning has gone on in the star
B) the star's evolutionary stage
C) how many planets have fallen onto the star in its lifetime
D) the chemical composition of the cloud from which the star formed
E) the amount of heavy elements the entire galaxy had when the star was formed

Question 54

Which of the following are generally NOT found in our galaxy's halo?

A) globular clusters
B) planetary nebulae
C) RR Lyrae stars
D) stars with high percentages of heavy elements
E) low-mass main-sequence stars

Question 55

If you found a halo star that was 13 billion years old and had the same amount of heavy elements as the Sun, would you be surprised?

A) No; many halo stars have high fractions of heavy elements.
B) No; this star probably escaped from a globular cluster.
C) No; this star has been around a long time, and has created a lot of heavy elements through nuclear fusion.
D) Yes, because most halo or globular cluster stars have few heavy elements.
E) Yes; halo stars mostly have ages less than 10 billion years.

Question 56

Globular clusters, when compared to open clusters, generally:

A) are located closer to the center of the Milky Way
B) are younger in age
C) have fewer amounts of heavy elements
D) are less massive
E) contain more dark matter

Question 57

In a star that has a chemical abundance similar to the Sun, what percentage of its mass is made of heavy elements?

A) 0.1 percent
B) 0.2 percent
C) 2 percent
D) 10 percent
E) 20 percent

Question 58

On average, we expect that __________ stars have the __________ percentage of heavy elements.

A) older; highest
B) disk; lowest
C) bulge; lowest
D) halo; highest
E) younger; highest

Question 59

Which objects in our Milky Way galaxy have orbits that are NOT similar to stars in elliptical galaxies?

A) disk stars
B) halo stars
C) bulge stars
D) globular clusters
E) satellite dwarf galaxies

Question 60

You measure the chemical abundances and space motions of stars near the Sun, and plot your data in the figure shown below. The stars separate into two distinct groups, which you label A and B. These groups are, respectively, in which two stellar components of the Milky Way?

A) disk and halo
B) bulge and disk
C) halo and disk
D) halo and bulge
E) globular clusters and halo

Question 61

Cosmic rays are:

A) photons with even higher energy than gamma rays
B) high-velocity particles produced in novae
C) elementary particles with very high energies
D) synchrotron emission from strong magnetic fields
E) dark-matter particles falling into the galaxy

Question 62

Magnetic fields in the Milky Way are concentrated in the disk because:

A) halo stars are incapable of producing strong magnetic fields
B) the fields are tied to the charged particles in dense molecular clouds
C) gravity forces them to sink to the center of the disk
D) supernovae explosions continually force them toward the middle of the disk
E) supernovae explosions eliminate magnetic fields away from the disk

Question 63

Based on the chemical abundances and velocities of stars in the Milky Way's disk and halo, we can conclude that the protogalactic fragments that came together to form the galaxy merged when the fragments had:

A) not yet formed stars
B) converted most of their gas into stars
C) begun forming stars
D) each formed a supermassive black hole at their center
E) gathered a lot of dark matter from surrounding space

Question 64

The presence of dwarf galaxies around the Milky Way supports what picture of our galaxy's formation?

A) The galaxy formed by the merger of two large galaxies, which scattered their dwarf companions to large distances.
B) The galaxy originally formed with no dark matter but gained its dark matter by the absorption of smaller dwarf systems.
C) The galaxy formed in a giant explosion caused by the violent collisions of clouds of dark matter.
D) The galaxy formed a long time ago and has been passively evolving since.
E) The galaxy formed by the merger of smaller systems.

Question 65

Describe the differences between the ages and chemical abundances of globular and open star clusters.

Question 66

If the central black hole started accreting a lot of gas, it would appear as what kind of an object?

A) a red supergiant
B) a pulsar
C) a planetary nebula
D) an AGN
E) a supernova remnant

Question 67

Sagittarius A*, the radio source located at the center of our galaxy, is believed to be:

A) a massive star cluster
B) a supernova remnant
C) a quasar
D) a Seyfert nucleus
E) a supermassive black hole

Question 68

How have astronomers measured the mass of the black hole at the center of our galaxy?

A) using the rotation curve derived from 21-cm emission
B) by observing the motions of stars near the center of the galaxy
C) by measuring the brightness of the quasar
D) by measuring the Doppler shift of Sagittarius A*
E) by counting the number of supernova explosions near the black hole during the last century

Question 69

A typical globular cluster is composed of 500,000 stars in a sphere whose radius is approximately 5 pc. The distance between stars in a globular cluster is given approximately by d $\approx$ r/N^1/3, where r is the radius and N is the number of stars in the cluster. Calculate this distance, and determine how many times smaller it is than the typical distance between stars in the solar neighborhood, which is 3 light-years.

Question 70

What are the main observational difficulties in observing the shape and spiral arm pattern of the Milky Way?

Question 71

Why do we know that at least one generation of stars formed and died before the Milky Way's globular clusters formed?

A) All globular cluster stars have some amount of heavy elements.
B) They are so old that nuclear fusion in globular cluster stars has altered their chemical abundances.
C) Globular clusters are 9 to 10 billion years old.
D) No globular cluster is older than 12 billion years.
E) All globular clusters reside in the disk of the Milky Way.

Question 72

What is the radius of the event horizon for the 4 *10⁶ M _$\odot$ black hole at the center of the Milky Way?

A) 0.02 AU
B) 0.08 AU
C) 0.17 AU
D) 0.35 AU
E) 0.60 AU

Question 73

Because of the ages of globular clusters in the Milky Way, we think our galaxy's early formation history was characterized by:

A) one single cloud of gas gently collapsing and star formation proceeding slowly within it
B) one single cloud of gas that rapidly collapsed and turned most of its gas into stars
C) violent merging of protogalactic fragments that stimulated a high rate of star formation
D) slow merging of protogalactic fragments after they had already turned most of their gas into stars
E) merging of two galaxies, each about half as massive as the Milky Way is today

Question 74

What is ripping apart the Sagittarius dwarf spheroidal galaxy?

A) a supermassive black hole at the center of the dwarf galaxy
B) pressure from its passage through the dark matter in the Milky Way
C) a violent episode of star formation
D) the gravitational tidal force of the Milky Way
E) supernova explosions occurring in the dwarf galaxy

Question 75

Cosmic rays in our galaxy can produce synchrotron radiation at what wavelengths?

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

Question 76

Describe the population of globular star clusters in the Milky Way: how many are there, where do they reside in the Milky Way, and how do their ages and chemical abundances compare to the Sun?

Question 77

How do we think the central black hole in the galaxy primarily grew to its current mass?

A) ordinary stellar evolution of massive stars
B) merger of close binary stars
C) shock waves from supernova explosions
D) dark matter clumping together
E) accretion of nearby gas

Question 78

Cosmic rays in the Milky Way are important because:

A) these energetic photons easily penetrate the Earth's atmosphere
B) they can be collected and used to generate electricity
C) they have about the same energy as that contained in magnetic fields and in the kinetic energy of gas clouds
D) they carry information about AGN in external galaxies
E) they can influence the motion of stars in the halo

Question 79

The oldest disk stars are both __________ and __________ compared to halo stars.

A) younger; higher in chemical elements
B) older; higher in chemical elements
C) younger; lower in chemical elements
D) older; lower in chemical elements
E) younger; equal in chemical element abundances

Question 80

If the Large Magellanic Cloud is orbiting the Milky Way in a circular orbit with a speed of 175 km/s and a distance of 50 kpc from the center, how long would it take for the Large Magellanic Cloud to complete one orbit around the Milky Way?

A) 500 million years
B) 2 billion years
C) 5 billion years
D) 9 billion years
E) 12 billion years