Deck 20: The Milky Waya Normal Spiral Galaxy

Question 1

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 distance 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.

Answer 1

Cluster A is 10 times farther away than Cluster B.

Question 2

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 proportions of heavy elements have shorter lifetimes

Answer 2

low; they were formed before much chemical enrichment had taken place

Question 3

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

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

Answer 3

barred spiral

Question 4

The figure shown below immediately reveals that

A) disk material concentrates along the spiral arms but is completely absent between the arms.
B) spiral arms are sites where ongoing star formation happens efficiently.
C) disk galaxies do not contain any red, old stars.
D) the space between spiral arms must be filled with thick layers of invisible dark matter.
E) star formation happens only in the spiral arms of galaxies without bars.

Question 5

The dark bands pictured in the figure below are strong evidence that the Milky Way is a(n) ________ galaxy.

A) spiral
B) elliptical
C) irregular
D) active
E) radio

Question 6

The most distant globular clusters are found in which part of the galaxy?

A) the disk
B) the halo
C) the bulge and the bar
D) near the Sun's position
E) the outskirts of the disk

Question 7

Spiral arms are

A) solid features.
B) clumps of gas and dust that travel together.
C) tracks that stars follow to leave the galaxy.
D) density fluctuations in a galaxy's disk.
E) bipolar outflows from the galactic nucleus.

Question 8

Why is 21-cm radiation one of the best ways to map the spiral arms of 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, are not blocked by 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 9

If dust is present between Earth and a distant globular cluster, the cluster will appear to be ________ than it is in reality.

A) more distant
B) larger
C) more massive
D) more heavy metal-rich
E) smaller

Question 10

What type of standard candles did Shapley use to determine distances to globular clusters?

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

Question 11

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 12

Any single star in the Milky Way Galaxy has the same elemental abundances as

A) all other stars in the Milky Way.
B) all other stars in the galactic disk.
C) the cloud of gas it formed from.
D) all other stars of the same age.
E) the stars nearest to it in space.

Question 13

Somebody shows you the picture in the figure below and comments that it is the most recent photo of the Milky Way taken with an instrument aboard a spacecraft that has just recently left the boundaries of our home galaxy and is on its way to Andromeda. You would immediately react by saying: "This cannot be the Milky Way because

A) we live in an elliptical galaxy."
B) our galaxy does not have a bar."
C) the Sun is at the very center of the galaxy."
D) the Milky Way is way too big for a spacecraft to go that far."
E) all galaxies in the universe are identical and therefore we cannot be so sure this is the Milky Way."

Question 14

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

A) low heavy-element abundances and low relative velocities
B) high heavy-element abundances and high relative velocities
C) low heavy-element abundances and high relative velocities
D) high heavy-element abundances and low relative velocities
E) equal heavy-element abundances and high relative velocities

Question 15

Spiral arms are generated by

A) bipolar jets from a galaxy's nucleus.
B) expanding gas from supernova explosions.
C) sudden bursts of star formation.
D) gravitational disturbances to a galaxy's disk.
E) random motions of stars.

Question 16

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 17

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 number of heavy elements the entire galaxy had when the star was formed.

Question 18

In a star that has heavy-element abundance similar to the Sun's, what percentage of the star's mass is made up of elements heavier than hydrogen and helium?

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

Question 19

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 elliptical patch 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 galactic plane

Question 20

Globular clusters, when compared with open clusters, generally

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

Question 21

X-ray observations indicate that around the Milky Way there may be an extended halo of

A) hot and diffuse gas at 2 million K, similar to that in the solar corona.
B) cold and dense gas where new stars form abundantly.
C) cosmic rays captured from other galaxies.
D) ions accelerated by strong magnetic fields.
E) gas heated by the very active supermassive black hole at the center.

Question 22

Compared to the Sun's stellar neighborhood, globular clusters have

A) higher heavy-element abundances.
B) much higher stellar densities.
C) mainly younger stars.
D) large amounts of dust present between stars.
E) larger amounts of dark matter present.

Question 23

If you find a halo star that is 13 billion years old and has the same amount of heavy elements as the Sun, should you be surprised?

A) No, because many halo stars have high fractions of heavy elements.
B) No, because this star probably escaped from a globular cluster.
C) No, because 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, because most halo stars are less than 10 billion years old.

Question 24

The motion and chemical composition of stars in the outer halo may suggest that

A) it had formed long ago as a product of merger with a small galaxy.
B) it is very young and luminous.
C) it is filled with cosmic rays.
D) its composition is altered by galactic fountains.
E) it does not contain any dark matter.

Question 25

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

A) The stars of the oldest globular cluster have some heavy elements.
B) They are so old that nuclear fusion in globular cluster stars has altered their chemical abundances.
C) Globular clusters have a much higher hydrogen content than expected.
D) The RR Lyrae stars found in globular clusters must be second-generation stars.
E) All globular clusters reside in the disk of the Milky Way.

Question 26

Where are the stars with the highest fraction of heavy elements found in the Milky Way?

A) in the disk near the Sun
B) in the inner parts of the galactic disk
C) in the halo
D) in globular clusters
E) in old open clusters

Question 27

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

A) The thin-disk stars are, on average, younger than the thick-disk stars.
B) Molecular clouds are found in the thick disk, but not the thin disk.
C) The thick disk is made up of stars that were stripped from small dwarf galaxies as they merged with the Milky Way.
D) The thick-disk stars may be older than disk stars that have higher upward velocities, because they have gravitationally interacted more with molecular clouds in the spiral arms.
E) All of these statements are true.

Question 28

You measure the heavy element abundances and motions of stars near the Sun.When plotting your data, you find that the stars separate into two distinct groups, which you label A and B.Group A has velocities and heavy-element abundances similar to the Sun's, while Group B has high velocities and low heavy-element abundances.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 29

Astronomers expect stars near the edge of the galactic disk to have smaller amounts of heavy elements than other parts of the disk because

A) there is more star formation but fewer supernovae at the edge.
B) there is less star formation and fewer supernovae at the edge.
C) there is less star formation even though there are more supernovae at the edge.
D) there is more star formation and more supernovae at the edge.
E) None of these apply; there is no reason to expect the disk to be different based on location.

Question 30

Most of the stars near the Sun have

A) element abundances similar to those of the Sun.
B) velocities much higher than that of the Sun.
C) element abundances that are much higher than those of the Sun.
D) velocities much lower than that of the Sun.
E) element abundances that are much lower than those of the Sun.

Question 31

Open star clusters primarily inhabit which part of spiral galaxies?

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

Question 32

If you want to look for old stars with low amounts of heavy elements, the best place to look out of the following locations is

A) the galactic bulge.
B) the central bar.
C) the stellar halo.
D) the thin disk.
E) the thick disk.

Question 33

Which objects in our Milky Way Galaxy have orbits that are similar to the orbits of stars in elliptical galaxies?

A) molecular clouds
B) halo stars
C) thin disk stars
D) thick disk stars
E) open clusters

Question 34

Scientists have found many stars in the outer halo that ________ the rotation of the galactic disk and have a very ________ fraction of heavy elements.

A) move in a direction opposite to; high
B) move in a direction opposite to; low
C) move in the same direction as; high
D) move in the same direction as; low
E) don't move at all with respect to; low

Question 35

The oldest disk stars are both ________ and ________ compared with halo stars.

A) younger; have higher fractions of heavy elements
B) older; have higher fractions of heavy elements
C) younger; have lower fractions of heavy elements
D) older; have lower fractions of heavy elements
E) younger; similar fractions of heavy elements

Question 36

Observations show that the halo of the Milky Way contains

A) globular clusters.
B) dwarf galaxies.
C) stars orbiting the galaxy by themselves.
D) tidally disrupted objects.
E) all of these

Question 37

The location, motion, and heavy-element enrichment of interstellar gas can be strongly affected by what?

A) dark matter
B) low-mass stars
C) pulsars
D) supernovae
E) halo stars

Question 38

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 39

The extended hot "halo" of gas surrounding our galaxy is thought to contain ________ mass as the sum of the Milky Way's stellar mass.

A) the same amount of
B) 1/10 as much
C) 10 times as much
D) 1/1,000 as much
E) 1,000 times as much

Question 40

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

A) the oldest; highest
B) disk; lowest
C) bulge; lowest
D) halo; highest
E) the youngest; highest

Question 41

The Large Magellanic Cloud is about 50 kpc from the center of the Milky Way and moves around it at about 170 km/s, as shown in the figure below.What is the inferred mass of the galaxy enclosed within this radial extent?

A) 3.4 *10¹¹ M_SUN
B) 9.4 * 10¹⁰ M_SUN
C) 2.0 * 10¹⁰ M_SUN
D) 3.1 * 10¹² M_SUN
E) 1.7 * 10⁷ M_SUN

Question 42

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) 1.8 billion years
C) 5.3 billion years
D) 9.2 billion years
E) 12.7 billion years

Question 43

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

A) by 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 44

What observed property of the Milky Way suggests that it must contain a large amount of matter not in the form of stars, gas, or dust?

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 supermassive black hole at the galactic center

Question 45

The orbits of a few dozen stars have been carefully monitored over a few decades in the proximity of the supermassive black hole at the center of the Milky Way (see the figure shown below).One example is the so-called star SO-1, whose semimajor axis is 3,300 AU long and has a period of 94.1 years.What is the estimated mass of the central supermassive black hole?

A) 3.4 *10⁶ M_SUN
B) 7.6 * 10⁹ M_SUN
C) 1.5 * 10⁸ M_SUN
D) 4.0 * 10⁶ M_SUN
E) 3.4* 10¹¹ M_SUN

Question 46

The gamma-ray bubbles at the poles of our galaxy are evidence that the nucleus of the Milky Way was once a(n)

A) very high-mass star.
B) pulsar.
C) Herbig-Haro object.
D) active galactic nucleus (AGN).
E) supernova.

Question 47

Most cosmic rays are

A) photons with energy even higher than that gamma rays.
B) high-velocity antiparticles produced in novae.
C) charged particles moving at relativistic speeds.
D) synchrotron radiation from strong magnetic fields.
E) dark-matter particles falling onto the galaxy.

Question 48

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

A) Gravitational scattering of 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 49

In the figure below, we expect the absorption lines in the indicated star's spectrum to be

A) normal/unchanged.
B) redshifted.
C) blueshifted.
D) wider than usual.
E) narrower than usual.

Question 50

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

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

Question 51

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 from the halo.

Question 52

In the figure below, we expect the absorption lines in the indicated star's spectrum to be

A) normal/unchanged.
B) redshifted.
C) blueshifted.
D) wider than usual.
E) narrower than usual.

Question 53

Cosmic rays in the Milky Way are important to study because

A) these energetic photons easily penetrate Earth's atmosphere.
B) they can be collected and used to generate electricity.
C) they provide information about the most energetic events in the universe.
D) they are leftover particles from the formation of the Galaxy.
E) they can influence the motion of stars in the halo.

Question 54

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

A) unlike; makes up a smaller 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; makes up 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 55

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, and it orbited with a velocity of 250 km/s, what would be the total mass enclosed within this radius?

A) 1.4 * 10¹¹ M_SUN
B) 7.2 * 10¹¹ M_SUN
C) 9.5* 10¹¹ M_SUN
D) 4.2 * 10¹² M_SUN
E) 8.0 *10¹² M_SUN

Question 56

Which of the following statements about the galactic magnetic fields is true?

A) Cosmic rays spiraling magnetic fields produce the radio "glow" of the galactic disk.
B) They are confined mostly to the galactic disk.
C) They trap cosmic rays and interact with them.
D) They are anchored in molecular clouds.
E) All of these are true.

Question 57

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 58

Imagine you discovered a barred spiral galaxy that was a "Milky Way twin" because of the size of the bulge and disk, the arrangement of the spiral arms, and other characteristics just like those in our galaxy.Then you measured its rotation curve and plotted the data as shown in the figure 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 59

Which of the following is evidence that the object known as Sagittarius A* is a supermassive black hole?

A) The Milky Way currently has a powerful active nucleus (AGN).
B) Star orbits imply a large, invisible mass.
C) Gravitational waves from the central region are consistent with a supermassive black hole.
D) A large accretion disk of gas can be resolved in visible light.
E) The event horizon can be viewed through powerful telescopes.

Question 60

The highest-energy cosmic rays that reach Earth

A) could actually originate outside the Milky Way Galaxy.
B) are produced in particle accelerators here on Earth.
C) emerge from the supermassive black hole at the center of the Milky Way.
D) are in fact X-ray and gamma-ray photons.
E) are the best candidates for explaining dark matter.

Question 61

These three barred spirals are the largest members of the Local Group:

A) Andromeda, Milky Way, Sagittarius
B) Milky Way, Phoenix, Leo
C) Andromeda, Triangulum, Sagittarius
D) Andromeda, Milky Way, Triangulum
E) Milky Way, Triangulum, Sagittarius

Question 62

Outline the major differences between the thin and the thick disks of the Milky Way Galaxy.

Question 63

Describe, for each component of the Milky Way Galaxy labeled in the figure below, the typical heavy-element abundances found in that region, and what causes that region to have those abundances.

Question 64

What are spiral arms, and what is thought to cause them?

Question 65

Astronomers expect life to be rare (and maybe impossible) in the Milky Way's bulge because

A) there are so many stars that no material is left over to make planets.
B) the bulge is too poor in heavy elements.
C) the tidal force of the central black hole would strip stars of their planets.
D) the lack of green and blue photons from younger stars would make photosynthesis impossible.
E) the amount of radiation in the central regions is very high.

Question 66

Suppose it was discovered that the galaxy has constantly been absorbing large amounts of pristine gas that never had any stars form in it.Would this complicate our interpretation of ages and chemical enrichment?

Question 67

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) supernovae explosions occurring in the dwarf galaxy

Question 68

Astronomers have identified examples of extraordinarily massive globular clusters (in both the Milky Way and Andromeda) that contain several million solar masses worth of stars.Moreover, these clusters also show evidence of multiple stellar populations.What would be a reasonable interpretation for such atypical clusters?

Question 69

Suppose that scientists in the future are able to observe a star orbiting the central region of the Andromeda Galaxy and find that it has an orbital period of 31 years and a semimajor axis of 4,500 AU.The mass of Andromeda's central black hole must then be around

A) 4.0 * 10⁶ M_SUN
B) 9.5 *10⁶ M_SUN
C) 4.0 * 10⁷ M_SUN
D) 9.5 * 10⁷ M_SUN
E) 4.0 * 10⁸ M_SUN

Question 70

By comparing globular clusters, you find that Cluster A's RR Lyrae stars are 225 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 cluster has the larger distance, and what is the ratio of the clusters' distances?

Question 71

How would we explain why are there are no known disk stars with exceptionally low abundances of massive elements?

Question 72

The most distant galaxy observable with the naked eye from Earth is the

A) Large Magellanic Cloud.
B) Sombrero Galaxy.
C) Sagittarius Dwarf.
D) Andromeda Galaxy.
E) Small Magellanic Cloud.

Question 73

The Milky Way and Andromeda are on a collision course.When will the initial collision happen?

A) in about 4 million years
B) in about 4 billion years
C) in about 2.5 million years
D) in about 400,000 years
E) in about 13.7 billion years

Question 74

What is expected to happen to our Solar System when the Andromeda and Milky Way galaxies collide?

A) The Sun is expected to collide with another star, destroying our Solar System.
B) The Sun will be unaffected, but the planets will be scattered into space or destroyed.
C) The Sun will have exploded by then, and Earth will already be uninhabitable.
D) The Sun may be thrown to a different orbit within the merged galaxies, but the solar system will be relatively unchanged.
E) The Sun may be thrown to a different orbit within the galaxies, and the planets will be scattered off into space.

Question 75

What would be the apparent angular size of a 100 light-year-diameter globular cluster in the Andromeda Galaxy (2.5 x10⁶ light-years away), as measured from Earth?

Question 76

How do we know that globular cluster stars are not the first stars that have ever formed? Would this understanding change if some or all globular clusters came into the galaxy by accretion of small satellite galaxies?

Question 77

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

A) The galaxy formed through the merger of two large galaxies, which scattered their dwarf companions across large distances.
B) The galaxy originally formed with no dark matter but gained its dark matter through 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 was formed by the merger of smaller protogalactic clumps.

Question 78

The Andromeda Galaxy is

A) in agreement with Hubble's law, because its spectrum shows redshift.
B) a giant elliptical galaxy.
C) a rare galaxy that has little dark matter.
D) the largest member of the Local Group.
E) on a path that will separate it from the Milky Way forever.

Question 79

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

Question 80

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) the merging of protogalactic fragments that stimulated a high rate of star formation.
D) the merging of globular clusters that were already fully-formed.
E) the merging of two galaxies, each about half as massive as the Milky Way is today.