TEST BANK 21ST CENTURY ASTRONOMY THE SOLAR SYSTEM 5TH EDITION BY KAY
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Chapter 20: The Milky
Way—A Normal Spiral Galaxy
Learning
Objectives
Define the bold-faced vocabulary terms within the chapter.
20.1 Astronomers Have Measured the Size
and Structure of the Milky Way
Summarize the evidence that the Milky Way is a giant barred
spiral galaxy.
Multiple Choice: 1, 2, 8
Short Answer: 1
Explain why spiral arms are prominent even though they do not
contain significantly higher concentrations of stars than the rest of the
galaxy’s disk.
Multiple Choice: 15
Illustrate why spiral arms exist.
Multiple Choice: 10
Show how the distribution of globular clusters around our
galaxy indicates its size and our location within it.
Multiple Choice: 3, 4, 5, 6, 7, 9, 11, 12, 13, 14
Short Answer: 2, 3, 4
20.2 The Components of the Milky Way
Provide Clues about the Formation of Spiral Galaxies
Explain why the heavy-element abundances in stars are
snapshots of the chemical makeup of the gas out of which the stars formed.
Multiple Choice: 27, 34, 35
Short Answer: 16, 22
Illustrate the process of chemical evolution in our galaxy
over time.
Multiple Choice: 22, 25
Short Answer: 17
Explain why chemical abundances of stars differ from place to
place within a galaxy’s disk.
Multiple Choice: 21, 24, 29, 45, 49
Short Answer: 9, 12, 21
Compare and contrast the components and origins of the disk
and the halo of the galaxy.
Multiple Choice: 19, 26, 28, 30, 31, 40, 42, 43, 44, 50
Short Answer: 10, 11, 13, 20
Explain how we distinguish between halo and disk stars, and
between stars in the thin disk and thick disk.
Multiple Choice: 18, 20, 23, 36, 37, 46, 47, 48
Short Answer: 7, 8, 14, 15, 18
Summarize the types and origins of cosmic rays.
Multiple Choice: 32, 33, 38, 39, 41
Short Answer: 19
20.3 Most of the Milky Way Is Unseen
Illustrate the Doppler shifts we expect to observe as we look
at stars in different parts of the galaxy.
Short Answer: 27
Describe the observational evidence that the Milky Way is
primarily composed of dark matter.
Multiple Choice: 53, 57, 58, 60, 61
Short Answer: 24, 26, 28
Describe the observational evidence that Sagittarius A* is a
supermassive black hole.
Multiple Choice: 54, 55, 56, 59
Short Answer: 23, 25
20.4 The History and Future of the Milky
Way
Describe the components making up our Local Group.
Multiple Choice: 63, 64, 66, 67, 70
Short Answer: 29, 31
Summarize the evidence that the Milky Way formed when a huge
clump of dark matter collapsed.
Multiple Choice: 68, 69
Illustrate the future of the Milky Way and Andromeda system.
Multiple Choice: 62, 65
Short Answer: 30
Working It Out 20.1
Use our orbital speed around the galactic center to measure
the mass of the Milky Way out to the Sun’s position.
Multiple Choice: 16, 17, 51
Short Answer: 5, 6
Working It Out 20.2
Use orbital properties of stars around the galactic center to
measure the mass of the Milky Way’s supermassive black hole.
Multiple Choice: 52
MULTIPLE CHOICE
1.
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
2.
The Milky Way Galaxy is
a(n) __________ galaxy.
a.
irregular
b.
elliptical
c.
nonbarred spiral
d.
barred spiral
e.
lenticular
3.
Globular clusters are
found in which part of the galaxy?
a.
the disk
b.
the halo
c.
the bulge and the bar
d.
both the halo and the
disk
e.
the outskirts of the
disk
4.
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
5.
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
6.
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.
7.
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.
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.
9.
If a globular cluster
were 1,000 pc away from us and an RR Lyrae star in the cluster 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
10.
Which of the following
statements about the formation of spiral structure in disk galaxies is not
true?
a.
Spiral arms can be
produced by gravitational interactions with other galaxies.
b.
Spiral structure can be
triggered by bursts of star formation.
c.
Bars can disturb the
disk and initiate the formation of spiral arms.
d.
A single disturbance in
the disk would suffice in forming and maintaining stable, long-lived spiral
arms.
e.
Elongated bulges can
contribute to the formation of spiral arms.
11.
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.”
12.
The major cause that
made Shapley overestimate the size of the Milky Way was the
a.
lack of knowledge about
effects of interstellar material on starlight.
b.
very low number of
globular clusters measured.
c.
confusion between
various types of standard candles.
d.
confusion between
globular clusters and dwarf galaxies external to the Milky Way.
e.
lack of knowledge about
dark matter.
13.
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
14.
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 also know that Cluster A’s stars are 1.5 times
more luminous than Cluster B’s stars. 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 7 times
farther away than Cluster B.
e.
Cluster A and Cluster B
are at approximately the same distance.
15.
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.
16.
The Large Magellanic
Cloud is about 50 kpc from the center of the Milky Way and moves around it at
about 170 km/s. What is the inferred mass of the galaxy enclosed within this
radial extent?
a.
3.4 × 1011 M⊙
b.
9.4 × 1010 M⊙
c.
2.0 × 1010 M⊙
d.
3.1 × 1012 M⊙
e.
1.7 × 107 M⊙
17.
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 250 km/s, what would be the total
mass enclosed within this radius?
a.
1.4 × 1011 M⊙
b.
7.2 × 1011 M⊙
c.
9.5 × 1011 M⊙
d.
4.2 × 1012 M⊙
e.
8.0 × 1012 M⊙
18.
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
19.
Open star clusters
primarily inhabit which part of spiral galaxies?
a.
disk
b.
halo
c.
Bulge
d.
Nucleus
e.
satellite galaxies
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.
21.
On average, we expect
that __________ stars have the __________ percentage of heavy elements.
a.
the oldest; highest
b.
disk; lowest
c.
bulge; highest
d.
halo; highest
e.
the youngest; highest
22.
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 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
only 9 billion 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.
23.
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
24.
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
25.
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, 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, most halo stars
are less than 10 billion years.
26.
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
27.
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.
28.
Which objects in our
Milky Way Galaxy have orbits that are not similar to the orbits
of stars in elliptical galaxies?
a.
disk stars
b.
halo stars
c.
bulge stars
d.
globular clusters
e.
satellite dwarf
galaxies
29.
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
30.
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
31.
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.
32.
Which of the following
statements about the galactic magnetic fields is not true?
a.
They are much stronger
than the Earth’s magnetic field.
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.
Cosmic rays spiraling
magnetic fields produce the radio glow of the galactic disk.
33.
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.
34.
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
35.
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
36.
The ratio between the
diameter and the thickness of the thin disk in a spiral galaxy is about
a.
10 to 1.
b.
30 to 1.
c.
100 to 1.
d.
1,000 to 1.
e.
10,000 to 1.
37.
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
are, on average, younger than the thick disk stars.
b.
Molecular clouds are
distributed more like the thin disk stars than the thick disk stars.
c.
The thin disk stars
have, on average, lower abundances of heavy elements than the thick disk stars.
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.
The thick disk may have
formed when small dwarf galaxies merged with the Milky Way.
38.
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.
39.
Cosmic rays in the
Milky Way are important to study 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.
40.
Which of the following
is not true about globular clusters?
a.
Stars populating the
globular clusters account for only 1 percent of the stars in the galactic halo.
b.
They have much higher
stellar densities than the local solar neighborhood.
c.
Their stars typically
have low abundances of heavy elements.
d.
Their measured
positions in the sky and distances from us allow scientists to infer that the
Sun is located at the very center of the Milky Way Galaxy.
e.
Their orbits would take
them periodically through the galactic disk.
41.
The highest energy
cosmic rays that reach Earth
a.
could actually
originate outside the Milky Way galaxy.
b.
are produced in
particles 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 mater.
42.
Observations of our
galaxy seem to indicate the presence of an extended halo of _____________ with
a mass that is ________________________.
a.
very hot gas; similar
to the stellar mass of the Milky Way
b.
very cold gas; comparable
to the dark matter content of the Milky Way
c.
cosmic rays; much
greater than the mass of the whole Local Group
d.
neutrinos; similar to
the mass of all globular clusters of the Milky Way
e.
numerous young stars
with high content of heavy elements; much less than the mass of the Milky Way
43.
Observations show that
in the halo of the Milky Way
a.
there is nothing else
besides the globular clusters.
b.
the magnetic field is
stronger than in the disk.
c.
there is vigorous
ongoing star formation.
d.
there is a high rate of
occurrence for Type II supernovae.
e.
there are many stars
outside the globular clusters.
44.
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.
45.
Choose the incorrect
statement.
a.
There is a smooth
decline in abundances of heavy elements from the center of the disk outward,
along the radius.
b.
Older stars in the
central parts of the Milky Way can have higher abundances of heavy elements
than younger stars in the outskirts of the disk.
c.
Some globular clusters
are younger than certain open clusters within the disk.
d.
The stars in the halo
typically have very low abundances of heavy elements.
e.
There is essentially no
star formation happening in the halo.
46.
The stars in the disk
of the Milky Way near the Sun have
a.
heavy element
abundances similar to the Sun’s.
b.
velocities much higher
than that of the Sun.
c.
heavy element
abundances that are on average 10 times higher than the Sun’s.
d.
velocities much lower
than that of the Sun.
e.
heavy element
abundances that are on average 10 times lower than the Sun’s.
47.
You measure the heavy
element abundances and motions of stars near the Sun, and you plot your data as
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
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.
49.
Which of the following
is not correct about the galactic disk?
a.
The star formation in
the disk started after the formation of the stars in globular clusters.
b.
Star formation is more
intense in the inner parts of the galaxy than in its outer disk.
c.
Ongoing star formation
in the disk happens only around the bulge and bar.
d.
The distribution of
heavy elements throughout the disk is affected by accretion, merging events,
and galactic fountains.
e.
Recurring perturbations
can lead to the formation of long-lasting spiral structure.
50.
Which of the following
are generally not found in our galaxy’s halo?
a.
globular clusters
b.
cool, red stars
c.
RR Lyrae stars
d.
stars with high
percentages of heavy elements
e.
low-mass main-sequence
stars
51.
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 about 8.3 kpc from the center of the
galaxy.)
a.
180 thousand years
b.
7 million years
c.
35 million years
d.
230 million years
e.
620 million years
52.
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 stars SO-1, whose semimajor axis is
3300 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⊙
b.
7.6 × 109 M⊙
c.
1.5 × 108 M⊙
d.
4.0 × 106 M⊙
e.
3.4 × 1011 M⊙
53.
Most of the mass in our
galaxy is in the form of
a.
stars.
b.
gas.
c.
dust.
d.
dark matter.
e.
globular clusters.
54.
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.
.
55.
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
56.
If the central black
hole started accreting a lot of mass, it would manifest as what kind of an
object?
a.
a red supergiant
b.
a pulsar
c.
a planetary nebula
d.
an AGN
e.
a supernova remnant
57.
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
58.
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 galaxy
b.
unlike; makes up a
larger fraction of the total mass than in other galaxy
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
59.
What is the radius of
the event horizon for the 4 × 106 M⊙ 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
60.
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
61.
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.
62.
The Milky Way and
Andromeda are on a collision course. When will the 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
63.
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
64.
Which of the following
does not describe the Andromeda Galaxy?
a.
It is a spiral galaxy.
b.
It is the largest
member of the Local Group.
c.
It is on a collision
course with the Milky Way.
d.
It obeys Hubble’s law,
because its spectrum shows redshifts.
e.
It is currently about
770 kpc from the Milky Way.
65.
Which of the following
statements about the imminent collision between the Andromeda and Milky Way
galaxies in the distant cosmic future is not true?
a.
It could lead to the
formation of a big elliptical galaxy.
b.
The supermassive black
holes of the two galaxies could merge into a more massive one.
c.
The merger could
trigger a luminous quasar phenomenon.
d.
The frequency of
collisions between stars would reach extremely violent levels.
e.
The Sun could suffer a
slingshot effect and end up in a very different place within the disturbed
galaxy.
66.
The Andromeda Galaxy
harbors a supermassive black hole with a mass that is 25−50 times greater
than the mass of the black hole that lurks at the center of the Milky Way. The
event horizon of the Andromeda black hole would therefore be about
a.
2−4 AU.
b.
2−4 ly.
c.
75−150 km.
d.
8.3−50 kpc.
e.
10−12 miles.
67.
The most distant galaxy
observable with the naked eye from planet Earth is the
a.
Large Magellanic Cloud.
b.
Triangulum galaxy.
c.
Sagittarius Dwarf.
d.
Andromeda Galaxy.
e.
Small Magellanic Cloud.
68.
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 protogalactic clumps.
69.
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
protogalactic fragments after they had already turned most of their gas into
stars.
e.
the merging of two
galaxies, each about half as massive as the Milky Way is today.
70.
How many times would
the disk diameter of the Milky Way fit along the estimated diameter of the
Local Group?
a.
10
b.
1000
c.
20
d.
50
e.
100
SHORT ANSWER
1.
What are the main
observational difficulties in observing the shape and spiral arm pattern of the
Milky Way?
2.
Explain how a standard
candle allows you to determine the distance to an object. What is it that you
have to measure or know about the standard candle to derive its distance?
3.
What would be the
apparent angular size of a 100 ly- diameter globular cluster in the Andromeda
Galaxy, as measured from Earth?
4.
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?
5.
The Sun revolves about
the center of the galaxy at roughly 220 km/s at a distance of about 8.3 kpc.
How long will it take to complete one revolving cycle around the galactic
center? How many times has the Sun orbited the galaxy in its lifetime?
6.
You find a star
orbiting about the center of the galaxy at a distance of 15 kpc. Using the
rotation curve in the figure shown below, estimate how much mass is enclosed
within the orbit of that star? You can assume the star is on a circular orbit
around the center of the Milky Way.
7.
Describe a scale model
of the disk of the Milky Way using a scale in which 1 cm = 1000 light-years.
Note that the diameter of the disk is approximately 100,000 light-years, the
thickness of the thin disk of younger is about 1000 light-years, and the
thickness of the think disk of older stars is about 12,000 light-years.
8.
A typical globular
cluster consists of about 500,000 stars in a sphere whose radius is
approximately 10 pc. The average distance between stars in a globular cluster
is given approximately by 
, where r is the radius and N
is the number of stars in the cluster. Calculate this distance, and compare it
to the distance from the Sun to the nearest star.
, where r is the radius and N
is the number of stars in the cluster. Calculate this distance, and compare it
to the distance from the Sun to the nearest star.
9.
Outline some major
differences between the thin and the thick disks of the Milky Way galaxy.
10.
How do we know that
globular cluster stars were not the first stars that formed in our galaxy?
Would this still be true if some or all globular clusters came into the galaxy
by accretion of small satellite galaxies?
11.
Briefly explain how
scientists measure the ages of globular clusters.
12.
How would we explain
that there are no known disk stars with exceptionally low abundances of massive
elements?
13.
The Milky Way Galaxy
contains a reservoir of about 10 billion solar masses of interstellar material
(interstellar medium, or ISM, unincorporated in stars). Assuming that our
galaxy has about 200 billion solar masses worth of stars and the total estimated
mass of the galaxy is about 1 trillion solar masses, what fraction is still
available for star formation?
14.
A typical globular
cluster consists of about 500,000 stars in a sphere with a radius that is
approximately 10 pc. Assuming that the typical star is the size of our Sun,
estimate what fraction of the sphere is physically occupied by stars.
15.
What are the major
differences between globular and open star clusters?
16.
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?
17.
Where in the Milky Way
would we find the stars with the highest abundance of massive elements? Where
would we find those with the lowest abundance of massive elements? Why?
18.
If you wanted to study
the properties of nearby stars that were some of the first stars to form in the
Milky Way, but were not members of a globular cluster, how would
you go about finding them?
19.
The energy of a cosmic
ray is given by γmc2, where m is the mass of the particle, c
is the speed of light, and γ is the Lorentz factor 
. If a cosmic ray proton is traveling at a
speed of ν = 0.999999c, what is its total energy? How does that
compare with the kinetic energy of a grape tossed up in the air and caught in
your mouth? Assume the grape has a mass of about 5 grams and a speed of 0.5 m/s
when you catch it. The mass of a proton is 1.7 × 10−27 kg.
. If a cosmic ray proton is traveling at a
speed of ν = 0.999999c, what is its total energy? How does that
compare with the kinetic energy of a grape tossed up in the air and caught in
your mouth? Assume the grape has a mass of about 5 grams and a speed of 0.5 m/s
when you catch it. The mass of a proton is 1.7 × 10−27 kg.
20.
Outline the basic
scenario of formation and evolution of the Milky Way, including a description
of how the halo and disk formed.
21.
Explain why within the
galactic disk we can find older stars that have higher abundances of massive
elements than younger stars at larger radii from the center.
22.
Astronomers have
identified examples of extraordinarily massive globular clusters (in both the
Milky Way and Andromeda), which 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?
23.
Why is the supermassive
black hole in the center of our galaxy not an AGN right now? Could it be active
in the future? Why or why not?
24.
The “normal” mass of
the Milky Way (stars and interstellar medium together) is estimated at 100−
200 billion solar masses. The total mass of the galaxy,
however, is estimated in the interval of 1−1.5 trillion solar
masses. What is the fraction of “invisible” dark matter?
25.
Calculate the size of
the event horizon of the supermassive black hole at the center of the Milky
Way, whose mass is 4 × 106 M⊙. Give
your answer in units of AU.
26.
What objects could be
used to determine that the galaxy is mostly made of dark matter?
27.
What are the advantages
of observing the galaxy at long wavelengths like radio and IR?
28.
As technology is
advancing, scientists find more and more systems of very faint, dwarf galaxies
in the Local Group (some bound to the Milky Way). Could such systems represent
in fact the “unseen” dark matter scientists are trying to understand?
29.
How big does Andromeda
appear on a deep CCD image compared with the Moon’s disk? Note that deep CCD
images indicate a projected apparent angular size of 190 arcmin (× 60 arcmin for the
Andromeda Galaxy (an impressive visual rendition is available at the Astronomy
Picture of the Day website for the December 28, 2006 entry).
30.
Andromeda and the Milky
Way are approaching at a measured relative speed of 110 km/s. If Andromeda is
currently separated from the Milky Way by about 770 kpc, provide a simple
estimate for how far in the cosmic future the two galaxies will collide.
31.
What are the typical
distances between galaxies in the Local Group? What are the typical distances
between stars in the neighborhood of our Sun? Compare the two in a meaningful
way to explain whether collisions between stars are expected to be more or less
frequent than the collisions between galaxies.
32.
How the mass inside a
given radius must scale with that radius to produce a flat rotation curve in a
galaxy like Milky Way?
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