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Chapter 14: Our Star—The
Sun
Learning
Objectives
Define the boldfaced vocabulary terms within the chapter.
14.1 The Sun Is Powered by Nuclear Fusion
Describe hydrostatic equilibrium.
Multiple Choice: 3, 4, 5, 6, 7
Short Answer: 1, 2
Relate how a change in internal characteristics of a star
(e.g., core temperature, energy generation) will change its surface
characteristics (e.g., surface temperature, luminosity, size).
Multiple Choice: 15
Short Answer: 7
Illustrate the process of nuclear fusion between two nuclei.
Multiple Choice: 8, 10, 13, 14, 21
Explain why nuclear fusion is able to generate energy.
Multiple Choice: 1, 2, 9
Describe the conditions needed for nuclear fusion to occur.
Multiple Choice: 11, 18, 19, 20
Short Answer: 5
Illustrate the steps by which hydrogen nuclei are fused into
helium in the Sun in the proton-proton chain.
Multiple Choice: 12
Short Answer: 6
14.2 Energy Is Transferred from the
Interior of the Sun
Compare and contrast the conditions under which energy in the
sun is transported by radiation and by convection.
Multiple Choice: 24, 25, 26, 27, 28, 31
Short Answer: 8, 9, 11
Describe how opacity affects the speed at which energy is
radiated out of the Sun.
Multiple Choice: 22, 23, 29, 30, 32
Discuss how neutrino detection was used to test our theory of
nuclear fusion in the Sun, and how that led to a better understanding of
neutrinos themselves.
Multiple Choice: 33, 36, 37
Short Answer: 10, 13
Summarize the solar-neutrino problem and solution.
Multiple Choice: 34, 38
Short Answer: 12
Explain how helioseismology has been used to probe the
structure of the Sun.
Multiple Choice: 35, 39, 40
Short Answer: 14
14.3 The Atmosphere of the Sun
Illustrate the effect of limb darkening.
Multiple Choice: 41, 42
Short Answer: 15
Determine why sunlight has an absorption spectrum even though
we treat it as a blackbody.
Multiple Choice: 44, 45
Short Answer: 17
Characterize the different layers of the solar atmosphere.
Multiple Choice: 43, 46, 47, 48, 49, 50, 51, 52
Short Answer: 16, 18
14.4 The Atmosphere of the Sun Is Very
Active
Describe how magnetic effects in the sun create its solar
activity.
Multiple Choice: 53, 55, 57, 58, 60, 61, 62, 63, 66
Short Answer: 19, 21, 28
Summarize our theory explaining the sunspot cycle.
Multiple Choice: 56, 64, 65, 68
Short Answer: 22
Characterize the extent to which solar activity creates
measurable effects on Earth.
Multiple Choice: 54, 67, 69, 70
Short Answer: 23, 24, 25, 26, 27, 29, 30
Working It Out 14.1
Compute the fusion rate of a star.
Multiple Choice: 16
Short Answer: 3
Relate the mass, luminosity, fuel consumption, and lifetime
of a star powered by nuclear fusion.
Multiple Choice: 17
Short Answer: 4
Working It Out 14.2
Use the Stefan-Boltzmann law to compare the temperature and
flux of a star’s surface to its sunspots.
Short Answer: 20
MULTIPLE CHOICE
1.
The Sun is not
responsible for which of the following?
a.
daylight
b.
plant photosynthesis
c.
weather
d.
plate tectonics
2.
The Sun has a mass of
a.
2 × 1010
kg.
b.
2 × 1025
kg.
c.
2 × 1030
kg.
d.
2 × 1035
kg.
e.
2 × 1045
kg.
3.
Hydrostatic equilibrium
is a balance between
a.
heat and centrifugal
force.
b.
core temperature and
surface temperature.
c.
pressure and gravity.
d.
radiation and heat.
e.
centrifugal force and
gravity.
4.
Where does hydrostatic
equilibrium exist in the Sun?
a.
only in the core, where
energy production via fusion can balance gravity
b.
in the outer layers of
the atmosphere, where most of the visible light is produced
c.
just outside the core,
where heat from nuclear fusion is transported outward
d.
throughout the Sun
5.
Density, temperature,
and pressure increase as you move inward in the interior of the Sun. This means
that the weight of the star pushing inward at a given radius _________ as you
move inward toward the core.
a.
increases
b.
decreases
c.
stays the same
d.
There is not enough
information to answer.
6.
Which of the curves in
the figure shown below best matches the shape of a graph of the density of
material inside the Sun (in thousands of kg/m3) as you move farther
away from the center?
a.
A
b.
B
c.
C
d.
D
e.
E
7.
The balance of energy
in the solar interior means that
a.
energy production rate
in the core equals the rate of radiation escaping the Sun’s surface.
b.
the source of energy in
the core is stable and will sustain the Sun for millions of years.
c.
the outer layers of the
Sun absorb and re-emit the radiation from the core at increasingly longer
wavelengths.
d.
radiation pressure
balances the weight of the overlying solar layers.
e.
the core of the Sun has
pressure that is higher than that of the outer layers.
8.
Which force is
responsible for holding the protons and neutrons in the nucleus of an atom
together?
a.
gravity
b.
strong nuclear force
c.
electric force
d.
magnetic force
e.
electrons pushing them
together
9.
The majority of the
Sun’s energy comes from
a.
gravitational
contraction.
b.
nuclear fission of
uranium.
c.
hydrogen fusion.
d.
helium burning.
e.
burning material as in
a fire.
10.
Nuclei of atoms are
held together by
a.
gravity.
b.
the electric force.
c.
the strong nuclear
force.
d.
the weak nuclear force.
11.
What is the approximate
temperature at the center of the Sun?
a.
106 K
b.
1.5 × 107 K
c.
2.5 × 107 K
d.
10,000 K
12.
The net result of the
proton-proton chain of nuclear reactions is that four protons are converted
into
a.
one helium nucleus as
well as energy, electrons, and neutrinos.
b.
one helium nucleus as
well as deuterium, electrons, and energy.
c.
one helium nucleus, as
well as energy, positrons, and neutrinos.
d.
two helium nuclei, as well
as neutrinos and positrons.
13.
What do astronomers
mean when they say that the Sun makes energy by hydrogen burning?
a.
The Sun is combusting
hydrogen in a fire and releasing energy.
b.
The Sun is fusing
hydrogen into uranium and releasing energy.
c.
The Sun is made of
mostly hydrogen at very high temperature.
d.
The Sun is fusing
hydrogen into helium and releasing energy.
e.
The Sun is accumulating
hydrogen from the solar wind and releasing energy.
14.
When two atomic nuclei
come together to form a new species of atom, this is called
a.
nuclear fission.
b.
nuclear recombination.
c.
nuclear splitting.
d.
nuclear fusion.
e.
ionization.
15.
Suppose by some
mysterious process that the nuclear fusion rate in the core of the Sun were to
increase. What would happen to the appearance of the Sun?
a.
It would shrink so that
the higher gravity could balance the increased pressure from the core.
b.
It would grow larger
and hotter, making it more luminous.
c.
It would grow larger
but stay at the same temperature, making it more luminous.
d.
It would grow larger
but cooler.
16.
If the Sun converts 5 × 1011
kg of H to He per second and the mass of a single hydrogen nucleus is 1.7 × 10−27 kg, how many net
proton-proton reactions go on per second in the Sun? What is the luminosity
produced if the mass difference between a single helium nucleus and four
hydrogen nuclei is 4 × 10_29 kg? Note that 1 Watt = 1 m2
kg/s3.
a.
7 × 1037
reactions per sec; 4 × 1026 Watt
b.
3 × 1038
reactions per sec; 1027 Watt
c.
3 × 1038
reactions per sec; 4 × 1026 Watt
d.
7 × 1037
reactions per sec; 5 × 1025 Watt
e.
3 × 1037
reactions per sec; 6 × 1024 Watt
17.
If the Sun converts 5 × 1011
kg of H to He per second and 10 percent of the Sun’s total mass is available
for nuclear burning, how long might we expect the Sun to live?
a.
104 years
b.
108 years
c.
1010 years
d.
1011 years
e.
1014 years
18.
If the core of the Sun
were hotter than it is now, how would the Sun’s energy production change?
a.
It would produce less
energy per second than it does now.
b.
It would produce more
energy per second than it does now.
c.
Its energy production
would vary more than it does now.
d.
Its energy production
would be more stable than it is now.
e.
The Sun’s energy
production would not change.
19.
The energy that fuels
the Sun is generated
a.
only on its surface.
b.
only in its core.
c.
only in the solar wind.
d.
both in its core and on
its surface.
e.
in its core, on its
surface, and in the solar wind.
20.
Why is hydrogen burning
the main energy source for main-sequence stars?
a.
Hydrogen is the most
common element in stars.
b.
Hydrogen nuclei have
the smallest positive charge.
c.
Hydrogen burning is the
most efficient of all fusion or fission reactions.
d.
Hydrogen can fuse at
temperatures lower than other elements.
e.
All the above are valid
reasons.
21.
The net effect of the
proton-proton chain is that four hydrogen nuclei are converted to one helium
nucleus and _________ are released.
a.
visible wavelength
photons
b.
gamma ray photons,
positrons, and neutrinos
c.
ultraviolet photons and
neutrinos
d.
X-ray photons,
electrons, and neutrinos
e.
infrared photons and
positrons
22.
In the radiative zone
inside the Sun, photons are transported from the core to the convective zone
over a time of
a.
many thousands of
years.
b.
many millions of years.
c.
seconds.
d.
a few hours.
e.
months.
23.
If the Sun stopped
nuclear fusion in its core, how long would it take for its luminosity to change
significantly?
a.
months
b.
a few hours
c.
seconds
d.
about 100,000 years
24.
Which of the following
method(s) is (are) not used to transport energy from the core of the Sun
to its surface?
a.
radiation
b.
convection
c.
conduction
d.
All of the above are
important in the solar interior.
e.
None of the above are
important in the solar interior.
25.
If you hold onto one
end of a metal spoon after placing the other end in a pot of boiling water, you
will burn your hand. This is an example of energy being transported by
a.
radiation.
b.
convection.
c.
conduction.
d.
convection and
radiation.
e.
radiation and
conduction.
26.
Some restaurants place
food under infrared heat lamps so that it stays warm after it has been cooked.
This is an example of energy being transported by
a.
radiation.
b.
convection.
c.
conduction.
d.
convection and
conduction.
e.
radiation and
conduction.
27.
The interior zones of
the Sun are distinguished by
a.
jumps in density
between zones.
b.
their temperature
profiles.
c.
pressure differences
inside each zone.
d.
their modes of energy
transport.
e.
all of the above
28.
Which of the following
layers of the Sun makes up the majority of its interior?
a.
the core
b.
the radiative zone
c.
the convective zone
d.
the photosphere
e.
the chromosphere
29.
Approximately how long
does it take the photons released in nuclear reactions in the core of the Sun
to exit the Sun?
a.
8 minutes
b.
16 hours
c.
1,000 years
d.
100,000 years
e.
4.6 billion years
30.
Light from the Sun
reaches Earth approximately _________ times faster than photons released in
fusion in the core.
a.
1,000
b.
600,000
c.
1 million
d.
6 billion
e.
10 billion
31.
When you turn on the
heater in a car, the passengers in the front seat warm up first, and then
eventually the warm air gets to the passengers in the back seat. This is an
example of energy being transported by
a.
radiation.
b.
convection.
c.
conduction.
d.
convection and
conduction.
e.
radiation and conduction.
32.
Which of these can
travel directly from the center of the Sun to Earth in about 8 minutes?
a.
photons
b.
electrons
c.
protons
d.
neutrons
e.
neutrinos
33.
What makes neutrinos so
different from other particles of matter?
a.
They interact very
weakly with other particles.
b.
They interact very
strongly with other particles.
c.
They are the only
particles that move quickly.
d.
They move very slowly.
34.
How was the solar
neutrino problem solved?
a.
by postulating that
neutrinos have a very large mass
b.
by postulating that
neutrinos oscillate between three different types
c.
by postulating that
some neutrinos become photons during their journey
d.
by postulating that
some neutrinos interact more strongly with matter such that they are absorbed
locally inside the Sun
35.
How does the fact that
the surface of the Sun rings like a bell help us better understand the Sun?
a.
The ringing tells us
how quickly the Sun is expanding with time.
b.
The ringing helps us
understand the solar interior better.
c.
The ringing reveals how
rapidly the Sun’s magnetic field is changing.
d.
The ringing helps us
determine the surface temperature of the Sun.
36.
The detection of solar
neutrinos confirms that
a.
the Sun’s core is
powered by proton-proton fusion.
b.
energy transport by
radiation occurs throughout much of the solar interior.
c.
magnetic fields are
responsible for surface activity on the Sun.
d.
convection churns the
base of the solar atmosphere.
e.
sunspots are cooler
than the rest of the photosphere.
37.
If neutrinos oscillated
between five different types of neutrino during their transit from the Sun to
Earth and we could only detect one type of neutrino, then how many neutrinos
would we have detected compared with what was emitted by the Sun?
a.
one-half as many
b.
one-third as many
c.
one-fourth as many
d.
one-fifth as many
e.
We would detect no
neutrinos.
38.
The solar neutrino
problem was solved by
a.
adjusting the rate of
hydrogen burning in solar models.
b.
improving detector
efficiencies so more neutrinos were observed.
c.
postulating that
neutrinos had mass and oscillated between three different types.
d.
lowering the percentage
of helium in models of solar composition.
e.
correctly measuring the
density of the Sun’s interior.
39.
By studying how the
surface of the Sun vibrates like a struck bell we can determine its
a.
age.
b.
interior density.
c.
total mass.
d.
size.
e.
temperature.
40.
We can determine how
the density changes with radius in the Sun using
a.
radar observations.
b.
neutrino detections.
c.
high-energy (gamma ray)
observations.
d.
helioseismology.
e.
infrared observations.
41.
The surface of the Sun
appears sharp when we look at it in visible light because
a.
the photosphere is
cooler than the layers below it.
b.
the photosphere is thin
compared with the other layers in the Sun.
c.
the photosphere is much
less dense than the convection zone.
d.
the photosphere is
transparent to radiation.
e.
the Sun has a distinct
surface.
42.
Imagine that you
observed the Sun and measured the brightness of the face of the Sun at the
locations marked in the figure below. At which of these locations would you
measure the lowest brightness?
a.
A
b.
B
c.
C
d.
D
e.
They would all have the
same brightness.
43.
The hottest layer of
the solar atmosphere is the
a.
outer convection zone.
b.
photosphere.
c.
chromosphere.
d.
corona.
44.
The solar spectrum is
an example of a(n) _________ spectrum.
a.
emission
b.
absorption
c.
continuum
d.
blackbody
e.
X-ray
45.
Which of the following cannot
be measured from the optical absorption spectrum of the Sun?
a.
the temperature of the
photosphere
b.
the composition of the
Sun
c.
the temperature of the
corona
d.
the density of the
photosphere
46.
The Sun’s chromosphere
appears red because
a.
it is hotter than the
photosphere.
b.
as the Sun rotates, the
chromosphere appears to move away from us radially.
c.
it has a higher
concentration of heavy metals.
d.
it is made of mostly
helium.
e.
its spectrum is
dominated by Hα emission.
47.
The figure below shows
the Sun during a solar eclipse at visible wavelengths. Which part of the Sun is
visible around the shadow of the Moon?
a.
chromosphere
b.
photosphere
c.
radiative zone
d.
convective zone
e.
corona
48.
The best wavelength to
use to observe a solar prominence is
a.
550 nm, green visible
light.
b.
656 nm, a red hydrogen
emission line.
c.
16 mm, an ultraviolet
emission line.
d.
21 cm, microwave
emission.
e.
0.02 nm, X-ray
emission.
49.
The Sun’s corona has a
temperature of approximately 1 million degrees. At what wavelength and in what
part of the electromagnetic spectrum does its radiation peak?
a.
550 nm, visible
b.
2 × 10_5
m, infrared
c.
4 × 10_7
m, ultraviolet
d.
3 × 10_9
m, X-rays
e.
6 m, radio
50.
Which of the layers of
the Sun is located the farthest from the center of the Sun?
a.
chromosphere
b.
photosphere
c.
radiative zone
d.
convective zone
e.
corona
51.
We know the Sun’s
corona is very hot because
a.
we observe it emitting
radiation at visible wavelengths.
b.
the chromosphere and
the photosphere are that hot, too.
c.
we observe absorption
from highly ionized atoms of iron and calcium in its spectrum.
d.
the gas emits most of
its radiation at radio wavelengths.
e.
all of the above
52.
Suppose coronal holes
covered a larger fraction of the Sun’s surface than they currently do. Which of
the following consequences would result?
a.
The solar wind would
contain a higher density of particles.
b.
The solar wind would
become hotter.
c.
The solar wind would
move faster.
d.
The composition of the
solar wind would change.
53.
What keeps the gas in
the Sun’s corona from flying away from the Sun?
a.
gravity
b.
strong nuclear force
c.
the Sun’s magnetic
field
d.
the solar wind
e.
sunspots
54.
Which of the following
is not a result of an increase in solar activity?
a.
The altitudes of
orbiting satellites decrease.
b.
Airplanes have trouble
navigating.
c.
Stronger auroras are
seen.
d.
Power grids can be
damaged.
e.
None of the above; all
of these are caused by increased solar activity.
55.
The figure shown below,
taken at visible wavelengths, shows a section of the Sun with sunspots visible.
Which of the labeled regions is the lowest temperature?
a.
region A
b.
region B
c.
region C
d.
They are all the same
temperature.
e.
There is not enough
information to determine their relative temperatures.
56.
In a sunspot, the umbra
is
a.
hotter than the
penumbra.
b.
cooler than the
penumbra.
c.
the same temperature as
the penumbra.
d.
less dense than the
penumbra.
57.
The solar magnetic
field
a.
returns to the same
polarity every 11 years.
b.
switches polarity every
22 years.
c.
switches polarity every
11 years.
d.
retains the same
polarity during the entire solar activity cycle.
58.
Sunspots appear dark
because they have _________ than those of the surrounding gases.
a.
densities that are
higher
b.
densities that are
lower
c.
pressures that are
higher
d.
temperatures that are
lower
e.
temperatures that are
higher
59.
If a sunspot appears
one-quarter as bright as the surrounding photosphere, and the average
temperature of the photosphere is 5800 K, what is the temperature of the gas in
this sunspot?
a.
3625 K
b.
4100 K
c.
4500 K
d.
5200 K
e.
5500 K
60.
Which of the following
are created by solar magnetic activity?
a.
sunspots
b.
prominences
c.
coronal mass ejections
d.
solar flares
e.
all of the above
61.
The darkest part of a
sunspot is called the
a.
penumbra.
b.
umbra.
c.
granule.
d.
photosphere.
e.
magnetic field.
62.
The magnetic field of
the Sun is continuously produced and deformed by
a.
its differential
rotation.
b.
the solar wind.
c.
changes in the rate of
nuclear fusion in the core.
d.
a liquid conducting
layer in the interior.
e.
This is a trick
question. The solar magnetic field is primordial.
63.
The Sun’s internal
magnetic field becomes tangled up over time because of
a.
coronal holes.
b.
coronal mass ejections.
c.
differential rotation.
d.
temperature changes in
the Sun’s core.
e.
all of the above
64.
If you observe a
maximum number of sunspots right now, how long would you have to wait to see
the next solar maximum?
a.
24 hours
b.
6 months
c.
1 year
d.
11 years
e.
22 years
65.
The Maunder Minimum was
a 60-year period when
a.
debris from a comet
collision blanketed the Sun.
b.
almost no sunspots
occurred on the Sun.
c.
the Voyager 2
spacecraft traversed the heliopause.
d.
very few dust storms
occurred on Mars.
e.
very few volcanic
eruptions occurred on Mars.
66.
The Sun’s magnetic
field reverses direction every
a.
24 hours.
b.
27 days.
c.
12 months.
d.
11 years.
e.
22 years.
67.
If a coronal mass
ejection occurs on the Sun that expels material at a speed of 800 km/s, how
long will it take these charged particles to reach the Earth?
a.
0.7 day
b.
1.4 days
c.
1.8 days
d.
2.2 days
e.
3.5 days
68.
When is the Sun most
luminous?
a.
when there are a
maximum number of sunspots
b.
when there are an
average number of sunspots
c.
when there are a
minimum number of sunspots
d.
The Sun’s luminosity
does not change.
e.
The Sun’s luminosity
changes, but it has no relation to the number of sunspots.
69.
When solar activity is
very high, the Earth’s atmosphere will
a.
expand.
b.
contract.
c.
remain approximately
the same.
d.
repel charged
particles.
e.
block out sunlight.
70.
Solar wind particles
hit the surface of the Moon, but they don’t make it to the surface of the Earth
because the Earth
a.
is larger than the
Moon.
b.
is warmer than the
Moon.
c.
has an atmosphere while
the Moon does not.
d.
has a magnetic field
while the Moon does not.
e.
is farther from the Sun
than the Moon is.
SHORT ANSWER
1.
In addition to the laws
of physics and chemistry, what information do we need to know about our Sun to
calculate its internal structure and radius?
2.
Explain why hydrostatic
equilibrium results in the center of the Sun having the highest pressure and
temperature.
3.
Calculate the amount of
energy released by converting four hydrogen atoms into one helium atom. The
mass of a hydrogen atom is 1.67 × 10_24g;
the mass of a helium atom is 6.65 × 10_24
g. The speed of light is 3 × 108 m/s.
4.
Through hydrogen
fusion, the Sun loses approximately 4 million tons of mass each second. If it
burns hydrogen at this rate for 10 billion years, what percentage of its
original mass will it lose in all? (Note: The mass of the Sun is 1.99 × 1030
kg, and 1 ton = 1,000 kg.)
5.
Why is hydrogen burning
the main energy source for main-sequence stars? Give at least two reasons.
6.
In the proton-proton
chain, the net reaction is that 4 protons are converted into 1 helium nucleus.
What other byproducts are released in this reaction, and why?
7.
In the text we
considered the case of a “too-large” Sun. Show that a star with the same mass,
composition, radius, and luminosity as the Sun, but with a higher temperature
(that is, a “too-hot” Sun), also leads to a contradiction.
8.
List three methods of
energy transport in nature and explain how the energy is being transferred in
each of those methods. Which two are means by which energy is transported
inside the Sun?
9.
The figure below shows
a diagram of the Sun with zones labeled A, B, and C. Explain how energy is
being transferred in each of the three regions.
10.
Explain why the
solution to the solar neutrino problem is an excellent example of how
observations drive the evolution of science.
11.
Explain why the
radiative zone in the solar interior gives way to a convective zone
approximately two-thirds of the way to the surface.
12.
Describe the solar
neutrino problem and its solution.
13.
Why are neutrino
detectors located deep underground?
14.
Describe how
helioseismologists measure the opacity of the solar interior.
15.
What is “limb
darkening”? Explain why limb darkening occurs in the Sun.
16.
What makes the
chromosphere appear so red?
17.
What is the origin of
the absorption lines in the Sun’s spectrum?
18.
The temperature profile
of the solar atmosphere is shown in the figure shown below. What causes the
sharp increase in temperature when going from the chromosphere to the corona?
19.
Explain why magnetic
fields trap coronal gas over much of the solar surface but allow it to escape
in coronal holes.
20.
If a sunspot is half as
bright as the surrounding photosphere of the Sun, what is the approximate
temperature of the gas in the sunspot if the photosphere’s average temperature
is 5800 K?
21.
The Sun exhibits
differential rotation. Explain what differential rotation is. Which planets
also do this? Why don’t the others?
22.
When, during its
11-year cycle, is the Sun most luminous? What might this have to do with the
Maunder Minimum?
23.
Astronauts in space
could be harmed by the high-energy particles given off during a solar flare.
So, when a solar flare is observed, a warning can be given to astronauts to
tell them to get inside the space station for protection. Explain why there is
enough time between the first observation of a flare and the arrival of the harmful
particles for this system to work.
24.
If a coronal mass
ejection occurred on the Sun and ejected particles toward the Earth that
traveled at the speed of 1,000 km/s, how long would it take them to reach
Earth?
25.
If the Sun went through
a period where there were many sunspots for a number of decades straight, what
would happen to the climate of the Earth?
26.
How do periods of
strong solar activity affect near-Earth-orbiting spacecraft?
27.
Why is there increased
drag on spacecraft orbiting the Earth during periods of increased solar
activity?
28.
When gas in a sunspot
cools, it does not sink into the solar interior as one would expect for
material in an atmosphere. Why?
29.
Explain what the
heliosphere is and how it helps protect life on Earth.
30.
What eventually stops
the solar wind from expanding in the outer reaches of our Solar System, 100 AU
from the Sun?
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Chapter 15: Star Formation and the Interstellar Medium
Learning Objectives.
Define the bold-faced vocabulary terms within the chapter.
15.1 The Interstellar Medium Fills the
Space between the Stars
Describe
the observational signatures of interstellar dust.
Multiple
Choice: 2, 13, 14, 15, 16, 17, 18
Short
Answer: 3
Differentiate between interstellar extinction and reddening.
Multiple
Choice: 33
Short
Answer: 7
Compare and contrast the densities and temperatures of the
gas components of the interstellar medium.
Multiple
Choice: 1, 3, 4, 6, 7, 10, 11, 12, 20, 21, 28, 29, 31
Short
Answer: 1, 2, 4, 10, 11, 13, 14
Describe the observational signatures of each gas component
of the interstellar medium.
Multiple
Choice: 5, 8, 22, 23, 24, 25, 26, 27, 30, 32
Short
Answer: 5, 6, 8, 9
15.2 Molecular Clouds Are the Cradles of
Star Formation
Describe
the process of fragmentation during the collapse of a cloud.
Multiple
Choice: 35, 36, 37, 39, 40, 41, 42, 43
Short
Answer: 15, 16
Evaluate why molecular clouds are the cradles of star
formation.
Multiple
Choice: 34, 38
Short
Answer: 17, 18
15.3 Formation and Evolution of Protostars
Explain
why conversion of gravitational to thermal energy heats a collapsing gas.
Multiple
Choice: 47
Short
Answer: 20, 21, 23
Describe how hydrostatic equilibrium supports a
self-gravitating object.
Illustrate
the chain of events leading from molecular cloud to protostar.
Multiple
Choice: 9, 44, 45, 48, 53
Short
Answer: 19
Distinguish between the conditions under which a protostar
becomes a star and a brown dwarf.
Multiple
Choice: 49, 50, 51, 52
Short
Answer: 22
15.4 Evolution before the Main Sequence
Describe
how the H−
ion acts as a natural thermostat for a star.
Multiple
Choice: 54
Short
Answer: 24
Explain why a protostar’s temperature rises but its
luminosity drops as it settles onto the main sequence.
Multiple
Choice: 46, 55, 56, 57, 58, 59, 60, 62, 64
Short
Answer: 25
Illustrate the observational features and possible origins of
bipolar outflows.
Multiple
Choice: 67, 68
Short
Answer: 26
Establish why a protostar’s mass influences the rate at which
it collapses to become a star.
Multiple
Choice: 61, 63, 65, 66, 69
Short
Answer: 28
Describe the conditions necessary for planets to form around
protostars.
Multiple
Choice: 70
Short
Answer: 30
Working It Out 15.1
Compute
the peak wavelength of emission from dust grains.
Multiple
Choice: 19
Short
Answer: 12
Working It Out 15.2
Use
the blackbody luminosity, temperature, and size
relationship to relate how changing a protostar’s size changes its luminosity.
Short
Answer: 27, 29
MULTIPLE CHOICE
1.
The average density of the interstellar
medium is
a. much
denser than the Earth’s atmosphere.
b. much
less dense than the best vacuum on Earth.
c. about
the same density as air on the peak of Mount Everest.
d. zero.
2.
The dust in the interstellar medium comes
primarily from
a. the
stellar winds of main-sequence stars.
b. the
cooled material ejected from supernova explosions.
c. cold
cores of molecular clouds.
d. all
of the above
3.
The lowest-density gas in the interstellar
medium is also the
a. coldest.
b. least
ionized.
c. hottest.
d. most
localized, being found mostly around protostars.
4.
The interstellar medium is divided up into
three different kinds of gas clouds. These are
a. cold
gas at 100 K, warm gas at 8000 K, and hot gas at about 1 million K.
b. warm
gas at 8000 K, hot gas at 1 million K, and superhot gas at 10 million K.
c. warm
gas at 5000 K, warm-hot gas at 100,000 K, and hot gas at about 1 million K.
d. cold
gas at 100 K, cool gas at 1000 K, and warm gas at 8000 K.
5.
We observe neutral hydrogen gas using
a. X-ray
radiation from highly ionized atoms.
b. visible
radiation at 656.3 nm from re-combined hydrogen.
c. 21-cm
emission.
d. ultraviolet
radiation from helium and oxygen.
6.
Molecular hydrogen atoms are found
a. everywhere
throughout interstellar space.
b. only
in dense clouds where they are shielded from stellar radiation.
c. in
low density clouds of hot gas surrounding hot stars.
d. only
in the atmospheres of the giant planets, such as Jupiter.
7. The
coldest molecular clouds in our galaxy have temperatures of approximately
a. 1000
K.
b. 100
K.
c. 10
K.
d. 0
K.
8. Electronic
transitions from the H2 molecule are easily seen at
a. X-ray
wavelengths.
b. visible
wavelengths.
c. radio
wavelengths.
d. infrared
wavelengths.
9. If
you could watch stars forming out of a gas cloud, which stars would form first?
a. low-mass
stars
b. medium-mass
stars
c. high-mass
stars
d. stars
with low temperatures
e. stars
with more heavy elements
10. When
looking at the space between stars, what might you see?
a. nothing;
it is empty.
b. spacecraft
c. gas
d. dark
matter
e. none
of the above
11. The
average density of the interstellar medium is
a. 1
atom/cm3.
b. 1,000
atom/cm3.
c. 104
atom/cm3.
d. 106
atom/cm3.
e. 1012
atom/cm3.
12. If
you wanted to observe heavy elements in the interstellar medium, where would be
the best place to look?
a. dust
grains
b. cold
gas
c. hot
gas
d. warm
gas
13. When
radiation from an object passes through the interstellar medium,
a. the
object appears dimmer.
b. the
object appears bluer.
c. the
object appears bluer and dimmer.
d. the
object appears redder and dimmer.
e. the
object’s apparent velocity changes.
14. Dust
in the ISM appears dark in _________ wavelengths and bright in _________
wavelengths.
a. visible;
ultraviolet
b. infrared;
radio
c. infrared;
visible
d. radio;
ultraviolet
e. visible;
infrared
15. Dust
reddens starlight because it
a. re-emits
the light it absorbs at red wavelengths.
b. emits
mostly in the infrared due to its cold temperature.
c. is
made mostly of hydrogen, which produces the red H-alpha emission line.
d. preferentially
affects light at visible and shorter wavelengths.
e. primarily
moves away from Earth.
16. What
is the most likely explanation for the dark area in the figure shown below?
a. It
is a region where there are no stars.
b. It
is a region with lots of dark matter.
c. It
is a super-massive black hole.
d. It
is a region with thick dust blocking the starlight coming from behind.
e. It
is a dark star cluster.
17. The
figure below shows the spectrum of a star, along with five other spectra
labeled A through E. Which one of the labeled spectra shows what the spectrum
of that star would look like if it were viewed through a significant amount of
interstellar dust?
a. A
b. B
c. C
d. D
e. E
18. The
figure below shows three pictures of the disk of the Milky Way, taken in three
different wavelength ranges. Put the three pictures in order from shortest to
longest wavelength.
a. I,
II, III
b. II,
III, I
c. I,
III, II
d. II,
I, III
e. III,
I, II
19. Dust
that is heated to 30 K will emit a blackbody spectrum that peaks at
a. 1
µm.
b. 30
µm.
c. 50
µm.
d. 100
µm.
e. 500
µm.
20. Sitting
in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This
analogy illustrates why
a. interstellar
dust is dark at optical wavelengths but bright in the infrared.
b. supernovae
can heat their shells to such high temperatures.
c. an
astronaut would feel cold standing in the 106 K intercloud gas.
d. the
Solar System is immersed in a hot bubble of gas.
e. fusion
occurs only in the cores of stars.
21. Which
of the following is responsible for heating the bulk of the very hot intercloud
gas?
a. high-energy
radiation from stars
b. supernovae
c. young
O and B stars
d. planetary
nebulae
e. The
heating is an even mix of all of the sources above.
22. Warm
ionized gas in the interstellar medium appears _________ when imaged in the
optical region of the electromagnetic spectrum.
a. red
b. yellow
c. white
d. blue
e. dark
23. The
red emission in the figure shown below is due to
a. carbon
monoxide (CO).
b. warm,
neutral hydrogen.
c. molecular
hydrogen (H2).
d. ionized
hydrogen (H II region).
e. dust.
24. An
H II region signals the presence of
a. newly
formed stars.
b. young
stars.
c. ionized
hydrogen gas.
d. O-
and B-type stars.
e. all
of the above
25. If
you wanted to study regions where star formation is currently happening, you
could use
a. H-alpha
emission to look for O and B stars.
b. 21-cm
radiation to find neutral hydrogen clouds.
c. radio
emission from carbon monoxide (CO) to find molecular cloud cores.
d. infrared
emission to identify T Tauri stars.
e. all
of the above
26. 21-cm
radiation is important because it
a. allows
us to study the deep interiors of stars.
b. allows
us to image magnetic fields directly.
c. allows
us to study neutral hydrogen in the interstellar medium.
d. is
produced by every object in the universe.
e. is
the longest wavelength of light that can naturally be produced.
27. We
detect neutral gas in the interstellar medium by looking for radiation at 21 cm
that arises when
a. an
electron moves from the n = 1 to n = 2 state in a hydrogen atom.
b. an
electron is ionized from a hydrogen atom.
c. carbon
monoxide (CO) gas is excited by stellar radiation.
d. the
spin of an electron flips and aligns with the spin of a proton in a hydrogen
atom.
e. an
electron combines with a proton to make a hydrogen atom.
28. In
the interstellar medium, molecules survive only in regions with
a. low
temperatures.
b. high
densities.
c. lots
of dust.
d. all
of the above
29. Interstellar
clouds are
a. hydrogen
gas, condensed out of the interstellar medium, like water clouds in the Earth’s
atmosphere.
b. regions
where hydrogen tends to be denser than the surrounding gas.
c. regions
where water condenses out of the interstellar medium.
d. oxygen
gas, condensed out of the interstellar medium, like water clouds in the Earth’s
atmosphere.
e. regions
where hydrogen combines with oxygen to create water molecules.
30. What
primarily makes it difficult to observe the process of star formation?
a. They
occur in dusty regions.
b. They
have low luminosities.
c. They
do not shine at any wavelength until they become T Tauri stars.
d. The
star formation process happens so quickly.
e. They
are too small to be seen.
31. A
typical molecular cloud has a temperature of approximately
a. 0.3
K.
b. 10
K.
c. 80
K.
d. 300
K.
e. 1000
K.
32. Molecular
clouds, which have temperatures of around 10 K, are best observed at _________
wavelengths.
a. X-ray
b. ultraviolet
c. optical
d. infrared
e. radio
33. Interstellar
extinction compared to interstellar reddening is like _______ as opposed to
_______
a. viewing
the Sun through a fog in Earth’s atmosphere; viewing the Sun through a cloud of
haze from a forest fire.
b. viewing
the Sun through a cloud of haze from a forest fire; viewing the Sun looking
outward from underwater.
c. viewing
the Sun through a cloud of haze from a forest fire; viewing the Sun through a
fog in Earth’s atmosphere.
d. viewing
the Sun looking outward from underwater; viewing the Sun through a prism.
34. Molecular
cloud cores are places where you might find
a. protostars
b. Herbig-Haro
objects.
c. molecular
hydrogen (H2).
d. carbon
monoxide (CO).
e. all
of the above
35. For
an object in hydrostatic equilibrium, if the temperature inside the object were
to increase, the object would
a. expand.
b. contract.
c. implode.
d. remain
the same size.
e. explode.
36. Because
angular momentum must be conserved, as a gas cloud contracts due to gravity it
will also
a. spin
slower.
b. spin
faster.
c. increase
in temperature.
d. decrease
in temperature.
e. stay
the same temperature.
37. Star
formation in a molecular cloud can be slowed by
a. the
presence of dust.
b. the
strength of its magnetic field.
c. turbulence
caused by supernovae and stellar winds from massive stars.
d. all
of the above
38. Stars
forming in molecular clouds tend to form first in
a. the
low-density periphery.
b. the
high-density core.
c. random
locations.
d. any
location where the temperature is highest.
39. Of
the following processes at work in molecular clouds, which is the one that
inevitably dominates the clouds’ evolution?
a. magnetic
fields
b. conservation
of angular momentum
c. pressure
d. gravity
e. turbulence
40. Magnetic
fields inside a molecular cloud act to
a. inhibit
gravitational collapse.
b. fragment
the cloud into numerous cores.
c. modulate
the temperature of the molecules.
d. increase
the formation of dust grains.
e. increase
the formation of protostars.
41. The
entire process of star formation is really just an evolving balance between
a. heat
and rotation.
b. core
temperature and surface temperature.
c. pressure
and gravity.
d. radiation
and heat.
e. luminosity
and distance.
42. Which
of the following traits does not
help slow or prevent the collapse of a gas cloud?
a. high
mass
b. high
temperature
c. turbulence
d. magnetic
fields
e. angular
momentum
43. An
accretion disk forms around a collapsing protostar because infalling material
must conserve
a. energy.
b. centrifugal
force.
c. gravity.
d. velocity.
e. angular
momentum.
.
44. As
a protostar evolves, its temperature
a. decreases
because it is radiating.
b. decreases
because of gravitational contraction.
c. decreases
because of angular momentum.
d. increases
because of nuclear fusion.
e. increases
due to the kinetic energy of infalling material.
45. A
protostar is
a. in
hydrostatic equilibrium as it collapses.
b. far
out of hydrostatic equilibrium when it collapses.
c. heated
to millions of degrees as it collapses.
d. flattened
into a disk as it collapses.
46. A
young protostar is _________ than the Sun even though its surface temperature
is _________
a. less
luminous; hotter.
b. larger;
cooler.
c. smaller;
the same.
d. more
luminous; cooler.
e. smaller;
hotter.
47. The
source of energy for a contracting protostar comes from
a. thermonuclear
energy.
b. kinetic
energy.
c. chemical
energy.
d. gravitational
potential energy.
e. radiation
energy.
48. What
happens as a protostar contracts?
a. Its
density rises.
b. Its
temperature rises.
c. Its
radius decreases.
d. Its
pressure rises.
e. All
of the above are true.
49. What
critical event transforms a protostar into a normal main-sequence star?
a. Planets
form in the accretion disk.
b. The
star grows suddenly larger in radius.
c. Triple
alpha reactions begin in the core.
d. Nuclear
fusion begins in the core.
e. Convection
begins throughout the star’s interior.
50. Stars
with a mass from 0.01 M⊙ to 0.08 M⊙ are
very different from the Sun because they
a. do
not have strong enough gravity to form planets.
b. have
much higher temperatures than the Sun.
c. cannot
successfully execute the proton-proton chain reactions.
d. form
much faster than the Sun did.
e. do
not exhibit sunspots.
51. A
_________ is a failed star that shines primarily because of energy derived from
its gravitational collapse rather than nuclear burning.
a. black
hole
b. brown
dwarf
c. Herbig-Haro
object
d. protostar
e. T
Tauri star
52. Brown
dwarfs are considered failed stars because
a. they
never reach masses larger than 50 Jupiter masses.
b. hydrogen
fusion never begins in their cores.
c. convection
never plays a role in their energy transport.
d. they
shine primarily at infrared wavelengths.
e. they
are never as luminous as the Sun.
53. The
H−
atom is important in protostars because it acts as a
a. source
of friction, stopping the cloud from collapsing too rapidly.
b. source
of infrared radiation, causing the cloud to cool off rapidly.
c. temperature
regulator.
d. source
of buoyancy, allowing the atmosphere of the protostar to expand.
54. The
H−
ion is very important in protostars because it
a. reacts
with oxygen to produce water.
b. undergoes
fusion and produces energy.
c. helps
make the protostars denser.
d. acts
as a temperature regulator.
e. reduces
angular momentum.
55. A
protostar’s evolutionary “track” in the H-R diagram traces out
a. only
how the protostar’s radius changes with time.
b. how
the protostar’s luminosity, temperature, and radius change with time.
c. only
how the protostar’s luminosity changes with time.
d. only
how the protostar’s spectral type changes with time.
e. the
protostar’s location in the molecular cloud.
56. The
Hayashi track of a low-mass protostar in the H-R diagram is a path of
approximately constant
a. density.
b. luminosity.
c. age.
d. temperature.
e. radius.
57. Use
the figure shown below to complete the following statement. A high-mass
protostar remains roughly constant in _________ and increases in _________ as
it follows its evolutionary track.
a. temperature;
luminosity
b. radius;
temperature
c. luminosity;
radius
d. luminosity;
temperature
e. radius;
luminosity
58. Use
the figure shown below to complete the following statement. A low-mass
protostar remains roughly constant in _________ and decreases in _________ as
it follows its evolutionary track.
a. temperature;
luminosity
b. radius;
temperature
c. luminosity;
radius
d. luminosity;
temperature
e. radius;
luminosity
59. Use
the figure shown below to complete the following statement. At the start of the
evolution of a protostar, the radius of a 60 M⊙ protostar is roughly _________ that of a 1 M⊙ main-sequence star.
a. 10
times bigger than
b. 100
times bigger than
c. 10
times smaller than
d. 100
time smaller than
e. the
same as
60. Use
the figure shown below to complete the following statement. As a protostar
contracts,
a. the
luminosity decreases.
b. the
luminosity increases.
c. the
temperature increases.
d. the
temperature decreases.
e. either
the luminosity decreases or the temperature increases.
61. Star
formation is an inefficient process, with only a few percent of the initial
cloud fragment ending up as stars. This means the initial mass of a molecular
cloud fragment that formed a 2 M⊙ star
was probably close to
a. 1
M⊙.
b. 50
M⊙.
c. 100
M⊙.
d. 5000
M⊙.
e. 1,000,000
M⊙.
62. If
a 1 M⊙ protostar starts out on the Hayashi track with
a temperature of 3300 K and a luminosity of 320 L⊙, what is its
approximate radius?
a. 5
R⊙
b. 50
R⊙
c. 100
R⊙
d. 200
R⊙
e. 500
R⊙
63. Which
of the following stars spend the longest time on their Hayashi tracks?
a. 100
M⊙ stars
b. 10
M⊙ stars
c. 1
M⊙ stars
d. 0.1
M⊙ stars
e. 0.08
M⊙ stars
64. A
surprising fact about a 1 M⊙ protostar is that, even though nuclear
reactions have not yet started in their cores, they are _________ than the Sun
a. hotter
b. rotating
faster
c. smaller
d. denser
e. more
luminous
65. How
long does it typically take for a protostar to form a 1 M⊙ star?
a. 3
× 107 years
b. 3
× 105 years
c. 3,000
years
d. 300
years
e. 30
years
66. The
most common types of stars in our galaxy are
a. high-mass
stars.
b. low-mass
stars.
c. an
equal mix of high- and low-mass stars.
d. low-mass
stars near the Sun and high-mass stars far away.
e. We
do not yet know which types of stars are most common in our galaxy.
67. Herbig-Haro
objects are almost always found
a. in
pairs on either side of a young protostar.
b. far
away from molecular clouds where stars form.
c. close
to planets that are forming around protostars.
d. deep
inside molecular clouds.
68. When
winds blow the gas away from a forming protostar, the protostar
a. expands
rapidly to 100 times its original size.
b. is
revealed as a main-sequence star.
c. becomes
visible as a T Tauri star.
d. is
unable to reach the main sequence.
69. When
a molecular cloud fragments,
a. the
least massive stars are the first to form, while the most massive stars take
longer.
b. the
most massive star are the first to form, while the least massive star take
longer.
c. the
most massive stars promptly explode as supernovae, destroying all remaining
gas.
d. the
stars form at the same rate, regardless of their mass.
70. Where
have astronomers observed the existence of planets?
a. in
our Solar System
b. orbiting
stars other than the Sun
c. orbiting
stars in binary systems
d. traveling
on their own through the Milky Way, not orbiting a star
e. all
of the above
SHORT ANSWER
1.
Compare the volume of the Sun with the
volume of interstellar space it occupies. Is the occupied percentage large or
small? Consider the volume around the Sun to be a sphere whose radius is equal
to the distance to the nearest star, which is equal to 5 light-years. (Note:
the radius of the Sun is 7 ×
105 km, and 1 light-year = 9.5 × 1012 km.)
2.
What is the interstellar medium made of?
Give rough percentages of each.
3.
Why can we see dust in the interstellar
medium better at far-infrared wavelengths than we can at optical wavelengths?
4.
How are H II regions and the hot intercloud
gas heated?
5.
How are each of the following types of ISM
detected by astronomers: hot intercloud gas, H II regions, neutral hydrogen
gas, and molecular clouds.
6.
At what wavelength are H II regions most
clearly visible, and why do H II regions mark the regions where new stars are
currently being formed?
7.
What is the difference between interstellar
extinction and interstellar reddening?
8.
Suppose the 21-cm photon of neutral hydrogen
were instead emitted at 500 nm (i.e., a visible blue photon). Would it still be
a useful probe of the Milky Way’s structure? Why?
9.
Why do H II regions mark the regions where
new stars are currently being formed?
10. How
are typical interstellar gas clouds different from the clouds that we see in
the Earth’s sky?
11. Why
do molecules readily exist in Earth’s atmosphere but not in most of
interstellar space?
12. Suppose
we observe two molecular clouds containing dust. The dust in Cloud 1 peaks in
emission at 50 mm, while the dust in Cloud 2 peaks in emission at 78 mm. How
much warmer is the dust in Cloud 1 compare to Cloud 2?
13. In
the densest molecular clouds, the average density is approximately 300 atoms/cm3.
If a cube of molecular cloud gas with this density contained 100 M⊙ of
material (the amount needed to make a 1 M⊙ star), what would be the length of a side of
the cube in units of AU? For reference, the mass of the Sun is 2 × 1030 kg,
the mass of a hydrogen atom is 1.7 × 10−27
kg, and 1 AU =
1.5 × 1011 m.
14. Why
is it possible for self-gravity to dominate pressure in molecular clouds but
not in most interstellar clouds?
15. Some
molecular clouds have so much internal pressure that it exceeds their
self-gravity. What keeps them from expanding and dissipating?
16. Why
do molecular clouds collapse from the inside out?
17. Why
do many stars form from a single molecular cloud?
18. Why
do stars form most often within molecular clouds?
19. Describe
the general process of how the interstellar medium can create a star.
20. Why
can’t very bright protostars be seen in visible light?
21. Why
does a protostar continue to collapse as it is forming?
22. What
is the energy source that powers brown dwarf stars?
23. Explain
why a star of higher mass must have a higher core temperature.
24. Why
does the surface temperature of a low-mass protostar remain nearly constant as
its core contracts?
25. In
the figure shown below, the portion of the H-R diagram corresponding to the
Hayashi track of a 1 M⊙ star is shown. Temperature increases toward
the right, and luminosity increases toward the top of the diagram. Even though
the temperature of the protostar is hardly changing as it approaches the main
sequence, its luminosity is decreasing. Why?
26. How
are Herbig-Haro objects related to T Tauri stars?
27. When
a 3 M⊙ protostar forms, it starts out at the top of
the Hayashi track with a luminosity of 4,000 L⊙ and a
temperature of 3600 K. What is its radius at this point (give the answer in
units of R⊙),
and how many times larger is it at this stage compared to its radius as a
main-sequence star, which is about 2.5 R⊙? For reference, the Sun’s temperature is
5800 K.
28. Astronomers
cannot observe the entire process of star formation during a human lifetime.
What property of star clusters allows them to circumvent much of this problem?
29. Suppose
a protostar shrinks in size from 100 R⊙ down to 20 R⊙,
while maintaining a constant temperature. By what factor does its luminosity
decrease?
30. Name
four conditions necessary for planets to exist with conditions suitable for
life.
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