Download Process of Science: PreMainSequence Stellar Life Tracks on the HR

Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Process of Science: Pre­Main­Sequence Stellar Life Tracks on the H­R Diagram
Learning Goal:
To understand how to read, interpret, and make inferences from the life tracks of protostars on an H­R diagram, and to consider how these tracks are known.
Introduction. The diagram shows life tracks (also called "evolutionary tracks") of five
protostars on the H­R diagram. Each track is labeled with the time it takes the star to go from
the beginning of its protostar stage to the start of its main­sequence stage.
Part A
The five colored curves on the diagram have arrows pointing to the left. Each of these five curves represents a star of a different __________.
Hint 1. What is a protostar?
A protostar is a __________.
ANSWER:
star that is very low in mass
very massive star
star that is still in the process of forming
Hint 2. What is the main sequence?
Recall that a main­sequence star is what we usually think of as a "normal" star, meaning one that generates energy through hydrogen fusion in its core. On the H­
R diagram shown in this activity, main­sequence stars can be found __________.
ANSWER:
along the well­defined curve running from the upper left to the lower right
along one of the five colored track along the right side of the diagram
only near the lower right of the diagram
Hint 3. What determines a star’s position on the main sequence?
The position of a star along the main sequence is determined by the star’s __________.
ANSWER:
mass
distance from Earth
age
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
age
distance from Earth
surface temperature
mass
Correct
Notice that each star’s mass is indicated by the purple numbers to the left of where each curve touches the main sequence.
Part B
The arrows on each protostar’s curve on the diagram’s indicate that __________.
Hint 1. What does a star’s (or protostar’s) position on the H­R diagram mean?
The position of a star (or protostar) on the H­R diagram tell us its __________.
ANSWER:
velocity and age
surface temperature and luminosity
location relative to the Sun
ANSWER:
protostars change in mass as they age
protostars move through space and come to rest only when they become main­sequence stars
protostars change in surface temperature and luminosity as they develop
Correct
Each curve is a life track (or evolutionary track) showing how the luminosity and temperature of a single protostar changes as it gradually develops into a main­
sequence star.
Part C
Which protostars maintain nearly the same luminosity throughout the time that they are protostars?
Hint 1. How do you recognize constant luminosity on the H­R diagram?
A group of stars with the same luminosity would form a _____ line on the H­R diagram.
ANSWER:
horizontal
vertical
diagonal
ANSWER:
protostars with masses about 10 or more times that of the Sun
protostars that eventually become main­sequence stars
red protostars
protostars with masses about that of the Sun or less
Correct
Notice that the life tracks for the 9­ and 15­solar­mass protostars are nearly horizontal, indicating that these high­mass stars maintain nearly constant luminosity
throughout their protostar stages.
By this point you should understand how to read the diagram. The remaining questions check to see whether you can extend this understanding to make inferences from the
information shown on the diagram.
Part D
Based on the protostar tracks on the diagram, which statement must be true about the Sun?
Hint 1. What is the meaning of vertical position on the H­R diagram?
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Suppose one star is directly above another on the H­R diagram. What can you conclude about the two stars?
ANSWER:
Both stars have the same luminosity, but the one located higher up (vertically) on the diagram has a higher surface temperature.
Both stars have the same surface temperature, but the one higher up is more luminous.
Both stars have the same mass.
ANSWER:
The Sun is 30 million years old.
The Sun was much more luminous when it was a protostar than it is today.
The Sun was cooler and dimmer when it was a protostar than it is today.
The Sun was much hotter when it was a protostar than it is today.
Correct
You can see this fact by looking at the life track for the Sun’s protostar stage. Note that it starts (at the right) much higher up at than it ends (where it reaches the
main sequence), and higher on the H­R diagram means more luminous.
Part E
Suppose two protostars form at the same time, one with a mass of 0.5M Sun and the other with a mass of 15M Sun . Which of the following statements are true?
Check all that apply.
Hint 1. Lifetime of a 15­solar­mass star
The main­sequence lifetime of a 15­solar­mass star is about 10 million years. Now, compare this main­sequence lifetime to the lengths of the protostar stages
that are shown on the diagram.
ANSWER:
The 15M Sun star will end its main­sequence life before the 0.5M Sun star even completes its protostar stage.
The 15M Sun protostar will orbit the Milky Way Galaxy at much higher speed than the 0.5M Sun protostar.
The 15M Sun protostar will be much more luminous than the 0.5M Sun protostar.
Correct
High­mass stars proceed through all stages of their lives much faster than low­mass stars—so much faster that when a star cluster forms, the high­mass stars can
live and die before the low­mass stars are "born" at the ends of their protostar stages.
Part F
The life tracks shown on the diagram for different mass protostars are based on computer models. Observationally, how can astronomers test whether these models are
correct?
Hint 1. The role of star clusters in astronomy
Star clusters are extremely useful to astronomers for two key reasons:
1. All the stars in a cluster lie at about the same distance from Earth.
2. All the stars in the cluster formed from the same interstellar cloud and therefore began to form at about the same time.
Astronomers can therefore use star clusters as laboratories for comparing the properties of stars at about the same distance or of about the same age.
ANSWER:
By observing and comparing protostars and stars of different masses within a single star cluster.
By observing and comparing protostars and stars of different masses at different distances from the center of the Milky Way Galaxy.
By monitoring protostars of different mass over many years and comparing the changes during those years to the predictions of the computer models.
Correct
A star cluster forms from a large molecular cloud, so all the stars in a cluster begin to form at nearly the same time. We can therefore learn about how stars
progress through their lives by comparing different stars within a cluster.
Ranking Task: Exploring the Different Stages of Star Birth
Part A
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
The following figures show four stages that occur during the formation of a one­solar­mass star. Rank these stages based on the order in which they occur, from first to
last.
Hint 1. What is a molecular cloud fragment?
A molecular cloud fragment is __________.
ANSWER:
the small portion of a molecular cloud that is ejected as collapse starts during star formation
a piece of a larger star­forming cloud in which one or a few stars will form
a piece of an interstellar cloud that is composed only of complex molecules such as water and carbon dioxide
Hint 2. What causes a cloud to heat up as it contracts?
In general, a contracting cloud will heat up only if __________.
ANSWER:
it is also rotating
it starts out extremely cold
it is dense enough to trap light inside it
ANSWER:
Correct
Continue on to Parts B and C to examine how temperature and rotation rate change as the star formation process progresses.
Part B
The following figures show four stages that occur during the formation of a one­solar­mass star. Rank these stages based on the central temperature, from highest to
lowest.
Hint 1. What happens to the cloud’s gravitational potential energy as it contracts?
A contracting cloud loses gravitational potential energy as it contracts in size. What happens to this energy?
ANSWER:
It causes the cloud to spin more rapidly.
It is converted to light and to heat within the cloud.
It simply disappears.
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
Notice that the central temperature increases as the formation process progresses. This is a consequence of the law of conservation of energy: The cloud loses
gravitational potential energy as it shrinks, and some of this energy is converted to thermal energy within the cloud (the rest is radiated away as light).
Part C
The following figures show four stages that occur during the formation of a one­solar­mass star. Rank these stages based on their rotation rate, from fastest to slowest.
(Assume that the angular momentum of the forming star is conserved throughout the formation process, though in fact it may shed some angular momentum by ejecting
material into interstellar space.)
Hint 1. What happens to the cloud’s rotation rate as it contracts?
Angular momentum must be conserved as a cloud contracts. Therefore, in a contracting cloud, __________.
ANSWER:
the rotation rate must increase as it shrinks in size
the rotation rate must decrease as it shrinks in size
the rotation rate must stay the same at all times
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
Notice that the rotation rate increases as the formation process progresses. This is a consequence of the law of conservation of angular momentum: The cloud
inevitably starts with some small net rotation, so as it contracts (reducing the radius of the rotation) the rotation must speed up to keep the total angular momentum
the same.
Sorting Task: Protostar or Main­Sequence Star
Part A
Each item following is a characteristic of a one­solar­mass star either during its protostar phase or during its main­sequence phase. Match the items to the appropriate
phase.
Hint 1. What you learn about the protostar and main­sequence phases on an H­R diagram
The figure shows the progression of a one­solar­mass star ­­ the same mass as our Sun ­­ from the time it begins to collapse from an interstellar cloud until it
becomes a main­sequence star. Studying this diagram should allow you to sort most of the preceding items. If you are having difficulty interpreting the diagram,
open the next two hints.
Hint 2. How do you determine luminosity during the protostar stage, from the diagram in Hint 1?
The following figure is the same as the figure in Hint 1; it shows the progression of a one­solar­mass star from the beginning of its formation until it reaches the
main sequence. The star has its greatest luminosity just to the right of the stage labeled _____.
In the following box, type in one of the numbers 1 through 4, based on the numbered stages shown in the figure.
ANSWER:
2
Hint 3. How do you determine surface temperature during the protostar stage, from the diagram in Hint 1?
The following figure is the same as the figure in Hint 1; it shows the progression of a one­solar­mass star from the beginning of its formation until it reaches the
main sequence. At which stage has the star’s surface temperature stopped increasing?
In the following box, type in one of the numbers 1 through 4, based on the numbered stages shown in the figure.
ANSWER:
4
Hint 4. How do you determine the energy source of a protostar, from the diagram in Hint 1?
The following figure is the same as the figure in Hint 1; it shows the progression of a one­solar­mass star from the beginning of its formation until it reaches the
main sequence. During Stage 2 on the figure, the protostar shrinks and heats as gravitational potential energy is converted into thermal energy. This represents a
source of energy for the star that we call _____.
ANSWER:
nuclear fusion
gravitational contraction
luminosity
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
Visual Activity: The Life Track of a One­Solar­Mass Protostar
First, launch the animation below. Explore the interactive figure before beginning to answer the questions. Notice that as the animation plays, a red dot shows you the forming
star’s changing position on the H­R diagram while the box illustrates how it would look if you could see it up close.
Part A
As a clump of interstellar gas contracts to become a main­sequence star, its changing position on the H­R diagram tells us __________.
Hint 1. What information does the H­R diagram show directly?
From the H­R diagram we can directly read off __________.
ANSWER:
an object’s surface temperature and luminosity
an object’s location in our galaxy
when an object existed
ANSWER:
how its outward appearance is changing
the time during which it existed in the history of the universe
how it moves through the galaxy
Correct
An object’s position on the H­R diagram tells us its surface temperature and luminosity, which are essentially its outward appearance.
Part B
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Watch the red dot representing the protostar in the animation. After it reaches its highest point on the diagram, how do the protostar's surface temperature and luminosity
change as it approaches the main sequence?
Hint 1. How is temperature shown on the H­R diagram?
On the H­R diagram, hotter objects are plotted __________ than cooler objects.
ANSWER:
lower down
farther to the left
higher up
farther to the right
Hint 2. How is luminosity shown on the H­R diagram?
On the H­R diagram, more luminous objects are plotted __________ on the graph than less luminous objects.
ANSWER:
farther to the right
lower down
higher up
farther to the left
ANSWER:
Its surface temperature decreases, but its luminosity increases.
Its surface temperature increases, but its luminosity decreases.
Its surface temperature and luminosity both decrease.
Its surface temperature and luminosity both increase.
Correct
Notice that in the red dot in the animation moves left and down after it passes its highest point and begins to approach the main sequence. Left means increasing
surface temperature, and down means decreasing luminosity.
Part C
When does a newly forming star have the greatest luminosity?
Hint 1. How does an object’s maximum luminosity value appear on the H­R diagram?
In the interactive figure, the red dot represents an object at its maximum luminosity when the dot appears __________.
ANSWER:
at its highest point on the H­R diagram
at its point farthest to the right on the H­R diagram
at its point farthest to the left on the H­R diagram
at its lowest point on the H­R diagram
ANSWER:
when it first becomes a main­sequence star
when it is a shrinking protostar with no internal fusion
when its internal temperature becomes high enough for nuclear fusion
when its surface temperature is the highest
Correct
If you watch the position of the red dot on the H­R diagram as the interactive figure plays, you will see that the dot is highest — meaning the object is most
luminous— when it is a protostar and therefore does not yet have internal fusion. This fact can be a little surprising, but do not forget that luminosity depends on
both surface temperature and size. Protostars are always much larger than the main­sequence stars they will eventually become, which is why they can be so
luminous even though they are still cool.
Part D
When a newly forming star is at its greatest luminosity, what is its energy source?
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Hint 1. What is the energy source for a protostar?
Protostars shine with energy generated by _____.
ANSWER:
radioactive decay
gravitational contraction
nuclear fusion
ANSWER:
radioactive decay of unstable isotopes
A newly forming star has no energy source, because it does not shine at all until it becomes a true main­sequence star.
gravitational contraction
nuclear fusion of hydrogen into helium
Correct
From Part C, you know that a newly forming star is most luminous during its protostar stage. The source of energy for protostars is the energy released by
gravitational contraction.
Concept Quiz
Part A
Which two processes can generate energy to help a star or gas cloud maintain its internal thermal pressure?
Hint 1.
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
nuclear fission and supernova
nuclear fusion and gravitational contraction
nuclear fusion and nuclear fission
nuclear fusion and supernova
Correct
Gravitational contraction is most important before the star finishes forming, while fusion is most important during the star's lifetime.
Part B
About what percentage of the mass of a molecular cloud is in the form of dust?
Hint 1.
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
98%
1%
10%
50%
Correct
Elements heavier than helium constitute a total of about 2% of the mass, and about half of the atoms of heavy elements in a molecular cloud are found in dust
grains.
Part C
How do we learn the chemical composition of the interstellar medium?
Hint 1.3
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
By studying spectra of interstellar gas clouds.
We use computer simulations of the interstellar medium.
We make an educated guess based on the Sun's composition.
We collect samples of gas and dust from interstellar space.
Correct
We see emission lines when the clouds are glowing, or absorption lines when the clouds absorb light from stars behind them. Because each chemical element has
its own unique set of spectral lines, we can use the spectra to identify the elements that have created the different lines.
Part D
What happens to the visible light radiated by stars located within a dusty gas cloud?
Hint 1.
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
It is reflected by dust back to the star from whence it came.
It is blocked by dust and its energy is thereby lost.
It is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light.
It passes through the cloud unaffected.
Correct
That is why clouds that appear dark in visible­light photos often glow in the infrared.
Part E
Under which circumstances can you be sure that the thermal pressure within a gas cloud is increasing?
Hint 1.
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
the cloud's temperature is increasing and its density is decreasing
the cloud's temperature and density are both increasing
the cloud's temperature is decreasing and its density is increasing
Impossible to say.
the cloud's temperature and density are both decreasing.
Correct
Thermal pressure is proportional to temperature and density, so with both increasing, the thermal pressure must also be increasing.
Part F
Which process is required to allow a gravitationally­collapsing gas cloud to continue to collapse?
Hint 1.
Study Section 16.1 of The Cosmic Perspective.
ANSWER:
The cloud must trap most of its thermal energy.
New dust particles must continually be made in the cloud.
The cloud must collide with other clouds.
The cloud must radiate away much of its thermal energy.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
Otherwise, the cloud would heat up, which would increase the thermal pressure and halt the gravitational collapse.
Part G
According to current understanding, how did the first generation of stars differ from stars born today?
Hint 1.
Study Sections 16.1 of The Cosmic Perspective.
ANSWER:
They contained much more hydrogen and helium than stars born today.
They were much more massive than most stars born today.
They were much more likely to be members of binary star systems than stars are born today.
They were much cooler in temperature than most stars born today.
Correct
Because there were no heavy elements at that time, these stars formed in clouds that had no dust and no molecules like carbon monoxide. This made it more
difficult for the clouds to radiate energy away, so they needed larger mass to form stars
Part H
Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's
angular momentum?
Hint 1.
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
the existence of protostellar jets
the formation of a protostellar disk
the onset of core hydrogen fusion
the strength of protostellar winds
Correct
The time to onset of hydrogen burning is mainly affected by a protostar's mass, not by its rotation (or angular momentum).
Part I
Close binary star systems are thought to form when _____.
Hint 1.
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
two interstellar gas clouds happen to contract so close together that there's no room for a disk or planets.
the protostellar disk around a protostar has enough material to form a second star.
a protostar emits two jets, each of which turns into a star
gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than colliding.
Correct
That is the leading hypothesis.
Part J
Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main­sequence star it
becomes?
Hint 1.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
A main­sequence star is cooler and dimmer than it was as a protostar.
A main­sequence star is cooler and brighter than it was as a protostar.
A main­sequence star is hotter and brighter than it was as a protostar.
A main­sequence star is hotter and dimmer than it was as a protostar.
Correct
The protostar is actually brighter than the main­sequence star because it is so much larger in size (radius).
Part K
Where does a 1­solar­mass protostar appear on an H­R diagram?
Hint 1.
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
to the right of the main sequence, and higher up than the Sun
to the right of the main sequence, and lower down than the Sun
nowhere ­ only stars that have fusion in their cores can be shown on H­R diagrams.
to the left of the main sequence, and higher up than the Sun
Correct
It is to the right because it is cooler than the main sequence, and above the Sun because it is more luminous.
Part L
Why does the rotation of a protostar slow down over time?
Hint 1.
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.
All rotating objects slow down over time.
The onset of fusion causes the rotation rate to slow dramatically.
Magnetic fields of other stars interact with the magnetic fields of the protostars, slowing its rotation.
Correct
Angular momentum transfer must happen for rotation to slow down.
Part M
The surface of a protostar radiates energy while its core ________.
Hint 1.
Study Section 16.2 of The Cosmic Perspective.
ANSWER:
expands and cools
shrinks and cools
shrinks and maintains a constant temperature
shrinks and heats
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
Gravity continues to compress the core, which grows hotter until the onset of fusion.
Part N
The core of a protostar that will eventually become a brown dwarf shrinks until ______.
Hint 1.
Study Section 16.3 of The Cosmic Perspective.
ANSWER:
the type of pressure called degeneracy pressure becomes important
it forms a rocky core
its central temperature is high enough to support fusion reactions
it radiates brown light
Correct
Degeneracy pressure is what halts the contraction of a brown dwarf.
Part O
If a star is extremely massive (well over 100 solar masses), why isn't it likely to survive for long?
Hint 1.
Study Section 16.3 of The Cosmic Perspective.
ANSWER:
It explodes as a supernova after just a few dozen years.
It may blow itself apart because of radiation pressure.
Its great mass will cause it to suck itself into becoming a black hole.
It eventually divides into two lower­mass stars.
Correct
Excess radiation pressure is probably why stars cannot be more massive than 150 solar masses or so.
Part P
Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?
Hint 1.
Study Chapter 16 of The Cosmic Perspective.
ANSWER:
All the stars in the cluster will have approximately the same luminosity and surface temperature.
A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
All the stars in the cluster will become main­sequence stars at about the same time.
All the stars in the cluster will be of about the same mass.
Correct
Very massive stars can form in a million years or less, and then live just a few million years. Low­mass stars take tens of millions of years ­­­ or more ­­­ just to
reach the beginning of their main­sequence lives.
Part Q
We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be
true?
Hint 1.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
ANSWER:
Brown dwarfs would be responsible for most of the overall luminosity of our Milky Way Galaxy.
Brown dwarfs would be extremely rare.
Brown dwarfs would outnumber all ordinary stars.
Most of the brown dwarfs in the Milky Way Galaxy would be quite young in age.
Correct
Less massive stars are more common than more massive stars, so if this trend continues then the very low mass brown dwarfs would be more common than any
type of main­sequence star.
Part R
Where would a brown dwarf be located on an H­R diagram?
Hint 1.
Study Section 16.3 of The Cosmic Perspective.
ANSWER:
below and to the right of the lowest part of the main sequence
in the lower left corner of the H­R diagram
in the upper right corner of the H­R diagram
above and to the left of the highest part of the main sequence
Correct
They are dimmer and cooler than the smallest main­sequence stars.
Problem 16.36
Choose the best answer.
Part A
Which kinds of stars are most common in a newly formed star cluster?
ANSWER:
O stars
G stars
M stars
Correct
Problem 16.52: Internal Temperature of the Sun
The Sun is essentially a gas cloud in which the forces of pressure and gravity balance each other.We can therefore use the equation in Mathematical Insight Gravity Versus
Pressure in the textbook to estimate the interior temperature of the Sun from its mass and particle density.
Part A
What is the average number density of particles within the Sun, given that the average mass per particle is about 10 −24 gram? (Hint: The volume of a sphere of radius r
is equal to 4πr 3 /3 .)
ANSWER:
n
= 1.42×1024 particles/cm3
Correct
Part B
What is the approximate temperature necessary for gas pressure to balance gravity within the Sun, given the average particle density from part A?
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
T
= 1.64×107 K Correct
Part C
How does your estimate compare with the internal core temperature of the Sun?
Express your answer using two significant figures.
ANSWER:
TSun
core
= 1.5×107 K Correct
Visual Quiz
Part A
The dark area stretching from the center of this picture to the upper right is about 50 light­years long and lies in the plane of the Milky Way Galaxy. What is it?
ANSWER:
A black hole.
A star­forming cloud.
The expanding remains of a supernova.
A region of the galactic plane in which there are very few stars.
Correct
It is dark because it contains cool, molecular gas ­ the type of gas cloud in which stars can form.
Part B
Both photos show the same field of view containing a star­forming molecular cloud. One of the photos was taken in visible light and the other in infrared light. Which one
is the visible­light photo, and how do you know?
ANSWER:
B is the visible­light photo, and we can tell because more stars are visible in the molecular cloud.
A is the visible­light photo, and we can tell because dust in the molecular cloud absorbs visible light, making the cloud appear dark.
B is the visible­light photo, and we can tell because the molecular cloud has a reddish tint.
A is the visible­light photo, and we can tell because its stars are more colorful.
Correct
Notice that we can see stars within the cloud in Photo B, because infrared light can penetrate through dusty clouds.
Part C
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
The main photo and the smaller inset both show the same field of view containing a star­forming molecular cloud. One of the photos was taken in visible light and the
other in infrared light. Which one is the infrared photo, and how do you know?
ANSWER:
The main (larger) photo is the infrared one, which we can tell because star­forming clouds generally are pink in color.
The smaller inset photo is the infrared one, which we can tell because edge of the cloud glows red in the image.
The smaller inset photo is the infrared one, which we can tell because the cloud appears dark and infrared light is invisible.
The main (larger) photo is the infrared one, which we can tell because molecular clouds emit infrared light but not visible light.
Correct
Molecular clouds are too cool to emit visible light. Notice that the bright region in the large, infrared image is the dark region in the smaller, visible inset.
Part D
This radio image shows emission from carbon monoxide molecules in the Milky Way; it spans a huge piece of the sky, nearly as large in angular extent as the Big
Dipper. What are the bright regions in this image?
ANSWER:
Cool, molecular clouds.
Very bright stars.
Patterns of the constellations.
The explosive debris from supernovae.
Correct
Carbon monoxide molecules can exist only at low temperatures like those in molecular clouds, so they trace out cool gas in the disk of the galaxy.
Part E
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light­year across. What is
happening as time passes (from left to right)?
ANSWER:
The cloud is developing hot spots that shine more brightly within it.
The cloud has been hit by a shock wave from a nearby stellar explosion, which causes the gas to move violently around.
Magnetic fields are forcing the gas to align in intricate ways along twisted magnetic field lines.
The cloud is fragmenting into smaller pieces that will form stars.
Correct
This type of fragmentation is why most clouds give birth to many stars.
Part F
This photo shows gas associated with a protostar, hidden from view at the location indicates at the left. What is this gas doing?
ANSWER:
It is gas blown off the protostar's surface as a protostellar wind.
It is part of a protostellar disk in which planets may someday form.
It is gas that is gradually accreting onto the protostar.
It is jet of gas flowing outward from the protostar.
Correct
The long, mostly straight path shows that it is a jet.
Part G
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
This figure shows the life track of a one­solar­mass star from its beginnings in a collapsing cloud fragment until it becomes a main­sequence star. At which of the
numbered points is the object generating energy primarily from gravitational contraction?
ANSWER:
1, 2, and 3.
2 only.
3 only.
1 and 2 only.
4 only.
1 only.
Correct
Until it reaches the main­sequence, gravitational contraction is its only source of energy.
Part H
This figure shows the life track of a one­solar­mass star from its beginnings in a collapsing cloud fragment until it becomes a main­sequence star. At which of the
numbered points is the object generating energy primarily from hydrogen fusion?
ANSWER:
1 and 2 only.
1, 2, and 3.
1 only.
3 only.
4 only.
2 only.
Correct
Hydrogen fusion marks the beginning of the star's main­sequence life.
Part I
This diagram shows the life tracks from protostar to the main sequence for several stars of different masses. Which track represents the star that takes the longest
amount of time to reach the main sequence?
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
1.
2.
4.
5.
3.
Correct
The lowest mass star takes the longest time.
Part J
What is the main idea captured by this graph?
ANSWER:
Low­mass stars are much more common than higher­mass stars.
Low­mass stars are smaller and redder than higher­mass stars.
Stars are best understood by dividing them into four categories by mass.
Objects with mass below 0.08 MSun are brown dwarfs rather than true stars.
Correct
For example, it shows that for every star with mass above 10 MSun , there are 200 stars with mass less than 0.5 MSun .
Problem 17.51: Roasting the Earth
During its final days as a red giant, the Sun will reach a peak luminosity of about 3000LSun . Earth will therefore absorb about 3,000 times as much solar energy as it does
now, and it will need to radiate 3,000 times as much thermal energy to keep its surface temperature in balance.
Part A
Estimate the temperature Earth's surface will need to attain in order to radiate that much thermal energy. You will need to use the formula for emitted power per unit area.
(Assume that Earth's temperature today is around 300 K .)
ANSWER:
TEarth
= 2220 K Correct
Reading Quiz
Part A
Which of the following stars will live longest?
Hint 1.
Study Section 17.1 of The Cosmic Perspective.
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
3 solar­mass star
2 solar­mass star
4 solar mass star.
1 solar­mass star
Correct
The lowest mass star lives longest.
Part B
In the context of understanding stellar lives, "high­mass" stars have masses:
Hint 1.
Study Section 17.1 of The Cosmic Perspective.
ANSWER:
more mass than our Sun
more than about 8 times the mass of our Sun
more than about 3 times the mass of our Sun
all stars, since all stars are far more massive than planets
Correct
These stars are massive enough that they will die in supernova explosions.
Part C
Which of the following lists the stages of life for a low­mass star in the correct order?
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
protostar, main­sequence star, red giant, supernova, neutron star
main­sequence star, white dwarf, red giant, planetary nebula, protostar
protostar, main­sequence star, red giant, planetary nebula, white dwarf
protostar, main­sequence star, planetary nebula, red giant
Correct
But remember that these stages are not equal in length; the star spends much more time as a main­sequence star than in any other stage of life. (Once dead, it
remains a white dwarf, though it gradually cools and therefore dims with time.)
Part D
What happens when a main­sequence star exhausts its core hydrogen fuel supply?
Hint 1.
Study Sections 17.2 and 17.3 of The Cosmic Perspective.
ANSWER:
The star becomes a neutron star.
The core shrinks while the rest of the star expands.
The entire star shrinks in size.
The core immediately begins to fuse its helium into carbon.
Correct
Gravity shrinks the core until hydrogen shell burning begins; the shell burning produces so much energy that the outer layers of the star expand, and the star
becomes a subgiant and then a red giant.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Part E
The main source of energy for a star as it grows in size to become a red giant is ______.
Hint 1.
Study Sections 17.2 and 17.3 of The Cosmic Perspective.
ANSWER:
hydrogen fusion in a shell surrounding the central core
hydrogen fusion in the central core
gravitational contraction
helium fusion in the central core
Correct
Hydrogen shell burning produces energy at an even greater rate than it was produced by core burning when it was on the main­sequence. This excess energy
causes the star's outer layers to expand.
Part F
The overall helium fusion reaction is:
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
Four helium nuclei fuse to form one oxygen nucleus.
Three helium nuclei fuse to form one carbon nucleus.
Two hydrogen nuclei fuse to form one helium nucleus.
Two helium nuclei fuse to form one beryllium nucleus.
Correct
Each helium nucleus has an atomic mass number of 4 (2 protons and 2 neutrons). The resulting carbon nucleus therefore has an atomic mass number of 3 x 4 = 12
(6 protons and 6 neutrons).
Part G
What is a helium flash?
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
A sudden brightening of a low­mass star, detectable from Earth by observing spectral lines of helium.
It is another name for the helium fusion reaction.
The ignition of helium shell burning in a high­mass star with a carbon core.
The sudden onset of helium fusion in the core of a low­mass star.
Correct
Note that helium flash occurs only in low­mass stars, not in high­mass stars.
Part H
An H­R diagram for a globular cluster will show a horizontal branch ­ a line of stars above the main­sequence but to the left of the subgiants and red giants. Which of the
following statements about these horizontal branch stars is true?
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
In a particular star cluster, all horizontal branch stars have the same spectral type.
Their sole source of energy is hydrogen shell burning.
They generate energy through both hydrogen fusion and helium fusion.
They have inert (non­burning) carbon cores.
Correct
This is true: Horizontal branch stars are those in which helium fusion has already begun in the core, while hydrogen fusion continues in a shell around the core.
Part I
What is a planetary nebula?
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
The remains of a high­mass star that has exploded.
Gas ejected from a low­mass star in the final stage of its life.
Interstellar gas from which planets are likely to form in the not­too­distant future.
Gas created from the remains of planets that once orbited a dead star.
Correct
Note that, despite its name, it has nothing to do with planets.
Part J
The ultimate fate of our Sun is to _____.
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
become a black hole
become a white dwarf that will slowly cool with time
explode in a supernova
become a rapidly spinning neutron star
Correct
This will happen about 5 billion years from now.
Part K
Which low­mass star does not have fusion occurring in its central core?
Hint 1.
Study Chapter 17 of The Cosmic Perspective.
ANSWER:
a helium burning star
a main sequence star
a red giant
Correct
A red giant has fusion occurring in shells around the core.
Part L
How are low­mass red giant stars important to our existence?
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Hint 1.
Study Section 17.2 of The Cosmic Perspective.
ANSWER:
These stars provide most of the light that reaches us from globular clusters.
These stars manufactured virtually all the elements out of which we and our planet are made.
These stars generate the energy that makes life on Earth possible.
These stars manufactured most of the carbon atoms in our bodies.
Correct
While most heavier elements come from higher­mass stars, carbon is thought to come primarily from red giants: the carbon is produced by helium fusion in the core,
then dredged up to the surface by convection and blown out into space by winds and by the ejection of a planetary nebula.
Part M
Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
helium and helium
hydrogen and deuterium
hydrogen and helium
hydrogen and hydrogen
Correct
These two nuclei each have two protons (and two neutrons ­ but neutrons are electrically neutral) and therefore each have a charge of +2. Since these two nuclei
have the greatest total charge between them, they will feel the strongest electromagnetic repulsion.
Part N
Which of the following stars will certainly end its life in a supernova?
Hint 1.
Study Sections 17.1­17.3 of The Cosmic Perspective.
ANSWER:
a red giant star
a neutron star
the Sun
a 10 solar mass star
Correct
According to present understanding, high­mass stars always die in supernovae.
Part O
What is the CNO cycle?
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
the process by which carbon is fused into nitrogen and oxygen
the process by which helium is fused into carbon, nitrogen, and oxygen
the set of fusion reactions that have produced all the carbon, nitrogen, and oxygen in the universe
a set of steps by which four hydrogen nuclei fuse into one helium nucleus
Correct
The CNO cycle is one of two primary pathways to hydrogen fusion. The proton­proton chain is the other pathway and it is used by low­mass stars, while high­mass
stars fuse hydrogen via the CNO cycle.
Part P
In order to predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
its overall abundance of elements heavier than helium
its mass
its luminosity
how much oxygen it now has in its core
Correct
Mass determines whether the star's core will ever be compressed enough to fuse oxygen.
Part Q
Why is iron significant to understanding how a supernova occurs?
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
Iron cannot release energy either by fission or fusion.
Iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.
Supernovae often leave behind neutron stars, which are made mostly of iron.
The fusion of iron into uranium is the reaction that drives a supernova explosion.
Correct
Therefore, a star with an iron core has no way to generate the energy needed to counteract the crush of gravity. This is the crisis that, in a fraction of a second,
initiates the supernova.
Part R
After a supernova explosion, the remains of the stellar core ______.
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
will always be a neutron star
will always be a black hole
may be either a neutron star or a black hole
may be either a white dwarf, neutron star, or black hole
Correct
Correct.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Part S
Why is Supernova 1987A particularly important to astronomers?
Hint 1.
Study Section 17.3 of The Cosmic Perspective.
ANSWER:
It was the first supernova detected in nearly 400 years.
It occurred only a few light­years from Earth.
It provided the first evidence that supernovae really occur.
It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
Correct
Many supernovae are detected every year, but generally in distant galaxies. Supernova 1987A was in the Large Magellanic Cloud, about 150,000 light­years away,
making it the nearest one detected since Kepler's supernova in 1604.
Part T
Algol consist of a 3.7 MSun main­sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
Hint 1.
Study Section 17.4 of The Cosmic Perspective.
ANSWER:
It doesn't make sense to find a subgiant in a binary star system.
The two stars should be the same age, so we'd expect the subgiant to be more massive than the main­sequence star.
The two stars in a binary system should both be at the same stage of life; that is, they should either both be main sequence stars or both be subgiants.
A star with a mass of 3.7 MSun is too big to be a main sequence star.
Correct
A subgiant has already completed its main­sequence life, and massive stars live shorter lives. Thus, we'd expect the subgiant to be more massive than a star that
is still a hydrogen­burning, main­sequence star.
Part U
Where does gold (the element) come from?
Hint 1.
Study Chapter 17 of The Cosmic Perspective.
ANSWER:
it was produced during the Big Bang
it is produced by mass transfer in close binaries
it is produced during the supernova explosions of high­mass stars
it is produced during the late stages of fusion in low­mass stars
Correct
Fusion in the core produces elements as heavy as iron; heavier elements like gold are produced during the supernova itself.
Process of Science: Evidence for How the Elements Were Created
Learning Goal:
To read and interpret a graph that provides important evidence in favor of current models of element creation by stars and supernovae.
Introduction. The diagram shows the measured relative abundances of elements compared to the abundance of hydrogen in the Milky Way Galaxy. Note that the relative
abundance compares the numbers of atoms of each element. For example, the graph shows that the abundance of nitrogen is about 10 −4 , which means there are about −4
10
= 0.0001 times as many nitrogen atoms in the galaxy as there are hydrogen atoms.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Part A
The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____.
Hint 1. Interpreting the powers of 10 on the vertical axis
Notice that the numbers on the vertical axis (relative abundance) are written as powers of 10. Recall that, for negative powers of 10, the exponent tells us the
number of decimal places to the right of the decimal point. For example, 10 −2 = 0.01 = 1/100 , 10 −4 = 0.0001 = 1/10, 000, −6
10
= 0.000001 = 1/1, 000, 000 , and so on. You should also interpret values in between the labeled points as powers of 10; for example, a point on the
vertical axis halfway between 10 −6 and 10 −4 represents an abundance of 10 −5 .
Hint 2. Understanding why the horizontal axis shows atomic number
The horizontal axis shows atomic number, which is defined as the number of protons in an atom’s nucleus. Each different chemical element has a different atomic
number. Therefore, by showing atomic number, the graph allows us to compare the abundances different chemical elements.
Hint 3. Which direction on this graph represents greater relative abundance?
On this graph, the points representing elements that are more abundant are located __________.
ANSWER:
farther to the right
lower
farther to the left
higher
ANSWER:
hydrogen
boron
helium
lithium
oxygen
Correct
The vertical axis represents relative abundance, so the fact that oxygen has the third­highest point on the diagram (after hydrogen and helium) means that it is the
third most abundant element.
Part B
According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________.
Hint 1. Interpreting the powers of 10 on the vertical axis
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Notice that the numbers on the vertical axis (relative abundance) are written as powers of 10. Recall that, for negative powers of 10, the exponent tells us the
number of decimal places to the right of the decimal point. For example, 10 −2 = 0.01 = 1/100 , 10 −4 = 0.0001 = 1/10, 000, −6
10
= 0.000001 = 1/1, 000, 000 , and so on. You should also interpret values in between the labeled points as powers of 10; for example, a point on the
vertical axis halfway between 10 −6 and 10 −4 represents an abundance of 10 −5 .
ANSWER:
1/10 that of hydrogen
1/100 that of hydrogen
1/1000 that of hydrogen
1/10,000 that of hydrogen
Correct
Looking along the vertical axis, you can see that the point for oxygen lies about halfway between the points labeled 10 −4 and 10 −2 , which means about 10 −3 ,
which is 0.001 or 1/1000. Therefore, the abundance of oxygen atoms is about 1/1000 that of hydrogen.
Part C
According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20?
Hint 1. Where on the graph are the elements with atomic number greater than 20?
On the given graph, the points representing elements that have atomic number greater than 20 are located __________.
ANSWER:
to the right of the number 20 along the horizontal axis
to the left of the number 20 along the horizontal axis
directly above the number 20 along the horizontal axis
ANSWER:
nickel
hydrogen
oxygen
calcium
iron
Correct
In this question you were asked to consider only data points at or to the right of atomic number 20 along the horizontal axis. In this region, the data point for iron is
the highest on the graph, which means it has the greatest relative abundance. Note that the iron abundance is only a little more than 10 −6 = 1/1, 000, 000 that of
hydrogen. In other words, for every atom of iron in the Milky Way Galaxy, there are nearly 1 million atoms of hydrogen.
Part D
Based on the diagram, which of the following statements best describes the observed pattern of abundances for elements with an atomic number between 6 and 20?
Hint 1. Identifying elements with even atomic numbers
The graph has a data point for each atomic number along the horizontal axis, so you can find the even atomic numbers (2, 4, 6, etc.) just by counting. For
example, you’ll find that carbon has atomic number 6, oxygen has atomic number 8, neon has atomic number 10, and so on.
ANSWER:
There is a general trend of decreasing abundance with increasing atomic number, but elements with even atomic numbers tend to be more abundant than those
with odd atomic numbers.
There is a general trend of increasing abundance with increasing atomic number, but elements with odd atomic numbers tend to be more abundant than those
with even atomic numbers.
All of the abundances are about the same.
These abundances decrease smoothly from atomic number of 6 to atomic number of 20.
These abundances fluctuate randomly.
Correct
The even­numbered peaks are higher than their neighboring points, for reasons that we will explore in the remaining questions.
Parts A through D have tested your ability to read the data on the graph. The remaining questions explore the interpretation of these data. You may wish to refer to your
textbook to review how elements are created by nuclear fusion in stars.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Part E
According to current understanding, the two most abundant elements in the universe were made __________.
Hint 1. What are the two most abundant elements?
The graph shows that the two most abundant elements in the galaxy are __________.
ANSWER:
hydrogen and helium
the two points farthest to the right on the graph
lithium and beryllium
ANSWER:
by nuclear fusion in the cores of stars
during supernova explosions
by nuclear reactions on interstellar dust grains
in the Big Bang
Correct
Hydrogen and most of the helium in the universe formed during the first 5 minutes of the universe’s birth in the Big Bang.
Part F
In Part D, you saw that elements with even atomic numbers tend to be more abundant than neighboring elements with odd atomic numbers. What nuclear process
explains why this is the case?
Hint 1. What element has an atomic number of 2?
The element with two protons, or having an atomic number of 2, is _____.
ANSWER:
hydrogen
helium
deuterium
Hint 2. Does adding a neutron to a nucleus change its atomic number?
Adding a neutron __________ the atomic number of a nucleus.
ANSWER:
adds 1 to
does not change
subtracts 1 from
ANSWER:
Starting from lithium (atomic number is 3), the most common nuclear reactions involve the fusion of an additional hydrogen nucleus.
Nuclei with odd numbers of protons tend to be unstable and undergo radioactive decay.
Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus.
Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional hydrogen nucleus.
Correct
Helium nuclei have two protons—which means it has an atomic number of 2—so fusing a helium nucleus into some other element increases the atomic number by
two. Carbon is formed by the fusion of three helium nuclei, which is why carbon has an atomic number of 6. Fusing another helium nucleus to carbon makes
oxygen, with an atomic number of 8; fusing a helium nucleus to oxygen makes neon, with an atomic number of 10; and so on. That is why even­numbered elements
tend to be more common.
Part G
The observational data for the element abundances agree quite well with what we expect based on our current understanding of nuclear fusion and stellar evolution. But
imagine the data had turned out to be different. Which of the following differences, if it had actually been observed, would have forced us to rethink our entire picture of
stellar evolution?
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Hint 1. The formation of elements heavier than iron
As discussed in your textbook, iron is the heaviest element that is formed by a major fusion pathway in high­mass stars, and the formation of iron leads to a
supernova explosion. Elements heavier than iron are formed primarily by nuclear reactions associated with supernova explosions, and as a result these elements
are quite rare compared to iron.
ANSWER:
The abundances of elements heavier than iron turned out to be much smaller than we’ve actually observed.
All the abundances after hydrogen and helium turned out to be smaller by a factor of 2 than what we’ve actually observed.
The abundance of elements heavier than uranium turned out to be greater than the abundance of carbon.
Correct
We expect carbon to be relatively common because it is formed by the fusion of three helium nuclei, and helium is the second most abundant element. We expect
uranium and heavier elements to be quite rare because they form only in rare reactions associated with supernova explosions. Therefore, if elements heavier than
uranium turned out to be more common than carbon, we would be forced to conclude that our model of element creation is incorrect.
Visual Activity: Stellar Lifetimes
First, launch the animation below. Explore the interactive figure before beginning to answer the questions. Note that the animation is schematic and not precise. Real stellar
cores always begin with some helium already present and gradually shrink as hydrogen fuses into helium.
Part A
How do the properties of long­lived stars compare to those of short­lived stars?
Check all that apply.
Hint 1. What factors determine stellar lifetime?
A star’s lifetime depends on which of the following factor(s)?
ANSWER:
mass only
luminosity only
surface temperature only
both mass and luminosity
both mass and surface temperature
Hint 2. How does mass affect luminosity for main­sequence stars?
True or False? More massive main­sequence stars are less luminous than less massive main­sequence stars.
ANSWER:
True
False
ANSWER:
Long­lived stars begin their lives with more mass and a larger amount of hydrogen fuel.
Long­lived stars begin their lives with less mass and a smaller amount of hydrogen fuel.
Long­lived stars are more luminous during their main­sequence lives.
Long­lived stars are less luminous during their main­sequence lives.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Correct
The key point is that longer lifetime goes with less massive stars. Stellar lifetime depends on how much fuel is available for fusion and how rapidly that fuel is
burned. More massive stars have more fuel, but they burn through it at far higher rates and hence have shorter lives.
Part B
A main­sequence star twice as massive as the Sun would last __________.
Hint 1. How do the luminosities for main­sequence stars with different masses compare?
ANSWER:
one­tenth as luminous.
half as luminous.
A main­sequence star twice as massive as the Sun will be about just as luminous.
twice as luminous.
ten times as luminous.
ANSWER:
about half as long as the Sun
much less than half as long as the Sun
about twice as long as the Sun
much longer than twice as long as the Sun
Correct
Notice (with the interactive figure) that a main­sequence star with twice the mass of the Sun has a luminosity close to ten times that of the Sun — which means it
burns through its hydrogen fuel at a rate ten times faster than the Sun’s. As a result, even though it starts with twice as much fuel, its lifetime is much less than half
the Sun’s lifetime.
Part C
If stars A and B are both main­sequence stars and star A has a greater fusion rate than star B, which of the following statements hold(s)?
Check all that apply.
Hint 1. Why do main­sequence stars have different rates of fusion in their cores?
True or False? The fusion rate in the core of a main­sequence star will be greater if the core temperature and density are higher.
ANSWER:
True
False
ANSWER:
Star A must be more luminous than star B.
Star A must be less luminous than star B.
Star A must be more massive than star B.
Star A must be less massive than star B.
Correct
A more massive main­sequence star has a higher rate of fusion, which makes it more luminous.
Ranking Task: The Life of a High­Mass Main Sequence Star
Part A
Provided following are various stages during the life of a high­mass star. Rank the stages based on when they occur, from first to last.
Hint 1. How do the life stages of a high­mass star compare to those of a low­mass star?
High­mass stars live much shorter lives than low­mass stars and differ in the details of many of the life stages. Nevertheless, low­ and high­mass stars share
some things in common. Which of the following statements is true?
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
ANSWER:
Both low­ and high­mass stars end their lives in supernovas and leave behind neutron stars or black holes.
Both low­ and high­mass stars are protostars until they can fuse hydrogen in their cores, then become hydrogen­burning main­sequence stars, and near
the ends of their lives expand to become giants or supergiants.
Both low­ and high­mass stars start lives as huge giant or supergiant stars, then become protostars before they reach their main­sequence lives.
Hint 2. What is a protostar?
Protostar is the name we give to __________.
ANSWER:
a very high­mass star with a strong stellar wind
a star that has recently died
a star that has not quite reached its “birth,” meaning its core is not yet hot enough to sustain nuclear fusion
Hint 3. What makes a supergiant shine?
A supergiant shines with energy released by __________.
ANSWER:
nuclear fusion of elements heavier than hydrogen (as well as of hydrogen in the shell)
gravitational contraction
the explosion of the entire star
ANSWER:
Correct
Remember also that high­mass stars progress through all these stages at a much faster rate than lower­mass stars. The highest­mass stars may be born, live, and
die in only a few million years. Note also that while this particular high­mass star leaves behind a neutron star after its supernova, an even higher­mass star may
instead leave behind a black hole.
Part B
Provided following are various elements that can be produced during fusion in the core of a high mass main sequence star. Rank these elements based on when they are
produced, from first to last.
Hint 1. How heavy are these elements?
Which of the following lists the elements in order of increasing atomic mass?
ANSWER:
helium, carbon, oxygen, iron
helium, oxygen, carbon, iron
helium, iron, carbon, oxygen
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
ANSWER:
Correct
During their main­sequence lives, all stars fuse hydrogen into helium in their cores. During the late stages of their lives, massive stars fuse helium into carbon, and
ongoing reactions create successively heavier elements, including oxygen. Iron is the last product of fusion in a massive star’s core; iron fusion does not release
energy, so the production of iron is the event that provokes the stellar crisis that ends (within seconds) in a supernova.
Sorting Task: High­ and Low­Mass Stars
Part A
Listed following are characteristics that describe either high­mass or low­mass stars. Match these characteristics to the appropriate category.
Hint 1. A strategy for completing this task
The only way to complete this sorting task successfully is to know the different ways in which low­mass and high­mass stars live their lives. To get started on
this task, first remember that high­mass stars proceed through all their life stages in a much shorter time than low­mass stars. Second, remember that most of
the elements of which we and Earth are made were produced by fusion in high­mass stars. Beyond that, you’ll need to review the life stages of low­ and high­
mass stars as discussed in your textbook.
ANSWER:
Correct
A long­lived star such as the Sun eventually ejects its outer layers as a planetary nebula, leaving behind a white dwarf.
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Mandatory Assignment 4 ­ Testing material from Chapters 16 & 17
Vocabulary in Context: High­Mass Stars
Part A
Match the words in the left­hand column to the appropriate blank in the sentences in the right­hand column. Use each word only once.
Hint 1. Description of a supernova
A supernova is the complete explosion of a star.
Hint 2. Description of a neutron star
A neutron star is the compact corpse of a high­mass star left over after a supernova. It typically contains a mass comparable to the mass of the Sun in a volume
just a few kilometers in radius.
Hint 3. Description of a supernova remnant
A supernova remnant is a glowing, expanding cloud of debris from the explosion of a star.
Hint 4. Description of a high­mass star
A high­mass star is a star born with a mass above about 8 M Sun , which will end its life by exploding as a supernova.
Hint 5. Description of the CNO cycle
The CNO cycle is the cycle of reactions by which intermediate­ and high­mass stars fuse hydrogen into helium.
Hint 6. Description of a helium­capture reaction
A helium­capture reaction is a fusion reaction that fuses a helium nucleus into some other nucleus. Such reactions can fuse carbon into oxygen, oxygen into
neon, neon into magnesium, and so on.
Hint 7. Description of Supernova 1987A
Supernova 1987A was a supernova witnessed on Earth in 1987. It was the nearest supernova seen in nearly 400 years and helped astronomers refine their
theories of supernovae.
ANSWER:
Reset
Help
1. The Crab Nebula is the result of a supernova that was witnessed on Earth in the year 1054.
2. Betelgeuse is a supergiant star that will eventually supernova, which means that by mass it is
classified as a high­mass star .
3. The debris from the death of a high­mass star forms a supernova remnant several light years
across.
4. A neutron star has a density higher than the density of a white dwarf.
5. Supernova 1987A actually occurred about 150,000 years ago in the Large Magellanic Cloud.
6. The CNO cycle is the process by which hydrogen fusion proceeds in high­mass stars.
7. Carbon can be converted into oxygen in the cores of high­mass stars if carbon nuclei undergo a helium­capture reaction .
Correct
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