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1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
Stellar Evolution

• Chapter 12

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Guidepost
This chapter is the heart of any discussion of
astronomy. Previous chapters showed how
astronomers make observations with telescopes and
how they analyze their observations to find the
luminosity, diameter, and mass of stars. All of
that aims at understanding what stars are. This
is the middle of three chapters that tell the
story of stars. The preceding chapter told us how
stars form, and the next chapter tells us how
stars die. This chapter is the heart of the
storyhow stars live. As always, we accept
nothing at face value. We expect theory to be
supported by evidence. We expect carefully
constructed models to help us understand the
structure inside stars. In short, we exercise our
critical faculties
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Guidepost (continued)
and analyze the story of stellar evolution rather
than merely accepting it. After this chapter, we
will know how stars work, and we will be ready to
study the rest of the universe, from galaxies
that contain billions of stars to the planets
that form around individual stars.
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Outline
I. Main-Sequence Stars A. Stellar Models B. Why
There Is a Main Sequence C. The Ends of the Main
Sequence D. The Life of a Main-Sequence Star E.
The Life Expectancies of Stars II.
Post-Main-Sequence Evolution A. Expansion into a
Giant B. Degenerate Matter C. Helium Fusion D.
Fusing Elements Heavier than Helium
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Outline (continued)
III. Evidence of Evolution Star Clusters A.
Observing Star Clusters B. The Evolution of Star
Clusters IV. Evidence of Evolution Variable
Stars A. Cepheid and RR Lyrae Variable Stars B.
Pulsating Stars C. Period Changes in Variable
Stars
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Main Sequence Stars
The structure and evolution of a star is
determined by the laws of

• Hydrostatic equilibrium

• Energy transport

• Conservation of mass

• Conservation of energy

A stars mass (and chemical composition)
completely determines its properties.
Thats why stars initially all line up along the
main sequence.
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Maximum Masses of Main-Sequence Stars
a) More massive clouds fragment into smaller
pieces during star formation.
Mmax
b) Very massive stars lose mass in strong stellar
winds
100 solar masses
h Carinae (Eta Carinae)
Example h Carinae Binary system of a 60 Msun
and 70 Msun star. Dramatic mass loss major
eruption in 1843 created double lobes.
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Minimum Mass of Main-Sequence Stars
Mmin 0.08 Msun
At masses below 0.08 Msun, stellar progenitors do
not get hot enough to ignite thermonuclear fusion.
Gliese 229B
? Brown Dwarfs
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Brown Dwarfs
Hard to find because they are very faint and
cool emit mostly in the infrared.
Many have been detected in star forming regions
like the Orion Nebula.
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Evolution on the Main Sequence (1)
Main-Sequence stars live by fusing H to He.
MS evolution
Zero-Age Main Sequence (ZAMS)
Finite supply of H gt finite life time.
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Future of the Sun
(SLIDESHOW MODE ONLY)
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Evolution on the Main Sequence (2)
A stars life time T energy reservoir /
luminosity
Energy reservoir M
Luminosity L M3.5
T M/L 1/M2.5
Massive stars have short lives!
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Evolution off the Main Sequence Expansion into a
Red Giant
Hydrogen in the core completely converted into He
? Hydrogen burning (i.e. fusion of H into He)
ceases in the core.
H burning continues in a shell around the core.
He Core H-burning shell produce more energy
than needed for pressure support
Expansion and cooling of the outer layers of the
star ? Red Giant
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Expansion onto the Giant Branch
Expansion and surface cooling during the phase of
an inactive He core and a H- burning shell
Sun will expand beyond Earths orbit!
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Degenerate Matter
Matter in the He core has no energy source left.
? Not enough thermal pressure to resist and
balance gravity
? Matter assumes a new state, called degenerate
matter
Pressure in degenerate core is due to the fact
that electrons can not be packed arbitrarily
close together and have small energies.
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Red Giant Evolution
H-burning shell keeps dumping He onto the core.
He-core gets denser and hotter until the next
stage of nuclear burning can begin in the core
4 H ? He
He
He fusion through the Triple-Alpha Process
4He 4He ? 8Be g 8Be 4He ? 12C g
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Helium Fusion
He nuclei can fuse to build heavier elements
When pressure and temperature in the He core
become high enough,
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Red Giant Evolution (5 solar-mass star)
C, O
Inactive He
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Fusion Into Heavier Elements
Fusion into heavier elements than C, O
requires very high temperatures occurs only in
very massive stars (more than 8 solar masses).
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The Life Clock of a Massive Star (gt 8 Msun)
Lets compress a massive stars life into one day
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H ? He
1
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Life on the Main Sequence Expansion to Red
Giant 22 h, 24 min. H burning
2
10
9
3
4
8
5
7
6
H ? He
He ? C, O
12
1
11
2
10
He burning (Red Giant Phase) 1 h, 35 min, 53 s
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3
4
8
5
7
6
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The Life Clock of a Massive Star (2)
He ? C, O
H ? He
12
1
11
C ? Ne, Na, Mg, O
2
10
3
9
4
C burning 6.99 s
8
5
7
6
C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
Ne burning 6 ms
235959.996
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The Life Clock of a Massive Star (3)
C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
O ? Si, S, P
O burning 3.97 ms
235959.99997
C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
O ? Si, S, P
Si ? Fe, Co, Ni
The final 0.03 msec!!
Si burning 0.03 ms
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Summary of Post Main-Sequence Evolution of Stars
Supernova
Fusion proceeds formation of Fe core.
Evolution of 4 - 8 Msun stars is still uncertain.
Mass loss in stellar winds may reduce them all to
lt 4 Msun stars.
M gt 8 Msun
Fusion stops at formation of C,O core.
M lt 4 Msun
Red dwarfs He burning never ignites
M lt 0.4 Msun
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Evidence for Stellar Evolution Star Clusters
Stars in a star cluster all have approximately
the same age!
More massive stars evolve more quickly than less
massive ones.
If you put all the stars of a star cluster on a
HR diagram, the most massive stars (upper left)
will be missing!
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HR Diagram of a Star Cluster
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Cluster Turnoff
(SLIDESHOW MODE ONLY)
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Example HR diagram of the star cluster M 55
High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence
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Estimating the Age of a Cluster
The lower on the MS the turn-off point, the older
the cluster.
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Evidence for Stellar Evolution Variable Stars
Some stars show intrinsic brightness variations
not caused by eclipsing in binary systems.
Most important example d Cephei
Light curve of d Cephei
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Cepheid Variables The Period-Luminosity Relation
The variability period of a Cepheid variable is
correlated with its luminosity.
The more luminous it is, the more slowly it
pulsates.
gt Measuring a Cepheids period, we can determine
its absolute magnitude!
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Cepheid Distance Measurements
Comparing absolute and apparent magnitudes of
Cepheids, we can measure their distances (using
the 1/d2 law)!
The Cepheid distance measurements were the first
distance determinations that worked out to
distances beyond our Milky Way!
Cepheids are up to 40,000 times more luminous
than our sun gt can be identified in other
galaxies.
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Pulsating Variables The Instability Strip
For specific combinations of radius and
temperature, stars can maintain periodic
oscillations.
Those combinations correspond to locations in the
Instability Strip
Cepheids pulsate with radius changes of 5 10
.
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Pulsating Variables The Valve Mechanism
Partial He ionization zone is opaque and absorbs
more energy than necessary to balance the weight
from higher layers. gt Expansion
Upon expansion, partial He ionization zone
becomes more transparent, absorbs less energy gt
weight from higher layers pushes it back inward.
gt Contraction.
Upon compression, partial He ionization zone
becomes more opaque again, absorbs more energy
than needed for equilibrium gt Expansion
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Period Changes in Variable Stars
Periods of some Variables are not constant over
time
because of stellar evolution.
? Another piece of evidence for stellar
evolution.
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New Terms
conservation of mass law conservation of energy
law stellar model brown dwarf zero-age main
sequence (ZAMS) degenerate matter triple alpha
process helium flash open cluster globular
cluster turnoff point horizontal branch variable
star intrinsic variable Cepheid variable star RR
Lyrae variable star
periodluminosity relation instability strip  
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Discussion Questions
1. How do we know that the helium flash occurs if
it cannot be observed? Can we accept an event as
real if we can never observe it? 2. Can you
think of ways that chemical differences could
arise in stars in a single star cluster? Consider
the mechanism that triggered their formation.
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Quiz Questions
1. Which of the following is NOT considered in
making a simple stellar model? a. Hydrostatic
equilibrium. b. Energy transport. c. Magnetic
field. d. Conservation of mass. e. Conservation
of energy.
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Quiz Questions
2. According to Figure 12-1, what is the
approximate radius of the Sun's nuclear fusion
zone? a. 0.10 solar radii b. 0.30 solar radii c.
0.50 solar radii d. 0.70 solar radii e. 0.90
solar radii
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Quiz Questions
3. Why is there a lower mass limit of 0.08 solar
masses for main sequence stars? a. This is an
unsolved astronomical mystery. b. Objects below
this mass can only form in HI clouds. c. Objects
below this mass are not hot enough to fuse normal
hydrogen. d. They form too slowly and hot stars
nearby clear the gas and dust quickly. e. Our
telescopes do not have enough light gathering
power to detect dim objects.
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Quiz Questions
4. Why is there an upper mass limit for main
sequence stars of about 100 solar masses? a.
Giant molecular clouds do not contain enough
material. b. General relativity does not allow
such massive objects to exist. c. The rotation
rate is so high that such an object splits into a
pair of stars. d. Objects above this mass fuse
hydrogen too rapidly and cannot stay together. e.
Objects above this mass do form in molecular
clouds however, they emit no light and are not
considered stars.
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Quiz Questions
5. Why are lower main sequence stars more
abundant than upper main sequence stars? a. More
low-mass main sequence stars are formed in
molecular clouds. b. Lower main sequence stars
have much longer lifetimes than upper main
sequence stars. c. High-mass main sequence stars
lose mass and become lower main sequence
stars. d. Both a and b above. e. All of the above.
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Quiz Questions
6. Why does a star's life expectancy depend on
mass? a. Mass determines the amount of fuel a
star has for fusion. b. More massive stars can
fuse hydrogen for a longer time. c. Mass
determines the rate of fuel consumption for a
star. d. Both a and b above. e. Both a and c
above.
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Quiz Questions
7. Which of the following observable properties
of a main sequence star is a direct indication of
the rate at which energy is produced inside that
star? a. Surface temperature. b. Luminosity. c.
Diameter. d. Distance. e. Age.
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Quiz Questions
8. Why does an expanding giant star become
cooler? a. Less energy is produced in the star's
interior. b. More energy is produced in the
star's interior. c. Thermal energy is converted
into gravitational energy. d. Both a and b
above. e. Both a and c above.
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Quiz Questions
9. Of the following, which main sequence star has
a longer life expectancy than the Sun? a.
Spectral type B9. b. Spectral type K2. c.
Spectral type A7. d. Spectral type O5. e.
Spectral type F4.
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Quiz Questions
10. How does the main sequence lifetime of a star
compare to its entire fusion lifetime? a. Stars
spend about 10 of their fusion lifetimes on the
main sequence. b. Stars spend about 30 of their
fusion lifetimes on the main sequence. c. Stars
spend about 50 of their fusion lifetimes on the
main sequence. d. Stars spend about 70 of their
fusion lifetimes on the main sequence. e. Stars
spend about 90 of their fusion lifetimes on the
main sequence.
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Quiz Questions
11. Why does an expanding giant star become more
luminous? a. Less energy is produced in the
interior. b. More energy is produced in the
interior. c. Thermal energy is converted into
gravitational energy. d. Both a and b above. e.
Both a and c above.
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Quiz Questions
12. What increases the temperature of an inert
helium core inside a giant star? a. Hydrogen
shell fusion. b. Helium shell fusion. c.
Gravitational contraction. d. The triple-alpha
process. e. Both a and b above.
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Quiz Questions
13. Twice during the late stages of the Sun's
life it will move upward and ascend the giant
branch on the H-R diagram. What will be going on
in the Sun's core while it is climbing the giant
branch? a. The Sun's core will fuse hydrogen to
make helium during both ascents of the giant
branch. b. The Sun's core will fuse helium to
make carbon and oxygen during both ascents of the
giant branch. c. The Sun's core will fuse
hydrogen to make helium during the first ascent,
and fuse helium to make carbon and oxygen during
the second ascent of the giant branch. d. The
Sun's core will fuse helium to make carbon and
oxygen during the first ascent, and is inert
during the second ascent of the giant branch. e.
The Sun's core will be inert during both ascents
of the giant branch.
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Quiz Questions
14. Why will a helium flash never occur in some
stars? a. Some stars will never leave the main
sequence. b. Some stars do not develop degenerate
helium cores. c. Some stars have a hydrogen flash
in place of a helium flash. d. Some stars contain
no helium. e. All of the above.
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Quiz Questions
15. Why are lower-mass stars unable to ignite
more massive nuclear fuels such as carbon? a.
They never get hot enough. b. They did not
accumulate enough carbon when they formed. c.
Beryllium is highly unstable. d. Carbon has too
many neutrons in its nucleus. e. Both a and d
above.
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Quiz Questions
16. How do star clusters confirm that stars are
evolving? a. The H-R diagram of a star cluster
is missing the upper part of the main
sequence. b. The H-R diagram of a star cluster is
missing the lower part of the main sequence. c.
The relative motion of stars in a cluster can be
estimated by their Doppler shifts. d. Pulsating
variable stars in globular clusters display a
period-luminosity relationship. e. Star clusters
occasionally lose members.
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Quiz Questions
17. How are the ages of star clusters related to
their turn-off points? a. The age of a cluster
is the life expectancy of stars at its turn-off
point. b. The higher the turn-off point, the
older the star cluster. c. The lower the turn-off
point, the older the star cluster d. Both a and b
above. e. Both a and c above.
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Quiz Questions
18. What is the general trend in the ages of the
two types of star clusters? a. Globular clusters
are young and open clusters are old. b. Globular
clusters are old, and open clusters are both
young and old. c. All star clusters are very
young d. All star clusters are very old. e. The
two types of star clusters have both very young
and very old members.
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Quiz Questions
19. From Figure 12-13, what is the absolute
magnitude of a Type II Cepheid with a period of
30 days? a. -5 b. -4 c. -3 d. -2 e. -1
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Quiz Questions
20. The period of a Cepheid variable star and the
time of one recent maximum can be used to predict
the time of a future maximum. Suppose that you
calculate the time of future maximum brightness
and then make measurements to observe this
maximum. After the correction for Earth's
orbital position has been made, you find that the
maximum occurred a few minutes later than
predicted. What does this tell you about this
star? a. The star is moving toward Earth. b. The
star is moving away from Earth. c. The star is
slowly contracting. d. The star is slowly
expanding. e. The star is not a Cepheid variable.
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Answers
1. c 2. b 3. c 4. d 5. d 6. e 7. b 8. c 9. b 10. e
11. b 12. c 13. e 14. b 15. a 16. a 17. d 18. b 19
. d 20. d
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Evolution of Stars
(SLIDESHOW MODE ONLY)