Stellar Evolution

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Stellar Evolution
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• The Main Sequence Life of a Star
• Stars spend 90 of their lifetime burning
  hydrogen on
• the main sequence.
• Main sequence stars are stable because of a
  built-in
• Pressure-Temperature thermostat

The Pressure-Temperature Thermostat Star
Contracts ? T, P ? ? Nuclear energy
generation increases, ? star expands, restoring
balance Star Expands ? T, P ? ? Nuclear energy
generation decreases, ? star contracts,
restoring balance
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• How do Astronomers know about conditions in the
• interiors of stars?
• We can construct stellar models from known laws
  of Physics
• Relevant Laws of Physics
• Hydrostatic Equilibrium
• Laws of Energy Transport
• Continuity of Mass (no gaps in the interiors of
  stars!)
• Conservation of Energy
• The Ideal Gas Law
• The laws of Nuclear Fusion

Stellar model tells you how T, ?, P vary
from center to edge.
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Nuclear Fusion Low mass main-sequence stars
burn hydrogen using the Proton-Proton Chain.
High mass main-sequence stars use a different set
of reactions, called the CNO cycle C12 H1
? N13 ? N13 ? C13 e ? C13 H1
? N14 ? N14 H1 ? O15 ? O15 ? N15 e
? N15 H1 ? He4 C12 Net effect 4H ? He
energy. C12 acts as a catalyst.
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• What do these stellar models tell us?
• Hot, massive main-sequence stars burn hydrogen
  much
• faster than cool, low mass main-sequence
  stars.
• This means that massive stars have shorter
  lifetimes.
• Mass Main Sequence Lifetime
• 30 M? 5 Million years
• 5 M? 65 Million years
• 1 M? 9 Billion years
• 0.1 M? 200 Billion years

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• Stars with M lt 0.08 M? will never ignite fusion
  reactions
• in their cores. These objects are called Brown
  Dwarfs.

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3) The interior structure of main sequence stars
depends on their mass
M gt 1.2 M?
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0.4 M? lt M lt 1.2 M?
M lt 0.4 M?
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What do you think the interior of a B-type
main-sequence star is like?
A.
Completely convective throughout
B.
A radiative core with a convective envelope
C.
A convective core with a radiative envelope
D.
Completely radiative

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• Stellar Evolution
• Main Sequence stars fuse hydrogen in their cores,
  producing
• Helium as a waste product. Eventually, the
  hydrogen
• in the core is exhausted, leaving behind a helium
  core.
• When this happens
• Helium core contracts and heats up
• Hydrogen burning begins in a shell around the
  core
• The outer envelope expands and cools ? Red Giant

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The core continues to collapse until temperatures
and pressures are high enough for helium to begin
nuclear fusion. Helium ignition occurs when the
star is in the Red Giant region. The helium
fusion reactions are called the Triple-?
Process He4 He4 ? Be8 ? Be8 He4 ? C12
?
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When do you think helium ignition will take
place in the core of the sun?
It has already taken place
A.
B.
Tomorrow
In a few billion years from now
C.
Never!
D.

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Helium Ignition (Red Giant Stage)
High Mass Stars M gt 3 M? Helium burning
begins gently Intermediate Mass Stars 0.4 M? lt
M lt 3 M? Helium ignition occurs explosively in
a degenerate core. This leads to a Helium
flash. Low Mass Stars M lt 0.4 M? Helium
burning never begins the star evolves directly
to the White Dwarf stage.
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Degenerate Matter
The cores of intermediate-mass red giants are
degenerate because the densities are very high.
The Pauli Exclusion Principle tells us that
electrons can only be packed so closely before
they form an incompressible fluid.
Under degenerate conditions, pressure is
decoupled from temperature.
This can lead to runaway nuclear reactions ?
Helium Flash!
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The Evolution of Intermediate Mass Stars
3 M? gt M gt 0.4 M?
Once hydrogen is exhausted in the core, the
helium core begins to collapse, hydrogen shell
burning begins, and the envelope expands ? Red
Giant.
In the Red Giant stage, helium ignition takes
place in the degenerate helium core. This leads
to an explosive Helium flash.
Effects of the helium flash cannot be observed on
the surface of the star.
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Evolution of Intermediate Mass Stars (continued)
Helium burning continues while the star is in the
red giant stage. Helium burning produces carbon.
At the end of helium burning, the star has a
carbon core.
Once helium is exhausted in the core, the carbon
core begins to collapse, and helium burning
ignites in a shell around the core.
The envelope once more expands, and the star
climbs the red giant branch again.
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Evolution of Intermediate Mass Stars (continued)
In intermediate-mass stars, this helium-shell
burning is unstable. This leads to thermal
pulses, and the stellar envelope is thrown off,
producing a Planetary Nebula. The exposed core
evolves quickly to become a White Dwarf.
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Planetary Nebulae
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What will the final fate of the sun be?
white dwarf
A.
black hole
B.
neutron star
C.
supernova explosion
D.
nova
E.
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What are White Dwarfs?
White dwarfs represent the end stage in the
evolution of all but the most massive stars.

• Inert, no energy production
• High densities 106 107g/cm3 ? matter is
  degenerate
• Supported by pressure of degenerate electrons
• White dwarfs cool to become black dwarfs
• Radius ? 1R?
• Maximum mass 1.4 M?

If the mass of a white dwarf exceeds 1.4 M?
(known as the Chandrasekhar Limit), the white
dwarf will collapse into a Neutron star.
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The Evolution of Low Mass Stars (M lt 0.4 M?)
Low mass stars are completely convective, and
thus most of the mass of the star is available
for hydrogen fusion. As a result, they are very
long-lived.
The helium core thus constitutes nearly the
entire star. Thus, on hydrogen exhaustion and
core collapse, a hydrogen shell does not ignite.
Helium ignition never happens in these stars.
Instead, the star evolves directly to the white
dwarf stage.
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How do we know that our theories of
stellar evolution are correct?
Stars evolve over millions and even billions of
years much longer than a human lifetime, or
even the history of human civilization. So how
can we test our theories of stellar evolution?
Answer observe star clusters. Star clusters
have the advantage that all of their member stars
were born at the same time and at the same place.
Hence, we can use star clusters of different
ages to test our theories of stellar evolution.
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Stellar Evolution and Cluster HR diagrams
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Cluster HR diagrams
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The Final Stages in the Evolution of Massive Stars
4 M? lt M lt 9 M? Once helium exhaustion has
occurred, the carbon cores of these stars
collapse. This leads to the ignition of a helium
shell, and the star expands to become a red
supergiant.
In these stars, carbon ignition takes place in a
degenerate core ? Carbon Flash ? Supernova??
However, we believe that stars in this mass range
manage to lose enough mass while they are red
giants that they avoid this fate and end up as
white dwarfs.
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When stars with M gt 3M? evolve off the
main- sequence, they move through
the Instability Strip where they begin to
pulsate. These yellow supergiant stars are
called Cepheid Variable
Stars and have played a very important role in
the study of distances in the Universe.
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Cepheid Variable Stars follow a
Period-Luminosity relationship. What this means
is that if we know their period of oscillation,
then we can determine their luminosity. Knowing
their luminosity, we can find their distances.
Since Cepheids are luminous supergiants, they
can be seen in external galaxies, and thus we can
use them to find distances to other galaxies.
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Evolution of stars with M gt 9 M?
Carbon ignition is gentle in these stars, and
these stars proceed to more advanced stages of
nuclear fusion
Carbon burning ? Neon, Magnesium Oxygen Neon
burning ? Magnesium Oxygen Oxygen burning ?
Silicon, Sulfur, Potassium, etc. Silicon burning
? Iron
Iron burning is Endothermic ? catastrophic
collapse of the core ?
Supernova!!!
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The Core of a Massive Star about to become
a Supernova
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Collapse of the Core of a Massive Star
If the mass of the core is less than 1.4
M?, degenerate electron pressure halts the
collapse and the core becomes a white dwarf.
If the mass of the core gt 1.4 M?, gravity
over- comes degenerate electron pressure and
the collapse continues until degenerate
neutron pressure halts the collapse ? Neutron
Star.
If the mass is greater than about 2 3 M?,
the collapse is unstoppable, and leads to the
formation of a Black Hole.
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Collapse of the Core of a Massive Star (continued)
If the core of the star collapses to a neutron
star, the electrons combine with protons to
produce neutrons and a flood of neutrinos.
e- p? n ?
This flood of neutrinos is responsible for
expelling the Outer layers of the star, producing
a supernova.
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Supernova 1987A

• First supernova visible to the naked eye since
  Keplers
• supernova in 1604
• Occurred in the Large Magellanic Cloud (LMC),
• 150,000 light years away.
• Progenitor was a Blue Supergiant (unexpected)
• Discovered by Ian Shelton, University of Toronto

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Light curve of SN1987A
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Historical Supernovae The supernova of 1054AD
This supernova was observed around the world
(except possibly Europe). It was recorded by the
Chinese who noted "In the 1st year of the period
Chih-ho, the 5th moon, the day chi-ch'ou, a
guest star appeared... After more than a year it
gradually became invisible. It may also have
been observed by the Anasazi, in the
Four-Corners area. Its remnant is the Crab
Nebula.
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In our Milky Way Galaxy, the progenitors of
core-collapse supernovae are red supergiants.
Which star(s) do you think are likely future
supernovae?
Alpha Persei F5 Ib
A.
Arcturus K2 III
B.
Betelgeuse M2 Ia
C.
Procyon F5 V
D.
Gliese 15B M3.5 V
E.
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Nucleosynthesis in Type II Supernovae
Many reactions in the final nuclear fusion phases
of massive stars produce neutrons. These
neutrons can be captured by atomic nuclei to
produce heavier nuclei.
Elements with atomic numbers between 24 and 56
are primarily produced in this way. This process
is called s-process (slow process) neutron
capture. Example
Cd114 n1 ? Cd115 Cd115 ? In115 e- ? In115
n1 ? In116 In116 ? Sn116 e- ?
The s-process can also occur in intermediate-mass
stars, just before the Planetary nebula stage.
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Nucleosynthesis (continued)
On the other hand, at the time of the
catastrophic collapse of the iron core, a flood
of neutrons is released.
This makes possible a rapid capture of neutrons
by heavy nuclei, leading to the production of
many unstable isotopes, and all elements up to
atomic number 92 (Uranium).
This is called the R-process (rapid process)
of neutron capture.
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The difference between the s-process and the
r-process is
the type of subatomic particle involved in
the nucleosynthesis
A.
the rate at which individual nuclei are
involved in collisions with neutrons
B.

the type of nuclear reaction involved
proton-proton chain or CNO cycle
C.

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Types of Supernovae
Type I No hydrogen can be seen in the
spectrum. Type II Strong lines of hydrogen
present in the spectrum

• Type II supernovae come from young, hydrogen-
• rich massive stars.
• ?Type I supernovae originate from stars in which
  the
• hydrogen is depleted.

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Interacting Binary Stars The Roche Lobe
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Or, maybe through the merger of a binary white
dwarf!
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Binary stars can become interacting when one
or both of the stars fill their Roche Lobes.
When one of the stars fills its Roche lobe,
mass can be transferred from one star to
the other. This can profoundly affect
the evolution of the two stars.