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Lecture
19: Stellar Evolution
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| Astronomy
101/103 |
Terry
Herter, Cornell University
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Lecture
Topics
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- The
Interiors of Stars
- How
stars "evolve"
- Main-sequence
evolution
- Giants
and Supergiants
- Nucleosynthesis
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M-S
Evolution
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- Fusion
is occurring in the cores of stars
- H
is being converted into He
- Since
4 particles are converted to 1, the pressure drops.
- The
core collapses and heats up.
- This
heats the outer layers which expand outward.
Stars
Evolve, even on the Main-Sequence
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The Sun
on the
M-S
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- 5
billion years ago:
- Beginning
of its life on main-sequence
- Sun
had 1/3 luminosity it has now.
- 5
billion years from now:
- End
of its life on main-sequence
- Sun
will have twice the luminosity it has now.
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Stellar
Evolution
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- When
H is exhausted, the core shrinks.
- It
heats up but can not yet burn He, which needs 100,000,000
K!
- The
high temperatures ignites a shell of H around the core.
- The
increased pressure drives the envelope of the star outward.
- Creating
a giant or supergiant.
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Giant
and
Supergiant Stars
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- Expanded
stars: very large radii
- =>
large luminosity ( L = 4*pi*R2 sigma*T4
)
- Uneasy
stellar evolutionary stage
- Variability
- Mass
loss
- Very
high temperature in the core
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Late
Stages of
Stars
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- The
Helium Flash:
- When
Tcore ~ 108 K, He begins to burn.
-
Be8 is highly unstable - another He4 must
come along in 10-8 seconds!
- Triple
Alpha process: 3He4 => C12
- First realized by Ed Salpeter (Cornell)
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Late
Stages of
Stars II
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- Onset
of He4 fusion is explosive in solar mass
stars
- Core
was largely supported by electron degeneracy
pressure (more on this in Lecture 20) - not dependent
on temperature
- As
T -> 108 K, reactions take off
=> temperature
increases further, but the pressure stays the
same!
- Reactions
run away since they are proportional to T40 -
like a bomb
- L
~ 1011 Lsun - but we don't
see this!
- This
increased luminosity lifts the core and enables
stable He4 fusion at the core: Luminosity
stabilizes at L ~ 30 to 100 Lsun.
- H
fusion continues in the shell continues but at
a much slower rate => L goes down.
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Late
Stages of
Stars III
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- Eventually
He in the core is exhausted
- Contraction
of the core raises the temperature further
- Ignites
He shell burning around the core
- We
now have twin layers of He and H shell burning at
ever increasing rates
- Eventually
for solar mass stars the core stabilizes under electron
degeneracy pressure
- Envolope
is ejected as a "planetary nebula"
- The
core remains as a "white dwarf"
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Evolution
of the
Sun
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Path
in the H-R diagram of a 1 Msun Star 
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Evolution
of a Massive
Star
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Path
in the H-R diagram of a 20 Msun Star
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Time Scales
of Stellar
Evolution
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Mass
(Msun)
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Formation
(years)
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Main-seq
(years)
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Giant
Phase (years)
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1
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1x108
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9x109
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109
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5
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5x106
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6x107
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107
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10
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6x105
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1x107
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106
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Star
Stuff
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- The
conversion of H into He is not the only nuclear reaction
that can take place in stars.
- All
elements other than H and He are produced from stars
(or explosions of stars.)
The
material in you was formed by a star!
The
process of building up heavy elements from light ones is called
nucleosynthesis.
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Nucleosynthesis
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.
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.
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4
H1
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=>
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He4
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He4
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+
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He
4
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=>
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Be8
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Be8
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+
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He
4
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=>
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C12
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C12
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+
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He
4
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=>
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O16
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O16
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+
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He
4
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=>
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Ne20
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Ne20
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+
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He
4
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=>
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Mg24
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.
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.
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.
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.
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...up
to Fe56
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Energy
Release
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Energy
from Fusion and Fission
Energy
is released when elements lighter than iron are fused together.
Likewise energy is released when element heavier than iron
are split apart (fission). Conversely, to fuse heavier elements
or split light elements requires extra energy.
The
plot below shows the "binding energy" per nucleon
versus mass number. Iron is the most tightly bound nucleus.
This means that moving towards iron releases energy. It
is like a ball rolling to the lowest point.
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Turning to
Iron
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- The
most stable element is Iron (26Fe56).
- Need
energy to split up Fe or to add to Fe.
- For
elements lighter than iron:
- For
elements heavier than iron:
- The
universe is slowly turning to iron!
The
Core of an Evolved Star
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Most
Common
Elements
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Element
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Atomic
Weight (amu)
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Relative
Number
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H
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1
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1.0
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He
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4
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0.16
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O
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16
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9.0x10-4
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Ne
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20
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5x10-4
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C
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12
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4x10-4
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N
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14
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1.1x10-4
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Si
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28
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3.2x10-5
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Mg
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24
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2.5x10-5
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Fe
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56
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4.0x10-6
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Ni
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59
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1.0x10-6
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...
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>60
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1.0x10-7
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Abundances
of Chemical
Elements
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Life Cycle
of Stars
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- Birth:
- Gravitational
Collapse of Interstellar Clouds
- "Hayashi
Contraction" of Protostar
- Life:
- Stability
on Main-Sequence
- Long
life - energy from nuclear reactions in the core
(E = mc2)
- Death:
Lack of fuel, instability, variability
- expansion
(giants, supergiants),
- explosions!!
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