Download Evolution of Stars

Evolution of Stars
(Part I: Solar-type stars)
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Death of the Sun Parts I and II
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Learning goals: Be able to ….
! summarize the future of the Sun on a rough timescale;
! apply the basics of the conservation of energy and the
battle between gravity and outward pressure to what
“drives” a star to evolve at each major stage of evolution;
! explain what is meant by main sequence, subgiant, red
giant branch, electron degeneracy, helium flash,
horizontal branch, asymptotic giant branch, planetary
nebula, white dwarf.
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Be able to summarize the future of the Sun in a rough timescale.
Apply the basics of the conservation of energy and the battle between gravely and pressure to what “drives” a star to evolve at each major stage of evolution.
Explain what is meant by red giant branch, electron degeneracy, helium flash, horizontal branch, planetary nebula, white dwarf
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Brown Dwarf
http://chandra.harvard.edu/xray_sources/browndwarf_fg.html
"In between a star and a
planet
"Jupiter < BD mass < 0.08
Sun
"Radiates in infrared due to
low temperature.
"Mass too low to start
fusion in core.
"Slowly cools over trillions
of years.
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Core producing energy--Fusing H to He
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A review of what we know the Sun is doing as a main-sequence star
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Number of particles in core is decreasing.
NET RESULT
IN: 6 H
OUT: 1 He + 2 H
FUSION RATE MUST INCREASE TO OFFSET
DECREASE IN PARTICLE PRESSURE!
Sun is slowly
becoming more and
more luminous.
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Explain how the Sun maintains a constant balance in its interior (solar thermostat)
Part of the pressure depends on the number of particles
Pressure decreases, core shrinks slightly, pressure evens out, fusion rate must increase to offset the smaller pressure provided by the number of particles present.
Luminosity of the Sun must increase!
Summary for #1 on flowchart
Stars on the main sequence:
What is happening in the core?
How does the rest of the star support itself?
Describe the interior of the star.
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Expanding Subgiant
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MS very stable long lasting stage but H can fuse only in very center of star
Why does core have to contract?
Why doesn’t it keep on contracting?
GPE-->KE-->thermal energy
Discuss shell fusion and luminosity generated
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Summary for #2 on flowchart
Stars on the sub giant branch:
Core runs out of hydrogen to fuse.
What is happening in the core?
How does the rest of the star support itself?
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Surface of star very
far away!
Core collapse stops
H # He fusion shell
Helium ash
Degenerate
He Core
Electron
degeneracy
supports the core.
[No two electrons can
occupy identical
states.]
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Degeneracy is a strange state of material. The pressure in the core does not depend on temperature. The core cannot expand and cool. Neither can it shrink any more.
“Human” (molecular?) “Degeneracy”
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Explain degeneracy. Illustrate the core gaining mass.
Summary: star changes dramatically!
Future Sun
Today’s Sun
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“climbing the red giant branch”
What’s happening in the star ?
Source of luminosity causes star’s radius to increase.
Because surface is so far away from source of luminosity and there is such a huge area over which radiation escapes, star’s surface is cool.
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Summary for #3 on flowchart
Stars going up the red giant branch:
Core core stops contracting; no fusion is
occurring. What supports the core?
How does the rest of the star support itself?
Are gravity and outward pressure balanced?
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No support from fusion, the core contracts.
Potential energy converted to thermal energy.
Thermal energy released initiates shell fusion of H to He.
Extreme luminosity expands the rest of star
Outward pressure greater than inward pull of gravity
Core continues to contract until electron degeneracy sets in.
Rest of star supported by thermal pressure.
Shell fusion dumps helium ash on core.
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What initiates helium fusion?
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Helium flash!
Core does not
expand and cool
so nuclear
runaway occurs
- the fusion of
3He -> carbon
begins.
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Compare the sizes from main sequence star to red giant to horizontal branch star.
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What is happening in the core? What supports the star now?
Helium - to - carbon fusion (triple-alpha fusion)
Only 2 gamma rays produced. Why does helium to carbon
fusion have to have high temperatures and a high fusion
rate?
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Where is the
star on HRD?
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After He flash, temperature and luminosity depend on how much mass the star lost as it “climbed” the red giant branch
Core gets enough mass that temperature reaches He fusion limit.
Luminosity drops as equilibrium starts again.
Core supported by He to C fusion.
Rest of star supported by radiation and thermal pressure.
Star is on horizontal branch; second longest stage of its life.
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Summary for #5 on flowchart
Stars is a horizontal branch star:
What is happening in the core?
Anything happening around the core?
How does the rest of the star support itself?
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Core runs out of helium.
"Core: contracting until electron
degeneracy sets in again; giving off
energy GPE
TE
" Shells start fusion
" H#He
" He#C
C
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Fusion of He to c operates at very high Pressures and temps and in a much smaller volume than H to He. Eventually the supply gives out.
Explain what happens in the core, shells, atmosphere, etc.
Draw a diagram.
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Summary for #6 on flowchart
Star leaves the horizontal branch:
Core runs out of helium. What happens in
the core? What happens around the core?
How does the rest of the star support itself?
Are outward pressure and gravity balanced?
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"Rest of star responding to huge luminosity coming
from shell fusion
" Outer parts of star start to reach escape velocity –
star is unstable!
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Summary for #7 on flowchart
Star is at the tip of the “asymptotic” branch:
Core stops contracting; no fusion is
occurring. What supports the core?
How does the rest of the star support itself?
Are gravity and outward pressure balanced?
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End of the road for Sun-like stars
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Fusion stops at carbon for solar-mass stars;
Atmosphere is not in gravitational equilibrium;
~ 30% of mass of the star gets expelled
Central remnant called a white dwarf -- size of the Earth -- EXTREMELY dense; Gaseous object called a Planetary
;
nebula
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"
White dwarf will
slowly cool to a cold,
black carbon cinder
of about 0.7 solar
masses
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“Mass sink” #
material does not go
back into ISM.
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“WHITE DWARF”
.
http://news.bbc.co.uk/2/hi/science/nature/3492919.stm
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End of the road for our Sun and similar stars. Material does not go back into the ISM.
Core contracts and heats up; thermal energy ignites shell fusion.
Double-shell fusion is very unstable process.
Rest of star expands enormously due to extreme luminosity.
Thermal pulses give outer regions escape velocity.
Carbon core eventually supported by electron degeneracy.
Rest of star expands away into interstellar space.
Fusion of sun-like stars stops with carbon.
Core never gets hot enough to fuse carbon.
Core turns becomes a white dwarf.
White dwarfs cool over trillions of years.
Evolution of the Sun
Notice “rapid”
increase in
luminosity
Trouble
starts here!
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Evolutionary tracks of the Sun
SUMMARY
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