Announcements

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Announcements

• Star Assignment 3, due Wednesday March 17
• READ chapter 16,
• Do Angel quiz,
• Do Astronomy Place tutorial Measuring Cosmic
  Distances Lesson 2, Stellar Parallax
• Global Warming Project, due Wednesday March 17
• Sample 4 web sites taking different positions on
  whether Global Warming is occurring whether
  people are responsible.
• For each site, evaluate the science - Is it good
  or poor?
• Criteria (list from class discussion)
• Submit via email

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Gravitational equilibrium The outward push of
pressure balances the inward pull of gravity
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Pressure
Pressure is force exerted by colliding particles

• Higher density --gt particles closer together --gt
  more collisions --gt higher pressure
• Higher temperature --gt particles move faster --gt
  more harder collisions --gt higher pressure

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Equilibrium

• Pressure balances Gravity

• Pressure weight of overlying material
• Pressure increases toward center
• to balance larger gravity toward center

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Energy SourceNuclear Fusion4 1H --gt 1 4He 2
Energy
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Fusion occurs ONLY in the Suns CORE ?

• Nuclear fusion
• Lighter nuclei are fused into heavier nuclei
• all nuclei are positively charged
• Electromagnetic force causes nuclei to repel each
  other.
• for fusion to occur, nuclei must be moving fast
  enough to overcome E-M repulsion
• this requires high temperatures
• When nuclei touch, the nuclear force binds them
  together

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Electric Barrier
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Answers to Explain the Thermostat

• Good - gives details
• Gravitational equilibrium acts as the Suns
• internal thermostat. If there is a small rise
• in the core temperature, the fusion rate will
• rise dramatically. The increased energy
• creates increased thermal pressure that can
• overcome gravity, causing the core to
• expand and cool which restores the fusion
• rate to normal. If there is a small decrease
• in the core temperature, there will be a
• dramatic decrease in the fusion rate. The
• decrease in energy will allow gravity to
• compress the core. This will heat up the
• core and in turn restore the fusion rate to
• normal.

• Bad - does not answer question
• Nuclear fusion transforms hydrogen into helium
  and creates extra energy. This energy is so
  great it heats up to temperatures no spacecraft
  could survive if it ever attempted to journey to
  its core.

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Energy Loss

• Heat is produced in the core
• Heat is lost (radiated away to space) from the
  surface
• How is heat transported from the hot core to the
  (relatively) cool surface?

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Inner 2/3 of Sun Heat is transported by RADIATION
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Radiative zone Energy gradually diffuses outward
(in about a million years) by randomly bouncing
photons. Each hot core gamma ray photon becomes
many visible cool surface photons.
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Random Walk Activity

• Each person in center of classroom take a
  balloon. Blow it up and tie it. Tap it up.
• Everyone tap balloons UP when one comes to you
• Time how long it takes half of balloons to reach
  a wall
• Now compare with time it takes if balloon are
  tapped directly towards a wall.

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Rate of energy loss

• Determined by bulk of star outside of core
• If star is good insulator -gt rate of energy loss
  smaller
• If star is poor insulator -gt rate of energy loss
  greater

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Why does energy take so long to reach the surface?

• Good - gives detailsGamma-ray photon collide
  with electrons often, sending them bounced in
  random directions. With each bounce the photon
  drifts farther from its initial location. As a
  result it has to go a much longer distance than a
  straight line and so takes a very long time to
  reach the surface

• Bad - no details
• Energy generated in the core takes about a
  million years to reach the surface, because the
  transportation process (via photons and
  convection) is so complex.

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Outer 1/3 of Sun Energy is transported by
CONVECTION
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Granule fountainHot gas rises, spreads out,
falls back
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Convection zone Hot gas rises, radiates away its
heat at the surface, becomes cool gas and sinks,
pulled down by gravity.
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Convectionpatternat solarsurfact
Hot gas rises (floats up) -gt Brighter Cool
gas sinks (pulled down by gravity) -gt
Darker
1000 km
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Convection patternat thesolar surface
Sunspots Magnetic fields inhibit
convection -gt cooler -gt Darker
(Fig. 15.15)
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EquilibriumThe Structure of a Star is
determined by two balance conditions

1. Energy Production (by nuclear fusion in core)
   Energy Loss(by radiation from surface)
2. Pressure (pushing out) Gravity (pulling in)

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Feedback - the Solar Thermostat
(Fig. 15.8)
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The Solar Thermostat

• Temperature increase
• increased fusion
• energy production greater than energy loss
• core heats up
• pressure increases
• pressure greater than gravity
• core expands
• core cools (convert KE to PE)
• energy generation decreases

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Tests Compare predictions of models with
observations

• 1. Observations of resonant sound wave
  oscillations (helio-seismology)
• 2. Neutrino observations

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Sound Waves in the Sun
If wave comes back on itself, it is
reinforced resonant
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Soundwavespenetratedifferentdistancesprobe
thesolarinterior.
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Global Oscillation Network Group
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Observed as Doppler Shiftat the solar surface
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Test Solar Models

• Compare
• frequencies
• of observed
• resonant
• oscillation
• modes with
• frequencies
• calculated
• from solar
• models
• Agreement Excellent

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Model vs. Helioseismic Observations
Density water 1 g/cm2
Fig. 15.10
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Sun in Neutrinos
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Solar Neutrino Flux Theory vs. Observation
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Theory is biased by Experiment
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Conclusioneither

1. Something wrong with models of the Sun
2. Something wrong with theory of neutrinos

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Resolution (recent)

• 3 types of neutrinos - e, mu, tau
• Sun produces only e type
• SNO observing only e-type, sees 0.35 predicted
  number
• Super-Kamiokanda, sensitive to e some mu tau,
  sees 0.46 predicted number
• Conclusion e-type lt-gt mu tau - types
• Hence, see only 1/3 predicted if observe only
  e-type

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