Chapter 10 Our Star

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Chapter 10Our Star
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10.1 A Closer Look at the Sun

• Our Goals for Learning
• Why does the Sun shine?
• What is the Suns structure?

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Why does the Sun shine?
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Is it on FIRE?
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Is it on FIRE?
Chemical Energy Content
10,000 years
Luminosity
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Is it on FIRE? NO!
Chemical Energy Content
10,000 years
Luminosity
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Is it CONTRACTING?
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Is it CONTRACTING?
Gravitational Potential Energy
25 million years
Luminosity
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Is it CONTRACTING? NO!
Gravitational Potential Energy
25 million years
Luminosity
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E mc2 - Einstein, 1905
Is it CONTRACTING? NO!
Gravitational Potential Energy
25 million years
Luminosity
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Is it powered by NUCLEAR ENERGY?
Nuclear Potential Energy (core)
10 billion years
Luminosity
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Is it powered by NUCLEAR ENERGY? YES!
Nuclear Potential Energy (core)
10 billion years
Luminosity
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Gravitational equilibrium The outward push of
pressure balances the inward pull of gravity
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Weight of upper layers compresses lower layers
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Gravitational equilibrium Energy provided by
fusion maintains the pressure
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Gravitational contraction Provided energy that
heated core as Sun was forming Contraction
stopped when fusion began
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What is the Suns structure?
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Radius 6.9 x 108 m (109 times Earth) Mass
2 x 1030 kg (300,000 Earths) Luminosity
3.8 x 1026 watts
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Solar wind A flow of charged particles from the
surface of the Sun
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Corona Outermost layer of solar atmosphere 1
million K
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Chromosphere Middle layer of solar atmosphere
104 - 105 K
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Photosphere Visible surface of Sun 6,000 K
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Convection Zone Energy transported upward by
rising hot gas
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Radiation Zone Energy transported upward by
photons
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Core Energy generated by nuclear fusion 15
million K
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What have we learned?
Why does the Sun shine? The Sun shines because
gravitational equilibrium keeps its core hot and
dense enough to release energy through nuclear
fusion.
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What have we learned?
What is the Suns structure?
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10.2 Nuclear Fusion in the Sun

• Our Goals for Learning
• How does nuclear fusion occur in the Sun?
• How does the energy from fusion get out of the
  Sun?
• How do we know what is happening inside the Sun?

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How does nuclear fusion occur in the Sun?
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Fusion Small nuclei stick together to make a
bigger one (Sun, stars)
Fission Big nucleus splits into smaller
pieces (Nuclear power plants)
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High temperature enables nuclear fusion to happen
in the core
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Sun releases energy by fusing four hydrogen
nuclei into one helium nucleus
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Proton-proton chain is how hydrogen fuses into
helium in Sun
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IN 4 protons OUT 4He nucleus 2 gamma rays 2
positrons 2 neutrinos Total mass is 0.7 lower
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Thought Question

• What would happen inside the Sun if a slight rise
  in core temperature led to a rapid rise in fusion
  energy?
• A. The core would expand and heat up slightly
• B. The core would expand and cool
• C. The Sun would blow up like a hydrogen bomb

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Thought Question

• What would happen inside the Sun if a slight rise
  in core temperature led to a rapid rise in fusion
  energy?
• A. The core would expand and heat up slightly
• B. The core would expand and cool
• C. The Sun would blow up like a hydrogen bomb

Solar thermostat keeps burning rate steady
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Solar Thermostat
Temperature Decreases
Temperature Restored
Fusion Rate Decreases
Core compresses
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Solar Thermostat
Temperature Increases
Temperature Restored
Fusion Rate Increases
Core expands
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How does the energy from fusion get out of the
Sun?
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Energy gradually leaks out of radiation zone in
form of randomly bouncing photons
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Convection (rising hot gas) takes energy to
surface
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Bright blobs on photosphere are where hot gas is
reaching surface
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How do we know what is happening inside the Sun?
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We learn about inside of Sun by

• Making mathematical models
• Observing sun quakes
• Observing solar neutrinos

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Patterns of vibration on surface tell us about
what Sun is like inside Results agree very well
with mathematical models of solar interior
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Neutrinos created during fusion fly directly
through the Sun Observations of these solar
neutrinos can tell us whats happening in core
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Solar neutrino problem Early searches for solar
neutrinos failed to find the predicted number
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Solar neutrino problem Early searches for solar
neutrinos failed to find the predicted
number More recent observations find the right
number of neutrinos, but some have changed form
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What have we learned?

• How does nuclear fusion occur in the Sun?
• Fusion of hydrogen into helium, which occurs via
  the protonproton chain. Gravitational
  equilibrium acts as a thermostat that keeps the
  fusion rate steady.

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What have we learned?

• How does the energy from fusion get out of the
  Sun?
• Energy moves through the deepest layers of the
  Sunthe core and the radiation zonein the form
  of randomly bouncing photons. After energy
  emerges from the radiation zone, convection
  carries it the rest of the way to the
  photosphere, where it is radiated into space as
  sunlight.

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What have we learned?

• How do we know what is happening inside the
  Sun?
• theoretical models
• use known laws of physics and then check the
  models against observations and studies

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10.3 The Sun-Earth Connection

• Our Goals for Learning
• What causes solar activity?
• How does solar activity affect humans?
• How does solar activity vary with time?

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What is solar activity?
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Solar activity is like weather

• Sunspots
• Solar Flares
• Solar Prominences
• All related to magnetic fields

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Sunspots Are cooler than other parts of the
Suns surface (4000 K) Are regions with strong
magnetic fields
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Charged particles spiral along magnetic field
lines
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Loops of bright gas often connect sunspot pairs
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Loops trace magnetic field lines
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Magnetic activity causes solar flares that send
bursts of X-rays and charged particles into space
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Corona appears bright in X-ray photos in places
where magnetic fields trap hot gas
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How does solar activity affect humans?
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Coronal mass ejections send bursts of energetic
charged particles out through the solar system
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Charged particles streaming from Sun can disrupt
electrical power grid and can disable
communications satellites
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Energetic particles high in Earths atmosphere
cause auroras (Northern Lights)
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How does solar activity vary with time?
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Number of sunspots rises and falls in 11-year
cycle
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Sunspot cycle has something to do with winding
and twisting of Suns magnetic field
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What have we learned?

• What causes solar activity?
• Convection combined with the rotation pattern of
  the Sunfaster at the equator than at the
  polescauses solar activity because these gas
  motions stretch and twist the Suns magnetic
  field.

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What have we learned?

• How does solar activity affect humans?
• Bursts of charged particles ejected from the Sun
  during periods of high solar activity can hamper
  radio communications,disrupt electrical power
  generation,and damage orbiting satellites.

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What have we learned?

• How does solar activity vary with time?
• The sunspot cycle, or the variation in the number
  of sunspots on the Suns surface,has an average
  period of 11 years.The magnetic field flip-flops
  every 11 years or so, resulting in a 22-year
  magnetic cycle.