Our Star

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Our Star

• The Sun

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The Sun in Bulk

• Radius 109 Earth radii
• Mass 332,000 Earth masses
• Composition 92 Hydrogen 7 Helium (by vol)
• Surface Temperature 5780 K
• Rotational Period 25 to 36 days
• Differential rotation
• Luminosity 4x1026 W
• 1400 W/m2 at Earths surface
• 10 billion 1-Mton nuclear bombs per second
• Six seconds of energy could evaporate the oceans

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Our Knowledge of the Sun

• The sun is at hydrostatic equilibrium
• The inward pull of gravity is just balanced by
  the outward pressure of its interior
• This means the interior must be extremely hot
• Provided first clue that fusion might be
  happening
• Helioseismology
• The surface of the sun oscillates like a set of
  bells
• Caused by internal pressure waves
• Studies can reveal much about the interior much
  like earthquakes are used to study Earths
  interior

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Layers of the Sun

• Core 200,000 km thick
• Radiation Zone 300,000 km thick
• Convection Zone 200,000 km thick
• Photosphere 500 km thick (marks the surface)
• Chromosphere 1500 km thick
• Transition Zone 8000 km thick
• Corona 300,000 km thick
• Solar Wind 50 AU thick

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The Core

• Contains 40 of the Suns mass
• Site of nuclear fusion (source of energy)
• Temperatures near 15,000,000 K
• All atoms are completely ionized
• This makes them transparent to radiation
• Energy travels as photons unhindered until it
  reaches the radiation zone

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Radiation Zone

• Temperatures around 7,000,000 K
• This lower temperature allows some atoms to
  retain their electrons
• When a photon from the core encounters an atom,
  it is absorbed and the electron is excited to a
  higher energy level
• When the atom reverts to a lower energy level,
  the photon is released in a random direction
• Eventually the photon reaches the outer edge of
  the radiation zone (est. 10,000 to 170,000 years)
• The radiation zone is so dense it is completely
  opaque to radiation

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Convection Zone

• Temperatures around 2,000,000 K
• As the density decreases with increasing distance
  from the center, atoms are more able to move
• Convection currents get established
• Hot solar gas rises toward the surface
• Colder solar gas sinks to the interior
• Transports energy from the interior to the surface

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Photosphere
Radiation Zone
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Photosphere

• Temperature of 5780 K
• Density is low enough that a released photon is
  unlikely to encounter another atom
• This radiation is what we see as sunlight
• This is the layer of the Sun we can see
• The surface appears to be sharp because this
  layer is very thin

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An Energy Transfer Analogy
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Photospheric Properties

• Granulation
• The surface of the sun appears mottled
• These bubbles are the tops of convection cells
  in the layer below the photosphere
• The variations in brightness result from
  differences in temperature
• Each granule measures about 1000 km across
• Supergranulation
• Measure up to 30,000 km across
• Likely imprints of a deeper level of larger
  convective cells

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Chromosphere

• Coolest part of the Sun (4500 K)
• Temperature not high enough to emit visible
  radiation
• Composition determined by studying absorption
  spectra

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Transition Zone and Corona

• Temperature rises steeply, to over 1,000,000 K
• Coronal temperatures hold steady at near 3
  million Kelvin
• Cause of the rapid temperature rise is unknown,
  but probably related to magnetic disturbances in
  the photosphere
• The corona emits its own radiation and can
  clearly be seen against the blackness of space
  during a total solar eclipse

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Solar Wind

• Fast moving particles from the corona escape the
  Suns gravitation
• Photons travel at the speed of light and reach
  Earth in about 8 minutes
• Particles are much slower (500 km/s) and reach
  Earth in a few days
• The Sun is gradually evaporating at a rate of
  roughly 2 million tons every second