THE SUN

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THE SUN

• The star we see but seldom notice

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Goals

• Summarize the overall properties of the Sun.
• What are the different parts of the Sun and how
  do we know this?
• Where does the light we see come from?
• Solar activity and magnetic fields.

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The Sun, Our Star

• The Sun is an average star.
• From the Sun, we base our understanding of all
  stars in the Universe.
• No solid surface.

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Vital Statistics

• Radius 100 x Earth (696,000 km)
• Mass 300,000 x Earth (1.99 x 1030 kg)
• Surface temp 5780 K
• Core temp 15,000,000 K
• Luminosity 4 x 1026 Watts
• Solar Day
• 24.9 Earth days (equator)
• 29.8 Earth days (poles)

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Structure

• Surface
• Photosphere
• Atmosphere
• Chromosphere
• Transistion zone
• Corona
• Solar wind
• Interior
• Convection zone
• Radiation zone
• Core

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The Solar Interior

• How do we know whats inside the Sun?
• Observe the outside.
• Theorize what happens on the inside.
• Complex computer programs model the theory.
• Model predicts what will happen on the outside.
• Compare model prediction with observations of the
  outside.
• Scientific Method!

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Helioseismology

• Continuous monitoring of Sun.
• Ground based observatories
• Spacecraft (SOHO)
• Surface of the Sun is ringing
• Sound waves cross the the solar interior and
  reflect off of the surface (photosphere).

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Interior Properties

• Core 20 x density of iron
• Surface 10,000 x less dense than air
• Average density Jupiter
• Core 15,000,000 K
• Surface 5780 K

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Do you see the light?

• Everything in the solar system reflects light.
• Everything also absorbs light and heats up
  producing blackbody radiation.
• Q Where does this light come from?
• A The Sun.
• But where does the Suns light come from?

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Our Journey through the Sun

• Journey from the Suns core to the edge of its
  atmosphere.
• See where its light originates.
• See what the different regions of the Sun are
  like.
• See how energy in the core makes it to the light
  we see on Earth.

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In The Core

• Density 20 x density of Iron
• Temperature 15,000,000 K
• Hydrogen atoms fuse together
• Create Helium atoms.

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Nuclear Fusion

• 4H ? He
• The mass of 4 H atoms
• 4 x (1.674 x10-27 kg) 6.694 x 10-27 kg
• The mass of He atom 6.646 x 10-27 kg
• Where does the extra 4.8 x 10-29 kg go?
• ENERGY! ? E mc2
• E (4.8 x 10-29 kg ) x (3.0 x 108 m/s)2
• E hc/l ? l 4.6 x 10-14 m (gamma rays)
• So 4H ? He light!

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The Radiation Zone

• This region is transparent to light.
• Why?
• At the temperatures near the core all atoms are
  ionized.
• Electrons float freely from nuclei
• If light wave hits atom, no electron to absorb
  it.
• So Light and atoms dont interact.
• Energy is passed from core, through this region,
  and towards surface by radiation.

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The Convection Zone

• This region is totally opaque to light.
• Why?
• Closer to surface, the temperature is cooler.
• Atoms are no longer ionized.
• Electrons around nuclei can absorb light from
  below.
• No light from core ever reaches the surface!
• But where does the energy in the light go?
• Energy instead makes it to the surface by
  convection.

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Convection

• A pot of boiling water
• Hot material rises.
• Cooler material sinks.
• The energy from the pots hot bottom is
  physically carried by the convection cells in the
  water to the surface.
• Same for the Sun.

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Solar Cross-Section

• Progressively smaller convection cells carry the
  energy towards surface.
• See tops of these cells as granules.

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The Photosphere

• This is the origin of the 5800 K blackbody
  radiation we see.
• Why?
• At the photosphere, the density is so low that
  the gas is again transparent to light.
• The hot convection cell tops radiate energy as a
  function of their temperature (5800 K).
• l k/T k/(5800 K) ? l 480 nm (visible
  light)
• This is the light we see.
• Thats why we see this as the surface.

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The Solar Atmosphere

• Above the photosphere, transparent to light.
• Unlike radiative zone, here atoms not totally
  ionized.
• Therefore, there are electrons in atoms able to
  absorb light.
• Absorption lines in solar spectrum are from these
  layers in the atmosphere.

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Atmospheric Composition

• Probably same as interior.
• Same as seen on Jupiter.
• Same as the rest of the Universe.

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The Chromosphere

• Very low density
• But also very hot
• Same as the gas tubes we saw in class and lab.
• Energy from below excites the atoms and produces
  emission from this layer.
• Predominant element Hydrogen.
• Brightest hydrogen line Ha.
• Chromosphere color

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Prominences
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Ha Sun
Photo by Robert Gendler
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Corona

• Magnetic activity carry energy up to the
  Transition Zone.
• 10,000 km above photosphere.
• Temperature climbs to 1,000,000 K
• Remember photosphere is only 5800 K
• The hot, low density, gas at this altitude emits
  the radiation we see as the Corona.
• But corona very faint compared to photosphere.

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The X-Ray Sun

• Q At 1,000,000 K where does a blackbody spectrum
  have its peak?
• A X-rays
• Can monitor the Solar Coronasphere in the X-ray
  spectrum.
• Monitor Coronal Holes

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Courtesy of SOHO/LASCO/EIT consortium. SOHO is a
project of international cooperation between ESA
and NASA.
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Solar Wind

• At and above the corona
• Gas is very hot
• Very energetic
• Like steam above our boiling pot of water, the
  gas evaporates.
• Wind passes out through Coronal Holes
• Solar Wind carries away a million tons of Suns
  mass each second!
• Only 0.1 of total Suns mass in last 4.6 billion
  years.

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Coronal Mass Ejections

• Material ejected by the Corona.

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Aurorae

• The solar wind
• passes out
• through the
• Solar System.
• Consists of electrons, protons and other charged
  particles stripped from the Suns surface.
• When charged particles and magnetic fields
  interact light!

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The Active Sun

• Solar luminosity is nearly constant.
• Very slight fluctuations.
• 11-year cycle of activity.

Courtesy of SOHO/LASCO/EIT consortium.
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Solar Cycle

• Increase in Coronal holes
• Increase in solar wind activity
• - Coronal Mass Ejections
• Increase in Auroral displays on Earth
• Increase in disruptions on and around Earth.

Courtesy of SOHO/LASCO/EIT consortium.
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Sunspots

• 11-year sunspot cycle.
• Center Umbra 4500 K
• Edge Penumbra 5500 K
• Photosphere 5800 K

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• The Sun doesnt rotate as a solid body.
• Equator rotates faster.
• This differential rotation leads to complications
  in the Solar magnetic field.

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Magnetic fields and Sunspots

• At kinks, disruption in convection cells.
• Sunspots form.

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Magnetic fields and Sunspots

• Where magnetic fields pop out of Sun, form
  sunspots.
• Sunspots come in pairs.

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Sunspot Numbers
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Active Regions

• Areas around sunspots give rise to the prominences

Courtesy of SOHO/LASCO/EIT consortium.