The Sun

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The Sun
Our Star
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General Properties

• Average star

• Spectral type G2

• Only appears so bright because it is so close.

• 109 times Earths diameter

• 333,000 times Earths mass

• Consists entirely of gas (av. density 1.4
  g/cm3)

• Central temperature 15 million 0K

• Surface temperature 5800 0K

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Recall Which parts of the sun could only be seen
during a total solar eclipse?

• Prominences
• The solar corona
• Sun spots
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• All of the above.

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Structure of the Sun
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Only visible during solar eclipses
Apparent surface of the sun
Heat Flow
Temp. incr. inward
Solar interior
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The Suns Interior Structure
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Photosphere
Energy transport via convection (explained soon)
Flow of energy
Energy transport via radiation
Energy generation via nuclear fusion
Animation
Temp, density and pressure decr. outward
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Do we have a direct view of the suns energy
source?

• Yes, because the sun is just a transparent gas
  ball.
• Yes, because most of the energy is produced very
  close to the surface.
• Yes, because the suns center is so bright that
  the light is shining through any material.
• No, because the sun has a non-transparent solid
  surface.
• No, because the radiation produced in the center
  is scattered around many times on its way towards
  the surface.

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Energy Production
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Energy generation in the sun (and all other
stars)
Binding energy due to strong force on short
range, strongest of the 4 known forces
electromagnetic, weak, strong, gravitational
Nuclear Fusion
fusing together 2 or more lighter nuclei to
produce heavier ones.
Nuclear fusion can produce energy up to the
production of iron
For elements heavier than iron, energy is gained
by nuclear fission.
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How is energy produced in an H bomb?

• (Chemical) Burning of hydrogen.
• Nuclear fusion of hydrogen into heavier elements.
• Nuclear fission of hydrogen.
• Nuclear fission of heavier elements into
  hydrogen.
• Nuclear fission of heavier elements into elements
  heavier than hydrogen.

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Energy generation in the SunThe Proton-Proton
Chain
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Need large proton speed (? high temperature) to
overcome Coulomb barrier (electromagnetic
repulsion between protons).
Basic reaction 4 1H ? 4He energy
4 protons have 0.04810-27 kg ( 0.7 ) more mass
than 4He.
T 107 0K 10 million 0K

• Energy gain Dmc2
• 0.4310-11 J
• per reaction.

Sun needs 1038 reactions, transforming 5 million
tons of mass into energy every second, to resist
its own gravity.
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Energy Transport
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Energy generated in the suns center must be
transported to the surface.
Inner layers Radiative energy transport
Outer layers (including photosphere) Convection
Cool gas sinking down
Bubbles of hot gas rising up
Gas particles of solar interior
g-rays
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Granulation
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is the visible consequence of convection
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Which every-day phenomenon is another example of
convective energy transport?

• Gas bubbles rising up in a soda drink.
• Gas bubbles rising up in boiling water.
• Giant waves moving onto the sea shore.
• Earthquakes.
• All of the above.

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Which every-day phenomenon is another example of
radiative energy transport?

• The heat of a bonfire warming you when youre
  setting close to it.
• Heating food in the microwave oven.
• The air around a light bulb heating up when the
  light is on.
• None of the above.
• All of the above.

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Very Important Warning
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Never look directly at the sun through a
telescope or binoculars!!!
This can cause permanent eye damage even
blindness.
Use a projection technique or a special sun
viewing filter.
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Sun Spots (I)
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Sun Spots (II)
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Active Regions
Visible
Ultraviolet
Cooler regions of the photosphere (T 4240 K).
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Considering that sunspots are cooler regions on
the photosphere with a temperature of 4240 K,
how would you think a sunspot would appear if you
could put it on the night sky without the sun
surrounding it?

• It would be invisible.
• It would glow very faintly, similar to the faint
  red glow of the eclipsed moon.
• It would appear moderately bright, comparable to
  the brightest stars.
• It would appear very bright even brighter than
  the full moon.
• It would be almost as bright as the sun itself.

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Solar Activity, seen in soft X-rays
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What can we infer from the fact that we see the
gas above active regions (sun spots) mostly in
ultraviolet light and X-rays?

• The gas must be very dense.
• The gas must be very dilute.
• The gas must be very hot.
• The gas must be very cold.
• The gas must consist mostly of Helium.

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Sun Spots (III)
Magnetic North Poles
Magnetic South Poles
Related to magnetic activity.
Magnetic field in sun spots is about 1000 times
stronger than average.
In sun spots, magnetic field lines emerge out of
the photosphere.
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Magnetic Field Lines
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Magnetic North Pole
Magnetic South Pole
Magnetic Field Lines
Mass ejection from the sun often follow magnetic
field loops.
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Magnetic Loops
Magnetic field lines
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What does the term Solar Maximum refer to?

• A period of high luminosity of the sun.
• The culmination of the sun in the southern
  direction on the celestial sphere.
• A temporary maximum of the number of sun spots.
• A temporary maximum of the surface temperature of
  the sun.
• A temporary maximum of the risk to get sun burned.

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The Solar Cycle
Solar Maxima
11-year cycle
Reversal of magnetic polarity (during solar
minima)
After 11 years, North/South order of
leading/trailing sun spots is reversed
gt Total solar cycle 22 years
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The Solar Cycle (II)
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Maunder Butterfly Diagram
Sun spot cycle starts out with spots at higher
latitudes on the sun
Evolve to lower latitudes (towards the equator)
throughout the cycle.
Animation
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The Chromosphere
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Region of suns atmosphere just above the
photosphere.
Chromospheric structures visible in Ha emission
(filtergram)
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The Chromosphere (II)
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Spicules filaments of hot gas, visible in Ha
emission.
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The Solar Corona
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Very hot (T 1 million 0K), low-density gas
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Prominences
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Looped Prominences gas ejected from the suns
photosphere, flowing along magnetic loops
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Eruptive Prominences
Extreme events, called coronal mass ejections
(CMEs) and solar flares, can significantly
influence Earths magnetic field structure and
cause northern lights (aurora borealis).
(Ultraviolet images)
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Eruptive Prominences
(Ultraviolet images)