Announcements

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Announcements

• The next Observing nights are this Tuesday and
  Thursday_at_ 730
• You have until Wednesday at noon to make up the
  test if you missed it
• Due today
• Starry Night Workbook Stars and the HR Diagram
  p.43
• Chapter 13 Smartwork

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Chapter 14The Sun
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The Sun Today
Current view of the sun
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The Sun in other wavelengths
X-Ray
Extreme UV
Hydrogen Alpha
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Basic Solar Facts
Diameter 1.4 million km Age 4.5 billion years
Mass 330,000 x Earth Average Density 1.41
gm/cm3 Distance from Earth 149.6 million
km Average Solar Wind Speed 3 million
km/hr Luminosity 3.9x1036 watts Temperature at
surface 5,770 K Temperature at Core 15,000,000
K Rotation Period at Equator 25 Earth
days Rotation Period at Poles 35 Earth days What
you can view is only the surface of the sun,
there is a lot going on in the interior
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What is the interior of the Sun like? How do we
know?
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What is the interior of the Sun like? How do we
know?
We make models using what we know about Physics
and Mathematics
The model balances pressure above and below and
also gravity
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What is the interior of the Sun like? How do we
know?
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Using Hydrostatic Equilibrium scientists develop
model conditions inside the Sun
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We compare what the model predicts to what we can
see
If the model fits our observationswe know that
the model is correct. If it doesnt fit we must
adapt.
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• So, the Suns core is constantly spewing out lots
  of energy in the form of radiation.
• That radiation pushes the material above it
  outward.
• Gravity and the pressure above push material
  inward.
• The inward and outward forces balance at each
  point in the entire sun.

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But what if the forces dont balance
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So, if the Suns interior forces were out of
balance, it would balance itself and return to
hydrostatic equilibrium, which wouldmake it
identical to our true sun.
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There is only one possible way for the Sun to be
arranged and thatis the way we observe it.
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Where does the Sun get its energy?
Earliest idea chemical burning If sun was
highest quality chemical fuel it would exhaust
the fuel in less than 10,000 years!
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Later ideas for source of Suns energy
Gravitational Collapse
Kelvin-Helmholtz Contraction Whenever anything
shrinks it heats up.This could produce the
observed solar output for about 25 million years.
This was the original source of energy as the Sun
was forming.
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In 1905 Einstein proposed a new way to get
energy from Matter
The answer came from his famous equation E mc2
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The sun gets its energy from Matter energy
conversion via Thermonuclear Fusion
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The sun gets its energy from Matter energy
conversion via Thermonuclear Fusion
The same source of energy as the hydrogen bomb
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The sun converts hydrogen into helium in a multi
step process
It starts with two normal hydrogen nuclei fusing
to form a heavy hydrogen nuclei a deuterium. The
reaction also produces a positron and a neutrino.
When the positron annihilates with an electron
two gamma rays are produced
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The second step uses the product of the first
step plus another hydrogen
A deuterium and a hydrogen fuse to form a
helium-3 plus another gamma ray photon
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Final step of Proton-Proton cycle
Two helium-3 fuse to form a normal helium plus
two hydrogen nuclei
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Overall Proton Proton Cycle
4 1H Þ 4He 2e 2g 2n
Releases 4.3x10-12 Joules per helium atom
produced To get the observedenergy output of sun
requires converting 600,000,000 tonnes of H to He
every second! The sun has enough hydrogen in its
core to last another 5 billion years before it
runs out Energy is only produced in the core
region where the temperature and pressure are
high enough
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Fusion requires high temperatures to overcome the
electric repulsion of protons
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How does the energy get from the core to the
surface?
Most of the energy released in the core is in the
form of gamma ray photons Most of the energy
released at the surface is in the form of visible
photons
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How does the energy get from the core to the
surface?
Most of the energy released in the core is in the
form of gamma ray photons Most of the energy
released at the surface is in the form of visible
photons
Something must happen between the core and the
surface!
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Heat Transfer Energy can move by three methods
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Which method is most efficient is determined by
density, temperature and pressure at that location
The two methods that are important in the sun are
convection and radiation.
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The Solar Interior
Radiation dominates for just over 70 of the way
out. The last 30 is by convection. At the
surface, it once again becomes radiation out into
space
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Solar Granulation
Each granule is 1000 km across and lasts a
minute or two