Download Overview: The Sun The outer layers Photosphere: Visible Surface

10/24/11
Overview: The Sun
•  Properties of the Sun
•  Sun’s outer layers
The outer layers
•  Photosphere
–  Photosphere
–  Chromosphere
–  Corona
•  Chromosphere
•  Solar Activity
–  Sunspots & the sunspot cycle
–  Flares, prominences, CMEs, aurora
•  Corona
•  Sun’s Interior
–  The Sun as an energy source
–  Fusion in the Sun
Photosphere: Visible Surface
• Innermost visible
layer, only 400 km thick
(0.06% of radius)
• Temperature: 6,000 K
• Defines “size” of the
Sun
Photosphere isn’t actually
smooth!
Features called
granules are about
1000 km across
• Absorption lines
Convection causes granules
Granules Up-Close:
like boiling water
•  Convection: transfer of
energy through currents
•  Dark-colored areas =>
cooler gas
•  Light-colored areas =>
hotter gas
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Chromosphere: “sphere of color”
Middle layer, temperature 104 - 105 K
Chromosphere:
Hot hydrogen
atoms form a red
streak when
viewed during a
solar eclipse
Corona or “crown” of the Sun
•  Very low density
Solar activity is like “weather”
• 
• 
• 
• 
–  10 million to 10 billion
times less dense than
Earth’s atmosphere
•  However, corona is
extremely hot!
–  About 1 million K
–  Known from spectrum
Sunspots
Solar Flares
Solar Prominences
Coronal Mass Ejections (CMEs)
•  All are related to magnetic fields, vary
on an 11-year cycle
Sunspots
Dark spots
on the
photosphere
Cooler than
other parts of
the Sun’s
surface
(4000K vs.
5800K)
Sunspots
•  One of Galileo’s
major observations
•  More than 30
sketches from
summer of 1612
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Sunspot Number
• Number & locations change daily
• Can use observations to determine how fast
the Sun spins on its axis
Sunspot number peaks about
every 11 years: the sunspot
cycle
Data goes back to the 1600’s
Solar Maximum
Solar Minimum
Sunspot Cycle
Sunspots often come in pairs
•  Sunspots usually appear within 30º of equator
•  Where they form changes during a cycle
• One is positive, the other negative, like a magnet
“Butterfly Diagram”
• Connected by loops of bright gas
We think most solar activity is related
to changing magnetic fields
Fig. 7-14a, p. 135
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Other Solar Activity:
•  Solar Prominences
•  Solar Flares
•  Coronal Mass Ejections
All occur in “active regions”
associated with strong magnetic
fields
Fig. 7-14e, p. 135
Solar
Prominences
Solar Flares
Solar Flares
• Erupt high
above the
Sun’s surface
• Bursts of Xrays and
charged
particles
• Can be quite
stable (hoursdays)
• 5-10 minutes
• Usually form
near
sunspots!
Coronal Mass Ejections
Rare bursts,
more energetic
than flares or
prominences
Speeds up to
1,000 km/s
50,000 km
above Sun’s
surface
Solar Wind
•  Not moving air, like
on Earth
•  Stream of charged
particles from coronal
holes
•  900,000 mph,
reaches Earth in
about 4 days
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Solar Wind
Charged particles get trapped in Earth’s
magnetic field and cause auroras
The Sun Today
www.spaceweather.com
Structure of the Sun
Core
Corona
Core:
Energy
generated by
nuclear fusion
Inner 25%
~ 15 million K
Very dense!
Radiation Zone
Radiation
Zone: Energy
transported
upward by
photons
Convection Zone
Convection
Zone:
Energy
transported
upward by
rising hot gas
106-107 K
Outer 30%
Photons spend
a long time
here
104-106 K
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Convection causes granules
Convection (rising hot gas) takes energy to surface
How does the Sun shine?
What is the Sun’s structure?
• 
From inside out, the layers are:
•  Core
•  Radiation Zone
•  Convection Zone
•  Photosphere
•  Chromosphere
•  Corona
Luminosity of the Sun
•  Energy output: 3.9 x 1024 Joules/sec
•  A 100-Watt light bulb emits 100 Joules
per second
The Sun has its own energy source
–  Main difference between a star and a planet
–  Not well understood until 1940’s
•  Need to explain
lifetime & luminosity
Lifetime of the Sun
•  Need a vast, constant source of energy
•  Sun is at least 4.6 billion years old (from
fossils)
•  Most ideas could not sustain the energy
rate needed
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Nuclear Power on Earth
Nuclear Fission: breaking large nuclei
This is what happens in nuclear power plants.
Nuclear Fusion: combining small nuclei
This has only been used for weapons.
San Onofre Fission Plant, CA
Hydrogen (fusion) bomb, Pacific Ocean, 1962
Fission
Fusion
Big nucleus splits
into smaller pieces
Small nuclei stick
together to make a
bigger one
(Nuclear power
plants)
(Sun, stars)
High temperature
enables nuclear fusion
to happen in the core:
Energy from the Sun:
•  The Sun has hydrogen at its core, at ~15 million K
•  Basic Particles:
Overcome electrical
repulsion
–  Protons (in nucleus), positive charge
–  Neutrons (in nucleus), no charge
–  Electrons, negative charge
•  Particles in nucleus have
binding energy
Core is so hot, the
hydrogen is in plasma
form: the electrons
are “free”
Low Speed
Interaction
High Speed
Interaction
Nuclear Fusion
•  4 hydrogen nuclei (protons) must collide
–  Not very likely
•  Helium nucleus is built up in steps
•  This sequence of steps is called the protonproton chain
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Where Stars Get Their Energy
IN
4 protons
End product of fusion process is a helium nucleus
OUT
nucleus
2 gamma rays
2 positrons
2 neutrinos
1 He is less massive than 4 H by a factor of 0.007
(0.7%)
Where did that mass go?
4He
E =mc2
Total mass is
0.7% lower
c is the speed of light
c = 3 x 10 8 m/s
so c2 = 9 x 10 16 !!!!!
A small amount of mass can produce a lot of energy
Proton-proton chain
has 3 steps
Fusion in the Sun
•  P-P chain
•  3 steps:
2 1H -> 2H + e+ + ν
2H
+ 1H -> 3He + γ
3He
(14 billion years)
(6 seconds)
+ 3He -> 4He + 2 1H (1 million years)
Most of the stuff in the Sun has not yet undergone
fusion! (and that’s a good thing…)
How does the energy from
fusion get out of the Sun?
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