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Lecture 13
How Stars
Work
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The Sun’s Magnetic Field
• Magnetic fields apply a force on
charged particles that pushes
them along the direction of the
field lines.
• The charged particles in the
ionized gas also push back on the
magnetic field, altering its shape.
• Magnetic fields appear to play an
important role in the “storms” we
see on the Sun’s surface.
Chapter 15 (Sun)
Chapter 16
OWL #3 is open
Due 4/3/08
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Twisting Magnetic Fields
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PRS: Sunspots
Sunspots are dark because
1. They are cool relative to the gas around them.
2. They contain 10 times as much iron as
surrounding regions.
3. Nuclear reactions occur in them more slowly
than in the surrounding gas.
4. Clouds in the cool corona block our view of the
hot photosphere.
5. The gas within them is too hot to emit any light.
The Sun’s differential rotation “winds up” the
magnetic field, like twisting a rubber band
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Sunspots
• Cooler regions in Sun’s photosphere—still hot!
• Magnetic fields alter atomic energy levels slightly
• Magnetic fields in sunspots can be measured
spectroscopically.
Zeeman
splitting
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Sunspot Structure
• The strong magnetic field
should push the sunspot
apart.
• Like a storm on Earth, the
cool region draws in gas,
which drags along
magnetic field.
• The strong, tangled
magnetic field blocks gas
from rising from below
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The Solar Cycle
Magnetic Waves near Sunspot
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• The number of sunspots goes up and down
about once every 11 years on average.
• After each sunspot maximum, the Sun’s
magnetic field reverses.
• Geologists find that the Earth’s magnetic field
reverses once every ~250,000 years
In this filtered movie from the Big Bear Solar Observatory, you
can see magnetohydrodynamic waves emanating from the center
of the sunspot.
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Solar Flares
In regions of strong
magnetic fields,
sudden bursts of gas
may occur, and
intense X-rays and
visible light can be
generated by
electrons and
charged particles
impacting the
surface.
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More
Solar Flares
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After and
during the
flare, material
moves in the
confined
magnetic
tubes above
the surface.
Prominence
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Solar
Prominences
In X-Rays we
can observe hot
gas directly
from telescopes
in space.
The plasma moves along the magnetic field lines.
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Magnetic Loops
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Coronal Mass Ejections
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Occasionally, a large
blob of ionized gas is
ejected from the
Sun’s corona, leading
some weeks later to
auroras and
interference with
satellites here on
Earth.
Ionized plasma is
confined by magnetic
field lines looping out of
the solar surface.
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Long Term Changes in the Sun
• When Sun is more
active it generates
more power overall.
• This may affect
Earth’s climate.
Solar Wind
Aurora on Earth:
Excited O, N, H atoms
(4+ days after CME)
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PRS: Sunspots
How do strong magnetic fields make
sunspots dark?
1) They repel hydrogen atoms so the nuclear
fusion is less intense.
2) They stop convection in the plasma so heat
can’t rise through them.
3) They attract iron particles that block the light
coming from below.
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Calculating the Sun’s Luminosity
Imagine completely surrounding the Sun with a
sphere of radius 1 AU (1.5 x 1011 m), to capture all
of its power output. Each square meter would
receive 1300 Watts because the Sun’s radiation is
the same in all directions.
The Sun’s luminosity is therefore:
L = B × 4 π d2
= (1300 watts/m2 ) × 4 π (1.5 x 1011 m)2
~ 1.3 x 103 W/m2 × 2.6 × 1023 m2
~ 4 x 1026 Watts
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Human Energy Consumption
The Sun from the Inside Out
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Core
Humans worldwide use ~4 x 1020 Joules of energy per
year. (U.S. 25% of this.)
How many years worth of energy does the Sun generate
in one second?
• This is where the Sun’s
energy is generated.
1) 10-26
2) 10-20
3) 10-6
• Energy is transported by
photons
Radiation Zone
4) 106
5) 1014
6) 1026
Convection Zone
time = 4x1026 Joules / 4x1020 Joules/year = 106 years
The amount of sunlight that strikes the Earth is ~ 5 x
1024 Joules per year.
• Energy is transported by
upwelling gas (we see the
top as granulation)
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E=m
PRS: Size of Sun’s Core
If the Sun’s core has ¼ the Sun’s radius, how big is
its volume compared to the volume of the whole
Sun?
1)
2)
3)
4)
5)
6)
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1/4
1/12
1/16
1/32
1/64
1/256
c2
• Einstein’s relationship between mass and energy
implies that the Sun must be turning
m = E / c2
= 4 x 1026 Joules / (3 x 108 m/sec) 2
~ 4 x 109 kg = 4 million tons
of matter into energy every second!
• The Sun’s mass is so large that this is negligible.
• What’s providing all of this power?
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Nuclear Fusion
• One of the only processes which is known
to convert a significant fraction of mass into
energy is nuclear fusion.
• In the combining four hydrogen nuclei into
a helium nucleus, almost 1% of the mass is
released as energy.
• If the Sun fuses all of the hydrogen in its
core, it could burn for ~10 billion years at
its current luminosity.
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CPS: H, He, C burning
Fusing atomic nuclei are stripped of all of
their electrons in a plasma in the stellar
core. Therefore, the order of easiest to
hardest to fuse would be
(1) H, C, He
(3) He, C, H
(2) H, He, C
(4) C, He, H
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Fission vs. Fusion
Hydrogen Fusion
The P-P chain is made possible by the extremely high temperature
and density pressure at the Sun’s core.
Net reaction: P+P+P+P = He + neutrinos + E
Nuclei lower in mass than iron can combine to
release energy. Higher mass nuclei can split to
release energy. If the mass of the starting atoms is
greater than that of the products, energy is released.
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