Lec 14: 12 OCT 11
Chapter 16 - The SUN
LAST TIME - Formation of the Solar System
Why is the Sun Important to Us?
TODAY - The “Quiet” Sun
•  Why is it important? What can it tell us
about other stars, planetary systems, etc.
•  Why does it “shine”?
•  What is it made of? Internal Structure.
How do we know?
In Lab This Week - The “Active” Sun Next: “Debris” in the Solar System
Read Chapter 14-9, 14-10, and 15 for next week!
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contains nearly all the mass
in the solar system;
everything in s.s. orbits Sun
provides energy (heat)
through electromagnetic
radiation ==>Source of life
produces particles and
magnetic field that interact
with planetary
magnetospheres,
atmospheres, and surfaces
Next semester: “Rosetta Stone” to understand stars
Some Facts About the Sun
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ordinary star: middle age, middle size, middle
temperature, middle brightness, etc. One of 100
billion in our galaxy!
not a binary
huge, nearly constant output of energy; we only
receive a tiny portion of it 4 x 1026 Watts 1365 W/m2 How Does The Sun Generate Energy?
rotation period: 24 days
(at equator)
How Much Energy Does the Sun Produce?
• 
Sun is in a steady-state and in balance –  not varying (much)
–  not expanding or contracting (much)
•  And it has been for a very long time
(4.6 billion years)
Energy Emitted = Energy Produced
Luminosity = 4 x 1026 Watts
Almost all of the energy used on the
Earth comes originally from the Sun
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What Keeps It From Blowing Itself Apart?
•  Remember how solar system formed. Most of material
ended up in the center. •  Gravitational collapse -> temperature increases.
•  Once begun, the fusion
reactions generated energy
which provided an outward
pressure.
•  This pressure perfectly
balances the inward force of
gravity at all levels
•  This balance is called
hydrostatic equilibrium
The Sun’s energy is produced by hydrogen fusion, a
sequence of thermonuclear reactions in which four
hydrogen nuclei combine to produce a single helium
nucleus. This is known as the proton-proton chain.
How Does it Do It?
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gravitational contraction provided initial heat
source (along with accretion energy)
could only “fuel” present output of Sun for
~100,000 years!
of all known energy sources, only FUSION can
provide this energy at this rate for > 5 billion
years
fusion (of light nuclei into heavier nuclei)
requires high temperature and high pressure to
begin and to sustain; why?
The net effect is:
4H
1He + Energy
•  The mass of the 4 Hydrogen nuclei is greater
than the mass of the 1 Helium nucleus
•  The mass difference is converted to energy.
E = mc2
600 million tons of Hydrogen to Helium per second for 10 billion years
“Observing” the Solar Interior
•  The Sun’s interior is opaque, we can not see directly into it
•  We can construct mathematical computer models of it.
•  grids of temperature, pressure, & density vs. depth
•  these values are calculated using known laws of physics
•  they are tested against the Sun’s observable quantities
•  We can indirectly measure
sound waves moving through
the interior
•  these can be used to probe
conditions in the interior of the
Sun
Helioseismology
•  Helioseismology is the
study of how the Sun’s
“surface” vibrates up and
down
•  These vibrations have
been used to infer
pressures, densities,
chemical compositions,
and rotation rates within
the Sun •  There might be another
way to see all the way into
the core … neutrinos!
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Layers of the Sun Neutrinos
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Temperature Depth
produced in fusion reactions
“little neutral” particles;
energetic (fast)
don’t interact with atoms,
molecules, nuclei, very well
therefore fly straight out of
the Sun in all directions
almost impossible to stop, but
a tiny fraction can be caught
in a detector
only about 1/3 as many as
expected!
•  Core
•  1.5 x 107 K
0.25 R
•  Radiation Zone
•  > 2 x 106 K
0.70 R
•  Convection Zone
–  < 2 x
106
K
0.85 R
•  Photosphere
•  5.8 x 103 K
400 km thick
•  Chromosphere
•  1– 5 x 104 K
2,500 km thick
•  Corona
•  2 x 106 K
600,000 km thick
•  Solar Wind
•  > 106 K
beyond the Kuiper Belt
Energy Transfer in Sun
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gamma ray photons
produced by fusion in
core; neutrinos escape
radiative diffusion
convection zone
photosphere heated from
below by convection,
conduction, radiation
light - 1 million years to
get out then 8 minutes to
Earth
heavy elements made in
core but don’t mix
Convection in the
photosphere produces
granules
Photosphere The visible “surface” of the Sun. But it’s not a solid surface,
just the deepest we can see into the solar atmosphere
Solar Magnetic Activity
•  Photosphere:
visual
sunspots
•  Chromosphere:
H-alpha & UV
plage & spicules
•  Corona:
X-ray
loops & streamers
flares
•  Solar Wind:
charged particles
coronal mass ejections
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Sunspots: relatively cool
regions in the photosphere
Sunspots can be used to
measure solar rotation
Rotates about once per
month, but when it formed
it spinned a lot faster!
“Differential Rotation”:
equator rotates faster than
poles
The Solar Cycle
Sunspots are also regions of intense magnetic field
Strong magnetic field suppresses convection
"   11 year cycle of intensity,
sunspot number, etc.
"   spots emerge at higher
latitude at beginning of
cycle; at equator near end of
cycle
1992
1996
Minimum 
Maximum 
"   22 year cycle of polarity
Number of Sunspots
1850 1860
1870
1880 1890
1900 1910
1920
1930 1940
1950 1960 1970
1980 1990 2000
The solar “dynamo” is controlled by
convection and differential rotation
(but we don’t understand it very well)
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Above photosphere, Temperature INCREASES with height!
Chromosphere: Hotter than the Photosphere!
Where does energy
come from? •  magnetic field?
•  sound waves?
How does it get
deposited into the thin
plasma in the outer
atmosphere?
We know magnetic field
is related to rotation and
convection
The corona: almost as hot as the core!
Magnetic “Storms” in the Solar Corona
•  A solar flare is a brief
eruption of hot,
ionized gases from a
sunspot group
•  A coronal mass
ejection is a much
larger eruption that
involves immense
amounts of gas from
the corona
Coronal Mass Ejections
Earth Is Shown For
Size Comparison
Magnetic Cloud
Earth
SUN
Coronal
Mass
Ejection
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Coronal Holes
Effects of Solar Activity on Earth’s Environment
power grids
pipelines
radio communications
spacecraft health
astronaut health
airline passenger health
aurorae
evolution of life
climate???
Correlations between activity and climate are
highly suggestive, but what is the mechanism?
Some correlations could be spurious:
Number of Sunspots vs. # of Republicans in US Senate
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