1
Lecture 23
The Sun
January 10c, 2014
2
Basic Information
•
•
•
•
Mass = 330,000 M
Radius = 109 R
Density=1400 kg/m3
Spins differentially
– P = 25 days at equator
– P = 36 days at poles
• Closest star to the Earth.
• Very strong magnetic
field
3
Composition of the Sun
•
•
•
•
75% Hydrogen
24% Helium
1% Trace elements
How do we know?
– Spectroscopic measurements
– Analysis of solar wind
• Other important solar data comes from:
– Solar and Heliospheric Observatory (SOHO)
– Solar Dynamics Observatory (SDO)
4
Solar Atmosphere
Photosphere
Corona
Interior
Solar Wind
Chromosphere
5
Photosphere
• Layer of gas seen from the Earth
• Temp ~5800 K
• Granules -- patches of gas experiencing
convection
Cooler gas
sinking
Warmer gas
rising
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Chromosphere
• Layer of less dense gas
– Seen only during solar eclipses
– Rising jets of gas = spicules
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Corona
• Seen only during a solar
eclipse
• Thin, hot gas (over 1
million K)
• Why so hot???
– Possibly energized by
magnetic field.
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Solar Wind
•
•
•
•
Outflow of particles from Sun into space
Mainly protons and electrons.
Sun ejects 1 million tons of mass per second
Greatest wind is emitted through coronal holes
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Sunspots
•
•
•
•
Dark, cooler (~ 4300 K) spots on surface of Sun.
Powerful magnetic field 5000× stronger than Earth’s
Lifetime of ~2 rotations (8 weeks).
More numerous in an ~11 year cycle.
Sunspot
motion
movie
Seething
granules
movie
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[descriptions of movies on previous slide]
Sunspot Motion
• The Motion of a Small Sunspot
Group
• Sunspot groups, which may last for
about two months, are produced by
cycles in the Sun's magnetic field. To
measure the Sun's rotation, observers
can track the motion of sunspot
groups, much as Galileo did four
centuries ago. (This movie shows
these motions over a period of about
two weeks.) Such studies show that
the equatorial regions of the Sun
rotate somewhat faster than the polar
regions.
Granules
• Seething Granules Around
Sunspots
• This time-lapse movie shows three
and a half hours in the life of a
sunspot (note the clock at upper
right). A sunspot consists of a dark
central umbra and a feather-like
penumbra. Around them, granules
some 1000 km across rise and fall
in the hot photosphere. Each
granule releases enough energy in
its 8-minute lifetime to supply the
United States with its energy needs
for 300 years.
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Sunspot photo
Close up
image of
sunspot and
granules.
Click here for
more info on
this image
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Solar Cycle
• Last solar maximum = 2001
• Currently moving out of solar minimum = 2007
250
Smoothed Sunspot Numbers
July 1749 - June 2009
from www.ngdc.noaa.gov
Number of Sunspots
200
150
100
50
0
1745
1770
1795
1820
1845
1870
1895
1920
1945
1970
1995
http://www.ngdc.noaa.gov/stp/SOLAR/ftpsunspotnumber.html
2020
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Formation of Sunspots
–
–
–
–
Sun rotates faster at the equator than at the poles
Magnetic field under surface becomes wound up
Magnetic field gets pushed above surface.
Magnetic field inhibits warmer gas from moving to
those areas where field pushes through surface.
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Sunspots are dark because
A. they are cool relative to the gas around them.
B. they contain 10 times more iron than the
surrounding regions.
C. nuclear reactions occur in them at a slower rate
than in the surrounding gas.
D. they are clouds in the cool corona that block our
view of the solar surface.
E. absorption lines are clustered together there.
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Sunspots are dark because
A. they are cool relative to the gas around them.
B. they contain 10 times more iron than the
surrounding regions.
C. nuclear reactions occur in them at a slower rate
than in the surrounding gas.
D. they are clouds in the cool corona that block our
view of the solar surface.
E. absorption lines are clustered together there.
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Prominences and Solar Flares
• Prominences
– Hot gas lofted upwards by
the magnetic field.
– Associated with sunspots
• Solar Flares
– Gas and particles erupt off
of the surface
– Produce sun quakes
– Associated with sunspot
groups
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Prominence -- SOHO
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Sunquake -- SOHO
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Coronal Mass Ejection
• Gigantic event sometimes set off by flares
• Can create disruption of satellites,
communications, and power grids
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Flares and Coronal Mass Ejection Video
Click here to view
QuickTime movie
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Solar Activity – 2004 Prediction
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Solar minimum
• In 2007-2009 the Sun entered a quieter solar
activity period in more than a century
• Read NASA Solar Physics article
• NASA news articles in April 2009 and
September 2009
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Solar Activity – 2010 Update
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Solar Activity – 2010 Update
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The Energy of the Sun
• Sun produces energy through the process of
nuclear fusion
• 4 hydrogen atoms are combined to form one
helium atom
He
H
H
p
p
H p
H
p
n n
p p
Energy
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• During fusion, some of the mass is
transferred into energy through
E = energy (in Joules)
E  mc
2
m = mass (in kg)
c = speed of light
= 3.0×108 m/s
• Mass and energy are two manifestations of
the same thing: mass-energy
• Fusion can only occur in the core of the Sun
– high gravitational pressure
– high temperatures
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How much total mass-energy is
contained in a 55-gram candy bar?
A.
B.
C.
D.
170 (food) calories
5.0×1015 J
9.0×1016 J
5.0×1018 J
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How much total mass-energy is
contained in a 55-gram candy bar?
A.
B.
C.
D.
170 (food) calories
2
15
5.0×10 J E  mc
2
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9.0×10 J
  0.055 kg   3.0  10 m/s 
5.0×1018 J
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E  5.0  10 J
 1.18 trillion food calories!
Enough energy to power
62,700 homes for one year!
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Interior Structure of the Sun
• Core
– Nuclear
fusion
produces
energy
• Radiative
Zone
– Energy is
transported
by photons
of light
• Convective
Zone
– Warmer
gas moves
upward,
cool gas
sinks
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Hydrostatic Equilibrium
• Balance between pressure of out-flowing
energy pushing outward and inward pull of
gravity.
• Keeps Sun from collapsing due to gravity
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The Sun produces its energy from
A.
B.
C.
D.
E.
fusion of neutrinos into helium
fusion of positrons into hydrogen
fission of helium into hydrogen
fusion of hydrogen into helium
electric currents generated in its core
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The Sun produces its energy from
A.
B.
C.
D.
E.
fusion of neutrinos into helium
fusion of positrons into hydrogen
fission of helium into hydrogen
fusion of hydrogen into helium
electric currents generated in its core