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Chapter 9
The Sun
Sunspots
Our Sun
• Is the nearest star
• 8 light minutes away
• Next nearest star is 4.3 light-years away
(300,000X further than sun)
Figure 9.1
The Sun
Table 9.1
Some Solar Properties
Solar Rotation
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Differential rotation
Measure by timing sunspots
Faster at equator
Slower at poles
X-ray and visible light movie of sun
Figure 9.2
Solar Structure
Solar Structure
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Core
Radiation zone
Convection zone
Photosphere (“surface” we see)
Chromosphere
Transition zone
Corona
Solar wind
Solar Luminosity
• Solar constant 1400 W/m2 above earth’s
atmosphere
• 1000 W/m2 at earth’s surface
• Solar constant and distance to sun gives
luminosity of about 4 X 1026 W
Figure 9.3
Solar Luminosity
Stellar balance
• Outward pressure of hot gas
• Inward pull of gravity
• Balanced at every point in a star
Figure 9.4
Stellar Balance
Standard solar model
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Mathematical and physical model of sun
Based on observations and physical laws
Predicts density and temperature
Helioseismology allows knowledge of
interior structure
Figure 9.5
Solar Oscillations
Figure 9.6
Solar Interior
Sun’s temperature and density
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Core density 150,000 kg/m3 (20X iron)
Core temperature 15 million K
Core is a gas (plasma)
Photosphere is 0.0001X density of earth’s
atmosphere
• Photosphere temperature 5780 K
Discovery 9.1
SOHO: Eavesdropping on the Sun
SOHO
• Solar and Heliospheric Observatory
• European Space Agency
Solar energy transport
• Near core - very hot
• Gas is completely ionized (plasma)
• No photons captured - transparent to
radiation
• Outer edge of radiation zone cool enough
for electrons to re-combine with nuclei
• Photons all absorbed
Convection Zone
• Energy transported by rising hot gases
• Cooler gas sinks
• Convection cells vary in size with depth in
convection zone
• Tens of thousands of km to a thousand km
convection cells
• Photons from photosphere escape into
space
Figure 9.7
Solar Convection
Evidence for convection
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Solar granulation of photosphere
Granules size of a large US state
Last 5 to 10 minutes
Bright regions - hot gas rising
Dark regions - cool gas sinking
500 K difference between hot and cool
Figure 9.8
Solar Granulation
Solar Granulation movie
• Near infrared
• 60 minute sequence sped up
• At http://www.bbso.njit.edu or click here
Doppler shift
• Bright granules move up at about 1 km/s
• Dark granules move down at about 1 km/s
Supergranulation
• Larger scale flow beneath solar surface
• 30,000 km across
Figure 9.9
Solar Spectrum
Composition of solar
atmosphere
• Primarily H and He
• Also O, C, N, Si, Mg, Ne, Fe, S
• Similar to Jovian planets and rest of
universe
Table 9.2
The Composition of the Sun
Figure 9.10
Solar Chromosphere
Solar chromosphere
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Above photosphere and less dense
Pinkish hue from H emission
Expelling jets of hot matter - spicules
Last minutes
100 km/s
Figure 9.11
Solar Spicules
Corona
• Corona visible during total solar eclipse
• Emission spectrum visible against
blackness of space
• Ionized atoms - high coronal temperatures
Figure 9.12
Solar Corona
Transition Zone
• Minimum temperature of 4500 K in
chromosphere
• Temperature climbs through transition
zone
• Reaches several million K in corona
Figure 9.13
Solar Atmospheric Temperature
Solar wind
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Starts 10 million km above photosphere
Hot coronal gas escapes sun’s gravity
Millions of tons of sun ejected each second
Only lost 0.1% of mass in 4.6 billion years
X-rays in corona
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Photosphere emits primarily visible light
Hotter corona emits primarily X-rays
Coronal holes - visible in X-rays
Solar wind escapes in coronal holes
Related to magnetic fields
Figure 9.14
Sunspots
Figure 9.15
Sunspots, Up
Close
Sunspots
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In photosphere
Cooler (darker) than surrounding material
Dark umbra (4500 K)
Grayish penumbra (5500 K)
Typically 10,000 km across (size of earth)
Sunspot magnetism
• Magnetic field of photosphere stronger
than earth’s
• Magnetic field in sunspots is 1000X greater
than surrounding photosphere
• Field lines perpendicular to surface
• Strong fields interfere with convective flow
• Causes sunspots to be cooler
Sunspot magnetic polarity
• Sunspots in pairs at same latitude
• Pair members have opposite polarity N&S
• Leading spot in a hemisphere always has
same polarity
• Leading spot in other hemisphere has
opposite polarity
Figure 9.16
Sunspot Magnetism
Magnetic field wrapping
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Differential rotation “wraps” magnetic field
North-south re-oriented to east-west
Convection lifts field to surface
Twisting and tangling results
Some kinks rise out of photosphere
Forms sunspot pair
Figure 9.17
Solar Rotation
Analogy 9.1
A tangled garden hose
Figure 9.18
Sunspot Cycle
Maunder minimum
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Cycle varies from 7 to 15 years
Overall activity varies
Solar inactivity from 1645-1715
Maunder minimum caused “Little Ice Age”
Figure 9.19
Maunder Minimum
Solar prominences
• Loops or sheets of gas ejected into lower
corona
• Maybe due to magnetic fields near
sunspots
• Typically 100,000 km (10X diameter of
earth)
Figure 9.20
Solar Prominences
Solar prominence movie
• Big Bear Solar Observatory
• At http://www.bbso.njit.edu or click here
Solar flares
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More violent than prominences
Sweeps across active region in minutes
Temperature of millions of K
Material blasted into space
Figure 9.21
Solar Flare
Solar flare movie
• Big Bear Solar Observatory
• Go to http://www.bbso.njit.edu or click here
Corona activity
• Coronal mass ejection
• Several times per day during sunspot
maximum
• Can cause communication and power
disruption on earth
Figure 9.22
Coronal Mass Ejection
Figure 9.23
Coronal Hole
Figure 9.24
Active Corona
SOHO Coronal mass ejection
• White ring is size of sun
• Play movie or go to
• http://sohowww.nascom.nasa.gov/data/LA
TEST/current_c2.mpg
Sun’s energy source
• Nuclear fusion reactions in core
• Two nuclei combine forming 3rd nucleus
plus energy
• 3rd nucleus has less mass than sum of two
nuclei
• Mass converted to energy E = mc2
• Need high temperature to overcome
charge repulsion
Figure 9.25
Proton Interactions
Proton-Proton chain
• 4 protons  helium-4 + 2 neutrinos +
energy
• Neutrino is chargeless and virtually
massless particle
• Neutrinos easily pass through sun
Figure 9.26
Solar Fusion
Energy generated
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600 million tons of H fused into He every s
Sun can sustain this another 5 billion years
Energy produced in core as gamma rays
Neutrinos also carry off energy
Figure 9.27a
Neutrino Telescope in Japan
Figure 9.27b
Neutrino Telescope in Ontario, Canada
Neutrinos
• Less neutrinos detected at earth than
predicted by standard solar model
• Neutrinos oscillate into new types during
journey to earth
• Latest neutrino detectors can find all types
More Precisely 9.2
Energy Generation in the Proton–Proton Chain