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Chapter 14:
The Sun: A
Garden-Variety
Star
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The Sun
Biggest object in the
solar system
diameter 1,392,000 km
109x earth diameter
10x Jupiter diameter
Most Massive
333,000x earth mass
1000x Jupiter mass
so heavy, everything else
orbits around it!
so heavy, it makes its own
heat and light
Temperature 15,000,000
K in its core
nuclear power!
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The Sun: Profile
Mass
Equatorial Radius
Mean Density
1.989x1030 kg
332,830 MEarth
695,000 km
10897 REarth
1.410 gm/cm3
Rotational Period
25-36* days
Escape Velocity
618.02 km/s
Luminosity
3.827 x 1033 ergs/s
Magnitude
-26.8 Vo
Mean Core Temperature
Mean Surface Temperature
Age
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15,000,000 K
5,700 K (about 6000 ºC)
4.5 billion Earth years
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Chemical Composition
Same elements as on Earth Elements
Different proportions
Hydrogen
Helium
Discovered by Cecilia
Oxygen
Payne-Gaposchkin
1st woman to get PhD in AST
Mostly Hydrogen
Some Helium
+ traces
mostly atoms but some
molecules (H20, CO)
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Fraction
92.1%
7.8%
0.061%
Carbon
0.030%
Nitrogen
0.0084%
Neon
0.0076%
Iron
0.0037%
Silicon
0.0031%
Magnesium
0.0024%
Sulfur
0.0015%
All others
0.0015%
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Sun's Interior
from inside out:
Core
Radiative
Zone
Convection
Zone
Photosphere
Chromospher
e
Transition
Region
Corona
I. Sun's Core
•
•
•
•
innermost 10% of sun's
mass
generates energy from
nuclear fusion – highest
temperature and density
– temperature 15
million K
– density = 160x
density of water =
20x density of iron
at this temperature sun's
core is a gas
no molten interior
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How does heat from the core reach
us?
Three ways to transfer heat:
conduction: spoon in coffee cup gets
warm
convection: currents of warm gas flow
from hot to cold
hot air rises
boiling water, spaghetti sauce
radiation: photons travel from heat source
– absorbed by cooler material
space heater
hold hands close to electric stove
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II. Radiative Zone
radiation transfers heat from interior to "cooler" outer layers
core + radiation zone = 85% of sun
temperature drops from 15 million K inside to 2 million K at
edge of zone
photons too energetic to be absorbed by atoms
it takes 170,000 years for the energy to escape the radiation
zone!
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III. Convection Zone
matter above the radiative zone is "cooler" – only 2 million K – atoms can
absorb energy and hold on to it
convection: most efficient means of energy transfer
hotter material near the top of the radiation zone (the bottom of the
convection zone) rises while the cooler material sinks – heated below like a
pot of boiling water
it takes a week for the hot material to carry its energy to the top of the
convection zone.
convection zone
cool
hot
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radiative zone
Photosphere
Deepest layer one can see from the outside
photosphere means “light sphere”
Sun's “surface” of the Sun: photons can finally escape to space
About 500 kilometers thick
Surface is not something one could land or float on
However, the gas is so dense that you can’t see through it, The gas, in
fact, emits a continuous light spectrum
Features Sun spots
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Temperature of Photosphere
Photosphere Temperature about 5840 K
Temperature of several measurement methods.
Wien’s Law
wavelength of the peak emission, lpeak, in the Sun's continuous
spectrum.
T(Kelvin) = 2.9 x 106 nm/lpeak.
Energy Output
flux = amount of energy passing through a unit area (e.g., 1 meter2)
every second.
Inverse Square Law of Light Brightness
solar flux at the Earth's distance = the Sun's surface flux x (Sun's
radius/Earth's distance)2 = 1380 Watts/meter2.
Sun's photosphere is a thermal radiator, the flux of energy at its surface
=sx(the Sun's surface temperature)4, where s is the Stefan-Boltzmann
constant.
T = [(solar flux at Earth)/s) x (Earth distance/Sun's radius)2]1/4.
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Features of Photosphere
sunspots
dark spots, 1500 K,
cooler than surroundings
glow by themselves
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granules
tops of convection cells
700 to 1000 km diameter
last 10 minutes
centers ~ 100 K hotter
than edges
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Sun Spots
Discovered by Galileo Galilei.
Sun's surface sprinkled with
small dark regions - sunspots.
Sunspots are darker because
they are cooler by 1000 to 1500 K
than the rest of the photosphere.
Spots can last a few days or as long as a few
months.
Galileo used the longer-lasting sunspots to map the
rotation patterns of the Sun.
Sunspots number varies in a cycle
with an average period of 11 years.
Cycle starts with minimum and most of
them are at around 35° from the solar equator.
At solar maximum (number peaked), about 5.5
years later, most of the sunspots are within just
5° of the solar equator.
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Sun Spots and Magnetic Field
Sun Spots are regions of strong
magnetic field.
Confirmed by observation of
Zeeman effect.
Zeeman effect: one spectral line
splits into several in the presence
of a magnetic field.
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The Sun Rotates
Galileo
discovered sunspots
sunspots moved  sun rotates
Rotation: the speed varies with
latitude
equator: once/25 days
30º N: once/26.5 days
60º N: once/30 days
Jupiter has similar behavior
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Chromosphere
Visible during solar eclipses
as a thin pink layer at the
edge of the dark Moon.
• Colorful layer – “color sphere”.
– Color due to hydrogen bright
emission line.
– Also shows yellow emission due
to Helium – discovered in 1868 –
new element previously not seen
on Earth.
• Helium found on Earth, 1895.
• Chromosphere is only 2000 to 3000
km thick.
• Temperature rises outward away from
the photosphere – from 5700 K to
10000 K
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Transition Region
Temperature in the Solar Atmosphere
1.E+07
Corona
Temperature (K)
Thin region of the
atmosphere where
temperature changes
from 10000 K to
nearly 1000000 K.
10 km thick.
1.E+06
Transition
Region
1.E+05
Chromosphere
1.E+04
Photosphere
1.E+03
0
500
1000
1500
2000
2500
3000
Height in solar atmosphere (km)
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Solar Weather
The Sun has complex and
violent weather patterns.
Chromosphere contains
jet-like spikes of gas –
called spicules.
Spicules rise vertically
through the chromosphere.
Last 10 minutes
Consist of gas jets, at 30
km/s
Rise to heights of 5000 to
20000 km.
T ~ chromosphere.
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Corona
Visible during total solar eclipse
as pearly-white corona around
the dark Moon.
Rarefied upper atmosphere of the Sun.
Very high temperature ~ 1-2 million K.
Low amount of heat because very tenuous.
Known to be very hot because it contains multiply
Total solar eclipse (1973)
ionized atoms
At very high temperatures, atoms like iron can have 9 to 13 electrons
ejected - ionized.
9-times ionized iron is only produced at temperatures of 1.3 million K
13-times ionized iron means the temperature gets up to 2.3 million K!
During strong solar activity, the temperature can reach
3.6 million K and lines from 14-times ionized calcium are
seen.
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Corona
Most of the corona is trapped
close to Sun by loops of
magnetic field lines.
In X-rays, those regions appear
bright.
Some magnetic field lines
do not loop back to the Sun
and will appear dark in X-rays.
Called ``coronal holes''.
More details visible at
short wavelengths).
Solar eclipse photographed in extreme ultraviolet
taken by the SOHO spacecraft
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X-rays from the Corona
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The Corona is Hot!
emission spectra of highly
ionized gasses
Ni, Fe, Ar, Ca, ...
powered by Sun’s
magnetic field
gas heats up
current flowing through a
light bulb filament
wires in a toaster
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Prominences
Bright clouds of gas
formed above the
sunspots.
Quiet prominences
40,000 km above surface.
Last days to several weeks.
Eruptive Prominences
700 km/s
rare
Surge prominences
• Last up to a few hours
• shoot gas up to 300,000 km
• Gas speed ~ 1300 km/s
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Prominences follow
magnetic field loops
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Solar Flares
Solar flares are eruptions more powerful
than surge prominences.
Last from a few minutes to a few hours.
A lot of ionized material is ejected in a flare.
Unlike the material in prominences, the solar
flare material moves with enough energy to
escape the Sun's gravity.
When such a burst of ions reaches the Earth, it interferes with
radio communication.
Sometimes a solar flare will cause voltage pulses or surges in
power and telephone lines.
Brownouts or blackouts may result. Humans traveling outside
the protection of the Earth's magnetic field will need to have
shielding from the powerful ions in a flare.
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Solar Wind
Fast-moving (+/-) ions can escape the Sun's
gravitational attraction.
Moving outward at hundreds of kilometers/second,
(~400 km/s).
Travel to the farthest reaches of the solar system.
Called solar wind.
Solar wind particles passing close to a planet with a
magnetic field are deflected around the planet.
Some are deflected to the planet's magnetic poles.
Charged particles hit the planet's atmosphere, they make the
gas particles in the atmosphere produce emission spectra:
• aurora borealis in the north and
• aurora australis in the south
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coronal hole
magnetic lines
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14.4 Is the Sun a Variable Star?
What is more certain than that the Sun will rise
tomorrow?
We’ve already seen that sunspots follow an 11 year
cycle.
On longer time scales, the Sun undergoes changes in
overall activity.
Changes are only about 0.1%! Yet this is enough to
affect our climate.
In the mid 1600’s the Sun’s output was particularly low
 the “Little Ice Age”.
Other stars are seen to vary by 0.3%, up to 1%.
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Historical Sunspot Data
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