The Sun Today Visible lightHydrogen Alpha light The Sun nearing
the peak of its 11-year cycle. This cycle seems to be a low
activity one.
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The Sun in other wavelengths X-RayExtreme UVHydrogen Alpha
These show what the Sun can look like when it is active
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Basic Solar Facts Diameter: 1.4 million kmAge: 4.5 billion
years Mass: 330,000 x EarthAverage Density: 1.41 gm / cm 3 Distance
from Earth: 149.6 million km Average Solar Wind Speed: 3 million
km/hr Luminosity: 3.9x10 26 watts Temperature at surface: 5,770 K
Temperature at Core: 15,000,000 K Rotation Period at Equator: 25
Earth days Rotation Period at Poles: 35 Earth days
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What is the interior of the Sun like? How do we know?
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We make models based on hydrostatic equilibrium
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The models tell us the conditions inside the Sun
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We compare what the models predict to what we can see We
observe the Sun in as many different wavelengths to learn as much
as possible about how it works
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Where does the Sun get its energy? To the ancients who believed
the Earth was the center of the universe, the Sun was made of
quintessence, an element whose property was to glow. The concept of
energy wasnt even invented until the late 1600s.
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By the 1700s the best ideal for the source of the Suns energy
was chemistry If Sun was highest quality chemical fuel (i.e. pure
carbon coal) it would exhaust its fuel in less than 10,000
years!
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Later ideas for source of Suns energy: Gravitational Collapse
Kelvin-Helmholtz Contraction Whenever anything shrinks it heats up.
This could produce the observed solar output for about 25 million
years. This was the original source of energy as the Sun was
forming.
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In 1905 Einstein proposed a new way to get energy: from matter
The answer came from his famous equation: E = mc 2
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By the 1920s Sir Arthur Eddington proposed the Sun produced
energy by fusion The same source of energy as the hydrogen
bomb
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The Sun converts hydrogen into helium in a multi step process
It starts with two normal hydrogen nuclei fusing to form a heavy
hydrogen nuclei: a deuterium. The reaction also produces a positron
and a neutrino. When the positron annihilates with an electron two
gamma rays are produced Hans Bethe worked out the details of
hydrogen fusion in the 1930s
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The second step uses the product of the first step plus another
hydrogen A deuterium and a hydrogen fuse to form a helium-3 plus a
gamma ray photon
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Final step of the Proton- Proton cycle Two helium-3 fuse to
form a normal helium plus two hydrogen nuclei
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Overall Proton Proton Cycle 4 1 H 4 He + 2e + + 2 + 2 Releases
4.3x10 -12 Joules per helium atom produced The Sun converts
600,000,000 tonnes of H into 596,000,000 tonnes of He every second!
The difference in mass is the energy produced according to E = mc
2. This is only a 0.67% efficient conversion! The Sun has enough
hydrogen in its core to last another 5 billion years before it runs
out Energy is only produced in the core region where the
temperature and pressure are high enough Watch ClassAction
Proton-Proton Animation in Sun and Solar Energy Module
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Fusion requires high temperatures to overcome the electric
repulsion of protons The electric force between the protons is
repulsive. The strong nuclear force between them is attractive but
it is a very short range force so they have to get very close
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The solar neutrino problem Step 1 in the Proton-Proton cycle
Early measurements only detected 1/3 as many as predicted by solar
models. Either the models were wrong or we didnt really understand
the neutrino
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Detecting neutrinos is not easy Neutrinos come in three flavors
and early experiments could only detect one flavor
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We now know the neutrino changes flavor Current experiments can
detect all three flavors and we are now finding just as many as our
solar theories predict
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How does the energy get from the core to the surface? Most of
the energy released in the core is in the form of gamma ray photons
Most of the energy released at the surface is in the form of
visible photons Something must happen between the core and the
surface!
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Heat Transfer: Energy can move by one of three methods
Conduction: atomic & molecular vibrations in solids.
Examplecast iron skillet Convection: large scale motions in liquids
and gasses Exampleboiling water Radiation: electromagnetic
radiation Exampleheat lamp
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Which method works best is determined by pressure, density and
temperature
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Once again, we use hydrostatic equilibrium models to determine
which method works best at each layer of the Sun
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The Solar Interior The models tell us that radiation is the
means of heat transport for the first 70% of the way then
convection takes over near the surface before going back to
radiation at the surface.
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How can we verify our models for the interior of the Sun? GONG
stands for Global Oscillating Network Group
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Solar Surface Oscillations The surface of the Sun oscillates in
many different ways. This shows one of the ways it oscillates
(extremely exaggerated)
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Different ways sound bounces around inside Sun How different
waves travel depends on the density and temperature of the gas it
is travelling through
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Internal Structure from Surface Waves
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Actual Image of Oscillations
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Internal Differential Rotation
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The Photosphere The Sun is a ball of gas so there is no
surface. The surface is a layer of gas that is only about four
hundred kilometers thick. The density of the gas has to be just
right to emit a good blackbody spectrum. Too dense and the light
cant get out. Not dense enough and not enough light is
produced
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Solar Granulation Each granule is ~1000 km across and lasts a
minute or two
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Granulation Cells are Convection Cells The photosphere is the
top of the convection layer of the Sun where convection changes
back to radiation
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Note the decrease in temperature in the photosphere
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The Chromosphere
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Spicules
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The temperature in the chromosphere climbs slowly but then
jumps up in the corona
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The Corona The corona eventually fades into the solar wind.
There are other components to the solar wind as well but the corona
contributes the main wind.
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The corona is heated by coronal loops Note the size of Earth
for scale
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The solar wind moves outwards at 1 to 3 million kilometers per
hour At those speeds it can take two to six days to reach
Earth
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Eventually the Suns magnetic field plows into the galactic
field
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Sunspots
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A Sunspot Close-up Penumbra Umbra The umbra can be up to two
thousand degrees cooler than the surrounding photosphere
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Under a sunspot Heat flow is stopped by the sunspot like a cork
in a bottle. The heat has to go somewhere, though, so it squirts
out around the edges
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The Solar Cycle The number of sunspots increases and decreases
with about an eleven year cycle
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The location of sunspots changes during the solar cycle
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The cause of sunspots lies in differential rotation Solar
Rotation Period versus latitude and depth
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Sunspots are created by kinking in the Suns magnetic field It
starts with a global magnetic field but the differential rotation
causes it to wrap around and get kinked up after a few
rotations
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Activity Associated with Sunspots
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Filaments & Plages
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Solar Prominence Watch YouTube Magnificent Eruptive Solar
Prominence at
http://www.youtube.com/watch?v=rQ2Ad2nK_VMhttp://www.youtube.com/watch?v=rQ2Ad2nK_VM
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Solar Flare
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Coronal Mass Ejection (CME) CMEs can also occur when there are
no sunspots
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Coronal Holes
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Surprisingly, the Sun is brighter during periods of maximum
sunspots The difference is about 0.3%
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If the sunspots go away, Earth gets cold During the Maunder
Minimum Europe suffered through a Little Ice Age
