THE SUN
The star we see but seldom notice
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Goals
• Summarize the overall properties of the Sun.
• What are the different parts of the Sun and how do
we know this?
• Where does the light we see come from?
• Solar activity and magnetic fields.
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The Sun, Our Star
• The Sun is an average star.
• From the Sun, we base our understanding of all
stars in the Universe.
• Like Jovian Planets it’s a giant ball of gas.
• No solid surface.
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Vital Statistics
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Radius = 100 x Earth (696,000 km)
Mass = 300,000 x Earth (1.99 x 1030 kg)
Surface temp = 5780 K
Core temp = 15,000,000 K
Luminosity = 4 x 1026 Watts
Solar “Day” =
– 24.9 Earth days (equator)
– 29.8 Earth days (poles)
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Structure
• ‘Surface’
– Photosphere
• ‘Atmosphere’
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Chromosphere
Transistion zone
Corona
Solar wind
• ‘Interior’
– Convection zone
– Radiation zone
– Core
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The Solar Interior
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How do we know what’s inside the Sun?
Observe the outside.
Theorize what happens on the inside.
Complex computer programs model the
theory.
• Model predicts what will happen on the
outside.
• Compare model prediction with observations
of the outside.
• Scientific Method!
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Helioseismology
• Continuous monitoring of
Sun.
– Ground based observatories
– One spacecraft (SOHO)
• Surface of the Sun is
‘ringing’
• Sound waves cross the the
solar interior and reflect off
of the surface
(photosphere).
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Interior Properties
• Core = 20 x density of iron
• Surface = 10,000 x less dense
than air
• Average density = Jupiter
• Core = 15,000,000 K
• Surface = 5780 K
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Do you see the light?
• Everything in the solar system reflects light.
• Everything also absorbs light and heats up
producing blackbody radiation.
• Q: Where does this light come from?
• A: The Sun.
• But where does the Sun’s light come from?
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Our Journey through the Sun
• Journey from the Sun’s core to the edge of its
‘atmosphere.’
• See where its light originates.
• See what the different regions of the Sun are like.
• See how energy in the core makes it to the light
we see on Earth.
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In The Core
• Density = 20 x
density of Iron
• Temperature =
15,000,000 K
• Hydrogen atoms
fuse together
• Create Helium
atoms.
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Nuclear Fusion
• 4H  He
• The mass of 4 H atoms:
4 x (1.674 x10-27 kg) = 6.694 x 10-27 kg
• The mass of He atom: = 6.646 x 10-27 kg
• Where does the extra 4.8 x 10-29 kg go?
• ENERGY!  E = mc2
• E = (4.8 x 10-29 kg ) x (3.0 x 108 m/s)2
• E = hc/l  l = 4.6 x 10-14 m (gamma rays)
• So: 4H  He + light!
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The Radiation Zone
• This region is transparent to light.
• Why?
– At the temperatures near the core all atoms are ionized.
– Electrons float freely from nuclei
– If light wave hits atom, no electron to absorb it.
• So: Light and atoms don’t interact.
• Energy is passed from core, through this region,
and towards surface by radiation.
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The Convection Zone
• This region is totally opaque to light.
• Why?
– Closer to surface, the temperature is cooler.
– Atoms are no longer ionized.
– Electrons around nuclei can absorb light from below.
• No light from core ever reaches the surface!
• But where does the energy in the light go?
• Energy instead makes it to the surface by
convection.
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Convection
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A pot of boiling water:
Hot material rises.
Cooler material sinks.
The energy from the pot’s hot bottom is physically
carried by the convection cells in the water to the
surface.
• Same for the Sun.
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Solar Cross-Section
• Progressively smaller
convection cells carry the
energy towards surface.
• See tops of these cells as
granules.
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The Photosphere
• This is the origin of the 5800 K blackbody radiation
we see.
• Why?
– At the photosphere, the density is so low that the gas is
again transparent to light.
– The hot convection cell tops radiate energy as a function of
their temperature (5800 K).
l = k/T = k/(5800 K)  l = 480 nm (visible light)
• This is the light we see.
• That’s why we see this as the surface.
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The Solar Atmosphere
• Above the photosphere, transparent to light.
• Unlike radiative zone, here atoms not totally
ionized.
• Therefore, there are electrons in atoms able to
absorb light.
• Absorption lines in solar spectrum are from these
layers in the atmosphere.
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Atmospheric Composition
• Probably same
as interior.
• Same as seen on
Jupiter.
• Same as the rest
of the Universe.
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The Chromosphere
• Very low density
• But also very hot
• Same as the gas tubes we saw in
class and lab.
• Energy from below excites the
atoms and produces emission
from this layer.
• Predominant element –
Hydrogen.
• Brightest hydrogen line – Ha.
• Chromosphere = color
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Spicules and
Prominences
• Emission from the
atmosphere is very faint
relative to photosphere.
• Violent storms in the
Chromosphere.
• Giant curved prominances
• Long thin spicules.
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Prominences
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Ha Sun
Photo by Robert Gendler
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Corona
• Spicules and other magnetic activity carry energy
up to the Transition Zone.
• 10,000 km above photosphere.
– Temperature climbs to 1,000,000 K
– Remember photosphere is only 5800 K
• The hot, low density, gas at this altitude emits the
radiation we see as the Corona.
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The X-Ray
Sun
• Q: At 1,000,000 K
where does a
blackbody spectrum
have its peak?
• A: X-rays
• Can monitor the Solar
Coronasphere in the Xray spectrum.
• Monitor Coronal Holes
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Solar Wind
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At and above the corona:
Gas is very hot
Very energetic
Like steam above our boiling pot of water, the gas
‘evaporates’.
• Wind passes out through Coronal Holes
• Solar Wind carries away a million tons of Sun’s
mass each second!
• Only 0.1% of total Sun’s mass in last 4.6 billion
years.
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The Aurora
• The solar wind passes
out through the Solar
System.
• Consists of electrons,
protons and other
charged particles
stripped from the Sun’s
surface.
• Interaction with
planetary magnetic
fields gives rise to the
aurora.
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The Active Sun
• Solar luminosity is nearly constant.
• Very slight fluctuations.
• 11-year cycle of activity.
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Solar Cycle
• Increase in Coronal holes
• Increase in solar wind activity
- Coronal Mass Ejections
• Increase in Auroral displays on Earth
• Increase in disruptions on and around Earth.
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11-year sunspot cycle.
Center – Umbra: 4500 K
Edge – Penumbra: 5500 K
Photosphere: 5800 K
Sunspots
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• Can see that Sun
doesn’t rotate as
a solid body?
• Equator rotates
faster.
• This differential
rotation leads to
complications in
the Solar
magnetic field.
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Magnetic fields and Sunspots
• At kinks, disruption in convection cells.
• Sunspots form.
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Magnetic fields and Sunspots
• Sunspots
come in pairs.
• Opposite
orientation in
North and
South.
• Every other
cycle the
magnetic
fields switch.
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Sunspot Numbers
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Active Regions
• Areas around sunspots give rise to the prominences
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