The Sun

1
The Sun Our Star
2
General Properties

• Average star

• Spectral type G2

• Only appears so bright because it is so close.

• Absolute visual magnitude 4.83 (magnitude if
  it were at a distance of 32.6 light years)

• 109 times Earths diameter

• 333,000 times Earths mass

• Consists entirely of gas (av. density 1.4
  g/cm3)

• Central temperature 15 million K

• Surface temperature 5800 K

3
Physical Properties of the Sun
Interior structure of the Sun Outer layers are
not to scale. The core is where nuclear fusion
takes place.
4
Very Important Warning
Never look directly at the sun through a
telescope or binoculars!!!
This can cause permanent eye damage even
blindness.
Use a projection technique or a special sun
viewing filter.
5
The Solar Atmosphere
Heat Flow
Temp. incr. inward
Solar interior
6
The Photosphere

• Apparent surface layer of the sun

• Depth 500 km

• Temperature 5800 K

• Highly opaque (H- ions)

• Absorbs and re-emits radiation produced in the
  solar interior

The solar corona
7
Photosphere
8
WHY DOES OUR SUN APPEAR TO HAVE A WELL-DEFINED
SURFACE?

• The light we see from a star is radiated from a
  thin outer layer of gas called the photosphere.
• The gas inside the photosphere is opaque, that
  is, it is a plasma in which matter and radiation
  are strongly coupled.
• Gas outside the photosphere is transparent, that
  is, matter is neutral, hence uncoupled from
  radiation.
• The sharpness of this transition is why a star
  appears to have a surface, which is defined as
  the boundary between any two phases of matter.

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Granulation
is the visible consequence of convection
10
Granulation
The visible top layer of the convection zone is
granulated, with areas of upwelling material
surrounded by areas of sinking material
11
Energy Transport in the Photosphere
Energy generated in the suns center must be
transported outward.
In the photosphere, this happens through
Convection
Cool gas sinking down
Bubbles of hot gas rising up
1000 km
Bubbles last for 10 20 min.
12
Limb Darkening
The edges of the sun appear darker and slightly
redder.
When we look at the limbs, we see light rays
which must skim through the photosphere at a
shallow angle to reach the Earth. They originate
in the upper reaches of the photosphere, where
the temperature is somewhat lower.
13
The Chromosphere

• Region of suns atmosphere just above the
  photosphere.

• Temperature increases gradually from 4500 K to
  10,000 K, then jumps to 1 million K

Transition region
14
The Chromosphere (2)
Spicules Filaments of cooler gas from the
photosphere, rising up into the chromosphere.
Visible in Ha emission.
Each one lasting about 5 15 min.
15
Spicules on the Solar Limb
16
The Corona
17
The Magnetic Carpet of the Corona

• Corona contains very low-density, very hot (1
  million oK) gas

• Coronal gas is heated through motions of
  magnetic fields anchored in the photosphere below
  (magnetic carpet)

Computer model of the magnetic carpet
18
The Solar Wind
Constant flow of particles from the sun.
Velocity 300 800 km/s

• Sun is constantly losing mass
• 107 tons/year
• ( 10-14 of its mass per year)

19
Sun Spots
Cooler regions of the photosphere (T 4240 K).
Only appear dark against the bright sun. Would
still be brighter than the full moon when placed
on the night sky!
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Sun Spots (2)
Active Regions
Visible
Ultraviolet
21
The Solar Cycle
After 11 years, North/South order of
leading/trailing sun spots is reversed
11-year cycle
Reversal of magnetic polarity
gt Total solar cycle 22 years
22
The Solar Cycle (2)
The Sun has an 11-year sunspot cycle, during
which sunspot numbers rise, fall, and then rise
again
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The Solar Cycle (3)
Maunder Butterfly Diagram
Sun spot cycle starts out with spots at higher
latitudes on the sun
Evolve to lower latitudes (towards the equator)
throughout the cycle.
24
The Suns Magnetic Dynamo
The sun rotates faster at the equator than near
the poles.
This differential rotation might be responsible
for magnetic activity of the sun.
25
The Suns Magnetic Cycle
After 11 years, the magnetic field pattern
becomes so complex that the field structure is
re-arranged.
? New magnetic field structure is similar to the
original one, but reversed!
? New 11-year cycle starts with reversed
magnetic-field orientation
26
Magnetic Loops
Magnetic field lines
27
Sun Spots (3)
Magnetic field in sun spots is about 1000 times
stronger than average.
Magnetic North Poles
Magnetic South Poles
In sun spots, magnetic field lines emerge out of
the photosphere.
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The Maunder Minimum
The sun spot number also fluctuates on much
longer time scales
Historical data indicate a very quiet phase of
the sun, 1650 1700 The Maunder Minimum
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Prominences
Areas around sunspots are active large eruptions
may occur in photosphere. Solar prominence is
large sheet of ejected gas
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Prominences
Relatively cool gas (60,000 80,000 oK)
May be seen as dark filaments against the bright
background of the photosphere
Looped Prominences gas ejected from the suns
photosphere, flowing along magnetic loops
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Prominences
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Eruptive Prominences and Flares
(Ultraviolet images)
Extreme events (solar flares) can significantly
influence Earths magnetic field structure and
cause northern lights (aurora borealis).
33
Solar Flares
A Solar flare is a large explosion on Suns
surface, emitting a similar amount of energy to a
prominence, but in seconds or minutes rather than
days or weeks
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Flares Affect Earth
35
Physical Properties of the Sun
Solar constant amount of Sun's energy reaching
Earth is 1360 W/m2.
36
The Solar Interior
Energy transport The radiation zone is
relatively transparent the cooler convection zone
is opaque
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