1. Visible Image of the Sun
The Sun
•Our sole source
of light and heat in
the solar system
•A very common
star: a glowing ball of
gas held together by its
own gravity and powered
by nuclear fusion at its
center.

2. Pressure (from heat
caused by nuclear
reactions) balances the
gravitational pull
toward the Sun’s center.
Called “Hydrostatic
Equilibrium.
This balance leads to a
spherical ball of gas,
called the Sun.
What would happen if
the nuclear reactions
(“burning”) stopped?

3. Radius = 696,000 km
(100 times Earth)
Mass = 2 x 1030 kg
(300,000 times Earth)
Av. Density = 1410 kg/m3
Rotation Period =
Solar Properties
24.9 days (equator)
29.8 days (poles)
Surface temp = 5780 K
Element
Hydrogen 70.9%
Helium 27.4%
etc

4. Luminosity of the Sun
= LSUN
(Total light energy
emitted per second)
~ 4 x 1026W
100 billion one-megaton
nuclear bombs
every second!
Solar constant:
LSUN
/ 4pR2
(energy/second/area
at the radius of
Earth’s orbit)

5. The Solar Interior
“Helioseismology”
•In the 1960s, it was
discovered that the
surface of the Sun
vibrates like a bell
•Internal pressure
How do we kknnooww tthhee iinntteerriioorr
ssttrruuccttuurree ooff tthhee SSuunn??
waves reflect off the
photosphere
•Analysis of the
surface patterns of
these waves tell us
about the inside of the
Sun

6. The Standard Solar Model

7. Energy Transport within the Sun
• Extremely hot core - ionized gas
• Temperature falls further from core - more and more non-ionized atoms
capture the photons - gas becomes opaque to light in the convection zone
• The low density in the photosphere makes it transparent to light - radiation
takes over again

8. Convection
Convection takes over when
the gas is too opaque for
radiative energy transport.
Hot gas is less dense and
rises (or “floats,” like a hot air
balloon or a beach ball in a
pool).
Cool gas is more dense and
sinks

9. Solar Granulation
Evidence for Convection
Solar Granules are the tops of convection cells.
Bright regions are where hot material is upwelling
(1000 km across).
Dark regions are where cooler material is sinking.
Material rises/sinks @ ~1 km/sec (2200 mph; Doppler).

10. The Solar Atmosphere
The solar spectrum has
thousands of absorption
lines
More than 67 different
elements are present!
Hydrogen is the most
abundant element followed
by Helium (1st discovered
in the Sun!)
Spectral lines only tell us about the part of the Sun that forms
them (photosphere and chromosphere) but these elements are
also thought to be representative of the entire Sun.

11. Main Regions of the Sun
Visible
surface

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18. Granules
Convection from inside the sun causes the
photosphere to be subdivided into 1000-
2000km cells.
Energy rises to the surface as gas wells up in the cores of
the granules, and cool gas sinks around their edges.

19. Chromosphere

20. Chromosphere (seen during full Solar eclipse)
Chromosphere emits very little light because it is of low density
Reddish hue due to 3®2 (656.3 nm) line emission from Hydrogen

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23. Chromospheric Spicules:
warm jets of matter
shooting out at ~100 km/s
Spicules are thought to the
result of magnetic
disturbances
Ha light

24. Transition Zone and Corona

25. Transition Zone
Corona
Very low density,
T ~ 106 K
We see emission
lines from highly
ionized elements
(Fe+5 – Fe+13) which
indicates that the
temperature here is
very HOT
Why does the Temperature rise further from the hot light source?
® magnetic “activity” -spicules and other more energetic
phenomena (more about this later…)

26. Corona (seen during full Solar eclipse)
Hot coronal gas
escapes the Sun
® Solar wind

27. Solar Wind

28. Solar Wind
Coronal gas has enough heat (kinetic) energy to escape the
Sun’s gravity.
The Sun is evaporating via this “wind”.
Solar wind travels at ~500 km/s, reaching Earth in ~3 days
The Sun loses about 1 million tons of matter each second!
However, over the Sun’s lifetime, it has lost only ~0.1% of
its total mass.

29. Hot coronal gas (~1,000,000 K) emits mostly in X-rays.
Coronal holes
are sources of
the solar wind
(lower density
regions)
Coronal holes
are related to the
Sun’s magnetic
field

30. The Active Sun
UV light
Most of theSolar luminosity is continuous photosphere emission.
But, there is an irregular component
(contributing little to the Sun’s total luminosity).

31. Sunspots
Granulation around sunspot

32. Sunspots
• Typically about 10000 km
across
• At any time, the sun may
have hundreds or none
•• Dark color because they
are cooler than photospheric
gas (4500K in darkest parts)
• Each spot can last from a few days to a few months
• Galileo observed these spots and realized the sun is rotating
differentially (faster at the poles, slower at the equator)

33. Sunspots
Magnetic Fields
•The magnetic ffiieelldd iinn aa ssuunnssppoott
is 1000x greater than the
surrounding area
•Sunspots are almost always in
pairs at the same latitude with
each member having opposite
polarity
•All sunspots in the same
hemisphere have the same
magnetic configuration

35. The Sun’s differential rotation distorts the magnetic field lines
The twisted and tangled field lines occasionally get kinked, causing the field
strength to increase

36. Sunspot Cycle
Solar maximum is
reached every ~11 years
Solar Cycle is 22 years long – direction of magnetic field
polarity flips every 11 years (back to original orientation every 22 years)

38. Heating of the Corona
Charged particles (mostly
protons and electrons) are
accelerated along magnetic field
“lines” above sunspots.
This type of activity, not light
energy, heats the corona.

39. Charged particles follow magnetic fields between sunspots:
Solar Prominences
Sunspots are cool,
but the gas above
them is hot!

40. Earth
Solar Prominence
Cloudlike structure, loop shape
Large amount of gases
Looks like a huge flame
coming out of the sun
Typical size is 100,000 km
May persist for days or weeks

41. Very large solar prominence (1/2 million km across base,
i.e. 39 Earth diameters) taken from Skylab in UV light.

42. Solar Flares – much more violent magnetic instabilities
5 hours
Particles in the flare are so energetic, the magnetic field cannot bring them
back to the Sun – they escape Sun’s gravity

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Image at http://solarscience.msfc.nasa.gov/the_key.shtml

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147. The Sun at X-ray wavelengths

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Electromagnetic spectrum
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html
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====
NASA image at http://visibleearth.nasa.gov/view_rec.php?id=107 Credit: NASA GSFC Water and Energy Cycle
http://www.nasa.gov/centers/jpl/news/grace-20061212.html

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