4 - The Sun
NEVER LOOK DIRECTLY AT THE SUN!
EYE DAMAGE CAN RESULT!
Major Regions:
Interior (“Inner-sphere”)
•Core
•Radiative Zone
•Convective Zone
Atmosphere (“Outer-sphere”)
•Photosphere
•Chromosphere
•Corona
No real “surface” - it is gaseous all the way to the center!
Basic Facts:
Mass M = 2x1030 kg = 300,000M
Radius R = 7x108 m = 7x105 km = 111R
Mean Density = Mass/Volume = M/((4/3)R3)= 1400 kg/m3 = 1.4 g/cm3
Distance = 1.5x1011 m = 1.5x108 km = 8.3 light-minutes
Luminosity L = 3.9x1026 W (Solar Constant = 1350 W/m2)
Mean Surface Temperature = Teffective = 5780 K
Composition
By Mass
By Number
H
~68%
~90%
He
~31%
~10%
~1%
~1%
“Heavies”
ATMOSPHERE (general)
Photosphere - “sphere of light” - Taverage = Teff ~ 5780 K
~1000 km
across
Few
X
100
Km
thick
Granulation - light & dark regions in photosphere
•upwelling is hotter & brighter
•downwelling is cooler & darker
•Flowing motion “convection”
Also see “supergranulation”
Surface pulsations - 5 min - 1 hr (measured via Doppler Effect)
Limb Darkening
The photosphere is hotter as you go deeper
We see deeper when looking at the center of the “disk”
Brightness increases as T4 (Stefan-Boltzmann Law!)
Chromosphere - “sphere of color” - T drops to ~ 4300 K
When seen edge-on we see the
656.3 nm Balmer line H
This is where Helium was discovered
(Helios - Greek god of the Sun)
Special
H
filter
used to
image
whole
Sun
Solar Spectrum
Kirchhoff’s
Laws…
Corona - T rises to ~ 500,000 - 1,000,000 K !!
Highly ionized atoms, such as Fe+13 (also called Fe XIV)*
Light scattered by ions & electrons (inner) & dust grains (outer)
Heated by complex magnetic/particle activity
C0  C I
C+1  C II
*NOTE: spectroscopists use a special notation: C+2  C III
C+3 is C IV
Solar Wind
Outer atmosphere is flowing away from the Sun - the
solar wind
Speeds up to few million km/hr
Earth is inside the solar wind!
At Earth:
V = 400 - 700 km/s
T = 50,000-500,000 K*
density = 103 - 104/m3 (Earth @ sea level - ~2.5x1025/m3)
Solar wind expands outward,
creating the heliosphere
* Why don’t we cook?
Listen to Voyager 1 crossing
“dense” regions here
The Active Sun
Sunspots:
•Appear in groups
•T~4000-4500 K
•Darker than surrounding photosphere (WHY?)
•Large magnetic fields inhibit convection
umbra
penumbra
note polarity reversed in N and S
magnetic field

Solar Cycle:
Number of spots = ~ 11 yrs
Latitude of spots = ~ 11 years
Magnetic polarity of spots = 22 years
(to return to starting orientation)
Horace Babcock Magnetic Dynamo via
differential rotation
But the cycle
seems to be
getting
shorter!
Effects on Earth’s Climate
What fraction of
global warming is
due to the Sun?
cosmic rays can induce cloud formation & cover
cloud chambers
Maunder
Minimum
(1645-1715)
&
“Little Ice Age”
Can lead to errors in radio-carbon dating
See Wikipedia for more
on the "Little Ice Age"
cycle seen in sedimentary rock samples
650 million yrs old - due to variations in
deposition of glacial meltwater
Plages - heated by
rising magnetic field
- compresses gas
Filaments - cool
streamers of gas
above photosphere
Prominences filaments seen from
the side
Flares - sudden release of energy
equal to 100s of millions of megatons radio through gamma rays!
Temperatures exceed 10 million K
A couple (here & here) of nice
movies from the Big Bear Solar
Observatory
Magnetic field loops
trap hot gas, area
bright in X-rays
Open fields - gas flows
& area is dark in X-rays
- coronal holes
Look at this with 3D glasses (redgreen) - from Yohkoh spacecraft
Coronal Mass Ejections
Expel trillions of tons at hundreds of km/s!
Potential damage to:
•satellites & astronauts
•power grids
•radio communication
etc.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
INTERIOR
Sun produces 4x1026 W for 4x109 yrs. How?
•Chemical - would last only 104 yrs
•Gravitational - would last 25-50x106 yrs
•Thermonuclear - can last 1010 yrs!
1905 - Einstein finds that matter & energy are different
aspects of the same thing - E=mc2
1920’s - Eddingtom calculates that the core of the Sun
must have T ~ millions K, and proposes some type of
atomic/nuclear process fuels the Sun’s energy
Atkinson suggests it is fusion of H into He
1 kg H  0.993 kg He + ENERGY equivalent to the
missing 0.007 kg (i.e. 0.7% of the original amount) - this is
equivalent to burning 6000 tons of chemical fuel!
L = 3.9x1026 W  4.3x109 kg/s lost  6x1011 kg/s H He
IF THE SUN COULD USE ALL OF ITS H FUEL:

fuel supply
2x10 30 kg 
1yr
9
Lifetime 


100x10
yrs
11
1 
7
1 
rate of fuel use 6x10 kg s  3.6x10 s yr 
Later, we will see that the Sun will only be able to tap
~10% of its mass, but that’s still 10 billion years!
proton-proton chain
CNO cycle
Proton-proton chain in detail……
PPI
(91% of time)
PPII
(8% of time)
A
B
C 
3
4
7
D
He

He

Be  
 
E  7 Be  e  7 Li   e
F  7 Li  1H  8 Be*  
G 
PPIII
(1% of time)
H  1 H  2 H  e   e
e  e  
2
H  1H  3 He  
3
He  3 He  4 He  2 1H
1
H 
I 
J 
8
Be*  2 4 He
  excited state
*
Be  1H  8 B  
8
B  8 Be*  e   e
8
Be*  2 4 He
7
CNO Cycle
A
B
C  1H  13 N *  
13 *
N  13C  e   e
13
C  1H  14 N  
14
N  1H  15O  
15
O  15 N  e   e
15
N  1H  12C  4 He
12
SOLAR INTERIOR MODELS
Mathematical descriptions of what the Sun should be like inside
Hydrostatic Equilibrium - balance of gravity & pressure
Thermal Equilibrium - balance of heat energy everywhere
Energy Transport (how does it get from the core to the surface?
•conduction - direct transfer through physical contact
•convection - transfer of energy through fluid motion
•radiation - transfer via photons
Equation of State - relates pressure, density, temperature
Opacity - how photon transfer are affected by the gas
Energy Generation - energy production vs. temperature, density, etc.
“Boundary Conditions” - L, M, R, etc. to look like a real star!
Results: Tc~15x106K, Pc~3x1011 Atm, density c~160 g/cm3 !!
The Standard Solar
Model (SSM)
Solar Neutrino Problem
pp chain produces neutrinos
The measured rate of neutrinos is 1/3-1/2 that predicted
by SSM
Something is wrong………..
•Experiments? - but many give similar “wrong” results
•Solar Models? - helioseismology says “probably not”
•New physics? - who asked for that?
Helioseismology to the Rescue!
Measured Oscillation
Modes - Global
Oscillation Network
Group (GONG)
Convection deeper than
once thought
Differential rotation goes
deep!
But SSM not endangered
The Experiments
Ray Davis Homestake Mine - looks for  from pp III
37
Cl   e  37 Ar  e
- gets 1/3 predicted value
pp III very sensitive to T
Experiment only measures 1
“flavor” of  - the “electron ”
What if ’s change in flight????
Super-K - gets about 1/2 the predicted amount
Sudbury Neutrino Observatory (SNO) - sensitive
to all “flavors” - agrees with SSM!!
Sun only makes electron ’s
SNO Results show:
We detect ~1/3 of electron ’s predicted by SSM
We detect TOTAL number of  predicted by SSM
2/3 of the electron ’s were converted into other
flavors before they reached Earth (so the total number
is conserved)
Requires revising “standard physics” of subatomic
particles - New Physics!