1. Proton-proton cycle 3 steps
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2. 4 Layers of the Sun CORE : center, where fusion occurs
RADIATION: energy transfer by radiation
CONVECTION: energy transfer by convection
PHOTOSPHERE: what we see
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3. Layers of the Sun
Mostly Hydrogen with about 25% Helium. Small amounts of heavier elements
Gas described by Temperature, Pressure, and Density with P= kDT (mostly)
Larger temperature near Radius = 0
Inner radius is a PLASMA - gas where all atoms are ionized. T >100,000 degrees K – and so “free” electrons
H (48)
He (4)
electron
(56)
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4. PHYS 162

5. Equilibrium
Temperature of the Sun is constant for any given radius. It doesn’t change as heat flows out
Gravitational Force pulling in BALANCES the gas pressure (Electric force) pushing out
At center : highest gravitational pressure gives the highest temperature
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6. Temp is highest in the core  where nuclear fusion occurs
heat flows outward to surface, then radiated as light to (say) Earth
Convection Zone T = 6,000 – 100,000 K
Radiation Zone T = 100,000 – 5,000,000 K
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7. Core - Center of Sun High temperature ~15,000,000 degrees K
high density ~ 100 g/cm3
where fusion occurs
H  He
and heat flows out
source of neutrinos
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8. Core - changes with time
As heavier, Helium produced by fusion reaction tends to “float” to the center.
For now, He isn’t burning and there is a mini-core of (mostly) He with reduced fusion being built up
Red=H
green=He
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9. Radiation Layer temperature 100,000 to 5,000,000 degrees (plasma)
no fusion
electrons are not in atoms very, very opaque
Energy transferred by absorption and reradiation of light
photon
photon
photon
electron
electron
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10. Convection Layer temperature 6,000 to 100,000 degrees no fusion
electrons in atoms  less opaque
Energy transferred through convection. Movement of gas to/from surface (“hot” air rises)
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11. Convection and Radiation layers differ on how heat is transferred
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12. Photosphere photosphere Convection region
Sun  gas cloud  no true surface Light we see comes from a 200 km fairly transparent region  photosphere and top of convection region
temperature 4,500-6,000
photosphere cooler than convection region  dark line absorption spectrum
photosphere
Convection region
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13. Outer Atmosphere
Surface of the Sun  hot, turbulent with electric/magnetic storms which throw out energetic particles
CHROMOSPHERE
low density, high T
glows red (H atom)  seen in eclipse
CORONA
even lower density and higher T (over 1,000,000 degrees)
SOLAR WIND
protons escaping Sun’s gravity so large velocity. Can interact in Earth’s atmosphere
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14. Sunspots
Intense magnetic fields which inhibited convection currents to the surface  appear darker as at lower temperature
Solar storms/flares often associated with sunspots
Had been observed prior to Galileo’s time (and without telescopes) – Galileo gets credit as he had best explanation
Sunspot activity varies with time. 11 year cycle plus variation over hundreds (thousands) of years – change in Solar energy output
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15. Outer Atmosphere
Can see during eclipses. Interactions of solar wind with Earth’s magnetic field and atmosphere causes Aurora Borealis
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16. Aurora Borealis – Northern Lights
seen at high latitudes as magnetic fields are lower in the atmosphere. rarely seen in DeKalb. Photos are from Alaska and Maine
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17. Solar Storms
Large eruptions from Sun’s surface are called “flares” or “storms”
Will increase flow of charged particles to Earth, increase Northern Lights, and have (some) radiation impact (plane flights, on space station, radio signals)
Large one in January 2012
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18. Test 1 Guide for short answer questions
Motion of Sun, stars, planets through sky vs seasons
Galileo’s astronomical observations
Kepler’s Laws of planetary motion
Newton’s Laws of motion apply to Kepler’s (mostly F=ma)
how light is produced (accelerated charge) plus discrete vs. continuous
nuclear reactions in the Sun : p-p cycle
Layers in the Sun
4 forces with examples
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19. The Nature of Stars Measure properties of Stars Distance Mass
Absolute Brightness
Surface Temperature
Radius
Find that some are related
Large Mass  Large Brightness
Determine model of stellar formation and life cycle
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20. Distances to Stars
Important as determines actual brightness but hard to measure as stars are so far away
Closest Alpha Centauri
4.3 light years = 4 x km
(1 AU = distance Earth to Sun = 8 light minutes)
Close stars use stellar parallax (heliocentric parallax or triangulation  same meaning)
Can “easily” measure distance using parallax to a few 100 LY. Need telescope: first observed in Study close stars in detail. Other techniques for distant stars
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21. Distances to Stars - Parallax
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22. Shifting Star Positions
The orbit of the earth is used as the base.
Near stars appear to move more than far stars
distance = (base length)/angle
define: 1 parsec = 1/(angle of 1 second of arc) = 3.3 LY
site A
December angle
Sun
site B
June
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23. Stellar Parallax A photo of the stars will show the shift. July
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24. Nearest Stars
61 Cygni first parallax in 1838
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25. Nearest Stars
The larger the angle (T.Par. = trigonometric parallax) the closer the star
many stars come in groups like the 2 stars in the Sirius “binary cluster”  close together, within same “solar system”
Alpha Centauri and Procyon are close binary systems
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26. Parallax Data
200 BC, Hipparcos 850 stars, 1 degree; 1627, Brahe, 1000 stars, 1 arc-minute; 1725 (telescope) 3000 stars, 10 arc-seconds In 1900 only 60 stars had parallax measurements. photographic plates  0.01 arc-seconds
European satellite Hipparcos parallax measurements > 2,300,000 stars up to 500 LY distance  118,000 stars .001 arc-second resolution
OLD(1990): 100 stars with distance known to 5%. “NEW” (2005): 7000 such stars
ESA Gaia satellite: arc-second. Goal: measure 1 billion stars (2016 still analyzing)
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