lecture 19: the sun our star some facts about the sun distance from earth: 1.5 x 10 8 km ...
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Lecture 19: The Sun
Our Star
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Some Facts about the Sun
distance from Earth: 1.5 x 108 kmluminosity: 3.86 x 1026 Wmass: 1.98 x 1030 kg (3.33 x 105 Earth
masses)radius: 696,000 km (109 times the radius of
the Earth)surface temperature: 5,800 K
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What makes the Sun Shine?
chemical burning?– not enough atoms to sustain for lifetime of
Earth
gravitational contraction?– Sun would have been much larger in recent
past; could not shine for long enough
nuclear fusion?– should allow Sun to shine for 10 billion years
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Nuclear Fusion and Fission
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Nuclear Fusion in the Sun: the proton-proton chain
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Net effect of the p-p chain:in:
– 6 protons (1H nuclei)
out: – one 4He nucleus– two protons– two neutrinos– two positrons photons– two photons– kinetic energy
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How much energy is released?
mass of proton: 1.6726 x 10-27 kgmass of four protons: 6.690 x 10-27 kgmass of Helium-4 nucleus: 6.643 x 10-27 kg
mass difference: 4.7 x 10-29 kg(about 0.7 percent of original mass)
E = m c2 = (4.7 x 10-29 kg)(3 x 108 m/s)2 = 4.3 x 10-12 J
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How much mass is converted to energy each second in the Sun?
Luminosity of the Sun: 3.8 x 1026 J/s
E = m c2 m = E/c2
3.8 x 1026 J/s x 1 s
(3.0 x 108 m/s)2m =
= 4.2 x 109 kg
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Structure of the Sun
core (15 million K)radiation zone (10 million K)convection zone (2 million K)photosphere (6000 K)chromosphere (10,000 K)corona (1 million K)solar wind
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Radiative Diffusion
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Granulation of the Sun’s surface
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“Observing” the Solar Interior
helioseismology
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Gravitational Equilibrium
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The Sun’s Built-in Thermostat
the rate of nuclear fusion is very sensitive to temperature
core temperature increase increase in fusion rate
increase in pressure core expands and cools down
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The Sun is getting hotter…
4 H particles 1 He particle– causes solar core to shrink in size
fusion rate must increase to maintain gravitational equilibrium
solar core gets gradually hotterthe Sun is about 30 percent hotter now
than it was 4.6 billion years agotemperature on Earth has remained
constant – Earth has its own thermostat?
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Solar Neutrinos
Homestake mine,South Dakota
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Super-Kamiokande experiment
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The Solar Neutrino Problem
models of the Sun predict the number of neutrinos that we should see with these detectors – but actual number seen is only about half as large as the prediction.
either the models are wrong or something funny is going on…
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Neutrino Oscillations?
remember there are three different kinds of neutrinos, which go with the three kinds of leptons (electron, muon, and tauon)
Fusion produces only electron neutrinos, and detectors are only sensitive to electron neutrinos
could some of the electron neutrinos change into muon or tauon neutrinos during their journey out of the Solar core?
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Sunspots
Sunspots are regions where the surface of the Sun is much cooler than the rest (4000 K instead of 5800 K)
what keeps these spots cool?
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Magnetic Fields
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The presence of a Magnetic field changes the spectral lines.Some lines split into two or more lines.
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Magnetic fields trap the hot gas, and suppress convection.
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Solar Prominence Solar Flare
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Solar Activity and the Earth
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Aurora Borealis/Australis