1 The Sun the main show in the solar system 99.8% of the mass % of the energy 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley Homework due next time - will count best 5 of 6
2 The Sun Radius: m (109 times Earth) Mass: kg (1,000 Jupiters; 300,000 Earths) Luminosity: watts 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley That s about a billion big nuclear bombs every second
3 Why does the Sun shine?
4 Is it on FIRE?
5 Is it on FIRE? Chemical energy content Luminosity ~ 10,000 years
6 Is it on FIRE? NO! Energy Available Chemical energy content Rate of Energy Use Luminosity ~ 10,000 years
7 Is it CONTRACTING?
8 Is it CONTRACTING? Gravitational potential energy Luminosity ~ 25 million years
9 Is it CONTRACTING? NO! Gravitational potential energy Luminosity ~ 25 million years
10 It can be powered by NUCLEAR ENERGY! (E = mc 2 ) Nuclear potential energy (core) ~ 10 billion years Luminosity
11 Weight of upper layers compresses lower layers.
12 Stars are stable: pressure balances gravity. Hydrostatic equilibrium: Energy released by nuclear fusion in the core of the sun heats the surrounding gas. The resultant pressure balances the relentless crush of gravity.
13 Energy Balance: The rate at which energy radiates from the surface of the Sun must be the same as the rate at which it is released by fusion in the core.
14 initial gravitational contraction: Provided the energy that heated the core as Sun was forming Contraction stopped when fusion began.
15 Core: Energy generated by nuclear fusion ~ 15 million K Radiation zone: Energy transported upward by photons Convection zone: Energy transported upward by rising hot gas 10_01StructSun 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley Photosphere: Visible surface ~5,800 K
16 Chromosphere: Middle layer of solar atmosphere Corona: Outermost layer of solar atmosphere Solar wind: A flow of charged particles from the surface of the Sun 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley Little mass in these components
17 Why the Sun shines Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.
18 Solar structure From inside out, the layers are: Core Energy generation by fusion reactions Radiation zone Outward energy transport by photons Convection zone Outward energy transport by gas motions Photosphere Luminous surface (energy emitted to space)» absorption spectrum Chromosphere low density upper atmosphere» emission spectrum, but much fainter than photosphere Corona very hot, low density plasma tailing off into space
19 Photosphere is what we see limb darkening is a geometric effect 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley
20 (thermal emission without absorption lines)
21 The photosphere produces an absorption spectrum
22 Composition determined from absorption lines Abundance (by number relative to Hydrogen) Atomic number (number of protons) 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley By mass, the sun is 3/4 Hydrogen 1/4 Helium < 2% everything else This is typical of the universe; Earth is an exception. Helium discovered in the spectrum of the sun (hence the name)
23 solar corona solar chromosphere only visible during eclipse - greatly outshown by photosphere normally seen in Hα emission
24 emission line spectrum
25 Nuclear fusion powers the Sun
26 Fission Big nucleus splits into smaller pieces. (Example: nuclear power plants) Fusion Small nuclei stick together to make a bigger one. (Example: the Sun, stars)
27 High temperatures enable nuclear fusion to happen in the core.
28 High temperatures enable nuclear fusion to happen in the core. Positively charged protons repel each other. Fusion only happens when the strong nuclear force is stronger than this repulsion, which only happens at very small separations. High temperatures are required to move fast enough to get that close Pearson Education Inc., publishing as Pearson Addison-Wesley
29 High temperatures enable nuclear fusion to happen in the core. Positively charged protons repel each other. Fusion only happens when the strong nuclear force is stronger than this repulsion, which only happens at very small separations. High temperatures are required to move fast enough to get that close Pearson Education Inc., publishing as Pearson Addison-Wesley
30 Four fundamental forces Gravity e.g, planetary orbits falling objects Electromagnetism attraction and repulsion of electric charges magnets Strong nuclear force fusion: binds protons together in atomic nuclei Weak nuclear force fission; radioactive decay 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley
31 The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. Fusion is driven by the strong nuclear force after gravity heats a star s core enough to overcome the electrostatic repulsion of protons.
32 Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. Starting point is 4 protons. End point is 2 p + 2 n (a helium nucleus) + energy There are several steps required to make this happen Pearson Education Inc., publishing as Pearson Addison-Wesley
33 The proton proton chain is how hydrogen fuses into helium in Sun.
34 Proton proton chain is how hydrogen fuses into helium in Sun step 1: p + p makes D (deuterium) step 2: D + D makes 3 He (helium 3) step 3: 3 He + 3 He makes 4 He (helium 4) plus energy plus 2 spare protons and neutrinos. The first step is the hardest - on average, takes 10,000,000 years to occur in the sun Pearson Education Inc., publishing as Pearson Addison-Wesley
35 Net Result: IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos E = mc 2 : Total mass is 0.7% lower Pearson Education Inc., publishing as Pearson Addison-Wesley
36 Energy transport through the Sun Energy is produced in the core Transported through Radiative Zone by photons Convective Zone by gas motions Nature chooses whichever is more efficient
37 Energy transport through the Sun Radiative Zone Energy gradually leaks out of radiation zone in form of randomly bouncing photons.
38 Energy transport through the Sun Convective Zone Convection (rising hot gas) takes energy to surface.
39 Bright blobs on photosphere show where hot gas is reaching the surface.
40 Solar activity
41 Solar activity is like "weather". Sunspots Solar flares Solar prominences All these phenomena are related to magnetic fields.
42 Sunspots Are cooler than other parts of the Sun's surface (4000 K) Are regions with strong magnetic fields
43 Loops of bright gas often connect sunspot pairs.
44 Magnetic activity causes solar flares that send bursts of X rays and charged particles into space.
45 Magnetic activity also causes solar prominences that erupt high above the Sun's surface.
46 The corona appears bright in X- ray photos in places where magnetic fields trap hot gas.
47 Coronal mass ejections send bursts of energetic charged particles out through the solar system.
48 Charged particles streaming from the Sun can disrupt electrical power grids and can disable communications satellites.
49 How does solar activity vary with time?
50 The number of sunspots rises and falls in an 11-year cycle.
51 The sunspot cycle has something to do with winding and twisting of the Sun's magnetic field.
52 Probing the Solar Interior
53 We learn about the inside of the Sun by making mathematical models observing solar vibrations observing solar neutrinos
Today The Sun Events Last class! Homework due now - will count best 5 of 6 Final exam Dec. 20 @ 12:00 noon here Review this Course! www.case.edu/utech/course-evaluations/ The Sun the main show in the solar
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