2 A Closer Look at the Sun Our goals for learning Why was the Sun s energy source a major mystery? Why does the Sun shine? What is the Sun s structure?
3 Why was the Sun s energy source a major mystery?
4 Scientists knew the Sun had existed and shone for millions of years. Could calculate the Sun s daily energy output Could not work out what fuel source could provide so much power for so long
5 Is it on FIRE? NO! Chemical Energy Content Luminosity ~ 10,000 years Is it CONTRACTING? NO! Gravitational Potential Energy Luminosity ~ 25 million years It can be powered by NUCLEAR ENERGY! Nuclear Potential Energy (core) Luminosity E=mc 2 ~ 10 billion years
6 Why does the Sun shine?
7 Composition of the Sun The Sun is a plasma - an extremely hot gas The atoms electrons are ionized - stripped away from the nucleus The Sun is made mostly of: Hydrogen, the smallest element (~70%) Helium, the 2nd smallest element (~30%) Atom Ion Atom Ion
8 Fusion occurs in the innermost part of the Sun because the weight of upper layers compresses then crushes lower layers
9 Gravitational contraction: Provided energy that heated core as Sun was forming. Intense heat eventually forced fusion. Contraction stopped when fusion began
10 Stable Star Sun s core fuses hydrogen into helium, releasing energy into the outer layers of the Sun. The energy causes gas expansion, pushing outward against the gravity pulling inward. This expansion is called thermal pressure. When the pressures balance, a star is in gravitational equilibrium.
11 What have we learned? Why was the Sun s energy source a major mystery? Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years Why does the Sun shine? The Sun shines because gravitational equilibrium keeps its hydrogen core hot and dense enough to release energy through nuclear fusion.
12 Nuclear Fusion in the Sun Our goals for learning How does nuclear fusion occur in the Sun? How does the energy from fusion get out of the Sun? How do we know what is happening inside the Sun?
13 How does nuclear fusion occur in the Sun?
14 Fission Fusion Big nucleus splits into smaller pieces Small nuclei stick together to make a bigger one (Nuclear power plants) (Sun, stars)
15 Fusion occurs in the innermost part of a star because the weight of upper layers compresses then crushes lower layers
16 High temperatures cause particles to smash together. Enables nuclear fusion to happen in the core
17 Sun releases energy by fusing four hydrogen nuclei into one helium nucleus
18 The Complete Fusion Reaction Proton-proton chain is how hydrogen fuses into helium in Sun
19 IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos ENERGY Total mass is 0.7% lower
20 Solar Thermostat The energy released from fusion causes the sun s gases to expand (thermal expansion). The balancing act between thermal expansion pushing out and gravity contracting inward, forces the temperature to stay constant.
21 Solar Thermostat If temperature falls because fusion rate drops : Thermal pressure drops, gravity gains =>core contracts and heats back up If temperature rises because fusion rate increases: Thermal pressure increases over gravity. =>core expands, which causes core to cool down
22 Solar Thermostat Therefore, gravitational equilibrium, causes thermal equilibrium, as long as a star s core continues to fuse. Sun is estimated to do this for another 5 billion years!
23 What is the net fusion reaction that produces energy in the core of the Sun? a) 4 hydrogen nuclei form 1 helium nucleus plus energy. b) 2 hydrogen nuclei form 1 helium nucleus plus energy. c) 6 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus plus energy. d) 3 hydrogen nuclei form 1 helium nucleus plus energy. e) 4 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus, plus energy Pearson Education, Inc.
24 What balances the inward push of gravity inside the Sun? a) b) c) d) the rigidity of the solid central core electron degeneracy pressure thermal pressure of the gas neutron degeneracy pressure 2014 Pearson Education, Inc.
25 What have we learned? How does nuclear fusion occur in the Sun? The core s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature
26 What is the Sun s structure?
27 Sun Contains Core Radiation Zone Convection Zone Photosphere interior Chromosphere Corona Solar wind Atm.
28 Core: Energy generated by nuclear fusion ~ 15 million K
29 Radiation Zone: Energy transported upward by photons
30 Convection Zone: Energy transported upward by rising hot gas
31 Photosphere: Visible surface of Sun ~ 6,000 K
32 Chromosphere: Middle layer of solar atmosphere ~ K
33 Absorption lines due to the Sun s atmosphere
34 Corona: Outermost layer of solar atmosphere ~1 million K Emits x-rays and streamers that follow the magnetic lines
35 Solar wind: The corona dissipates, becoming a flow of charged particles from the surface of the Sun
36 How does the energy from fusion get out of the Sun?
37 Energy moves randomly around the radiation zone in form of randomly bouncing photons. Eventually it makes it to the edge of the radiative zone and into the convective zone
38 Convection (rising hot gas) takes energy to surface
39 Bright blobs on photosphere are where hot gas is reaching the surface (effect is called granules, or granulation)
40 How we know what is happening inside the Sun?
41 We learn about inside of Sun by Making mathematical models Observing solar vibrations Observing solar neutrinos
42 Patterns of vibration on surface tell us about what Sun is like inside The vibrations reflect, reinforce or cancel each other which changes the patterns and frequencies seen at the Sun s surface.
43 Data on solar vibrations agree very well with mathematical models of solar interior
44 Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what s happening in core
45 Which of the following layers of the Sun is farthest from the core? a) b) c) d) convection zone photosphere chromosphere corona 2014 Pearson Education, Inc.
46 What have we learned? What is the Sun s structure? From inside out, the layers are: Core Radiation Zone Convection Zone Photosphere Chromosphere Corona Solar wind
47 What have we learned? How does the energy from fusion get out of the Sun? Randomly bouncing photons carry it through the radiation zone Rising of hot plasma carries energy through the convection zone to photosphere How do we know what is happening inside the Sun? Mathematical models agree with observations of solar vibrations and solar neutrinos
48 The Sun-Earth Connection Our goals for learning What causes solar activity? How does solar activity vary with time? How does solar activity affect humans?
49 What causes solar activity?
50 Solar activity is like weather Sunspots Solar Flares Coronal Mass Ejections (CMEs) Solar Prominences All are related to magnetic fields
52 Sunspots Are cooler than other parts of the Sun s surface (4000 K) Are regions with strong magnetic fields
53 Charged particles spiral along magnetic field lines, emitting light as they go: x-ray light.
54 Sun at Multiple Wavelengths Microwave Visible Ultra-violet X-Ray
55 Magnetic field lines pierce the surface Loops of bright gas often connect sunspot pairs
57 The Corona appears bright in X-ray photos in places where magnetic fields trap hot gas
58 Magnetic activity can cause solar prominences that erupt high above the Sun s surface The particles associated with the prominance can be flung out into space making a Coronal Mass Ejection
59 Magnetic activity also causes solar flares that send bursts of X-rays into space
60 How does solar activity vary with time?
61 Number of sunspots rises and falls in 11-year cycle
62 Sun in X-Rays over Solar Cycle
63 Sunspot cycle has something to do with winding and twisting of Sun s magnetic field
64 The magnetic lines erupt out and can snap. The snapping lines fling particles or energy (or both) out through the solar system Bursts of charged particles are Coronal Mass Ejections Bursts of x-rays are Solar Flares
65 Coronal Mass Ejection or Solar Flare
67 How does solar activity affect humans?
69 Solar Flare Headed toward Earth
70 Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites
71 Alaska Aurora
72 What have we learned? What causes solar activity? Stretching and twisting of magnetic field lines near the Sun s surface causes solar activity. Activity detectable as sunspots How does solar activity vary with time? Activity rises and falls with an 11-year period How does solar activity affect humans? Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation
73 Live Solar Observation at the Big Bear Solar Observatory Daily Space weather and geostorm warning
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