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10/24/11 Overview: The Sun • Properties of the Sun • Sun’s outer layers The outer layers • Photosphere – Photosphere – Chromosphere – Corona • Chromosphere • Solar Activity – Sunspots & the sunspot cycle – Flares, prominences, CMEs, aurora • Corona • Sun’s Interior – The Sun as an energy source – Fusion in the Sun Photosphere: Visible Surface • Innermost visible layer, only 400 km thick (0.06% of radius) • Temperature: 6,000 K • Defines “size” of the Sun Photosphere isn’t actually smooth! Features called granules are about 1000 km across • Absorption lines Convection causes granules Granules Up-Close: like boiling water • Convection: transfer of energy through currents • Dark-colored areas => cooler gas • Light-colored areas => hotter gas 1 10/24/11 Chromosphere: “sphere of color” Middle layer, temperature 104 - 105 K Chromosphere: Hot hydrogen atoms form a red streak when viewed during a solar eclipse Corona or “crown” of the Sun • Very low density Solar activity is like “weather” • • • • – 10 million to 10 billion times less dense than Earth’s atmosphere • However, corona is extremely hot! – About 1 million K – Known from spectrum Sunspots Solar Flares Solar Prominences Coronal Mass Ejections (CMEs) • All are related to magnetic fields, vary on an 11-year cycle Sunspots Dark spots on the photosphere Cooler than other parts of the Sun’s surface (4000K vs. 5800K) Sunspots • One of Galileo’s major observations • More than 30 sketches from summer of 1612 2 10/24/11 Sunspot Number • Number & locations change daily • Can use observations to determine how fast the Sun spins on its axis Sunspot number peaks about every 11 years: the sunspot cycle Data goes back to the 1600’s Solar Maximum Solar Minimum Sunspot Cycle Sunspots often come in pairs • Sunspots usually appear within 30º of equator • Where they form changes during a cycle • One is positive, the other negative, like a magnet “Butterfly Diagram” • Connected by loops of bright gas We think most solar activity is related to changing magnetic fields Fig. 7-14a, p. 135 3 10/24/11 Other Solar Activity: • Solar Prominences • Solar Flares • Coronal Mass Ejections All occur in “active regions” associated with strong magnetic fields Fig. 7-14e, p. 135 Solar Prominences Solar Flares Solar Flares • Erupt high above the Sun’s surface • Bursts of Xrays and charged particles • Can be quite stable (hoursdays) • 5-10 minutes • Usually form near sunspots! Coronal Mass Ejections Rare bursts, more energetic than flares or prominences Speeds up to 1,000 km/s 50,000 km above Sun’s surface Solar Wind • Not moving air, like on Earth • Stream of charged particles from coronal holes • 900,000 mph, reaches Earth in about 4 days 4 10/24/11 Solar Wind Charged particles get trapped in Earth’s magnetic field and cause auroras The Sun Today www.spaceweather.com Structure of the Sun Core Corona Core: Energy generated by nuclear fusion Inner 25% ~ 15 million K Very dense! Radiation Zone Radiation Zone: Energy transported upward by photons Convection Zone Convection Zone: Energy transported upward by rising hot gas 106-107 K Outer 30% Photons spend a long time here 104-106 K 5 10/24/11 Convection causes granules Convection (rising hot gas) takes energy to surface How does the Sun shine? What is the Sun’s structure? • From inside out, the layers are: • Core • Radiation Zone • Convection Zone • Photosphere • Chromosphere • Corona Luminosity of the Sun • Energy output: 3.9 x 1024 Joules/sec • A 100-Watt light bulb emits 100 Joules per second The Sun has its own energy source – Main difference between a star and a planet – Not well understood until 1940’s • Need to explain lifetime & luminosity Lifetime of the Sun • Need a vast, constant source of energy • Sun is at least 4.6 billion years old (from fossils) • Most ideas could not sustain the energy rate needed 6 10/24/11 Nuclear Power on Earth Nuclear Fission: breaking large nuclei This is what happens in nuclear power plants. Nuclear Fusion: combining small nuclei This has only been used for weapons. San Onofre Fission Plant, CA Hydrogen (fusion) bomb, Pacific Ocean, 1962 Fission Fusion Big nucleus splits into smaller pieces Small nuclei stick together to make a bigger one (Nuclear power plants) (Sun, stars) High temperature enables nuclear fusion to happen in the core: Energy from the Sun: • The Sun has hydrogen at its core, at ~15 million K • Basic Particles: Overcome electrical repulsion – Protons (in nucleus), positive charge – Neutrons (in nucleus), no charge – Electrons, negative charge • Particles in nucleus have binding energy Core is so hot, the hydrogen is in plasma form: the electrons are “free” Low Speed Interaction High Speed Interaction Nuclear Fusion • 4 hydrogen nuclei (protons) must collide – Not very likely • Helium nucleus is built up in steps • This sequence of steps is called the protonproton chain 7 10/24/11 Where Stars Get Their Energy IN 4 protons End product of fusion process is a helium nucleus OUT nucleus 2 gamma rays 2 positrons 2 neutrinos 1 He is less massive than 4 H by a factor of 0.007 (0.7%) Where did that mass go? 4He E =mc2 Total mass is 0.7% lower c is the speed of light c = 3 x 10 8 m/s so c2 = 9 x 10 16 !!!!! A small amount of mass can produce a lot of energy Proton-proton chain has 3 steps Fusion in the Sun • P-P chain • 3 steps: 2 1H -> 2H + e+ + ν 2H + 1H -> 3He + γ 3He (14 billion years) (6 seconds) + 3He -> 4He + 2 1H (1 million years) Most of the stuff in the Sun has not yet undergone fusion! (and that’s a good thing…) How does the energy from fusion get out of the Sun? 8