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The Sun – Our Nearest Star Surrounding the visible photosphere is the chromosphere. What you see in this next video clip is the chromosphere. Each of the granules you see are about the size of Texas. Granules change in a churning kind of motion. Chromosphere Convection and Granules The surface of the sun is 5000o C , the temperature of the corona is 2,000,000o C. (It is not understood what causes this increase.) The process of joining small nuclei together to make a larger nucleus is called fusion. The process of breaking a large nucleus into smaller nuclei is called fission. Fusion Fission H Fe U Because it has the most binding energy per nucleon, iron (Fe) is the most stable element. H Fe U The Proton-Proton Chain – How Our Sun "Burns" The three step proton-proton cycle can be written symbolically as: 1) 11H 11H 21H e e 2) 21H 11H 23He and e e 3) 23He 23He 42 He 2 11H Notice that the first two reactions must occur twice in order to produce the two Helium-3 nuclei needed for the third reaction. We can write the net process as: 4 11H 42 He 2 2 ENERGY Proton-proton Cycle Solar Fusion In the proton–proton chain, a total of six protons (and two electrons) are converted to two protons, one helium-4 nucleus, and two neutrinos. Neutrinos are chargeless (i.e. neutral), very light – almost massless – particles. The two leftover protons are available as fuel for new proton–proton reactions, so the net effect is that four protons are fused to form one Helium-4 nucleus. Energy, in the form of gamma rays, is produced at each stage. The energy given off in this reaction can be found by determining the amount of mass converted to energy. This is found by finding the difference in mass between the two sides of the equation and multiplying it by 931.5 MeV per atomic mass unit (i.e. E=mc2). For example, the mass of a proton is 1.007825 u, while the mass of the helium-4 nucleus is 4.002603 u. So we have Q (mi mf ) c2 4 (1.007825 u) 4.002603 u 931.5 MeV 26.7 MeV Since the Sun radiates about 2 x 1039 MeV/s, that means there are about 1038 fusion reactions per second, "consuming" 4 x 1038 protons per second. So the mass of the Sun decreases by about 670 billion kilograms per second, or almost 1.5 trillion pounds, due to the fusion process! Don't worry…the Sun has about 1057 protons - enough to burn for the next few billion years. The Solar Neutrino Problem Back when your instructor was working on his undergraduate degree, one of the biggest puzzles in high energy physics and astrophysics was that the number of solar neutrinos reaching Earth was substantially less (by 30 to 50 percent) than the prediction of the Standard Solar Model. This discrepancy was known as the solar neutrino problem. If neutrinos do have a minute amount of mass, theory indicates that it may be possible for them to change their properties, even to transform into other particles, during their eight-minute flight from the solar core to Earth, through a process known as neutrino oscillations. So neutrinos could turn into something else (in particle-physics jargon, they are said to “oscillate” into other particles) on their way to Earth and so go undetected. In June 1998 the Japenese group operating the Super Kamiokande detector reported the first experimental evidence of neutrino oscillations. Then in June 2001, measurements made at the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, revealed strong evidence for the type of neutrinos into which the Sun’s neutrinos have been transformed. The total numbers of neutrinos observed were completely consistent with the Standard Solar Model. Apparently the solar neutrino problem has at last been solved! The dominant thermonuclear reaction in a star changes with temperature. For example, common reactions include: MIN. TEMP. REACTION 8 x 106 K proton-proton chain 20 x 106 K CNO cycle 100 x 106 K triple alpha 600 x 106 K carbon-helium fusion 109 K carbon burning The CNO Cycle These six steps are termed the CNO cycle. Aside from the products of radiation, positrons and neutrinos, notice that no carbon is actually consumed in this process! 1) 12 C 11H 13 N energy 2) 13 N 13 C e e 3) 13 C 11H 14 N energy 4) 14 N 11H 15 O energy 5) 15 O 15 N e e 6) 15 N 11H 12 C 4 42 He 12 C 4 11H 12 C 42 He 2 e 2 e ENERGY ( rays ) Triple-Alpha Process This process of Helium fusion in stars is called the triple-alpha process. The helium nucleus (4He) is known as an alpha particle. 1) 4 He 4 He 8 Be energy 2) 8 Be 4 He 12 C energy 3 4 He 12 C energy Carbon-Helium Fusion At much hotter temperatures, the star may actually combine carbon and helium to make oxygen. This is called carbon-helium fusion. 12 C 4 He 16 O ENERGY ( rays ) Carbon Burning Some very large stars may become hot enough and have the right conditions to facilitate the carbon burning process. For example: 12 C 12 C 24 Mg energy 12 C 12 C 23 Na 11H energy 12 C 12 C 20 Ne 4 He energy 12 C 12 C 23 Mg 01n energy 12 C 12 C 16 O 2 4 He energy Carbon burning requires a temperature of about a billion Kelvin to overcome the vary large repulsion between the two positively charged carbon nuclei. The Solar Cycle A sunspot is an Earth-sized dark blemish found on the surface of the Sun. The dark color of the sunspot indicates that it is a region of lower temperature than its surroundings. Sunspots are caused by magnetic disturbances that occur in the Sun. They are magnetic storms on the Sun. The average number of sunspots reaches a maximum every 11 or so years, then falls off almost to zero before the cycle begins afresh. The latitudes at which sunspots appear vary as the sunspot cycle progresses. Individual sunspots do not move up or down in latitude, but new spots appear closer to the equator as older ones at higher latitudes fade away. In fact, the 11-year sunspot cycle is only half of a 22-year solar cycle. For the first 11 years of the solar cycle, the leading spots of all sunspot pairs in the same solar hemisphere have one magnetic polarity, and spots in the other hemisphere have the opposite magnetic polarity. These polarities then reverse for the next 11 years. Solar Cycle Sunspot Cycle (a) Annual number of sunspots throughout the twentieth century, showing fiveyear averages of annual data to make long-term trends more evident. The (roughly) 11-year solar cycle is clearly visible. At the time of minimum solar activity, hardly any sunspots are seen. About four years later, at the time of maximum solar activity, as many as 200 spots are observed per year. (b) Sunspots cluster at high latitudes when solar activity is at a minimum. They appear at lower and lower latitudes as the number of sunspots peaks. They are again prominent near the Sun’s equator as solar minimum is again approached. The most recent solar maximum occurred in 2001. Prominences, Flares, and Cornonal Mass Ejections Sunspots are relatively gentle aspects of solar activity. However, the photosphere surrounding them occasionally erupts violently, spewing forth into the corona large quantities of energetic particles. The sites of these explosive events are known simply as active regions. A prominence is a loop or sheet of glowing gas ejected from an active region on the solar surface, which then moves through the inner parts of the corona under the influence of the Sun's magnetic field. Magnetic instabilities in the strong fields found in and near sunspot groups may cause the prominences, although the details are still not completely understood. Flares are another type of solar activity observed near active regions. Also the result of magnetic instabilities, flares are much more violent (and even less well understood) than prominences. So energetic are these cataclysmic explosions that some researchers have likened them to bombs exploding in the lower regions of the Sun’s atmosphere. Unlike the trapped gas that makes up the characteristic loop of a prominence, the particles produced by a flare are so energetic that the Sun’s magnetic field is unable to hold them and shepherd them back to the surface. Instead, the particles are simply blasted into space by the violence of the explosion. The Sun rotates about every 27 days. Solar Activity and Rotation Solar Activity I Solar Activity II Solar Prominence Solar Flares in an Active Region of the Sun A coronal mass ejection from the Sun is sometimes (but not always) associated with flares and prominences. These phenomena are giant magnetic “bubbles” of ionized gas that separate from the rest of the solar atmosphere and escape into interplanetary space. Carrying an enormous amount of energy, they can—if their fields are properly oriented—connect with Earth’s magnetic field, dumping some of their energy into the magnetosphere and potentially causing communications and power disruptions on our planet. Such ejections occur about once per week at times of sunspot minimum, but up to two or three times per day at solar maximum. Coronal Mass Ejection Auroral StormI Auroral Storm II Aurora