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Leacture 11.docx

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SCI 238
Mike Fich

Leacture 11 Why does the Sun shine?  Nuclear fusion requires extremely high temperatures and densities  Recall that our Sun was born about 41 billion years ago from a collapsing cloud of interstellar gas. The contraction of the cloud released gravitational potential energy, raising the interior temperature and pressure. With- out another source of energy to replace the energy output from its surface, the Sun continued to contract until its central temperature finally rose high enough to sustain nuclear fusion. Then the Sun was finally able to replace the energy lost from the surface and stopped contracting.  2 kinds of balance to make sun stable: o gravitational equilibrium (or hydrostatic equilibrium), is between the outward push of internal gas pressure and the inward pull of gravity. A stack of acrobats provides a simple example of gravitational equilibrium  The Sun’s internal pressure precisely balances gravity at every point within it, thereby keeping the Sun stable in size  the pressure must increase with depth. Deep in the Sun’s core, the pressure makes the gas hot and dense enough to sustain nuclear fusion. The energy released by fusion, in turn, heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity. o energy balance between the rate at which fusion releases energy in the Sun’s core and the rate at which the Sun’s surface radiates this energy into space.  without it the balance between pressure and gravity would not remain steady. If fusion in the core did not replace the energy radiated from the surface, thereby keeping the total thermal energy content constant, then gravitational contraction would cause the Sun to shrink and force its core temperature to rise.  Summary : 41 billion years ago gravitational contraction made the Sun hot enough to sustain nuclear fusion in its core. Ever since, energy liberated by fusion has maintained gravitational equilibrium and energy balance within the Sun, keeping it shining steadily and supplying the light and heat that sustain life on Earth. Sun’s structure (out to in) 1. solar wind—the stream of charged particles continually blown outward in all directions from the Sun. Recall that the solar wind helps shape the magnetospheres of planets and blows back the material that forms the plasma tails of comets 2. outermost layer of this atmosphere, called the corona a. highest temp b. emits most X-ray c. low density 3. Nearer the surface, the temperature suddenly drops to about 10,000 K in the chromosphere, the middle layer of the solar atmosphere that radiates most of the Sun’s ultraviolet radiation. a. Reddish color due to strong H emission b. Less dense and less bright than photosphere 4. Lowest layer of the atmosphere, the photosphere, which is the visible surface of the Sun. Although the photosphere looks like a well-defined surface from Earth, it consists of gas far less dense than Earth’s atmosphere. (6000K) a. Sunspots i. Migrate in latitude over the 11-year cycle b. Intense magnetic fields c. Granulation – convection (hotter gar rise, cooler gas sinks) 5. convection zone, where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection 6. calmer plasma of the radiation zo
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