L6GH09SunSmallSSIntro

L6GH09SunSmallSSIntro - The Sun structure, energy sources...

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January 20, 2009 Sun - overview The Sun – structure, energy sources Small SS Bodies - Intro Lecture topics: the sun – energy sources, internal structure; intro to small SS bodies Text readings: Chapter 4; again resource = mostly this lecture; chapter 6: pp 127-132; chapter 7: pp 155-162
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January 20, 2009 Sun - overview Radiation pressure • Force due to radiation is maximal for particles about 0.1-1 μ m in size •These ±part ic les ±are ± comparable in size to the wavelength of light, thus may scatter light rather than absorb it. • Thus radiation pressure is greater for dark particles, which are more likely to absorb photons. • Remember, the photons are exerting a force: more photons absorbed => greater force F
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January 20, 2009 Sun - overview The sun as a star The sun is a “middling” star – • not the biggest or smallest • not the brightest or faintest • not the hottest or coolest • not the most massive or least massive Range of properties for normal stars: 9 radius: ~10 -2 –10 3 R sun 9 luminosity: ~10 -4 –10 6 L sun 9 temperature: ~3000-30000K (sun is ~6000K) 9 mass: ~0.1-50 M sun
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January 20, 2009 Sun - overview Star Sizes measure: temperature from spectrum measure: distance using parallax measure: apparent magnitude This gives the luminosity L=4 π R 2 σ T 4 measure star’s temperature measure star’s luminosity calculate star’s radius more on properties of stars later in the course
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January 20, 2009 Sun - overview Hydrostatic equilibrium: structure of interiors The force of gravity is always directed toward the centre of the star. Why does it not collapse? ¾ The opposing force is the gas pressure. As the star collapses, the pressure increases, pushing the gas back out. • How must pressure vary with depth to remain in equilibrium?
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January 20, 2009 Sun - overview Hydrostatic equilibrium: interiors Consider a small cylinder at distance r from the centre of a spherical star. Pressure acts on both the top and bottom of the cylinder. ¾ By symmetry the pressure on the sides cancels out dr A dm F P,b F P,t 2 r GM dr dP r ρ = • It is the pressure gradient that supports the star against gravity • The derivative is always negative. Pressure must get stronger toward the centre
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Sun - overview Stellar Structure Equations 2 r GM dr dP r ρ = ρπ 2 4 r dr dM r = H m kT P μ = Hydrostatic equilibrium: pressure/gravity balance Mass conservation: “no “gaps” Equation of state: ideal gas for most stars Z Y X i 2 1 4 3 2 1 + + • These equations can be combined to determine the pressure or density as a function of radius, if the temperature gradient is known ¾ This depends on how energy is generated Y=%He; Z=%everything else
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This note was uploaded on 04/15/2009 for the course PHYS 275 taught by Professor Harris during the Winter '09 term at Waterloo.

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L6GH09SunSmallSSIntro - The Sun structure, energy sources...

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