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Unformatted text preview: Ast 307  Oct. 26 , 2010 ch. 16: Our Star, The Sun Thermal Equilibrium (from 10/12) Consider a solid body orbiting the Sun. It is heated by absorbing some of the sunlight that falls on it. What happens to the sunlight that it doesnt absorb? If it behaves like a blackbody (obeys the laws of bb radiation), it radiates energy at a rate given by: P out = Surface Area x ! T 4 = 4 ! R 2 ! T 4 (for a sphere) The temperature will come to equilibrium if the radiated energy output (energy per second = power) equals the absorbed energy per second. Calculating the equilibrium temperature The absorbed energy is the flux of sunlight multiplied by the projected area of the rock that faces the Sun. P in = A face x F sunlight = ! R 2 F sunlight The strength (flux) of the sunlight falling on it is determined by the inversesquare law of light: F sunlight = L sun / (4 ! d 2 ) where d is the distance from the Sun (in what units?) and L sun is a known quantity. To find T eq : Do the math For a sphere, A face /A surface = ! , because the incident light is virtually parallel &amp; sees a projected area of &quot; R 2 ....
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This note was uploaded on 04/30/2011 for the course AST 317 taught by Professor Dinerstein during the Fall '10 term at University of Texas at Austin.
 Fall '10
 Dinerstein
 Astronomy

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