Lecture5

# Lecture5 - Clearing of Small Bodies Radiation pressure from...

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Clearing of Small Bodies Radiation pressure from the Sun can overcome gravity for small, rocky particles, and push them out of the solar system ¾ More effective for dark particles, because bright particles tend to scatter light rather than absorb it. This is different from the behaviour of a (large) solar sail. Larger particles are not easily pushed by radiation pressure, but are subject to other effects: Poynting-Robertson effect: ¾ Particles moving through the solar system are continually impacted by photons, mostly on their leading face. ¾ Thus they lose orbital energy and slowly spiral into the Sun (but process is complex) Yarkovsky effect: ¾ Sunlight warms one side of a larger body. ¾ The warm side rotates away from the Sun and radiates thermal energy as photons which provide a “thrust” ¾ This can move particles either in or out

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Wave nature of Light Light travels as an electromagnetic wave, with a speed (in vacuum) of c=2.99792456×10 8 m/s Wavelength ( λ ) and frequency ( ν ) are related by λν = c ν h E = The energy of a photon is ¾ bluer light is more energetic A blackbody is an ideal object that absorbs all incident radiation ¾ To remain in equilibrium it must also be a perfect emitter. ¾ The spectral energy distribution of emitted light from a blackbody is a well-known function with the following properties: ¾ The wavelength λ at which most radiation is emitted by a blackbody is given by ¾ And the total energy rate (L) emitted by a blackbody with area A increases with temperature T according to , where σ =5.67x10 4 T A L σ = -8 kg s -3 K -4 is the Stefan-Boltzmann constant. K m 00290 . 0 max = T λ Spectroscopy can be used to find the chemical composition, and radial velocity of a source.
Absorption and Emission lines Spectra of real objects are generally not like pure blackbodies, but show absorption and/or emission lines Absorption and emission lines are caused by electrons changing orbital levels

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Lecture5 - Clearing of Small Bodies Radiation pressure from...

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