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Now we begin….. http://sdo.gsfc.nasa.gov/gallery/potw.php?v=item&id=87 Stellar Atmospheres I • • • The Description of the Radiation Field Stellar Opacity Radiative Transfer Stellar Atmosphere • Light comes from the Star’s Atmosphere • Atmosphere is the layers of gas overlaying opaque interior of star • Flood of photons pour from these layers • releasing energy produced by the thermonuclear reactions, gravitational contraction and cooling in the star’s center • • The temperature, density and composition of the atmospheric layers determine the features of the star’s spectrum No solid surface… Photosphere: Segment of star that emits light. Typically defined to be the region down to an optical depth of 2/3. Chromosphere: In the Sun, a thin layer just above the photosphere that is visually more transparent than the photosphere. The spectrum of the light generated here is dominated by Hwavelength. Temperature of Chromosphere is up to 20,000K. Transition Region: In the Sun, a region between the Chromosphere and Corona. Corona: In the Sun, a type of plasma atmosphere that extends millions of kilometers into space. High temperature. Temperatures/Densities in the Sun Results from Skylab http://history.nasa.gov/SP-402/p2.htm Region Height Density Temperature Interior <-2000km -500km 10-6 g/cm3 10-6 g/cm3 15,000K+ 8500K Photosphere -500km0km 10-6 g/cm3 0.5*10-6 g/cm3 8500K 5770K Chromosphere 0 km 2100km 0.5*10-6 g/cm3 10-13 g/cm3 5000K 5770K Transition Region 2100km 2200km 10-13 g/cm3 10-14 g/cm3 5770K 50,000K Corona 2200km 18000km 10-14g/cm3 5*10-18g/cm3 50,000K 1,800,000K Description of the Radiation Field Specific and Mean Intensity • Specific Intensity • • Energy Density Mean Intensity Blackbody radiation is isotropic and has Il = Bl http://spiff.rit.edu/classes/phys440/lectures/radfield/radfield.html Description of the Radiation Field The Specific Energy Density of a Black Body Radiation Field Specific Radiative Flux • Net Energy having a wavelength between andd that passes each second through unit area in the direction of the +z axis Resolved Source Flux independent of r Unresolved Source Flux decreases as 1/r2 Radiation Pressure • Photon momentum exerts pressure Blackbody Radiation Radiation Pressure is 1/3 of the Energy Density for Blackbody Radiation Stellar Opacity • Solar Spectrum What determines features of Spectrum? Stellar Opacity Temperature and Local Thermodynamic Equilibrium What is temperature anyhow???? What is Thermodynamic Equilibrium??? http://spiff.rit.edu/classes/phys440/lectures/lte/lte.html Stellar Opacity Definition of Opacity • • • • • Consider a beam of parallel light rays traveling through a gas. Any process that removes photons from this beam of light is called absorption Absorption includes Scattering!! True absorption is by electronic transitions in atoms (and sometimes molecules) Change in Intensity is Proportional to: distance traveled density of gas absorption coefficient Absorption/Attenuation Coeffiecient • Absorption coefficient -dI = aIdx I(x) = I0 exp(-ax) a = k lr Attenuation coefficient=(opacity) x (density) [density]=kg/m^3 [opacity]=m^2/kg [Attenuation coefficient]=1/m Optical Depth • • Unit-less Measure of amount of attenuation. Accounts for varying density [density]=kg/m^3 [opacity]=m^2/kg [Optical Depth]=[density][opacity][distance]=1 Optical Depth http://spiff.rit.edu/classes/phys440/lectures/optd/optd.html