Now we begin…..

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… PhotospherePhotosphere: Segment of star that emits light. Typically defined to be the region down to an optical depth of 2/3. ChromosphereChromosphere: 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 H  wavelength. Temperature of Chromosphere is up to 20,000K. Transition RegionTransition Region: In the Sun, a region between the Chromosphere and Corona. CoronaCorona: 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 RegionHeightDensityTemperature Interior<-2000km -500km g/cm 3 15,000K+ 8500K Photosphere-500km- 0km g/cm 3 0.5*10 -6 g/cm K 5770K Chromosphere0 km 2100km 0.5*10 -6 g/cm g/cm K 5770K Transition Region 2100km 2200km g/cm g/cm K 50,000K Corona2200km 18000km g/cm 3 5* g/cm 3 50,000K 1,800,000K

Description of the Radiation Field Specific and Mean Intensity Specific Intensity Mean Intensity Energy Density Blackbody radiation is isotropic and has

Description of the Radiation Field The Specific Energy Density of a Black Body Radiation Field

Specific Radiative Flux Net Energy having a wavelength between  and  d 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/r 2

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????temperature What is Thermodynamic Equilibrium???Thermodynamic Equilibrium

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 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