now we begin…
TRANSCRIPT
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 SunResults from Skylabhttp://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 -500km-
0km
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 FieldSpecific and Mean Intensity
• Specific Intensity
• Mean Intensity
• Energy Density
Blackbody radiation is isotropic and has
http://spiff.rit.edu/classes/phys440/lectures/radfield/radfield.html
Description of the Radiation FieldThe 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 SourceFlux independent of r
Unresolved SourceFlux 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 OpacityTemperature and Local Thermodynamic Equilibrium
http://spiff.rit.edu/classes/phys440/lectures/lte/lte.html
What is temperature anyhow????
What is 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 traveleddensity of gasabsorption 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
http://spiff.rit.edu/classes/phys440/lectures/optd/optd.html